http://www.diva-portal.org

This is the published version of a paper published in Ecological Applications.

Citation for the original published paper (version of record):

Jansson, R., Nilsson, C., Dynesius, M., Andersson, E. (2000) Effects of river regulation on river-margin vegetation: a comparison of eight boreal rivers. Ecological Applications, 10(1): 203-224 http://dx.doi.org/10.2307/2640996

Access to the published version may require subscription.

N.B. When citing this work, cite the original published paper.

Permanent link to this version: http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-20829 Ecological Applications, 10(1), 2000, pp. 203±224 ᭧ 2000 by the Ecological Society of America

EFFECTS OF RIVER REGULATION ON RIVER-MARGIN VEGETATION: A COMPARISON OF EIGHT BOREAL RIVERS

ROLAND JANSSON,1 CHRISTER NILSSON,1,2 MATS DYNESIUS,1 AND ELISABET ANDERSSON1 1Landscape Ecology Group, Department of Ecology and Environmental Science, UmeaÊ University, SE-901 87 UmeaÊ, Sweden 2Department of Applied Science, Mid Sweden University, SE-871 88 HaÈrnoÈsand, Sweden

Abstract. Regulation and fragmentation by dams belong to the most widespread de- liberate impacts of humans on the world's rivers, especially in the Northern Hemisphere. We evaluated the effects of hydroelectric development by comparing the ¯ora of vascular plants in 200-m-long reaches of river margin distributed along eight entire rivers in northern Sweden. Four of these rivers were free-¯owing, and four were strongly regulated for hy- droelectric purposes. First, we compared species diversity per site between entire free- ¯owing and regulated rivers. To reduce the effects of natural, between-river variation, we compared adjacent rivers. One regulated river had lower plant species richness and cover than two adjacent free-¯owing ones, whereas two other parallel rivers, one regulated and another free-¯owing, did not differ signi®cantly. Second, river-margin vegetation responded differently to different types of regulated water-level regimes. Both along run-of-river impoundments, with small but daily water-level ¯uctuations, and along storage reservoirs, with large ¯uctuations between low water levels in spring and high levels in late summer and fall, the number of species and their cover per site were lower than along the free- ¯owing rivers. Regulated but unimpounded reaches were most similar to free-¯owing rivers, having lower plant cover per site, but similar numbers of species. For reaches with reduced discharge, evidence was mixed; some variables were lower compared to free-¯owing rivers whereas others were not. However, for the last two types of regulation, statistical power was low due to small sample sizes. Third, we classi®ed all plant species according to their dispersal mechanisms and tested whether they respond differently to different types of regulated water-level regimes. Three out of four types of regulation had higher proportions of wind-dispersed species, and two out of four had lower proportions of species without speci®c mechanisms for dispersal, compared to free-¯owing rivers, suggesting that dispersal ability is critical for persistence following regulation. Run-of-river impoundments had high- er proportions of long-¯oating species and species with mechanisms for vegetative dispersal, suggesting that water dispersal may still be important despite fragmentation by dams. Fourth, plant species richness and cover varied with both local factors, such as water-level regime, and regional factors, such as length of the growing season. Presence of clay and silt in the river-margin soil, preregulation position of the contemporary river margin, non- reservoir sites, low altitudes, and long growing seasons were associated with high plant species richness and cover. Key words: dams; dispersal capacity of river-margin plants; disturbance; fragmentation; plant species richness vs. water-level regime; reservoirs; riparian vegetation; river regulation, effects on vegetation; seed dispersal; Sweden, northern; vegetative dispersal.

INTRODUCTION to predict, because rivers are complex, dynamic eco- About two thirds of the freshwater ¯owing to the systems and river regulation changes hydrological and oceans is estimated to be controlled by dams (Petts geomorphological as well as biological variables. 1984, Naiman et al. 1993), and in the United States, Therefore, to understand the combined and ultimate Canada, Europe, and the former Soviet Union, 85 of effects of river regulation, a combination of long-term the 139 largest river systems, or 77% of the ¯ow, are studies of postregulation conditions (e.g., Nilsson et al. moderately or strongly affected by regulation (Dyne- 1997) and large-scale, quantitative comparisons of af- sius and Nilsson 1994). River-margin communities, fected and unaffected river systems (e.g., Johnson given their dependence on river hydrology (e.g., Day 1994, Wootton et al. 1996) is needed. However, such et al. 1988, Hughes 1990, Gregory et al. 1991, Naiman studies are rare, and more knowledge is required to and DeÂcamps 1997), inevitably change when river ¯ow provide a basis for better management and rehabilita- changes. However, the speci®c responses are dif®cult tion of river systems affected by hydroelectric schemes (Nilsson and Brittain 1996). Manuscript received 24 November 1997; revised 9 No- In a previous study (Nilsson et al. 1991a), we ana- vember 1998; accepted 19 January 1999. lyzed the effects of river regulation on river-margin 203 204 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1 vegetation by comparing one free-¯owing and one (Brown and Kodric-Brown 1977, Shmida and Wilson strongly regulated river. Plant cover and species rich- 1985). ness were lower in the regulated river. Furthermore, Fragmentation of rivers by dams may also affect most functional groups of species were more species plant dispersal that is effective along river corridors poor and none more species rich in the regulated river. (Guppy 1891±1893, Schneider and Sharitz 1988, TheÂ- Species richness was higher in sites with remnants of baud and Debussche 1991, Brock 1994). An example preregulation vegetation, and decreased with increas- of effective dispersal is the rapid spread of exotic plant ing height of the river margin. Nilsson et al. (1991a) species along river margins around the world (Rich- attributed overall differences between rivers to the hy- ardson et al. 1992, DeFerrari and Naiman 1994, de drological disruption caused by dams. A major dis- Waal et al. 1994, Planty-Tabacchi et al. 1996). Plant advantage of such a study is that it does not allow dispersal by water, or hydrochory, is important in struc- general conclusions about the effects in other rivers. turing riparian plant communities along rivers (Nilsson Although preregulation documentation suggested that et al. 1991b, Johansson and Nilsson 1993, Johansson river-margin vegetation was similar between rivers, we et al. 1996), but is obstructed by hydroelectric devel- cannot be certain to what degree differences were due opment. Dams are barriers for waterborne diaspores, to river regulation or represented natural variation be- and the reservoir surfaces between dams tend to be tween rivers. One way of testing the generality of the effective traps, since diaspores are likely to be washed previous results would be to repeat the study by com- ashore by wind and wave action. Furthermore, ¯ood paring adjacent free-¯owing and regulated rivers in pulses, which are the major events for hydrochory in other areas (Keddy 1989, Hargrove and Pickering 1992, free-¯owing rivers (Schneider and Sharitz 1988, Nils- Primack and Miao 1992b). Therefore, we compared son et al. 1991b), are reduced or absent. To test whether river-margin vegetation along ®ve other rivers, three plants with different dispersal modes respond differ- free ¯owing and two regulated, to test if similar dif- ently to regulation, we compared the proportions of ferences in river-margin vegetation would emerge. species with different dispersal traits among the four major types of regulated water-level regime with sites Although the combined effects of river regulation in free-¯owing rivers. may be dif®cult to predict, the time since regulation The aims of the study were (1) to test the generality (Petts 1984, Church 1995), the disturbances from wa- and validity of the comparison (Nilsson et al. 1991a) ter-level ¯uctuations (Keddy and Reznicek 1986, Nils- of one free-¯owing and one regulated river by com- son and Keddy 1988), and the conditions for coloni- paring other free-¯owing and regulated rivers; (2) to zation and establishment on the regulated river margins test whether river-margin vegetation responds differ- (Fenner et al. 1985, Hughes 1990, Rood and Mahoney ently to different types of regulation; (3) to test whether 1990, Krahulec and LepsÏ 1994) are likely to be im- species with different mechanisms for dispersal re- portant factors. In a previous study we investigated the spond differently to river regulation and fragmentation; long-term development of river-margin vegetation and (4) to understand which environmental factors are along regulated waterbodies (Nilsson et al. 1997). Fur- the most important in governing river-margin vegeta- thermore, river-margin plant communities may respond tion along both free-¯owing and regulated rivers. Fi- differently to different kinds of water-level regimes. nally, the fundamental differences in river-margin veg- For example, changing the frequency or the timing of etation between free-¯owing and regulated boreal riv- water-level ¯uctuations may produce different effects. ers are outlined and their implications discussed. We tested this in the present study by comparing reg- ulated sites experiencing four different types of arti- STUDY AREAS ®cial water-level regimes with sites along free-¯owing Eight rivers in northern Sweden were selected for rivers. study. They rise on the border between Norway and The dispersal capacities of species may be important Sweden, run southeast for 360±510 km, and empty into to know in order to predict their responses to regulation the Gulf of Bothnia (Fig. 1), except for the Vindel River of the river's ¯ow and water levels. Many species were which joins the Ume River 35 km from the coast. The lost at the onset of regulation, especially when water rivers are similar in channel length, catchment area, levels were raised, leaving the former river margin per- and mean annual discharge (Table 1). The rivers ¯ow manently ¯ooded. In such situations riparian species from the Scandinavian mountain range through a mo- must recolonize after the onset of regulation. Further- nadnock plain, to undulating hilly land, ¯attening out more, regulated river margins are often subject to into a narrow coastal plain (Rudberg 1970). The bed- strong physical disturbance by the arti®cial water-level rock of the mountain range is complex and partly com- regimes, which may entail frequent local extinctions posed of amphibolites, schists, and sparagmites (Kull- of riparian plant populations (Nilsson 1981). Thus, for ing 1953). The remaining area consists of the Baltic long-term persistence along regulated rivers, some ri- shield of Precambrian origin with bedrock composed parian plant species may need to repeatedly establish of predominantly granite and gneiss (Hjelmqvist 1953). new populations or to ``rescue'' declining populations Biogeographically, the rivers run through the boreal February 2000 RIVER REGULATION AND MARGIN VEGETATION 205

FIG. 1. Map of northern Sweden with the eight studied rivers and their catchments. The catchments of free-¯owing rivers (F) are white, those of regulated rivers (R) are dark gray. The land area of Sweden outside these catchments is light gray, with national borders marked by dashed lines. This area corresponds to the black area in the inset map of Scandinavia.

TABLE 1. Hydrological data on the eight Swedish rivers, following Melin and Gihl (1957), except when another source is indicated.

Annual discharge at Number of major Main the river mouth (m3/s) dams³ channel Catchment Flow length area Mean Mean regulation Main River (km) (km2) Mean minimum maximum (%)² channel Tributary Free-¯owing rivers Torne River 510 30 330§ 350 45 2750 0 0 3 Kalix River 450 17 950 290࿣ 33 2890 0 0 0 Pite River 360 11 220 170 20 1170 4 2 2 Vindel River 445 12 650 190¶ 15 1660 0 0 1 Regulated rivers Lule River 450 25 250 510 52# 2650# 72 9 7 Skellefte River 405 11 600 160 20# 720# 62 15 1 Ume River 455 13 100 230¶ 72# 1800# 27 16 4 AÊ ngerman River 440 31 890 490 62# 1970# 43 16 30 ² Based on live storage, expressed as percentage of virgin mean annual discharge (Dynesius and Nilsson 1994). ³ Up to 1991, excluding smaller structures such as dams for mills and timber ¯oating. § Of which 9910 km2 is common with the Kalix River. ࿣ Of which ϳ90 m3/s comes from the Torne River through the TaÈrendoÈ River, a natural cross-channel connection. ¶ At the point where the Vindel River joins the Ume River. # Discharge records obtained before hydroelectric exploitation of the rivers. 206 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1 coniferous zone, with a cold-temperate climate (Walter 1985). The upland vegetation along the rivers ranges from subalpine birch forests dominated by Betula pu- bescens ssp. tortuosa, to coniferous forests dominated by Pinus sylvestris and Picea abies. The annual grow- ing season on non-riparian land (days with a mean temperature exceeding ϩ3ЊC) ranges from Ͻ130 d at the headwaters in the northernmost rivers, to ϳ170 d at the mouth of the southernmost AÊ ngerman River (AÊ ngstroÈm 1974). Four rivers (Torne, Kalix, Pite and Vindel) are large- ly free ¯owing with large seasonal water-level ¯uctu- ations (up to 6 m). The annual water-level maximum occurs during the spring or early summer (Fig. 2A) due to snowmelt in the mountains, and delays the devel- opment of river-margin vegetation until water levels recede. During this spring ¯ood, ice jams may cause additional ¯oods, especially in the two northernmost Torne and Kalix rivers (Raab and Zachrisson 1984). The mean water levels then recede gradually until the next spring ¯ood, except for a minor, rain-induced in- crease in the fall. The seasonal ¯ow variations are large: the annual maximum spring ¯ood is typically two or- ders of magnitude larger than the annual minimum dis- charge in late winter (Table 1). However, the variation in ¯ow among years is small. Close to the river mouths the coef®cients of variation of the maximum annual ¯ows (attained during spring ¯ood) ranged between 7 and 24% for the four rivers during 1937±1975 (Anon- ymous 1979). Such small variations suggest environ- mental predictability among years. Four rivers (Lule, Skellefte, Ume, and AÊ ngerman) are strongly developed for hydroelectric power. The main channels have been transformed by 9±16 dams in each river into chains of consecutive reservoirs and impoundments with very few unimpounded reaches (Table 1). There are also additional dams in the trib- utaries. Prior to regulation, these rivers exhibited free- ¯owing water-level ¯uctuations similar to the remain- ing rivers. Now they exhibit four major kinds of reg- ulated water-level regime: FIG. 2. Hydrographs for (A) a free-¯owing river and 1) In the high-capacity storage reservoirs of the (B)±(E) the four major types of regulated ¯ow in northern Sweden. Data are from the Swedish Meteorological and Hy- upper reaches, the water level is at its lowest in spring drological Institute (NorrkoÈping, Sweden) and Nordkraft Ser- and is then raised to reach its maximum in late summer vice (UmeaÊ, Sweden). (Fig. 2B). Thus most of the river margins become in- undated during the growing season, and most of the margin is barren (Fig 3B). The water level is subse- stream outlets of the power stations, but wave erosion quently lowered during fall and winter. The water-level may be strong (Fig. 3C). ¯uctuations range from a few to 34.5 m (Sundborg 3) In some sections of the rivers, sometimes several 1977). kilometers long, the river channel is dry or has very 2) In the low-capacity, run-of-river impoundments low discharge because of underground passage through of the middle and lower reaches, the water level ex- tunnels and hydroelectric power stations (Figs. 2D and hibits daily or weekly ¯uctuations between its statutory 3D). These reaches may receive occasional high ¯ows high and low levels (in most cases 0.5±1 m apart) during water release from upstream dams. throughout the year (Fig. 2C). The river margins re- 4) There are also reaches that are not impounded by semble shorelines in high, headwater lakes in that wa- a dam downstream, but where ¯ow is affected by up- ter-level amplitudes are small and erosion from water stream dams (Fig. 2E). These reaches maintain their ¯ow is unimportant except from the closest down- annual discharge but water-level ¯uctuations are often February 2000 RIVER REGULATION AND MARGIN VEGETATION 207

FIG. 3. River-margins experiencing four different kinds of water-level regime. The white lines mark the upper and lower limits of the sampled river-margin area. (A) Free-¯owing river. Note the distinct zonation of the river-margin vegetation, going from forest communities at the top, to shrub vegetation, to herbaceous communities, to amphibious species at the bottom. The shrub zone is poorly developed in this particular case due to wave disturbance. (B) Run-of-river impoundment with poor zonation of the river-margin vegetation. Note the stumps in the water from former forest. (C) Storage reservoir during low water level. (D) Reach with reduced ¯ow. reduced in height, although in many cases with a large- munities with Carex spp. and amphibious species such ly natural rhythm. as Ranunculus reptans at the bottom (Fig 3A). Plant All rivers representing the free-¯owing stage are not species richness is generally highest in the riparian for- unaffected by dams. The Vindel River joins the regu- est community (Nilsson 1983). The river margins along lated Ume River 10 km above a major dam that also the regulated rivers generally lack such a distinctive regulates the lowermost 10 km of the Vindel River zonation. The vegetation on the reservoir shorelines (Table 1). The ¯ow of the Pite River is regulated to a (i.e., storage reservoirs and run-of-river impound- degree of4%bythree storage reservoirs, and a dam ments) can be separated into a narrow strip without with a hydroelectric power station is situated on the clear dominants close to the high-water level, and be- main channel 38 km from the mouth (Table 1). The low this a sparse occurrence of amphibious species Torne and Kalix rivers are unaffected by river regu- such as Ranunculus reptans and Subularia aquatica lation, except for a few impoundments on the TengelioÈ (Fig. 3B and C). In the run-of-river impoundments, River, a Finnish tributary to the lowermost Torne River. plant species typical of lakes, such as Carex rostrata The Torne and Kalix rivers, interconnected by a natural and Phragmites australis, have colonized due to small cross-channel, the TaÈrendoÈ River, form the largest free- water-level amplitudes and slow water currents. The ¯owing river system of Europe outside Russia (Dy- river-margin ¯ora along all rivers studied is composed nesius and Nilsson 1994). of species indigenous to Sweden, i.e., exotics or non- The river-margin vegetation along the free-¯owing natives are largely absent. rivers is distinctly vertically zoned, from forest com- We infer the effects of river regulation by comparing munities at the top with Pinus sylvestris and Alnus river-margin vegetation along the free-¯owing and the incana among the dominant tree species, to shrub veg- regulated rivers, thus assuming that the river-margin etation of predominantly Salix spp., to herbaceous com- vegetation was similar between the two groups of river 208 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1 prior to regulation. The validity of this assumption can- the damming level and the level of summer drawdown. not be tested directly due to lack of quantitative pre- If all parts of this area were not available for plant regulation data, but is supported by historical docu- growth in the regulated rivers, this would indicate a mentation. The most important of this material are bo- major environmental change. To maximize the likeli- tanical surveys made during the planning of the hy- hood of encountering all species present at the site, we droelectric schemes (Lundqvist 1970, SjoÈrs 1973), sampled one large area at each site rather than many showing that preregulation characteristics such as plant small plots. In free-¯owing rivers, the limit of the sum- zonation and species richness were similar to that of mer low-water level was de®ned as the lower end of the remaining free-¯owing rivers. Species composition continuous Carex vegetation, and the high-water limit differed somewhat geographically, but was highly sim- was de®ned as the lower boundary of a continuous ilar between adjacent rivers (Lundqvist 1970, SjoÈrs Vaccinium myrtillus carpet (WasseÂn 1966, Nilsson 1973). Other documentary material includes photos 1979). The presence of debris deposited by the spring (examples in Grelsson and Nilsson [1982]), and rem- ¯ood often helped to de®ne the upper limit of the river nants of preregulation river-margin vegetation, now left margin. un¯ooded along regulated rivers (Grelsson and Nilsson At each site we recorded all vascular plant species 1980). River systems with high percentages of lakes present. The de®nition of a ``species'' follows the tax- and much of the altitudinal range concentrated into onomy in Krok and Almquist (1984). In the following steep rapids were chosen ®rst for regulation, but dif- cases two or more species were treated as one taxon: ferences between rivers were small and there are no Alchemilla vulgaris coll., Callitriche spp., Carex jun- indications that those differences affected plant species cella ϩ C. nigra, Dactylorhiza maculata ϩ D. fuchsii, richness. Equisetum variegatum ϩ E. scirpoides, Euphrasia fri- gida ϩ E. stricta, Hieracium spp., HierochloeÈ hirta ϩ METHODS H. odorata, Luzula multi¯orum ϩ L. sudetica, Poa ne- moralis ϩ P. palustris, Rhinanthus groenlandicus ϩ Vegetation R. minor, Sagittaria natans ϩ S. sagittifolia, Salix bo- The entire main channel of each river was divided realis ϩ S. myrsinifolia ϩ S. phylicifolia, Salix star- into equally long sections, with one study site located keana ϩ S. xerophila, Sparganium spp., Taraxacum in the middle of each section. The number of sites spp., and Thalictrum ¯avum ϩ T. simplex. We also differed among rivers, because data were collected on estimated the percentage cover of trees ϩ shrubs (in- different occasions. We investigated the Torne River dividuals of woody species Ͼ0.25 m) and dwarf shrubs in 1985, the Kalix River in 1986, the Vindel and Ume (Ͻ0.25 m) ϩ herbs. To reduce the error almost always Rivers in 1988, and the remaining rivers in 1991. Be- present in vegetation analyses (LepsÏ and Hadincova cause each study site covered a large area (see next 1992) every site was analyzed individually by two per- paragraph) and Ͼ90% of the river-margin plant species sons and results were combined (Nilsson 1992). in the area are perennial, between-year differences in species composition are small and considered unim- Environment portant. We sampled 46 sites in the Torne River, 37 At each site we determined the width and height of sites in the Kalix River, 25 sites in each of the Vindel the river margin, the number of substrates, and the and Ume Rivers, and 10 sites in each of the remaining percentage cover of each substrate, using nine grain rivers. Thus, the distance between study sites varied sizes (the Wentworth groups: clay, silt, sand, gravel, among rivers, from 10 km in the Torne and Kalix Rivers pebbles, cobbles, and boulders [Chorley et al. 1984], to 45 km in the Lule River. Study sites were always supplemented by bedrock and peat), width of the river, located on the northern side of the rivers, except from mean annual discharge, catchment area, and length of one site in the Lule River and two sites in the Pite the growing season. Recording the cover of each sub- River, where absence of roads made this side dif®cult strate, albeit not very precise, gives a good represen- to access. tation of the overall substrate composition of deeper Each site in the free-¯owing rivers encompassed a soil at the site (R. Jansson, personal observations). We

200-m-long stretch of riverbank or lakeshore, spanning calculated substrate ®neness by weighing log2-trans- the entire area between the summer low-water and formed values of mean particle size by percentage com- spring high-water levels, attained at least once every position of the river-margin substrate (Wright et al. two years. In free-¯owing rivers, this area is available 1984, Nilsson et al. 1989). Width of the river margin for plant growth unless the substrate is too coarse was measured at ®ve locations 50 m apart, as the dis- grained. We sampled an analogous area also in the reg- tance between summer low-water level and spring ulated rivers. In the storage reservoirs, this was iden- high-water level. Height of the river margin was mea- tical to the area between the statutory damming and sured as the vertical distance between these two levels, drawdown levels, whereas in most run-of-river im- using a rod and level. We calculated the area of each poundments it encompassed the uppermost 0.5±1 m site as the mean of the ®ve width measurements mul- vertical interval of the shoreline, i.e., the zone between tiplied by the length of the site (always 200 m). Width February 2000 RIVER REGULATION AND MARGIN VEGETATION 209 of the river and catchment area were measured on maps. sidered as lacking mechanisms for dispersal. Species Mean annual discharge was estimated for each site us- with ballistic dispersal were included in this group, ing data presented by the Swedish Meteorological and because seeds released by this mechanism usually trav- Hydrological Institute (Anonymous 1979), and length el very short distances (e.g., van der Pijl 1982). We of the growing season was estimated from AÊ ngstroÈm also judged whether the species have more or less dif- (1974). ferentiated vegetative structures that can be detached and dispersed (in this case mainly by water). Vegetative Data analysis dispersal is common in most aquatic and many riverine To test whether different species respond differently plants (e.g., Ranunculus spp. and Salix spp.; Bartley to regulation, we classi®ed the species into functional and Spence 1987, Cook 1987). groups, each group sharing a speci®c trait. Equivalent For each site, we calculated the number of species terms are ``guilds'' (Root 1967), ``strategies'' (Grime of the entire ¯ora and of the different functional groups. 1979, Tilman 1988) or ``life-history types'' (van der Although study sites were always 200 m long, sample Valk 1981). We made four different classi®cations of area varied among sites due to variation in height and the ¯ora: (1) morphology: trees ϩ shrubs, dwarf shrubs, slope of the river margins. To correct for differences forbs ϩ ferns, and graminoids; (2) naturalness: natural in species richness due to variation in sample area, we vs. ruderal species (following Lid 1987), where ruderal calculated species density by dividing the number of species represent species mainly occurring in human- species for each site by the log10 of the area sampled made habitats, e.g., wasteland, cultivated land, and (Whittaker 1972, Connor and McCoy 1979). Values meadows; (3) location: riparian and terrestrial vs. were then standardized by multiplying with the log10 aquatic species (following Nilsson 1983); and (4) life of the mean sample area for all rivers (4648.15 m2). cycle: perennial vs. annual ϩ biennial species (follow- Comparison of ®ve rivers.ÐTo test the generality of ing Lid 1987). a previous comparison of the Vindel and Ume rivers We also classi®ed species according to seven dis- (Nilsson et al. 1991a), we compared (1) the free-¯ow- persal mechanisms: dispersal by water (hydrochory), ing Torne and Kalix rivers with the regulated Lule Riv- wind (anemochory), vertebrates (zoochory: epizoic or er, and (2) the free-¯owing Pite River with the regulated endozoic), ants (myrmechory), no mechanisms for dis- Skellefte River. The rivers in each of these two sets of persal, and vegetative dispersal. All classi®cations in- rivers are adjacent with similar climates (Lundqvist clude the entire ¯ora, except for hydrochory, where 1953±1971), geology (Hjelmqvist 1953), and regional data were available for 234 of 508 species. Species species pools (Mossberg et al. 1992). Although there with seeds and fruits that ¯oat for 2 d or more were are some between-river differences in the size of the de®ned as ``long-¯oaters''; the remaining species were catchment area, the percentage of lakes, and the fea- ``short-¯oaters'' (data from Romell 1938, Danvind and tures of the river channel, the rivers in each set are Nilsson 1997, and E. Andersson, unpublished manu- equally long and exhibited similar water-level regimes script). When several data were available, we used the prior to regulation, including spring ¯ooding and low mean ¯oating time. However, most of the data on ¯oat- winter ¯ows (Table 1). The regulated AÊ ngerman River ing capacity come from a compilation of experimental was not compared with any river, because it is geo- studies by Romell (1938). The general method in these graphically separated from the other ®ve rivers (Fig. studies was to place a number of diaspores in water 1), and contains some species of mainly southern dis- and to record maximum ¯oating time when the last tribution that are absent further north. diaspore had sunk. Romell (1938) reports the longest To test for overall differences we compared variables ¯oating time in classes (hours, days, weeks etc.), often describing the vegetation and environment between the as a range (e.g., days to months), and we consistently Torne, Kalix and Lule rivers, using the Kruskal-Wallis used the average of this range. Many species ¯oat well test. We also compared the Pite and Skellefte rivers without having distinct morphological adaptations for using the Mann-Whitney U test. In addition, to test for water dispersal. Therefore, buoyancy records describe a general pattern, we compared regulated vs. free-¯ow- the ability of being water-dispersed more accurately ing sites in the ®ve rivers, using the Mann-Whitney U than morphological criteria. The germinability of the test. In these analyses 14 variables described the veg- buoyant diaspores in the buoyancy tests was generally etation: the total number of species and species density high, and in some cases even increased compared to per site, the species density of the various functional diaspores that were not tested (E. Andersson, unpub- groups, classi®ed by morphology, naturalness, location lished manuscript). and life cycle, and the percentage cover of trees ϩ Anemochores (e.g., Betula spp., most Asteraceae shrubs, and herbs ϩ dwarf shrubs. The physical factors spp.) and zoochores (endozoic dispersal: e.g., Vaccin- were described by the length of the growing season, ium spp., Sorbus spp.; epizoic dispersal: e.g., many mean annual discharge, width of the river, height and Poaceae spp.) and myrmechores (e.g., Viola spp. and width of the river margin, number of substrates, and Luzula spp.) were classi®ed following a scheme of substrate ®neness. Willson et al. (1990). The remaining species were con- Type of regulation.ÐTo test if the effects of river 210 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1

TABLE 2. Comparison of environmental and vegetation-cover variables between (1) the free-¯owing Torne and Kalix rivers and the regulated Lule River, and (2) the free-¯owing Pite River and the regulated Skellefte River (n ϭ no. of river-margin sites). The last column of P values refers to a previous comparison of the free-¯owing Vindel and the regulated Ume rivers (Nilsson et al. 1991a).

Comparison of three rivers Torne River Kalix River Lule River (n ϭ 46; (n ϭ 37; (n ϭ 10; free-¯owing) free-¯owing) regulated)

Variable Mean 1 SE Mean 1 SE Mean 1 SE P² Environmental factors Growing season (d) 144 1.5 144 1.9 146 4.1 0.77 Discharge (m3/s) 183a 19.1 137a 15.5 319b 42.6 0.0021 Width of river (m) 1080a 237 490a 93 1360a 529 0.039§ Height of river margin (m) 1.9 0.16 2.1 0.16 4.6 2.85 0.75 Width of river margin (m) 24 4.3 20 3.0 88 68.3 0.99 Substrate ®neness 1.0ab 1.01 3.2a 0.76 Ϫ2.7b 2.29 0.034§ Number of substrates 3.1a 0.16 4.6b 0.28 3.7ab 0.47 0.0001 Vegetation cover Herbs ϩ dwarf shrubs (%) 66a 3.9 ´´´ ´´´ 11b 7.2 Ͻ0.0001 Trees ϩ shrubs (%) ´´´ ´´´ ´´´ ´´´ 14 8.3 ´´´ Notes: Means with different lowercase superscript letters are signi®cantly different at P Ͻ 0.05 according to nonparametric multiple comparisons with unequal sample sizes (Zar 1996). Ellipses indicate lack of data. Signi®cant (P Ͻ 0.05) differences are boldfaced. ² Kruskal-Wallis test. ³ Mann-Whitney U test, two-tailed probability. § Not signi®cant after Bonferroni correction, using a sequential procedure (Holm 1979) of the Dunn-SÏ idaÂk method (Sokal and Rohlf 1995). regulation varied depending on the kind of regulated ently to river regulation. We did this by comparing the water-level regime, we compared variables describing proportion of species per site, grouped after mecha- the vegetation and environment between the four types nisms for dispersal, between the free-¯owing rivers and of regulation identi®ed above (storage reservoirs, run- the four major types of regulated water-level regime, of-river impoundments, reaches with reduced dis- using one-way ANOVA. In these analyses, we included charge, and unimpounded but regulated reaches) with all study sites from all eight rivers, giving a total of sites in the free-¯owing rivers, using one-way ANOVA 173 sites. or the Kruskal-Wallis test. In these analyses, we in- To ensure that the ``experiment-wise'' signi®cance cluded all study sites from all eight rivers, giving a level always was ␣ϭ0.05 irrespective of the number total of 173 sites. Four variables described the vege- of statistical tests, we always adjusted the Type I error tation: the total number of species and species density rate using sequential Bonferroni tests, i.e., a sequential per site, and the percentage cover of trees ϩ shrubs, procedure (Holm 1979) of the Dunn-SÏ idaÂk method (So- and herbs ϩ dwarf shrubs. Cover values were arcsine kal and Rohlf 1995). We also calculated the statistical transformed and species density was squared prior to power of the tests to address the risk of committing analysis. The physical factors were described by ®ve Type II errors. variables: width of the river, height and width of the Relationship between vegetation and environment.Ð river margin, number of substrates, and substrate ®ne- In the following analyses, we used data from all eight ness. rivers, but to give equal weight to all rivers we used In these and the following analyses we assume that only 10 sites from each river, i.e., a total of 80 sites, sites were independent, although there were several 40 from the free-¯owing and 40 from the regulated sites in each river. When free-¯owing and regulated rivers. In cases where there were Ͼ10 sites from a river, rivers were treated separately, river af®liation was a we chose subsets of sites to ensure that selected sites poor predictor of species numbers. It did not enter the lay at equal distances along the entire river. regression models to explain the variation in species We tested how much of the variation in the variables numbers (P Ͼ 0.05), but variables describing substrate describing river-margin vegetation could be explained composition and river-margin height were signi®cant by the environment. Because several of the environ- (P Ͻ 0.05). This suggests that local site conditions were mental variables were correlated, we used partial least- more important in determining species composition squares (PLS) regression (Geladi and Kowalski 1986, than some spatial dependency among sites within a Martens and Naes 1989). In PLS regression the prob- river. lem of correlation among the dependent x variables is Dispersal traits.ÐWe tested whether species with overcome by projecting the observed x variables onto different mechanisms for dispersal responded differ- a few ``latent'' variables (components), rather than by February 2000 RIVER REGULATION AND MARGIN VEGETATION 211

TABLE 2. Extended. were examined by removing them to test whether pat- terns changed. To enable comparisons with the previ- ous study (Nilsson et al. 1991a), we also performed the regressions excluding data from the Ume and Vin- Comparison of two rivers del rivers. Pite River Skellefte River In all statistical analyses we used the SPSS software Vindel vs. (n ϭ 10; (n ϭ 10; package (NorusÏis 1993), except for the PLS regres- free-¯owing) regulated) Ume rivers sions, which were made using the computer program Mean 1 SE Mean 1 SE P³ P³ SIRIUS (version 2.3, Pattern Recognition Systems A/ S, 5015 Bergen, Norway). The power analyses were 146 3.8 144 2.6 0.88 0.81 made using the computer program PASS 6.0 (NCSS, 107 11.7 107 13.6 0.71 0.071 Kaysville, Utah, USA). 570 199 600 251 0.49 0.37 1.2 0.13 3.2 1.49 0.14 0.29 9 2.3 16 5.0 0.26 0.14 RESULTS Ϫ0.8 1.88 Ϫ6.2 1.54 0.028³ 0.0021 3.5 0.56 4.7 0.40 0.16 0.052 First, we present the results from the comparisons of the Torne, Kalix, and Lule rivers, and the Pite and 25 4.2 13 6.2 0.058 Ͻ0.0001 Skellefte rivers. Second, we present the results from 27 5.2 9 4.0 0.0075³ Ͻ0.0001 the comparison of four different regulated-water-level regimes with sites in the free-¯owing rivers, and third the results from similar comparisons of species with selecting among variables as in stepwise multiple re- different dispersal mechanisms. Fourth, we present our gression. These latent variables or components are then analyses of the relationships between the river-margin used as independent variables in the regressions. The vegetation and the environment. We then compare the components are estimated consecutively and the sig- results with those of Nilsson et al. (1991a). ni®cance of each component in the model is determined by cross-validation, i.e., parameters are estimated on Comparison of ®ve rivers one part of the data matrix, and the prediction is tested on another part (Wold 1978). The contribution of each The regulated sites had more coarse-grained soils observed x variable in a component is given by its than the sites along free-¯owing rivers (Ϫ4.4 Ϯ 1.40 variable loading. A variable with little in¯uence on the vs. 1.7 Ϯ 0.63 [mean Ϯ 1 SE], P ϭ 0.00011, Mann- model is close to the perpendicular of the PLS com- Whitney U test), but the other environmental variables ponent and gets a loading close to 0. A positive loading did not differ signi®cantly (P Ͼ 0.05). The percentage means that the variable is positively correlated with y cover of both herbs ϩ dwarf shrubs (14 Ϯ 2.9 vs. 55 and vice versa for a negative loading. Prior to the PLS Ϯ 3.0) and of trees ϩ shrubs (12 Ϯ 5.2 vs. 27 Ϯ 5.2) modeling, all variables were standardized to have a were lower on the regulated sites (P Ͻ 0.0001). Both mean of 0 and a standard deviation of 1, by subtracting the number of species per site (63 Ϯ 6.9 vs. 91 Ϯ 1.7) the mean from each variable and multiplying with the and species density (55 Ϯ 5.3 vs. 84 Ϯ 1.7) were lower inverse of the standard deviation. This procedure is in the regulated than in free-¯owing rivers (P Ͻ recommended when no a priori assumption is made on 0.0001). the relative importance of the observed variables (Mar- The Torne, Kalix, and Lule rivers differed in some tens and Naes 1989). of the environmental variables. The Lule River had In the PLS modeling, we used 15 variables describ- higher mean annual discharge than the other two rivers ing the environment: length of the growing season (in (P ϭ 0.0021, Kruskal-Wallis test, Table 2). The Kalix days), latitude, catchment area (in square kilometers), River had more river-margin substrates than the Torne altitude (in meters above sea level), width of the river River (P ϭ 0.0001, Table 2). The percentage cover of (in meters), height and width of the river margin (in herbs and dwarf shrubs was higher in the Torne than meters), the percentage cover values of peat, clay ϩ in the Lule River (P Ͻ 0.0001, Table 2). On average, silt, sand ϩ gravel, stones, and bedrock, plus three herbs and dwarf shrubs covered 66% of the river mar- dummy variables. The dummy variables were any kind gins along the Torne River, but only 11% of the river of regulated water ¯ow vs. natural water ¯ow, any kind margins along the Lule River (data from the Kalix Riv- of reservoir (i.e., storage reservoirs and run-of-river er are missing, Table 2). impoundments) vs. non-reservoir, and the preregulation The total number of species and species density per position of the river margin. Width and height of the site were higher in the free-¯owing Torne and Kalix river margins and the substrate cover values were log10- rivers than in the regulated Lule River, (P ϭ 0.0002 transformed prior to the analysis to better ®t a normal and P ϭ 0.0015, respectively; Kruskal-Wallis test, Ta- distribution. In the PLS regressions, data from all eight ble 3). The species densities of functional groups of rivers were combined. We examined the data for non- species were higher in the Torne and Kalix rivers (P linearities and outliers, and the in¯uence of outliers Ͻ 0.05), except for tree and shrub species, ruderal spe- 212 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1

TABLE 3. Comparison of the number of species per site for all species and for different functional groups between (1) the free-¯owing Torne and Kalix rivers and the regulated Lule River, and (2) the free-¯owing Pite River and the regulated Skellefte River, (n ϭ no. of river-margin sites). The last column of P values refers to a previous comparison of the free- ¯owing Vindel and the regulated Ume rivers (Nilsson et al. 1991a).

Comparison of three rivers Torne River Kalix River Lule River (n ϭ 46; (n ϭ 37; (n ϭ 10; free-¯owing) free-¯owing) regulated)

Variable Mean 1 SE Mean 1 SE Mean 1 SE P² Species richness of entire ¯ora Number of species 85.0a 2.34 86.4a 2.11 55.0b 6.61 0.0002 Species density 91.5a 2.13 94.3a 2.51 64.2b 10.3 0.0015 Morphology Trees ϩ shrubs 8.6 0.25 9.4 0.38 9.8 2.00 0.37 Dwarf shrubs 7.3a 0.49 7.7a 0.51 4.3b 0.79 0.014 Forbs ϩ ferns 49.1a 1.59 49.2a 1.51 30.5b 5.50 0.0019 Graminoids 26.6a 0.87 28.0a 0.93 19.5b 2.97 0.010 Naturalness Ruderal species 4.7 0.81 4.1 0.86 2.7 1.01 0.35 Natural species 86.3a 2.00 89.5a 2.50 61.4b 9.85 0.0016 Location Aquatic species 9.2 0.82 8.0 0.75 6.1 1.92 0.17 Riparian ϩ terrestrial spp. 81.9a 2.04 85.2a 2.47 57.9b 9.41 0.0008 Life cycle Annuals ϩ biennials 6.2a 0.42 6.4a 0.48 3.1b 0.87 0.0060 Perennials 84.8a 1.92 87.4a 2.38 60.7b 9.81 0.0014 Notes: Means with different lowercase superscript letters are signi®cantly different at P Ͻ 0.05 according to nonparametric multiple comparisons with unequal sample sizes (Zar 1996). Signi®cant (P Ͻ 0.05) differences are boldfaced. ² Kruskal-Wallis test, two-tailed probability. ³ Mann-Whitney U test, two-tailed probability. cies, and aquatic species that were not signi®cantly Reaches with reduced discharge did not differ signif- different between rivers (P Ͼ 0.05, Table 3). icantly in cover from any of the other ¯ow regimes, The percentage cover of trees and shrubs was higher but the statistical powers of the comparisons were low and the river-margin substrates were more ®ne grained (power Ͻ 0.66). in the free-¯owing Pite River than in the regulated Regulated margins (including all four types of reg- Skellefte River, but differences were not signi®cant af- ulated ¯ow) had lower percentage cover of herbs and ter Bonferroni correction (P Ͼ 0.05, Mann-Whitney U dwarf shrubs than the free-¯owing rivers (P Ͻ 0.0001, test; Table 2). Neither the number of species, nor spe- one-way ANOVA, Table 4), but did not differ signif- cies densities per site, differed between the Pite and icantly among the different types of regulation (P Ͼ the Skellefte rivers (P Ͼ 0.05, Table 3). Furthermore, 0.05). However, the statistical power was too low in species density did not differ for any of the functional all comparisons to conclude that such differences do groups (P Ͼ 0.05, Table 3). However, the statistical not exist (power Ͻ 0.51), except for between storage power of these tests was too low to conclude that there reservoirs and run-of-river impoundments (power ϭ were no differences (power Ͻ 0.44). 0.93). The number of species per site was lower on regu- Type of regulation lated river margins (including all types of regulated All four types of regulated water-level regimes had ¯ow) than on free-¯owing river-margins, except for the more coarse-grained river-margin soils compared to the unimpounded reaches (P Ͻ 0.0001, one-way ANOVA, free-¯owing rivers (P Ͻ 0.0001, Kruskal-Wallis test, Table 4), but did not differ among types of regulation Table 4), but the number of substrates did not differ (P Ͼ 0.05, power Ͻ 0.20). When species-density values signi®cantly among ¯ow regimes (P Ͼ 0.05). were used, the differences between the free-¯owing The percentage cover of trees and shrubs was highest rivers vs. storage reservoirs and run-of-river impound- on river-margins along free-¯owing rivers, lower along ments remained signi®cant (P Ͻ 0.0001, one-way run-of-river impoundments, and lowest along storage ANOVA, Table 4), but not between free-¯owing rivers reservoirs (P Ͻ 0.0001, one-way ANOVA, Table 4). and reaches with reduced discharge (P Ͼ 0.05). How- The regulated but unimpounded reaches had lower per- ever, statistical power was too low to conclude that centage cover of trees and shrubs than free-¯owing they do not differ in species density (power ϭ 0.29). rivers, but did not differ signi®cantly from any of the The margins along regulated but unimpounded reaches other types of regulation (P Ͼ 0.05, power Ͻ 0.46). had signi®cantly higher species density than storage February 2000 RIVER REGULATION AND MARGIN VEGETATION 213

TABLE 3. Extended. Dispersal traits Compared to the free-¯owing rivers, the proportion of wind-dispersed species was higher for all types of regulated ¯ow regime, except for run-of-river im- Comparison of two rivers poundments (P Ͻ 0.0001, one-way ANOVA, Table 5). Pite River Skellefte River These increases were offset by lower proportions of (n ϭ 10; (n ϭ 10; Vindel vs. free-¯owing) regulated) Ume species with no special devices for dispersal along stor- Rivers age reservoirs and reaches with reduced discharge (P Mean 1 SE Mean 1 SE P³ P³ Ͻ 0.0001). Compared to the free-¯owing rivers, the proportion of species with endozoic dispersal increased 65.7 5.49 54.2 8.56 0.60 0.0001 79.7 7.13 62.0 9.57 0.20 0.0016 along storage reservoirs and decreased along regulated but unimpounded reaches (P Ͻ 0.0001). The run-of- 13.0 1.48 9.1 1.52 0.059 0.0001 river impoundments had higher proportions of species 8.2 1.14 6.5 1.06 0.21 0.0001 with long-¯oating diaspores and species with vegeta- 36.0 4.67 25.5 4.72 0.17 0.0026 tive dispersal compared to the free-¯owing rivers (P 22.5 2.57 21.0 3.28 0.82 0.028 Ͻ 0.0001, one-way ANOVA, Table 5). The different mechanisms for dispersal were not positively correlated 1.8 1.42 0.8 0.42 0.84 0.30 77.6 6.88 61.1 9.55 0.26 0.0022 (P Ͼ 0.05, Spearman's coef®cient of rank correlation). Relationships between vegetation and environment 4.1 0.91 6.2 1.78 0.34 0.36 75.0 6.92 55.5 8.31 0.13 0.0009 There were signi®cant correlations between envi- ronmental variables used in the partial least-squares 2.0 0.78 2.3 0.59 0.60 0.90 modeling (P Ͻ 0.05, Pearson product-moment corre- 77.5 6.73 59.5 9.22 0.20 0.0011 lations, Table 6). The length of the growing season, catchment area, and altitude were signi®cantly corre- lated. The length of the growing season and catchment reservoirs and run-of-river impoundments (P Ͻ 0.05), area decreased with increasing altitude (r ϾϪ0.83), but did not differ signi®cantly from free-¯owing rivers and the length of the growing season increased with and reaches with reduced discharge (P Ͼ 0.05, power increasing catchment area (r ϭ 0.76). Furthermore, the Ͻ 0.29). Species richness (the number of species and occurrence of clay and silt in the river-margin soils species density per site) did not vary linearly with the increased with increasing length of the growing season number of years regulation had been in operation for and decreasing altitude (r Ͼ ͦ0.38ͦ), re¯ecting that river any of the types of regulation (P Ͼ 0.05, linear least- valleys are more sediment laden toward the coast. The squares regression). percentage cover of clay and silt in the river-margin

TABLE 4. Comparison of environmental and vegetation variables between free-¯owing rivers and four major types of regulated ¯ow.

Run-of-river Reaches with Regulated but Free-¯owing Storage impound- reduced unimpounded rivers reservoirs ments discharge reaches (n ϭ 118) (n ϭ 15) (n ϭ 24) (n ϭ 7) (n ϭ 9)

Variable Mean 1 SE Mean 1 SE Mean 1 SE Mean 1 SE Mean 1 SE P Substrate ®neness 1.8a 0.53 Ϫ3.9bc 1.29 0.8ab 1.17 Ϫ7.7c 1.10 Ϫ5.0bc 1.56 Ͻ0.0001² Number of substrates 3.7 0.15 4.3 0.32 3.9 0.25 4.6 0.69 5.4 0.56 0.023²,³ Tree ϩ shrub cover (%)§ 39a 4.0 0.3b 0.15 19c 5.3 14abc 5.0 16bc 7.0 Ͻ0.0001࿣ Herb ϩ dwarf shrub cover (%)¶ 55a 3.0 1.1b 0.39 18b 4.53 19b 8.1 24b 9.2 Ͻ0.0001࿣ Number of species 86.6a 1.56 57.6b 9.23 67.0b 4.32 60.0b 11.59 76.2ab 8.34 Ͻ0.0001࿣ Species density 86.2a 1.37 50.0b 8.34 71.4b 4.00 65.2ab 14.07 85.4a 9.48 Ͻ0.0001࿣ Notes: Data are from four free-¯owing and four regulated Swedish rivers; n ϭ no. of river-margin sites. Values differing signi®cantly (P Ͻ 0.05) from free-¯owing rivers and signi®cant P values (P Ͻ 0.05) are boldfaced. Means with different lowercase superscript letters are signi®cantly different (P Ͻ 0.05) according to nonparametric multiple comparisons with unequal sample sizes (Kruskal-Wallis tests; Zar 1996) and Bonferroni multiple-comparison tests (one-way ANOVAs). ² Kruskal-Wallis test, two-tailed probability. ³ Not signi®cant after Bonferroni correction, using a sequential procedure (Holm 1979) of the Dunn-SÏ idaÂk method (Sokal and Rohlf 1995). § In the free-¯owing rivers, n ϭ 35. ࿣ One-way ANOVA, two-tailed probability. ¶ In the free-¯owing rivers, n ϭ 80. 214 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1

TABLE 5. The proportion of species per site among species groups with different mechanisms for dispersal, compared between free-¯owing rivers and four major types of regulated ¯ow.

Regulated Run-of-river Reaches with but unim- Free-¯owing Storage impound- reduced pounded rivers reservoirs ments discharge reaches (n ϭ 118) (n ϭ 15) (n ϭ 24) (n ϭ 7) (n ϭ 9)

Diaspore characteristic Mean 1 SE Mean 1 SE Mean 1 SE Mean 1 SE Mean 1 SE P² Floating time Short ¯oating (Ͻ2d) 20.4ab 0.43 18.2ab 1.11 19.3ab 0.83 15.5a 2.93 22.5b 1.56 0.016 Long ¯oating (Ն2d) 42.8a 0.38 44.3ab 2.29 48.5b 1.23 44.9ab 2.08 45.4ab 1.25 Ͻ0.0001 Wind dispersal 28.1a 0.35 35.8b 1.59 30.4ac 1.06 36.3b 3.55 34.1bc 1.13 Ͻ0.0001 Endozoic dispersal 10.5a 0.26 13.1b 1.14 9.6ac 0.81 11.9ab 1.22 6.4c 1.15 Ͻ0.0001 Epizoic dispersal 17.2 0.21 18.3 0.99 16.5 0.62 18.5 1.82 18.2 1.32 0.18 Ant dispersal 5.2 0.18 4.6 0.58 4.6 0.53 3.2 1.00 5.3 0.57 0.11 Vegetative dispersal 23.1a 0.49 21.9a 1.71 29.1b 1.79 26.7ab 2.69 21.0a 2.05 0.00012 No special device for dispersal 41.6a 0.46 31.2b 2.03 41.4ac 1.29 33.0bd 3.70 38.7acd 1.64 Ͻ0.0001 Notes: Data are from four free-¯owing and four regulated Swedish rivers; n ϭ no. of river-margin sites. Values differing signi®cantly (P Ͻ 0.05) from free-¯owing rivers and signi®cant P values (P Ͻ 0.05) are boldfaced. Means with different lowercase superscript letters are signi®cantly different (P Ͻ 0.05) according to Bonferroni multiple-comparison tests. ² One-way ANOVA, two-tailed probability. soil was negatively correlated with the cover of stones tistically signi®cant (P Ͻ 0.05) models for all depen- (r ϭϪ0.49). Furthermore, the height and width of the dent variables describing species richness and per- river margin were signi®cantly correlated (r ϭ 0.86), centage cover of the river-margin vegetation in the and both increased with increasing width of the river eight rivers (Table 7). In all models, only the ®rst PLS (r Ͼ 0.43). Variables describing regulated-¯ow regimes component was signi®cant (P Ͻ 0.05), as determined (i.e., any kind of regulated water ¯ow vs. natural water by cross-validation (Wold 1978). Using a set of 15 ¯ow, any kind of reservoir vs. non-reservoir sites, and environmental factors as predictor variables, the mod- preregulation positions of the river margins) were all els explained 28±50% of the variation in the dependent interrelated (r Ͼ 0.67). Furthermore, the occurrence of variables. Regional factors, as well as local, river-mar- regulated ¯ows decreased with latitude (r ϭ 0.42), be- gin factors were important to explain the variation in cause the position of regulated vs. free-¯owing rivers species richness and plant cover. According to the mean was biased (Fig. 1). rank of loadings (Table 7), the length of the growing Partial least-squares (PLS) regression generated sta- season and the altitude were the most important re-

TABLE 6. Pearson product-moment correlation matrix of environmental variables for four free-¯owing and four regulated rivers in northern Sweden. Local factors refer to the river margin.

Local factors Regulated ¯ow Regional factors Width Reser- New Growing Catchment of All voirs river Variable season Latitude area Altitude river sites only margin Regional factors Growing season 1.00 Latitude n.s. 1.00 Catchment area 0.76 n.s. 1.00 Altitude Ϫ0.86 n.s. Ϫ0.83 1.00 Local factors Width of river n.s. n.s. n.s. n.s. 1.00 Regulated ¯ow, all sites n.s. Ϫ0.42 n.s. n.s. n.s. 1.00 Regulated ¯ow, reservoirs n.s. n.s. n.s. n.s. n.s. 0.82 1.00 Regulated ¯ow, new river margin n.s. n.s. n.s. n.s. n.s. 0.67 0.72 1.00 Height n.s. n.s. n.s. n.s. 0.44 n.s. n.s. n.s. Width n.s. n.s. n.s. n.s. 0.43 n.s. n.s. n.s. Peat n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. Clay ϩ silt 0.38 n.s. n.s. Ϫ0.44 n.s. n.s. n.s. n.s. Sand ϩ gravel n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. Stones n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. Bedrock n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. Note: Signi®cant correlations after sequential (Holm 1979) Bonferroni correction (the Dunn-SÏ idaÂk method [Sokal and Rohlf 1995]) are shown; n.s. ϭ nonsigni®cant. February 2000 RIVER REGULATION AND MARGIN VEGETATION 215 gional factors, and the proportion of clay and silt the new positions, in reservoirs or in regulated reaches, most important local factor. Regulated ¯ows were as- were highly negative, having high loadings in the PLS sociated with low species richness and plant cover. Res- models for graminoids, forbs and ferns, perennial spe- ervoir vs. non-reservoir sites had higher, negative load- cies, natural species, and riparian and terrestrial spe- ings than regulated vs. non-regulated sites. Regulated cies. Variables describing downstream variation along situations, where river margins were created after rais- the rivers were also important in several PLS models. ing water levels to intersect former upland soils, were The length of the growing season, altitude, and catch- most negative for both species richness and plant cover ment area had the highest loadings in the models for (Table 7). trees and shrubs, dwarf shrubs, annual and biennial A one-component PLS model explained 35% of the species, ruderal species, and aquatic species. The load- variation in the total number of species per site along ings for length of growing season and size of catchment all eight rivers (Table 7). Two of the most important area were positive, and the loadings for altitude were variables, regulated river margins in new positions and negative in the PLS models, except for dwarf shrubs reservoir shorelines, had high negative loadings and that showed opposite values (i.e., they were more spe- described effects of regulation. Furthermore, the per- cies rich towards the mountains). centage cover of clay and silt had a high positive load- ing. Comparison with the previous study The PLS model for cover of herbs ϩ dwarf shrubs The results of the present comparisons of three free- explained 50% of the variation (Table 7). The three ¯owing and two regulated rivers (Tables 2 and 3) are most important variables described regulated ¯ows. largely consistent with those of the previous compar- Situations where river margins were in new positions, ison between one free-¯owing and one regulated river in reservoirs or in regulated reaches, were highly neg- (Nilsson et al. 1991a). Species richness per site was ative. The PLS model for the cover of trees and shrubs lower in the regulated than in the free-¯owing rivers, explained 41% of the variation (Table 7). The length and no functional group of species was more species of the growing season had a high positive loading, rich in the regulated rivers, in either study (Table 3). whereas altitude and regulated river margins in new However, although the free-¯owing Pite River was positions had high negative loadings. more species rich than the regulated Skellefte River, The PLS models for functional groups of plant spe- differences were not statistically signi®cant (Table 2), cies explained 28±38% of the variation (Table 7). In but the statistical power was low. There were also some the PLS models for functional groups that were more other differences. The species richness of trees and species poor in the regulated rivers (Table 3), variables shrubs did not differ between the regulated Lule River associated with the effects of regulation had the highest and the free-¯owing Torne and Kalix rivers, but was loadings. Thus, situations where river margins were in lower in the regulated river in the previous study (Table 2). Furthermore, the species richness of annuals and biennials was higher in the free-¯owing Torne and Ka- TABLE 6. Extended. lix rivers than in the regulated Lule River, but did not differ between rivers in the previous study (Table 3). Local factors The results of the PLS regression did not change when performed without the Ume and Vindel rivers. Clay Sand This is demonstrated by the fact that all the variable ϩ ϩ Bed- loadings had the same mean ranks and signs compared Height Width Peat silt gravel Stones rock to regressions including all eight rivers. Furthermore, the coef®cients of determination for the regression models remained similar (not shown). Although we used different regression methods and different sets of variables in the two studies, certain comparisons can be made. For example, in cases where the river margin remained in its preregulation position (note that this situation also included all free-¯owing rivers), species richness was favored in both studies. 1.00 In the former study (Nilsson et al. 1991a), increasing 0.86 1.00 the height of the river margin strongly suppressed the n.s. n.s. 1.00 species richness of most groups, whereas in this study n.s. n.s. n.s. 1.00 it was not important in any model. Instead, the exis- n.s. n.s. n.s. n.s. 1.00 n.s. n.s. n.s. Ϫ0.49 n.s. 1.00 tence of a reservoir shoreline played the same negative n.s. n.s. n.s. n.s. n.s. n.s. 1.00 role (Table 7). Considering that this study covers a larger area (and more rivers) than the previous study, it is not surprising that regional factors such as the 216 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1

TABLE 7. Partial least-squares (PLS) regression of species richness (species-density values) and plant cover in relation to environmental characteristics for four free-¯owing and four regulated rivers in northern Sweden. Local factors refer to the river margin.

Local factors Regulated ¯ow Regional factors New Growing Catchment Width of All Reservoirs river Dependent variable season Latitude area Altitude river sites only margin SPECIES RICHNESS Total 0.30 Ϫ0.13 0.27 Ϫ0.32 Ϫ0.27 Ϫ0.33 Ϫ0.37 Ϫ0.41 Morphology Trees ϩ shrubs 0.47 Ϫ0.28 0.38 Ϫ0.44 Ϫ0.25 Ϫ0.08 Ϫ0.13 Ϫ0.18 Dwarf shrubs Ϫ0.50 0.37 Ϫ0.43 0.43 0.13 Ϫ0.20 Ϫ0.15 Ϫ0.11 Graminoids 0.22 Ϫ0.09 0.18 Ϫ0.23 Ϫ0.30 Ϫ0.35 Ϫ0.40 Ϫ0.45 Forbs ϩ ferns 0.30 Ϫ0.13 0.28 Ϫ0.32 Ϫ0.28 Ϫ0.33 Ϫ0.39 Ϫ0.43 Life cycle Annuals ϩ biennials 0.45 Ϫ0.21 0.44 Ϫ0.47 Ϫ0.21 Ϫ0.17 Ϫ0.19 Ϫ0.22 Perennials 0.25 Ϫ0.11 0.21 Ϫ0.26 Ϫ0.29 Ϫ0.35 Ϫ0.41 Ϫ0.45 Naturalness Ruderal species 0.49 Ϫ0.25 0.47 Ϫ0.49 Ϫ0.19 Ϫ0.08 Ϫ0.12 Ϫ0.13 Natural species 0.19 Ϫ0.08 0.15 Ϫ0.20 Ϫ0.29 Ϫ0.38 Ϫ0.43 Ϫ0.48 Location Aquatic species 0.49 Ϫ0.30 0.42 Ϫ0.46 Ϫ0.23 Ϫ0.08 Ϫ0.11 Ϫ0.17 Riparian ϩ terrestrial spp. 0.18 Ϫ0.05 0.16 Ϫ0.21 Ϫ0.28 Ϫ0.39 Ϫ0.45 Ϫ0.48 PLANT COVER Herbs ϩ dwarf shrubs 0.30 Ϫ0.10 0.29 Ϫ0.33 Ϫ0.21 Ϫ0.38 Ϫ0.40 Ϫ0.41 Trees ϩ shrubs 0.36 Ϫ0.28 0.26 Ϫ0.32 Ϫ0.30 Ϫ0.25 Ϫ0.28 Ϫ0.31 Mean rank of loadings 4.9 9.9 7.0 4.5 7.1 7.3 5.7 5.1 Notes: The indicated values are the independent variables loading in the ®rst component. The three highest loadings in each model are boldfaced. length of growing season and latitude were important tween the free-¯owing and the regulated rivers (16% predictor variables (Table 7). Furthermore, the coef®- and 13%, respectively)Ðnot supporting convergence. cients of determination for the regressions were lower The present study con®rms the results of Nilsson et 2 in this study (Radj ϭ 0.28±0.50; Table 7) than in the al. (1991a) in that species richness in the regulated 2 previous one (Radj ϭ 0.21±0.77; Nilsson et al. 1991a). rivers was lower for most species groups, and in no case higher compared to free-¯owing rivers. One might DISCUSSION ask whether no plant life-history strategy bene®ts from First, we discuss the results of the two comparisons regulation, because in many terrestrial and aquatic eco- of rivers, then the likely mechanisms causing the dif- systems disturbance favors species with appropriate ad- ferences between free-¯owing and regulated rivers, and aptations, either to resist the disturbance or to escape last the factors controlling river-margin vegetation. it in space or time (White 1979, Gill 1981, Sousa 1984). The answer is probably that many regulated river mar- Comparison with the previous study gins are so different from natural environments that no The fact that results of the present and previous species are adapted to them. An alternative explanation (Nilsson et al. 1991a) comparisons were largely similar would be that species with a potential to proliferate on suggests generality, and hence predictability, of the ef- regulated river margins have not yet colonized after fects of regulation on riparian vegetation along boreal regulation. For example, in other parts of the world rivers. Although the ¯oristic differences between the both native and non-native species have invaded or free-¯owing Pite River and the regulated Skellefte Riv- increased in abundance on river margins following ¯ow er were not statistically signi®cant, this is more related regulation (Pautou et al. 1992, Johnson 1994, Busch to the naturally low species richness per site along the and Smith 1995, DeÂcamps et al. 1995, Friedman et al. Pite River, than to a lack of regulation effects. The 1997), but exotics are still largely absent from river Skellefte River had the same low values of local species margins in northern Sweden (Nilsson et al. 1993). richness as the other regulated rivers under consider- In the previous study (Nilsson et al. 1991a), the num- ation (Table 2, Nilsson et al. 1991a), suggesting that ber of annual and biennial species per site did not differ the magnitude of effect may vary with preregulation between the free-¯owing and the regulated rivers, but conditions, but that regulation leads to convergence in in this study the free-¯owing Torne and Kalix rivers species diversity. However, the coef®cients of variation had more such species per site than the regulated Lule for mean species richness per river were similar be- River. Organisms with short generation times are often February 2000 RIVER REGULATION AND MARGIN VEGETATION 217

TABLE 7. Extended.

Local factors

Clay ϩ Sand ϩ 2 Height Width Peat silt gravel Stones Bedrock Radj

Ϫ0.23 Ϫ0.20 0.00 0.37 Ϫ0.07 Ϫ0.18 Ϫ0.24 0.35

Ϫ0.21 Ϫ0.25 Ϫ0.11 0.29 0.04 Ϫ0.22 Ϫ0.20 0.35 0.02 0.08 0.23 Ϫ0.23 Ϫ0.20 0.17 Ϫ0.00 0.38 Ϫ0.31 Ϫ0.22 0.11 0.34 Ϫ0.11 Ϫ0.20 Ϫ0.29 0.32 Ϫ0.22 Ϫ0.21 Ϫ0.01 0.38 Ϫ0.06 Ϫ0.15 Ϫ0.21 0.28

Ϫ0.07 Ϫ0.08 Ϫ0.07 0.36 0.05 Ϫ0.26 Ϫ0.20 0.34 Ϫ0.28 Ϫ0.24 0.03 0.35 Ϫ0.11 Ϫ0.16 Ϫ0.25 0.32

Ϫ0.03 Ϫ0.10 Ϫ0.14 0.33 0.09 Ϫ0.22 Ϫ0.12 0.36 Ϫ0.31 Ϫ0.25 0.07 0.34 Ϫ0.13 Ϫ0.14 Ϫ0.26 0.30

Ϫ0.12 Ϫ0.10 Ϫ0.06 0.33 0.09 Ϫ0.27 Ϫ0.23 0.37 Ϫ0.29 Ϫ0.25 0.05 0.34 Ϫ0.14 Ϫ0.12 Ϫ0.23 0.30

Ϫ0.14 Ϫ0.11 0.05 0.32 Ϫ0.07 Ϫ0.24 Ϫ0.25 0.50 Ϫ0.25 Ϫ0.23 Ϫ0.01 0.30 Ϫ0.04 Ϫ0.26 Ϫ0.25 0.41 9.5 10.6 13.3 4.0 13.2 9.5 9.3 favored in human-made ecosystems, but such species responds differently to different types of regulation also bene®t from natural formation of disturbance (Table 4). Regulated but unimpounded reaches were patches (Grime 1979, Pickett and White 1985, Huston most similar to free-¯owing rivers, with nearly iden- 1994). In areas with recurrent ice formation, ice scour- tical species-richness values as those of free-¯owing ing may damage or remove plants and create patches rivers. In contrast, storage reservoirs and run-of-river of bare soil on lakeshores and river margins, predom- impoundments were most dissimilar in species richness inantly during the spring ¯ood (BeÂgin and Payette to the free-¯owing rivers, whereas the status of the 1991, BeÂlanger and BeÂdard 1994). Ice scouring of ri- reaches with reduced discharge is uncertain due to the parian zones occurs regularly in the Torne and Kalix low statistical power of the comparisons. rivers during the spring ¯ood (Raab and Zachrisson The hydrology has been modi®ed in fundamentally 1984, Nilsson et al. 1989), but is infrequent in the different ways in storage reservoirs (changed timing Vindel River. On the shorelines of storage reservoirs and increased intensity of water-level ¯uctuations) and and run-of-river impoundments, lacking spring ¯oods, run-of-river impoundments (increased frequency of ice scouring during ¯oods is unimportant, but ice push water-level ¯uctuations) compared to free-¯owing riv- may be considerable (Sundborg 1977). However, on ers. Despite that, these changes had similar effects on shorelines along storage reservoirs, annual species of- the shoreline vegetation, because both storage reser- ten do not have time to complete their life cycles before voirs and run-of-river impoundments had lower num- becoming submerged during reservoir ®lling in the ber of plant species per site and sparser plant cover, summer (Nilsson 1981). The difference in ice distur- compared to the free-¯owing rivers. This suggests that bance between rivers might also explain why tree and river-margin plants are sensitive both to increased fre- shrub species were more species rich in the free-¯ow- quency and changed timing of water-level ¯uctuations. ing river of the previous study, but did not differ in River margins comprise a gradient from rarely to fre- richness between groups of rivers in this study (Table quently ¯ooded habitats perpendicular to the channel. 3). In the northernmost rivers, recurrent ice disturbance Wetland and riparian plant species respond even to limits the development of woody plants, producing small changes in water levels between years (e.g., Ek- tree-less river margins in the most ice-disturbed parts zertzev 1979, Nilsson 1981, van der Valk 1981, Keddy of the rivers (Julin 1963, Arnqvist and Dynesius 1987). and Reznicek 1986). Thus, all changes in the ¯ow re- gime are likely to modify plant zonation, e.g., by con- Type of regulation tracting part of the land±water gradient. The result The results of the comparisons between types of wa- could be large changes in vegetation without changing ter-level regime suggest that river-margin vegetation species richness. Indeed, in both storage reservoirs and 218 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1 run-of-river impoundments the middle and lower parts parian soils (40 vs. 46%, n ϭ 18 and 6 river-margin of the shorelines are ¯ooded during most of the growing sites, respectively, P ϭ 0.021, t test, two-tailed prob- season, and the habitat availability for riparian and ter- ability). However, river margins on preupland soils may restrial plants thus reduced (Nilsson and Keddy 1988). also be more disturbed because they are prone to strong However, the observed differences in species richness erosion for several decades after regulation (Sundborg between types of ¯ow regimes suggest that the effects 1977). of ¯ow regulation go beyond adjustments in plant zo- The fact that the proportion of ``long ¯oaters'' in- nation. creased in run-of-river impoundments seems to con- All the four major types of regulation had more tradict the notion that hydrochores are unlikely to coarse-grained river-margin soils compared to free- spread between reservoirs. However, diaspores can still ¯owing rivers. Nilsson et al. (1991a) found a similar ¯oat around within reservoirs, and may do so for con- difference in their study and explained it as an effect siderable periods of time in the absence of currents of ¯ow regulation. Regulated water ¯ows increase the until washed ashore. ``Short ¯oaters'' will not be fa- erosion of the river margins, and deposit ®ner materials vored in their dispersal by situations that require long below the drawdown level and not on the ¯oodplain ¯oating times, and their chances to reach and invade (Petts 1984). Reservoirs and impoundments act as sed- reservoir shorelines will be smaller compared to the iment traps, and when clear water is released from long ¯oaters. Thus, the relative increase in long ¯oaters dams, it causes erosion because of its large potential may be explained by better colonization abilities com- to increase its load of suspended material (Galay 1983, pared to short ¯oaters. Alternatively, it could be that Williams and Wolman 1984, Church 1995). The erosion short ¯oating times are correlated with other traits that could reduce plant cover and species richness either are disfavored in regulated rivers. Note that an increase because colonizing plants are killed, or because there in the proportion of long ¯oaters should be offset by is less habitat for plant growth in coarse-grained soils. a decrease in short-¯oaters, because all species are ei- However, there are sites along free-¯owing rivers, e.g., ther long or short ¯oaters, but all species could not be along rapids, that have high species richness despite classi®ed. coarse substrates (R. Jansson, C. Nilsson, M. Dynesius, The negative effects of regulation on the local river- and E. Andersson, personal observations). This leaves margin habitat may thus be ampli®ed by restrictions of differences in hydrology as the most probable expla- dispersal pathways. Such modi®cations are entailed nation, acting either directly by drowning or desiccat- both by the fragmentation of the river corridor by dams, ing plants, or by making soils unstable due to erosion. increasing the distance between source populations and colonizable patches, and by the unnatural water-level Dispersal traits regimes with low ¯ow velocities and low capacities for The ®nding that species with good dispersal capacity dispersing organisms and litter (Nilsson et al. 1993). (i.e., wind-dispersed and long-¯oating species) were For example, spring ¯oods that may carry large quan- the best survivors/recolonizers on shorelines along tities of diaspores along with litter in northern free- storage reservoirs, run-of-river impoundments, and ¯owing rivers, are absent or rare (Nilsson and Grelsson reaches with reduced discharge may depend on their 1990). In the free-¯owing rivers, short ¯oaters may be ability to reinvade after frequent local extinctions dispersed by the high-velocity spring ¯oods, ¯oating caused by arti®cial water-level ¯uctuations (cf. Fahrig or adhered to ¯oating objects such as packs of organic 1991). This is not expected on margins along regulated litter (Nilsson et al. 1993). The high current velocity but unimpounded reaches, where water-level ¯uctua- of free-¯owing rivers results in a large capacity to carry tions often maintain natural rhythm and ¯ow peaks are objects heavier than water, and diaspores may be trans- often decreased. A similar case for the importance of ported long distances in a short time, making good colonization was described by Malanson and Kay ¯oating ability less important. Furthermore, establish- (1980) and Malanson (1982), who found that river- ment varies along gradients of elevation (Jones et al. margin cliffs had rapidly colonizing ferns and mosses 1994), substrate texture (Keddy and Constabel 1986), on the most frequently ¯ood-disturbed sites, and slower litter deposition (van der Valk 1986), wave disturbance colonizers, including herbs and trees, on more rarely (Wilson et al. 1985), and timing of ¯oods (Johnson ¯ooded sites. Alternatively, the poor dispersers could 1994). Thus, there is good reason to expect that species potentially survive on regulated river margins, but have with different dispersal strategies may respond differ- failed to recolonize after becoming extinct at the onset ently to river regulation. of regulation, due to dispersal barriers. If so, river mar- gins created at the onset of regulation should have the Relationships between vegetation and environment least poor dispersers. Indeed, margins along run-of- The importance in the regression models of both re- river impoundments created by raising water levels, gional factors, such as length of the growing season, which left the former river margin permanently ¯ood- and local factors, such as water-level regime, suggests ed, had lower proportions of species with no special that river-margin vegetation is governed by factors device for dispersal than did river margins on preri- working on many spatial and temporal scales (Shmida February 2000 RIVER REGULATION AND MARGIN VEGETATION 219 and Wilson 1985, Auerbach and Shmida 1987, Reed et The likelihood that ecological effects of similar al. 1993). Regional factors set the size of the pool of structures and operations of regulation vary with lo- species that potentially can survive locally. However, cation is little explored (but see Friedman et al. [1997]), species are often absent from suitable, local patches because regulation measures usually vary between because no propagules have arrived there (Primack and regions. The response to a similar type of development Miao 1992a, Tilman 1997). may also vary longitudinally along rivers (Ward and The monotonic increase in the number of species Stanford 1983). Nilsson and Jansson (1995) found that downstream along the rivers contradicts previous ®nd- plant species richness was most reduced in the middle ings that plant species richness peaks in the middle reaches of regulated rivers, and this result could not reaches of free-¯owing rivers (Nilsson et al. 1989, DeÂ- be explained by the type of regulation. camps and Tabacchi 1994), but is explained by the fact Although modi®cations of natural ¯ow regimes lead that only linear models were tested for. Some groups to changes in riparian vegetation, there is no consensus of species, such as dwarf shrubs, increased in species among studies on the direction of these changes (Nai- richness upstream. Most dwarf shrubs are evergreen, a man and DeÂcamps 1997). In many temperate regions, trait promoted in unproductive environments where ac- reductions in the depths and durations of ¯ow peaks quisition of mineral nutrients is limited (Chapin 1980, have caused riparian pioneer communities to be re- Chabot and Hicks 1982, Aerts 1995). Most dwarf placed by later successional stages, often dominated by shrubs (in this case species of the family Ericaceae) trees (Bren 1992, Johnson 1994, Scott et al. 1997). also have ericoid mycorrhiza, which is effective in ac- Most of these changes have occurred in reaches down- quiring nutrients in the nutrient-poor soils with accu- stream of dams. In northern Sweden such reaches are mulating, slowly decomposing humus layers charac- rare, and species richness and plant cover are lowest teristic of high altitudes and latitudes (Read 1991). This along reservoir shorelines, where ¯ood duration has may explain why dwarf shrubs increased in richness increased following regulation. Flow reductions are of- towards higher altitudes. ten accompanied by reductions of the riparian water Two other examples show that the relationship be- table at higher elevations, with losses of deciduous tween regional factors and vegetation may have dif- trees due to drought stress (Rood and Mahoney 1990, ferent mechanistic explanations for different functional Rood et al. 1995). Such effects are most pronounced groups. The number of ruderal species per site in- in arid regions (Stromberg and Patten 1990). Reduc- creased with growing season and catchment area, but tions in the load of suspended material downstream of this is most likely because ruderal species are associ- dams often lead to erosion and channel incision (Wil- ated with agricultural activity, and cultivation in the liams and Wolman 1984, Bradley and Smith 1986, river valley increases towards the coast (Nilsson and Johnson 1994, Friedman et al. 1997). Changes in chan- Jansson 1995). The increase in the number of aquatic nel geometry are comparatively unimportant in regu- species towards the coast probably relates to the in- lated rivers in northern Sweden, because most river creasing length of the growing season and to the de- reaches are impounded, and particle sizes are often too creasing duration and thickness of ice cover (Loham- large to be moved by the stream. mar 1965). The water level of free-¯owing rivers and Boreal rivers differ from temperate ones in that ice lakes in northern Sweden decreases gradually during disturbance may be important during winters and winter, and the lower parts of the river margins are spring ¯oods (Nilsson et al. 1993). Regulated ¯ows covered by ϳ1-m-thick ice that is lifted by the spring may increase the role of ice action in disturbing river ¯ood, pulling away vegetation frozen into the ice. Only margins. During winter drawdowns, ice subsides onto a few species can withstand these conditions, e.g., is- the shoreline that freezes, killing off plant colonists oetids such as Subularia aquatica, Ranunculus reptans, (Nilsson 1981). Frequent changes in water levels, as and Eleocharis acicularis (Renman 1989). Many aquat- in the run-of-river impoundments, cause ice breakage ic species can still be regionally present in areas with along the shorelines, with moving and displacement of long duration of ice cover but are con®ned to greater riparian soils. water depths without bottom freezing (Lohammar One of the most important factors at the onset of 1965), and thus do not occur on river margins or lake- regulation is whether the regulated river margin is lo- shores according to our de®nition. cated within, above, or below the preregulation riparian zone (Nilsson et al. 1997). The reason why situations Toward a general model where new margins develop in former uplands are the The ultimate responses of river-margin plant com- most negative to plant species richness is probably be- munities to river regulation depend on three factors: cause postregulation succession starts with few species (1) the preregulation environmental conditions, such as at hand, and with intense erosion of the river-margin variation in biogeography, climate, and soil; (2) the soil (Sundborg 1977). Although species richness did operations during the construction of dams, power sta- not vary with the time since regulation for any of the tions, etc.; and (3) the environmental conditions during types of regulation, we cannot conclude that time is the following successional trajectory. unimportant because most hydroelectric schemes in the 220 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1

TABLE 8. Suggested relative differences between boreal river margins along free-¯owing rivers and along reaches with four major types of regulated water-level regimes.

Reaches with Regulated but Free-¯owing Storage Run-of-river reduced unimpounded Variable rivers reservoirs impoundments discharge reaches Local habitat factors Heterogeneity high low low moderate moderate Complexity high low low moderate moderate Availability high low moderate moderate moderate Disturbance intensity moderate±high very high low low (occasion- low±moderate ally high) Disturbance frequency low low high low low Regional, between-site factors Corridor function good bad bad bad moderate±good Dominant colonization vector various wind water wind various Source of immigration upstream ϩ surrounding surrounding surrounding upstream ϩ surrounding populations populations populations surrounding populations populations Area of available species pool large moderate small moderate moderate±large study were of similar ages, or between 20 and 40 yr ¯uctuations, and these effects are strengthened by ice. old. In a chronosequence of regulated sites, Nilsson et In contrast, river margins along regulated reaches that al. (1997) found that there were few species and sparse are unimpounded or have reduced discharge may be vegetation cover on the margins soon after the onset less disturbed in the absence of ¯oods. However, mar- of regulation. In the following decades, many species gins along reaches with reduced discharge may occa- colonized, but species richness remained impoverished sionally be strongly disturbed during ¯ow peaks. The compared to free-¯owing rivers. relative importance of local habitat factors in explain- Table 8 summarizes the most important factors for ing the low plant species richness in regulated rivers river-margin plant species diversity during postregu- is not clear, because knowledge of how natural ¯uc- lation succession, and how these factors are hypothe- tuations in ¯ow and water level affect lakeshore and sized to differ between free-¯owing rivers and rivers riverbank vegetation (e.g. van der Valk 1981, Keddy with any of four major types of regulation, on local as 1983, Nilsson 1987, Day et al. 1988, Hughes 1990) well as regional scales. may not be applicable outside the range of natural ¯ows Local habitat conditions have been modi®ed differ- (but see Stevens et al. [1995]). The ¯oristic response ently depending on the type of ¯ow regulation. Natural to changes in water-level regime also depends on how river margins have high habitat heterogeneity because sediment and litter dynamics (Hupp and Osterkamp natural river ¯ows scour some margins and deposit 1985, Salo et al. 1986, Nilsson and Grelsson 1990, sediment on others (Hupp and Osterkamp 1985, Kal- Kalliola et al. 1991), plant establishment (Fenner et al liola and Puhakka 1988) and because ¯ow velocity var- 1985, Rood and Mahoney 1990, Hughes 1994), and ies along free-¯owing rivers (Nilsson 1987). The ri- riparian water tables (Rood et al. 1995) are affected. parian zone has high complexity, with zonation of Studies are needed in which the effects of these pro- plants along the ¯ood-duration gradient. River margins cesses are separated. For example, one might ask along regulated reaches that are unimpounded or have whether adding ®ne-grained soils to eroded river mar- reduced discharge may still be fairly heterogeneous and gins could mitigate the negative effects of regulated complex, with zonation of plants, as long as semi-nat- water levels. Another question would be about the rel- ural ¯oods and variation in ¯ow velocity remains. In ative effects of regulated water levels and changes in contrast, reservoir shorelines are spatially homoge- river-margin water tables. As a matter of fact, soils with neous, and habitat complexity is reduced, as plant zo- periodically ef¯uent groundwater are common in ri- nation is lost. The lower habitat heterogeneity is sup- parian zones and favor high plant species diversity (R. posed to reduce the differences in plant species com- Jansson, C. Nilsson, M. Dynesius, and E. Andersson, position among river-margin sites (i.e., lower ␤-diver- personal observations). When water levels are raised sity) subject to similar types of water-level regime. by damming, new river margins are created in terrace For most plant species, habitat availability is reduced or hill-slope soils where ef¯uent groundwater is usually on regulated river margins (Table 8), as shown by lower less frequent. percentage plant covers for all types of regulation. This River regulation also affects regional, between-site is probably because ®ne-grained soils are eroded, and factors (Table 8). Dams cause fragmentation of the ri- (on reservoir shorelines) ¯ood duration is increased. parian corridor and are barriers to long-distance hy- Habitat disturbance on margins along storage reser- drochory, thus reducing the corridor function. How- voirs and run-of-river impoundments is increased be- ever, corridor function may still be good along unim- cause of changed intensity or frequency of water-level pounded reaches downstream of dams if natural ¯ow February 2000 RIVER REGULATION AND MARGIN VEGETATION 221 patterns are retained. Modi®cations of the water-level of necessary ¯oods (Barinaga 1996, Wuethrich 1996, regime may change the importance of different colo- Collier et al. 1997). Dam removals (Schuman 1995) nization vectors and recruitment patterns. For example, and reregulation of ¯ows (Hesse 1995, Petts 1996) are reductions in ¯ood peaks may hamper recruitment of complementary strategies to test how much of the some riparian species, as in the case with cottonwoods river's ecological integrity can be rehabilitated. (Populus spp.) along streams in western North America The relationship between the dispersal capacities of (Fenner et al. 1985, Rood and Mahoney 1990). In the species and their responses to regulation highlights the present study, the proportion of anemochores increased need to consider large-scale spatial processes when in the shoreline ¯oras of storage reservoirs and reaches evaluating the effects of human exploitation. The out- with reduced discharge, while the proportions of spe- come is not only determined by the ability of biota to cies without any special device for dispersal decreased. survive in the local environment; the ability to get there This suggests that the ability to recolonize following is equally important. For example, species with good local extinction may be critical. In run-of-river im- dispersal capacities were the best survivors on reser- poundments, hydrochory may be effective, but pri- voir shorelines as well as on margins along reaches marily for long-¯oating species, which will be stranded with reduced discharge. This emphasizes the need for on the river margin, while short ¯oaters may sink. studying both the corridor functions of rivers and the Since dams block populations from dispersing down- relative importance of surrounding biota in supplying stream by water, surrounding populations should be the rivers with new organisms. Studies separating the ef- main source of immigration to regulated river margins, fects of river-corridor fragmentation per se from local and the similarity in species composition between river site disturbances are called for. Comparisons of the margins and nearby uplands likely increased. The effects of river regulation between biogeographic break-up of the river continuum suggests that differ- regions are also likely to contribute substantially to our ences in species composition between impoundments knowledge about riparian plant ecology. may be considerable. However, on river margins along ACKNOWLEDGMENTS unimpounded reaches, downstream dispersal may be We thank Francine Hughes, Rebecca Sharitz and anony- effective, but the distance to the nearest upstream dam mous reviewers who improved the manuscript with their com- determines the area of the available species pool (Zobel ments. BjoÈrn Carlberg, Alf Ekblad, Maria Gardfjell, Gunnel 1997). Where dispersal ability is likely to be important Grelsson, Mats E. Johansson, and Ulf Sperens assisted in the for local persistence, as in storage reservoirs and reach- 1985±1988 ®eldwork. Lena KjaÈllgren assisted in the pro- cessing of primary data, and Tom Korsman helped with the es with reduced discharge, the area of available species PLS regressions. This work was funded by the Swedish Nat- pool may still be fairly large, but constrained to species ural Science Research Council and the Swedish Society for with good dispersal ability. the Conservation of Nature (grants to C. Nilsson). Study implications LITERATURE CITED Aerts, R. 1995. The advantages of being evergreen. Trends Plant species richness and cover were reduced in run- in Ecology and Evolution 10:402±407. of-river impoundments and in storage reservoirs, which AÊ ngstroÈm, A. 1974. Sveriges klimat. Third edition. [In both experience water-level ¯uctuations that are rather Swedish.] Generalstabens Litogra®ska Anstalts FoÈrlag, Stockholm, Sweden. different from the natural situation. However, the Anonymous. 1979. Stream¯ow records of Sweden. Swedish smallest deviation from free-¯owing water-level re- Metereological and Hydrological Institute, Stockholm, gimes able to produce such reductions is unknown. To Sweden. maintain different structures of river-margin vegetation Arnqvist, G., and M. Dynesius. 1987. RaÊneaÈlven. Naturin- we need to know what durations and frequencies of ventering och bedoÈmning av vetenskapliga naturvaÈrden. [In Swedish.] LaÈnsstyrelsen i Norrbottens LaÈn, LuleaÊ, Sweden. ¯ooding they require (Richter et al. 1997). Up to now, Auerbach, M., and A. Shmida. 1987. Spatial scale and the hydroelectric schemes have not been designed to min- determinants of plant species richness. Trends in Ecology imize losses of ecological integrity. In the future, river and Evolution 2:238±242. managers will face increased demands to make river Barinaga, M. 1996. A recipe for river recovery? Science 273: 1648±1650. ¯ows available also for uses other than power produc- Bartley, M. R., and D. H. N. Spence. 1987. Dormancy and tion, such as conservation of biodiversity, drinking wa- propagation in helophytes and hydrophytes. Archiv fuÈr Hy- ter supply, irrigation, and recreation (Postel et al. drobiologie, Beihang 27:139±155. 1996). Presently, we do not know the relationships be- BeÂgin, Y., and S. Payette. 1991. Population structure of lake- tween deviations from natural ¯ows and losses of eco- shore willows and ice-push events in subarctic QueÂbec, Canada. Holarctic Ecology 14:9±17. logical integrity. The relationship may be more or less BeÂlanger, L., and J. BeÂdard. 1994. Role of ice scouring and linear, or there may be certain thresholds. If thresholds goose grubbing in marsh plant dynamics. Journal of Eco- are present, as described by Scott et al. (1997), it is logy 82:437±445. important to identify them, and to let regulated ¯ow Bradley, C. E., and D. G. Smith. 1986. Plains cottonwood recruitment and survival on a prairie meandering river regimes match those thresholds in order to reduce per- ¯oodplain, Milk River, southern Alberta and northern Man- turbations to the biota. Experimental ¯ooding may be itoba. Canadian Journal of Botany 64:1433±1442. extremely helpful in identifying the size and frequency Bren, L. J. 1992. Tree invasion of an intermittent wetland 222 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1 in relation to changes in the ¯ooding frequency of the River regulated water ¯ows on regeneration of Fremont cotton- Murray, Australia. Australian Journal of Ecology 17:395± wood (Populus fremontii). Journal of Range Management 408. 38:135±138. Brock, J. H. 1994. Tamarix spp. (salt cedar), an invasive Friedman, J. M., M. L. Scott, and G. T. Auble. 1997. Water exotic woody plant in arid and semi-arid riparian habitats management and cottonwood forest dynamics along prairie of western USA. Pages 27±44 in L. C. de Waal, L. E. Child, streams. Pages 49±71 in F. J. Knopf and F. B. Samson, P. M. Wade, and J. H. Brook, editors. Ecology and man- editors. Ecology and conservation of Great Plains verte- agement of invasive riverside plants. John Wiley & Sons, brates. Springer-Verlag, New York, New York, USA. Chichester, UK. Galay, V. J. 1983. Causes of river bed degradation. Water Brown, J. H., and A. Kodric-Brown. 1977. Turnover rates Resources Research 19:1057±1090. in insular biogeography: effects of immigration on extinc- Geladi, P., and B. R. Kowalski. 1986. Partial least squares tion. Ecology 58:445±449. regression: a tutorial. Analytica Chimica Acta 185:1±17. Busch, D. E., and S. D. Smith. 1995. Mechanisms associated Gill, A. M. 1981. Fire adaptive traits of vascular plants. with the decline of woody species in riparian ecosystems Pages 208±230 in USDA Forest Service General Technical of the southwestern U.S. Ecological Monographs 65:347± Report GTR-WO-26. 370. Gregory, S. V., F. J. Swanson, W. A. McKee, and K. W. Chabot, B. F., and D. J. Hicks. 1982. The ecology of leaf Cummins. 1991. An ecosystem perspective of riparian life cycles. Annual Review of Ecology and Systematics 13: zones. BioScience 41:540±551. 229±259. Grelsson, G., and C. Nilsson. 1980. Colonization by Pinus Chapin, F. S., III. 1980. The mineral nutrition of wild plants. sylvestris of a former middle-geolittoral habitat on the Annual Review of Ecology and Systematics 11:233±260. UmeaÈlven river in northern Sweden, following river reg- Chorley, R. J., S. A. Schumm, and D. E. Sugden. 1984. ulation for hydro-electric power. Holarctic Ecology 3:124± Geomorphology. Methuen, New York, New York, USA. 128. Church, M. 1995. Geomorphic responses to river ¯ow reg- Grelsson, G., and C. Nilsson. 1982. FoÈraÈndringar i en aÈlvdals ulation: case studies and time scales. Regulated Rivers: vegetation belysta genom upprepad fotografering. [In Research and Management 11:3±22. Swedish.] Svensk Botanisk Tidskrift 76:368±379. Collier, M. P., R. H. Webb, and E. D. Andrews. 1997. ``Ex- Grime, J. P. 1979. Plant strategies and vegetation processes. perimental ¯ooding in Grand Canyon.'' Scienti®c Ameri- John Wiley & Sons, Chichester, UK. can, January 1997:82±89. Guppy, H. B. 1891±1893. The River Thames as an agent in Connor, E. F., and E. D. McCoy. 1979. The statistics and plant dispersal. Journal of the Linnean Society 29:333±346. biology of the species±area relationship. American Natu- Hargrove, W. W., and J. Pickering. 1992. Pseudoreplication: ralist 113:791±833. a sine qua non for regional ecology. Landscape Ecology Cook, C. D. K. 1987. Dispersion in aquatic and amphibious 6:251±258. vascular plants. Pages 179±190 in R. M. M. Crawford, Hesse, L. W. 1995. Water allocation for ecosystem manage- editor. Plant life in aquatic and amphibious habitats. Black- ment of the Missouri River. Regulated Rivers: Research well Scienti®c, Oxford, UK. and Management 11:299±311. Danvind, M., and C. Nilsson. 1997. Seed ¯oating ability and Hjelmqvist, S. 1953. The bedrock of Sweden excluding the distribution of alpine plants along a northern Swedish river. Caledonian mountain range. Maps 7 and 8 in M. Lundqvist, Journal of Vegetation Science 8:271±276. editor. National atlas of Sweden. Generalstabens Litograf- Day, R. T., P. A. Keddy, J. McNeill, and T. Carleton. 1988. iska Anstalts FoÈrlag, Stockholm, Sweden. Fertility and disturbance gradients: a summary model for Holm, S. 1979. A simple sequentially rejective multiple test riverine marsh vegetation. Ecology 69:1044±1054. procedure. Scandinavian Journal of Statistics 6:65±70. DeÂcamps, H., A. M. Planty-Tabacchi, and E. Tabacchi. 1995. Hughes, F. M. R. 1990. The in¯uence of ¯ooding regimes Changes in the hydrological regime and invasions by plant on forest distribution and composition in the Tana River species along riparian systems of the Adour River, France. ¯oodplain, Kenya. Journal of Applied Ecology 27:475± Regulated Rivers: Research and Management 11:23±34. 491. DeÂcamps, H., and E. Tabacchi. 1994. Species richness in Hughes, F. M. R. 1994. Environmental change, disturbance vegetation along river margins. Pages 1±20 in P. S. Giller, and regeneration in semi-arid ¯oodplain forests. Pages A. G. Hildrew, and D. G. Raffaelli, editors. Aquatic eco- 321±345 in A. C. Millington and K. Pye, editors. Envi- logy. Scale, pattern and process. Blackwell Scienti®c, Ox- ronmental change in drylands. Biogeographical and geo- ford, UK. morphological perspectives. John Wiley & Sons, Chich- DeFerrari, C. M., and R. J. Naiman. 1994. A multi-scale ester, UK. assessment of the occurrence of exotic plants on the Olym- Hupp, C. R., and W. R. Osterkamp. 1985. Bottomland veg- pic Peninsula, Washington. Journal of Vegetation Science etation distribution along Passage Creek, Virginia, in re- 5:247±258. lation to ¯uvial landforms. Ecology 66:670±681. de Waal, L. C., L. E. Child, P. M. Wade, and J. H. Brook, Huston, M. A. 1994. Biological diversity. The coexistence editors. 1994. Ecology and management of invasive riv- of species on changing landscapes. Cambridge University erside plants. John Wiley & Sons, Chichester, UK. Press, Cambridge, UK. Dynesius, M., and C. Nilsson. 1994. Fragmentation and ¯ow Johansson, M. E., and C. Nilsson. 1993. Hydrochory, pop- regulation of river systems in the northern third of the ulation dynamics and distribution of the clonal aquatic plant world. Science 266:753±762. Ranunculus lingua. Journal of Ecology 81:81±91. Ekzertzev, V. A. 1979. The higher aquatic vegetation of the Johansson, M. E., C. Nilsson, and E. Nilsson. 1996. Do rivers Volga. Monographiae Biologicae 33:271±294. function as corridors for plant dispersal? Journal of Veg- Fahrig, L. 1991. Simulation methods for developing general etation Science 7:593±598. landscape-level hypotheses of single-species dynamics. Johnson, W. C. 1994. Woodland expansion in the Platte Riv- Pages 417±442 in M. G. Turner and R. H. Gardner, editors. er, Nebraska: patterns and causes. Ecological Monographs Quantitative methods in landscape ecology. Analysis and 64:45±84. interpretation of landscape heterogeneity. Springer-Verlag, Jones, R. H., R. R. Sharitz, P. D. Dixon, D. S. Segal, and R. New York, New York, USA. L. Schneider. 1994. Woody plant regeneration in four Fenner, P., W. W. Brady, and D. R. Patton. 1985. Effects of ¯oodplain forests. Ecological Monographs 64:345±367. February 2000 RIVER REGULATION AND MARGIN VEGETATION 223

Julin, E. 1963. Den isgaÊngsbetingade aÈngen vid Sundholmen north Swedish hydroelectric reservoir during a 5-year pe- i Torne aÈlvs mynningsomraÊde. [In Swedish.] Svensk Bo- riod. Acta Phytogeographica Suecica 69:1±96. tanisk Tidskrift 57:19±38. Nilsson, C. 1983. Frequency distributions of vascular plants Kalliola, R., and M. Puhakka. 1988. River dynamics and in the geolittoral vegetation along two rivers in northern vegetation mosaicism: a case study of the River Kamajoh- Sweden. Journal of Biogeography 10:351±369. ka, northernmost Finland. Journal of Biogeography 15: Nilsson, C. 1987. Distribution of stream-edge vegetation 703±719. along a gradient of current velocity. Journal of Ecology 75: Kalliola, R., J. Salo, M. Puhakka, and M. Rajasilta. 1991. 513±522. New site formation and colonizing vegetation in primary Nilsson, C. 1992. Increasing the reliability of vegetation succession on the western Amazon ¯oodplains. Journal of analyses by using a team of two investigators. Journal of Ecology 79:877±901. Vegetation Science 3:565. Keddy, P. A. 1983. Shoreline vegetation in Axe Lake, On- Nilsson, C., and J. E. Brittain. 1996. Remedial strategies in tario: effects of exposure on zonation patterns. Ecology 64: regulated rivers: introductory remarks. Regulated Rivers: 331±344. Research and Management 12:347±351. Keddy, P. A. 1989. Competition. Chapman & Hall, London, Nilsson, C., A. Ekblad, M. Gardfjell, and B. Carlberg. 1991a. UK. Long-term effects of river regulation on river-margin veg- Keddy, P. A., and P. Constabel. 1986. Germination of ten etation. Journal of Applied Ecology 28:963±987. shoreline plants in relation to seed size, particle size and Nilsson, C., M. Gardfjell, and G. Grelsson. 1991b. Impor- water level. Journal of Ecology 74:133±141. tance of hydrochory in structuring plant communities along Keddy, P. A., and A. A. Reznicek. 1986. Great Lakes veg- rivers. Canadian Journal of Botany 69:2631±2633. etation dynamics: the role of ¯uctuating water levels and Nilsson, C., and G. Grelsson. 1990. The effects of litter buried seeds. Journal of Great Lakes Research 12:25±36. displacement on riverbank vegetation. Canadian Journal of Krahulec, F., and J. LepsÏ. 1994. Establishment success of Botany 68:735±741. plant immigrants in a new water reservoir. Folia Geobo- Nilsson, C., G. Grelsson, M. Johansson, and U. Sperens. tanica et Phytotaxonomica 29:3±14. 1989. Patterns of plant species richness along riverbanks. Krok, T. O. B. N., and S. Almquist. 1984. Svensk ¯ora. Ecology 70:77±84. Fanerogamer och ormbunksvaÈxter, Twenty-sixth edition. Nilsson, C., and R. Jansson. 1995. Floristic differences be- [In Swedish.] Esselte Studium, Uppsala, Sweden. tween riparian corridors of regulated and free-¯owing bo- Kulling, O. 1953. Bedrock of the Caledonian range. Maps real rivers. Regulated Rivers: Research and Management 7 and 8 in M. Lundqvist, editor. National atlas of Sweden. 11:55±66. Generalstabens Litogra®ska Anstalts FoÈrlag, Stockholm, Nilsson, C., R. Jansson, and U. Zinko. 1997. Long-term Sweden. responses of river-margin vegetation to water-level regu- LepsÏ, J., and V. HadincovaÂ. 1992. How reliable are our veg- lation. Science 276:798±800. etation analyses? Journal of Vegetation Science 3:119±124. Nilsson, C., and P. A. Keddy. 1988. Predictability of change Lid, J. 1987. Norsk, svensk, ®nsk ¯ora. Fifth edition. [In in shoreline vegetation in a hydroelectric reservoir, north- Norwegian.] Det Norske Samlaget, Oslo, Norway. ern Sweden. Canadian Journal of Fisheries and Aquatic Lohammar, G. 1965. The vegetation of Swedish lakes. Acta Sciences 45:1896±1904. Phytogeographica Suecica 50:28±47. Nilsson, C., E. Nilsson, M. E. Johansson, M. Dynesius, G. Lundqvist, J. 1970. Botaniska data om norra Sveriges vat- Grelsson, S. Xiong, R. Jansson, and M. Danvind. 1993. tenomraÊden. Meddelande nr. 1. [In Swedish.] Bio-Data- Processes structuring riparian vegetation. Pages 419±431 Gruppen, Naturhistoriska Riksmuseet, Stockholm, Sweden. in J. Menon, editor. Current Topics in Botanical Research Lundqvist, M., editor. 1953±1971. National atlas of Sweden. 1. Council of Scienti®c Research Integration, Trivandrum, Generalstabens Litogra®ska Anstalts FoÈrlag, Stockholm, India. Sweden. NorusÏis, M. J. 1993. SPSS for Windows, version 6.0. SPSS, Malanson, G. P. 1982. The assembly of hanging gardens: Chicago, Illinois, USA. effects of age, area, and location. American Naturalist 119: Pautou, G., J. Girel, and J.-L. Borel. 1992. Initial repercus- 145±150. sions and hydroelectric developments in the French Upper Malanson, G. P., and J. Kay. 1980. Flood frequency and the Rhone Valley: a lesson for predictive scenarios proposi- assemblage of dispersal types in hanging gardens of the tions. Environmental Management 16:231±242. Narrows, Zion National Park, Utah. Great Basin Naturalist Petts, G. E. 1984. Impounded rivers. Perspectives for eco- 40:365±371. logical management. John Wiley & Sons, Chichester, UK. Martens, H., and T. Naes. 1989. Multivariate calibration. Petts, G. E. 1996. Water allocation to protect river ecosys- John Wiley & Sons, Chichester, UK. tems. Regulated Rivers: Research and Management 12: Melin, R., and T. Gihl. 1957. Watercourses and drainage 353±366. basins, territorial waters. Maps 37 and 38 in M. Lundqvist, Pickett, S. T. A., and P. S. White, editors. 1985. The ecology editor. National atlas of Sweden. Generalstabens Litograf- of natural disturbance and patch dynamics. Academic iska Anstalts FoÈrlag, Stockholm, Sweden. Press, Orlando, Florida, USA. Mossberg, B., L. Stenberg, and S. Ericsson. 1992. Den nor- Planty-Tabacchi, A.-M., E. Tabacchi, R. J. Naiman, C. diska ¯oran. [In Swedish.] WahlstroÈm och Widstrand, DeFerrari, and H. DeÂcamps. 1996. Invasibility of species- Stockholm, Sweden. rich communities in riparian zones. Conservation Biology Naiman, R. J., and H. DeÂcamps. 1997. The ecology of in- 10:598±607. terfaces: riparian zones. Annual Review of Ecology and Postel, S. L., G. C. Daily, and P. R. Ehrlich. 1996. Human Systematics 28:621±658. appropriation of renewable fresh water. Science 271:785± Naiman, R. J., H. DeÂcamps, and M. Pollock. 1993. The role 788. of riparian corridors in maintaining regional biodiversity. Primack, R. B., and S. L. Miao. 1992a. Dispersal can limit Ecological Applications 3:209±212. local plant distribution. Conservation Biology 6:513±519. Nilsson, C. 1979. Geolittoral extent of Vaccinium myrtillus Primack, R. B., and S. L. Miao. 1992b. ``Safe sites'' for and vegetational differentiation on a bank of the Muddus- germination using Plantago seeds: a repetition of a thrice- jokk River, North Sweden. Holarctic Ecology 2:137±143. published experiment. Bulletin of the Torrey Botanical Nilsson, C. 1981. Dynamics of the shore vegetation of a Club 118:154±160. 224 ROLAND JANSSON ET AL. Ecological Applications Vol. 10, No. 1 Raab, G., and G. Zachrisson. 1984. Isproblem i TorneaÈlven. instream ¯ow requirements: a case study from a diverted [In Swedish.] Vannet i Norden 17:4±19. stream in the Eastern Sierra Nevada, California, USA. En- Read, D. J. 1991. Mycorrhizas in ecosystems. Experientia vironmental Management 14:185±194. 47:376±391. Sundborg, AÊ . 1977. AÈ lv, kraft, miljoÈ. Vattenkraftutbyggna- Reed, R. A., R. K. Peet, M. W. Palmer, and P.S. White. 1993. dens miljoÈeffekter. [In Swedish.] Swedish Environmental Scale dependence of vegetation±environment correlations: Protection Agency, Stockholm, Sweden. a case study of a North Carolina piedmont woodland. Jour- TheÂbaud, C., and M. Debussche. 1991. Rapid invasion of nal of Vegetation Science 4:329±340. Fraxinus ornus L. along the Herault River in southern Renman, G. 1989. Distribution of littoral macrophytes in a France: the importance of seed dispersal by water. Journal north Swedish riverside lagoon in relation to bottom freez- of Biogeography 18:7±12. ing. Aquatic Botany 33:243±256. Tilman, D. 1988. Plant strategies and the dynamics and struc- Richardson, D. M., I. A. W. Macdonald, P. M. Holmes, and ture of plant communities. Princeton University Press, R. M. Cowling. 1992. Plant and animal invasions. Pages Princeton, New Jersey, USA. 271±308 in R. M. Cowling, editor. The ecology of fynbos. Tilman, D. 1997. Community invasibility, recruitment lim- Nutrients, ®re and diversity. Oxford University Press, Cape itation, and grassland biodiversity. Ecology 78:81±92. Town, South Africa. van der Pijl, L. 1982. Principles of dispersal in higher plants. Richter, B. D., J. V. Baumgartner, R. Wigington, and D. P. Springer-Verlag, Berlin, West Germany. Braun. 1997. How much water does a river need? Fresh- van der Valk, A. G. 1981. Succession in wetlands: a Glea- water Biology 37:231±249. sonian approach. Ecology 62:688±696. Romell, L.-G. 1938. VaÈxternas spridningsmoÈjligheter. Pages van der Valk, A. G. 1986. The impact of litter and annual 279±448 in C. Skottsberg, editor. VaÈxternas liv IV. [In plants on recruitment from the seed bank of a lacustrine Swedish.] Nordisk Familjeboks FoÈrlag Aktiebolag, Stock- wetland. Aquatic Botany 24:13±26. holm, Sweden. Walter, H. 1985. Vegetation of the earth and ecological sys- Rood, S. B., and J. M. Mahoney. 1990. Collapse of riparian tems of the geo-biosphere. Third edition. Springer-Verlag, poplar forest downstream from dams in western prairies: Berlin, West Germany. probable causes and prospects for mitigation. Environ- Ward, J. V., and J. A. Stanford. 1983. The serial discontinuity mental Management 14:451±464. concept of lotic ecosystems. Pages 29±42 in T. D. Fontaine Rood, S. B., J. M. Mahoney, D. E. Reid, and L. Zilm. 1995. III and S. M. Bartell, editors. Dynamics of lotic ecosystems. Instream ¯ows and the decline of riparian cottonwoods Ann Arbor Science, Ann Arbor, Michigan, USA. along the St. Mary River, Alberta. Canadian Journal of WasseÂn, G. 1966. Gardiken. Vegetation und Flora eines lap- Botany 73:1250±1260. plaÈndischen Seeufers. [In German.] Kungliga Svenska Ve- Root, R. B. 1967. The niche exploitation pattern of the Blue- tenskapsakademiens Avhandlingar i NaturskyddsaÈrenden grey Gnatcatcher. Ecological Monographs 37:317±350. 22:1±142. Rudberg, S. 1970. Geomorphology. Maps 5 and 6 in M. White, P. S. 1979. Pattern, process, and natural disturbance Lundqvist, editor. National Atlas of Sweden. Generalsta- in vegetation. Botanical Review 45:229±299. bens Litogra®ska Anstalts FoÈrlag, Stockholm, Sweden. Whittaker, R. H. 1972. Evolution and measurement of spe- Salo, J., R. Kalliola, I. HaÈkkinen, Y. MaÈkinen, P. NiemelaÈ, cies diversity. Taxon 21:213±251. M. Puhakka, and P. Coley. 1986. River dynamics and the Williams, G. P., and M. G. Wolman. 1984. Downstream ef- diversity of Amazon lowland forest. Nature 322:254±258. fects of dams on alluvial rivers. Geological Survey Pro- fessional Paper 1286:1±64. Schneider, R. L., and R. R. Sharitz. 1988. Hydrochory and Willson, M. F., B. L. Rice, and M. Westoby. 1990. Seed regeneration in a bald cypress±water tupelo swamp forest. dispersal spectra: a comparison of temperate communities. Ecology 69:1055±1063. Journal of Vegetation Science 1:547±560. Scott, M. L., G. T. Auble, and J. M. Friedman. 1997. Flood Wilson, S. D., P. A. Keddy, and D. L. Randall. 1985. The dependency of cottonwood establishment along the Mis- distribution of Xyris difformis along a gradient of exposure souri River, Montana, USA. Ecological Applications 7: to waves: an experimental study. Canadian Journal of Bot- 677±690. any 63:1226±1230. Shmida, A., and M. V. Wilson. 1985. Biological determinants Wold, S. 1978. Cross-validatory estimation of the number of species diversity. Journal of Biogeography 12:1±20. of components in factor and principal components models. Shuman, J. R. 1995. Environmental considerations for as- Technometrics 20:397±405. sessing dam removal alternatives for river restoration. Reg- Wootton, J. T., M. S. Parker, and M. E. Power. 1996. Effects ulated Rivers: Research and Management 11:249±261. of disturbance on river food webs. Science 273:1558±1561. SjoÈrs, H. 1973. Om botaniska skyddsvaÈrden vid aÈlvarna.[In Wright, J. F., D. Moss, P. D. Armitage, and M. T. Furse. 1984. Swedish.] Department of Ecological Botany, Uppsala Uni- A preliminary classi®cation of running-water sites in Great versity, Uppsala, Sweden. Britain based on macro-invertebrate species and the pre- Sokal, R. R., and F. J. Rohlf. 1995. Biometry. Third edition. diction of community type using environmental data. W. H. Freeman & Company, New York, New York, USA. Freshwater Biology 14:221±256. Sousa, W. P. 1984. The role of disturbance in natural com- Wuethrich, B. 1996. Deliberate ¯ood renews habitats. Sci- munities. Annual Review of Ecology and Systematics 15: ence 272:344±345. 353±391. Zar, J. H. 1996. Biostatistical analysis. Third edition. Pren- Stevens, L. E., J. E. Schmidt, T. J. Ayers, and B. T. Brown. tice-Hall, Upper Saddle River, New Jersey, USA. 1995. Flow regulation, geomorphology, and Colorado Riv- Zobel, M. 1997. The relative role of species pools in deter- er marsh development in the Grand Canyon, Arizona. Eco- mining plant species richness: an alternative explanation logical Applications 5:1025±1039. of species coexistence? Trends in Ecology and Evolution Stromberg, J. C., and D. C. Patten. 1990. Riparian vegetation 12:266±269.