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 q 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.