Sexual Reproductive Ecology of Carex Bigelowii an Arctic‐Alpine Sedge

::OGRAPHY 22; 305-313. Copenhagen 1999

Sexual reproductive ecology of Carex higelowiU an arctic-alpine sedge

[ina Stenstrom

Stenstrom, A, 1999, Sexual reproductive ecology of Carex bigduwii, an arcUc-alpine sedge. - Ecography 22: 305 313.

Care.x higeh)\ii is an arctic-alpine sedge that shows an extensive clonal growth, and in many years also has a high degree of flowering. To see which factors are most imporlani in determining the sexual reproductive success, the effects of self- and cross-pollination, pollen donor distance, amount of pollen, temperature, and time of flowering were studied. The study was conducted at Latnjajaure Field Station, northern Sweden and Thingvellir Natioiiid Park, Iceland during three field seasons. Experitnentally increased air temperature raised Ihe fruit set and the fruit weight, in two of the three seasons. Pollen donor distance did not influence eilher fruit set or fruit weight, and there was no differenee in fVuit weight between selfing and cross-pollination. An increased amount of pollen raised the Triiit set. but did not afTect fruit weight. The time of flowering was also important; there was a negative relationship between flowering time and fruit set and fruit weight, i.e. late flowering ramets had a lower fruit set and fruit weight. The results suggest that Carex higetowii is an outcrossed species where the reproduclive success is mainly determined by the weather, and that there are differences between the two study sites.

A. Slenslrdni (aniui.slc'[email protected]). Dept of Bolciny, Golebor Univ., Box 461. SE-405 30 Golehorg. Sxveckn.

Plants that reproduce sexually face many problems in These authors concluded that sexual reproduction is the arctic etivirontiient. They include pollen limitation, important in this species, or at least has been important irregular seed set, and low seedling survival because of in the past. However, to maintain genetic variation in low temperature, low nutrient availability, short grow- the long term, it is not only necessary fo rcprodtice iti^ seasons and mechanical disturbance (Scrensen sexually. The seeds should result from outcrossing IS4I). Accordingly, many studies have shown that there rather than selfing and seedling establishment must be is a high proportion of clonal plants in the Arctic and possible. In a clonal plant outcrossing may require Stabarctic compared with other biomes (Callaghan and transfer of pollen from a far distance. How far depends Emanuelsson 1985, Jonsdottir et al. 1996), as sexual on the clonal structure, which varies between species reproduction is not as common in clonal plants as in and which, can also vary between sites for the same non-clonal plants (Schmid 1990). However, in recent species (Jonsson 1995), years several studies on genetic variation within popula- Climate is an important factor influencing both flow- ticj^ns of clonal plants have shown their levels ol" varia- ering and seed set in many arctic species (Billings 1987). ti(j>n to be as high as in non-clonal plants (Silander In the climate change experiments of The Jnternalional 1985, Ellstrand and Roose 1987, Hamrick and Godt Tundra Experiment, 10 out of II species showed in- 1989). As one of the few arctic clonal plants investi- creased seed set or seed weight in response to a 2-3°C gated. Carex bigelowii has been shown to have a level warmer than ambient air temperature (Henry and Mo- of within-population variability comparable to sexually lau 1997). Due to the adverse climate many arctic reproducing outcrossing species (Jonsson et al. 1996), plants are thought to be pollen limited (Billings 1987),

EOXJRAPHY 22:3 305 which means that flowering at the right time becomes vellir, Iceland. The flowers each have two white, exerted very important (Callaghan and Emanuelsson 1985). stigmas, and a single ovule. The fruit is an achenc Since the weather varies strongly from year to year, (terms of flowering morphology follow Reznicek 1990). flowering is very variable among years in aretic plants Like almost all Carex species. C. higehwii is wind-polli- (e.g. Sorensen 1941, Kalela 1962). Seed production is nated. At Latnjajaure it is protogynous. i.e. the sligmas generally low, but in unusually favourable years a large develop before the anthers (Stenstrom and Jonsdottir number of seeds may be produced (Callaghan and 1997). In contrast, at Thingvellir it is protandrous with Emanuelsson 1985). Seedling recruitment in closed veg- the anthers developing first (Jonsson et al. 1996). At etation of arctic and subarctic clonal plants is usually Latnjajaure in 1995 it started to flower 46 d after absent or very low (e.g. Callaghan and Emanuelsson snowmelt (Stenstrom and Jonsdottir 1997), but the time 1985. Jonsdottir 1991, 1995, this study). However, since of snowmelt can vary over small distanees, thereby many of these plants are long-lived, often have a long- separating nearby flowering ramets in time. lived seedbank (McGraw 1980, Jonsdottir 1995). and are known to reproduce from seeds in disturbed areas (Gartner et al. 1983. Callaghan and Emanuelsson 1985, Jonsdottir 1995, this study) seedling estabhshment Study sites probably occurs, but is too infrequent to be observed in any study of normal length and size. The fieldwork was carricd-out at two sites: at Latnja- jaure Field Station, northern Sweden during 1995-1997 As sexual reproduction seems to play an important and at Thingvellir, Iceland, during 1996 1997 (Fig. I). role even in plants investing a lot in ctonal growth. I wanted to study which factors determine the sexual Latnjajaure Field Station (68°21'N. 18°30'E) is a reproductive success in Carex higelowii at two contrast- subarctic-alpine site situated at 1000 m a.s.l.. which is ing subarctic sites. The following questions are asked: ca 300 m above the treeline. The bedrock consists of 1) is there a difference in reproductive success between mica garnet schist mixed with acid moraine (Kling pers. self- and cross-pollinated ramets? 2) Does the pollen- comm.). The experimental site is a mesic heath commu- donor distance have any effect? 3) What effect does a nity with C. bigelowii, Carex vaginata, Salix redculala, variable amount of pollen have? 4) Is the time of Bisturia vivipara, and Dryas octopetala as the main flowering of any significance? 5) Does temperature af- species (plant names follow Mossberg et al. (1992)). fect reproductive success? I also studied seedling emer- The annual mean temperature is — 2.7°C. mean tem- gence in both undisturbed, closed vegetation and in perature June August 6.5°C (1990-97) and mean pre- disturbed plots. cipitation 803 mm yr" ' (1990 96). The variation, both in temperature and precipitation (Table I), was very large during the years of the study period. Thingvellir (64°17'N 21°03'W) is a subarctic site situ- Material and methods ated at 120 m a.s.l. on an 8000 yr old post-glacial lava-field. The vegetation consists of a 5-30 cm thick The species carpet of the moss Racomilriuni lanuginosum, and the main vascular plant species growing in the moss are C Carex bigelowii Torr. ex Schwein is a circumpolar, bigelowii^ Festuca rubra. and Festuca rivipara. The an- clonal sedge, which is common in the Arctic. Subarctic, nual mean tetnperature is + 3.5°C. mean temperature and in some mountain ranges further south (Murray 1994). It grows in many kinds of habitats, often forming large rhizome systems with large genets covering several square meters (Jonsson 1995). The ramets may flower when 2-4 yr old and the shoot apex dies after flowering, since the apical meristem is used up (Carlsson et al. 1990). The flowering bud is preformed the year before flowering (Sorensen 1941), but not all flowering buds will develop into flowers (Hansson and Jonsdottir unpubl). The proportion of flowering ramets varies greatly from year to year (Carlsson and Callaghan 1994, Slenstrom and Jonsdottir 1997), which has been attributed to variation in July temperature the year before flowering (Carlsson and Callaghan 1994). Carex higehwii is monoecious, forming one male spike on top of 1-5 female spikes. A flowering culm contains on average 31 ±2 female flowers at Latnjajaure, Swe- Fig. 1. Map ol UK- licld-siies. (a) is Thingvellir National Park, den and 41+3 (means + SE) female flowers at Thing- Iceland and (b) is Latnjajaure Field Station, Sweden.

306 F.COtiRAPHY 22:.l 11999) Table 1. Mean Junc-Augusl temperature and total precipitation for Latnjajaure, northern Sweden and Thingvellir, Iceland during the study years. For Latniajaure. all data arc from the climate station at the field-station. For Thingvetlir, temperature data arc from the site but precipitation data are from Stifiisdalur. \5 km west of Thingvellir.

Latnjajaure Thingvelhr

temperature ("C) precipitation (tnm — 3 months) temperature (°C) precipitation (mm = 3 months)

M95 5.2 268 ]')96 6.8 201 9.9 355: 1997 8.0 99 10.2 299 niean 6 5 173 lO.U 251

Jtine-August IO.O°C, and mean precipitation 1340 Pollen qnantity experiment mm yr"' (Einarsson 1992). During the study period the weather varied much less than at Latnjajaure The effect of the amount of pollen was assessed at {Table 1). both sites in 1997. Forty ramets were caged and emasculated with the same techniques as in the auto- deposition experiment. Flowering culms were col- lected, the pollen transferred to microscope slides, Auto-deposition experiment and then a certain area of pollen was transferred to To measure the effect of auto-deposition and cross-pol- the stigtnas with a thin brush. Half of the ramets lination on fruit set and fruit weight, an experiment was were pollinated with "much pollen" (380000 ± set up where twenty flowering ramets were caged, 102300 pollen grains) and half with "little pollen" twenty ramets emasculated, and twenty ramets left as (55000+16400 pollen grains), the two levels were ctuitrols. The ramets in the cages could thereby only chosen as they were easy to separate. The amount of receive auto-deposited pollen, the emasculated ramets pollen grains was estimated from three samples of only cross-pollen, and the controls both types. The each treatment. These were suspended in 70% alcohol experiment was repeated in all three years at Latnja- and 1 ml subsaniples were counted in a Blirker cham- jaure, and in 1996 and 1997 at Thingvellir. In 1995 and ber. 1996 twenty ramets were both emasculated and caged to check that the cages were pollen-proof and that there wore no aponiixis. The cages had a mesh width of !U Temperature experiment \im and were tubular, 5x3 cm. The cages were then sealed with tape around the ramet in the lower end and A long-term climate change experiment was established fastened to a stick, which was put in the ground. The at Latnjajaure in June 1994 by setting up 10 open-top cages did not raise the tetnperature. In 1995 1.5 cm chambers. The open-top chamber is a polycarbonate wide plastic lubes, sealed with cotton wool in the top hexagon with a ground surface area of 1 tii- that were used instead. Ramets were emaseulated by cutting increases air temperature by 2-3°C. For a more de- olf the whole male spike with a pair of scissors and tailed description of the open-top chambers, see Marion hand pollinated by brushing the stigmas with a pollen- et al. (1997). Since the open-top chambers disturb polli- releasing male spike from another ramet. nation in wind-pollinated species (Molau and Shaver 1997), I hand pollinated the ratnets in the open-top chambers. Pollen was taken from at least 20 m away and it was therefore very likely from a different genet. P<»llen donor distance experiment After hand pollination the ramets could receive more To measure the effect of pollen donor distance the pollen, which would not be a problem since the amount following experiment was conducted at Latnjajaure in of pollen applied by hand probably was much greater. 1996; 20 ramets per treatment were both caged and For every open-top chamber there were two plots of I emasculated before any stigmas had emerged, using the m- with no temperature enhancement, one with hand same teehniques as in the auto-deposition experiment. pollinated ramets as in the open-top chambers and one When the stigmas were receptive, the ramets were hand with untreated ramets. In all plots eight ramets were pc>llinated with pollen from ramets located I, 10, and measured, and the mean values per plot were used in 1000 m away from the target ramet. After hand pollina- the analyses. Since the shoot apex dies after flowering, ikm the ramets were caged until harvest. new ramets were selected randomly each year.

ECOGRAPHY 22 307 Time of flowering Results To assess the effect of time of flowering on fruit set and Auto-deposition experiment fruil weight, phenology was followed, and fruits from the ramets were harvested. This was done in Latnja- A three-way ANOVA (treatment, site, year) of jaure during 1995-1997 on both hand-pollinated fruit:flower ratio for the auto-deposition experiment ramets and controls. Start of flowering was recorded as revealed significant interactions (treatment x site x the day when the stigmas were clearly visible and bent year, F= 16,58, p = 0,0001) and therefore sites and to the sides. Cessation of flowering was recorded as the years were analysed independently {Underwood 1997). day when more than half of the surfaces of all stigmas Of the ramets that were both emasculated and caged, were brown. In the calculations, mean values from plots only three set fruit. All those three had, however, with 8 ramets were used. developed new anthers and therefore had the opportunity to self-pollinate. The cages were therefore consid- ered pollen-proof. None of the ramets that were only emasculated developed any new anthers. At Lalnjajaure Seedling emergence there were significant differences in fruit:flower ratio in two out of the three years (F2,5i=2,55, p = 0.0883 Twenty 25 x 25 cm squares were cleared from all vege- 1995; F..54 = 5,16, p = 0.0089 1996; F..,s4 - 10,22, p = tation at Latnjajaure in June 1995 to assess seedling 0,0002 1997; Fig. 2a). This difference depended mainly establishment in C, higelowii. Other seedlings and vege- on the cage treatment. There was no difference in tative shoots from all species were removed onee a fruit:flower ratio between eontrol ramets and emaseu- month during the vegetation period in all three years. T lated ramets in any of the three years, hut the between- also looked for seedlings in closed vegetation during all year variance was large (Fig, 2a). In the caged ramets, the study years at both sites. where only auto-deposition was possible. fruit:flower

Harvest and analysis In all experiments reproduetive success was measured as fruit set. (measured as fruit:flower ratio), and as fruit weight. The ramets were harvested in early September at Latnjajaure and in early November (1996) or Octo- ber (1997) at Thingvellir, All ramets were harvested after the flowering stalk had started to wither and therefore no further increase in fruit weight would occur. The flowering stalks were cut off and kept dry in paperbags until analysed. The mature and undeveloped fruits were counted using a stereo lens. The mature fruits were reeognised by their spherieal shape, undeveloped fruits being fiat. As each fruit contains only one seed the fruit:flower ratio and the seedrovule ratio become equal. As the perigynia had a rather constant weight regardless of fruit weight, the mature fruits were weighed together with their perigynia and the mean fruit weight per ramet was calculated. All data, except differences between regressions, were analysed by ANOVA using Statview 4.5.1 or Super- ANOVA 1.11 (Abacus Concepts) after having ehecked for normal distribution and homogeneous variances. Fisher's PLSD post-hoe test was used to test for dilTer- ences among treatments. To test for differences between regressions Student's t-test were used (Zar 1996), Fig. 2. (a) Fruit set (mean ± SE) for Carex bigelowii at Latnja- Fruitiflower ratios were arcsine transformed (Zar 1996), jaure 1995-1997 (b) and for Thingvenir 1996-1997 for con- In some experiments, especially for fruit weights, the trols, emiisculated ramets and caged ramets. Values with differeni letters are signiticantly different at the same level sample sizes were unequal. To achieve equal sample when tested within years. For Latnjajaure n=18 in !995. sizes, replicates were randomly removed (Underwood n= 19 in 1996 and 1997 and for Thingvellir n= 17 in i996 1997). and n= 18 m 1997.

308 ECOGRAPHY 12:} Table 2. Fruit weight (mg, undeveloped frtjits excluded) for Care.v higelowii (mean ± SE) for caged shoot.s, emasculated shoots and controls in Latnjajaure, northern Sweden 1995-1997 and Thingvellir, Iceland 1996-1997. Values with different letters are significantly different when tested within years and sites with one-way ANOVA. For Latnjajaure n = 5 in 1995, n = 12 in 1996 and n= 19 in 1997. For Thingvellir n = 4 in 1996 and too small to be tested in 1997.

Site Year Caged shoots Emasculated shoots Controls DF latnjajaure 1995 0.45 + 0,04^ 0.35 + 0.04^^ 0.40 + 0.03" 2,12 0.41 0.6755 latnjajaure 1996 0.61 -1-0.04^' 0,57 + 0.04^ 0.46 + 0.05" 2,33 3,25 0.0515 Latnjajaure 1997 0.53 ±0.04'' 0.55 + 0,03^ 0.56 + 0.03^* 2,54 0.17 0.8452 1 hingvellir 1996 1.06 +O.I P 0.89 + 0.09" 1,6 1.44 0.2750 Ihingvellir 1997 0.95 + 0.08 0.95 0.62 - - - ratio varied less over the study years. In two of the ture actually deereased the fruitiflower ratio. In a years it was higher than the other treatments and in one similar fashion, the temperature treatment inereased year lower (Fig, 2a). At Thingvellir the fruit:flower fruif weight in 1995 and 1996, but not in 1997 (F, i.- nitio was very low and there was no difference between 43.64. p = 0.0001 1995; F^.^ = 20,93, p-0.0001 19^96; any of the treatments in cither year (F2_4s = 2.77., p = F2,27 - 1.63, p = 0,2147 1997; Fig. 4b). Mere hand polli- 0.0725 1996; and F^g, = 0.87, p = 0.4271 1997; Fig, 2b), nation, with no temperature increase, did not affect The fruit weight did not differ between the treatments fruit weight (Fig, 4b), at Latnjajaure (Table 2). At Thingvellir there were only enough fruits to test for the difference between controls and emasculated ramets in 1996, but no difference was detected there either (Table 2), Time of flowering At Latnjajaure the fruit:flower ratio and fruit weight depend on the time of flowering of the ramet. The later Pollen donor distance experiment the ramets cease fiowering, the lower the fruitiflower ratio and the fruit weight becomes. This holds true for There were no significant differences between the differ- both hand-pollinated ramets (fruit:flower ratio: r^ = ent pollen donor distances either in fruit:flower ratio or 0.732, F|.j(-76,42, p< 0.0001; Fig. 5a and fruit fruit weight (Table 3). weight: r= = 0.814, F,_25 - 109,76, p< 0,0001; Fig. 5b), and for controls (fruit:fiower ratio: r- = 0.791. F, 2^ ^ 105,9, p< 0.0001; Fig. 5a and fruit weight: r-- 0,638, Pollen quantity experiment F|,22 = 38.71, p < 0.0001; Fig, 5b), As the length of the flowering period is constant, the start and cessation of The ramets receiving a large amount of pollen had a flowering are strongly correlated (r^ = 0.921, p < higher fruit:flower ratio, but not a higher fruit weight 0,0001), which allows either to be used in the calcula- than those receiving a small amount of pollen (Table 4), tions. The time for eessation of fiowering also shows a There was also an effeet of site, where the same amount strong negative correlation with the number of growing of pollen gave less, but heavier fruits at Thingvellir than degree days until fruits are ripe (r-^ = 0,931, p < 0.0001), at Latnjajaure (Fig, 3. Table 4). Because ol' the very low which means that the later a ramet flowers, the less heat fruit set in the "low pollen'" treatment at Thingvellir. is available for fruit maturation. The hand-pollinated the sample sizes for fVuit weight became low (n ^ 3). ramets have a higher fruil:flower ratio than the controls through out the season, but the difference decreases with the season (t2,5(j = 2.07, p < 0.05; Fig, 5a), There is Temperature experiment no difference in fruit weight between hand-pollinated and control ramets during the season (slope of regres- A two-way ANOVA (treatment, year) showed a signifi- sion: Xj.si ^ 0-51, p > 0.679 and level of regression cant interaction between treatment and year for both t2,48 = 0.26, p > 0,680; Fig, 5b). frait:f1ower ratio (p< 0,0001) and fruit weight (p < 0.0001), and the years were therefore analysed independently. Both temperature and hand pollination increased the fruit:flower ratio during two of the three Seedling establishment years (F^..^ = 30.30, p = 0,0001 1995; ¥2,21 = 20.93, p - 0,0001 1996; Fig. 4a). In 1997, the ramets that were In 1995 no seedlings of Carex bigelowii were found in only hand pollinated had a higher fruitiflower ratio the cleared squares at Latnjajaure. In 1996 there were than the untreated controls and the hand pollinated 11.2 + 4.3 seedlings m"- and in 1997 0.8+0.8 ramets in the open-top chambers (F2_27 =^ 7,25, p = (means + SE) seedlings m" -. No seedlings were found 0.0030 1997). This means that the enhanced tempera- in closed vegetation at either site or year. litOGRAPHY 113 (1999) 309 Table 3. Fruit set and fruit weighl for Care.x higelowii (mean = SE) for different pollen donor distances in Latnjajaure, northem Sweden 1996. n = 17, DF,,,,,,,^^.,^, = 2 and DF,,,,j.,,| = 47.

I m 10 m 1000 m F P rrLiitidower ratio 0.49 + 0.06 0.40 -h 0.06 0.49 + 0.06 0.K4 0.4375 fruit weight (mg) 0.58 ± 0.03 0.63 ± 0.04 0.65 ± 0.04 0.63 0.5396

Discussion able to disperse its genes over large distances (Hamrick et al. 1995). Optimal outcrossing distances should be Seed set in Howering plants can be limited by resources more common in species with restricted gene flow or conditions (e.g. Stcphenson 1981), amount of pollen (Waser and Price 1989). (e.g. Zimmerman 1988). or both (e.g. Agren and Will- A higher air temperature increased both fruit set and son 1992). This study shows that fruit set in Can'.\ fruit weight in C. higelowii in two out of three years of higeiowii is limited both by temperature and the study. Many arctic and alpine plants are limited by the amount of pollen. The fruit weight is also limited by temperature for their se.xual reproduction, e.g. Cas.siopc temperature., but not influenced by the amount of pol- telrugomt (Molau 1997), Dryiis oelopelala. (Wookey et len. On the other hand fruit weight, but not fruit set, al. 1993, Welker et al. 1997). Erioplion/m lagiiuitimi has been shown to increase in this species with nutrient (Moiau and Shaver 1997), Leonlodon muummilis (Tot- addition (Jonsdottir 1995). There were also differences land 1997) and Papacer radiealum (Molgaard and due to the time of flowering, between the years, and Christensen 1997). In 1997 there was no effect o^ heat- between the two study sites. This shows the importance ing either on fruit set or fruit weight. This was probably of conducting experiments in the Arctic and Subarctic due to the unusually high temperatures during July and during several years and in multiple sites. August at Latnjajaure this year (Table I). Other arctic Carex bigelowii is self-compatible and is able to set plants also show less response to experimentally in- seeds from auto-deposition at both Latnjajaure and creased temperature in unusually warm summers (Mo- Thingvellir (Jonsdottir 1995, this study), but has been lau 1997). As an example. Ranunculus iiicalis and reported as se!t"-iiicompatib!e from other sites (Faulkner Polvgomim viiipaium ( = Bislorla vivipara) increased 1973). The pattern for auto-deposition and cross-polli- their propagule weight as a response to increased tem- nation, with a rather constant fruit set by auto-deposi- perature in Latnjajaure in 1994-1996, but not in 1997 tion and equal fruit set in emasculated and control (Molau pers. comtn.). ramets, is consistent from year to year in Latnjajaure. It The amount of pollen dispersed in wind-pollinated is possible that ihe caged ramets received a much larger plants depends on the weather (Corbert 1990). This has amount of pollen than normal. If the ramets normally been demonstrated in Beiula aiha where both tempera- do not self pollinate, this may explain the pattern ture and humidity had great effects on pollen dispersal above. This in turn could e.g. be explained by the (Dahl and Strandhede 1996). In C. higelowii no pollen ramets own pollen mainly being distributed further is released during rain, fog, or snow (unpubl.). A higher away by the wind, but this requires further studies. In temperature increases the number o\' ramets m - (and both sites the male and female flowering phases are thereby the number of male spikes m" ") and the partly separated. C. higelowii is protogynous at Latnja- jaure and protandrous at Thingvellir. However, this varies among ramets, and in some ramets the phases Table 4. Two-way factorial ANOVA of dependence of anioLinl of pollen and site in a) fruit set (measured as are completely separated in time and sclflng is then not fruit:fiower ratio) and b) fruil weight for Carex higekmii in possible. For most ramets at both sites there would be 1997. The treatments are "much pollen" (380 000+ 102 300 opportunity for auio-deposition. but it somehow docs pollen grains/shooi) and "litlle pollen"' (55 000+ 16400 pollen grains/shoot) and the sites are Lainjajaure, northern Swe- not seem to occur to any greater extent. In contrast e.g. den and Thingvellir, Iceland, n = 17 for tVuit:flower ratio and Carex Jkna set seeds from selfing despite being protog- n = 3 for fruit weight. ynous (Schmid 1984). DF MS F P The pollen donor distance did not influence fruit set nor fruit weighl in C. higelowii. This was also the case a) Fruit:t1ower ratio in Bailsia alpina (Molau et al. 1989). There was thus no treatment 1 0.74 10-08 0.0023 site 1 2.12 28.76 < 0.0001 optimal outcrossing distance found as in Delphinium treatment x site 1 0^45 0.61 0.4395 nelsonii (Waser et al. 1987) and Ipomopsis aggregaia residual 63 0.07 (Waser and Price 1989). This is consistent with selflng b) Fruil weight neither influencing fruit set nor fruit weight, as the trealmenl 1 0.007 0.57 0.4709 shortest pollen donor distance probably sometimes site ! 0.40 31.01 0.0005 ireatmenl x site i 0.007 {).52 0-4915 meant transferring pollen within a clone. Furthermore, residual 8 O.Oi C. higelowii is a wind-pollinated species, and as stich is

310 ECOGRAPHY 22:3 jaure. "muchpollen"gaveahigherfruitsethan"littl bigelowii ashand-pollinate d rametsalsosetfewe r and in theseason.Thi s seemnotobetheas e forC. ECOGR.'\PIIV 22- 3 tion efficiencybecaus e oflowerfloweringdensitie s late pollinated plantseould,however , havealowerpollina- which doesnotapplyforwind-pollinate d plants.Wind- partly explainedblowerpollinato r visitationrate, lower seedsetafteflowering lateinthseasowas adoneus (GalenandStanto1991) . Inthesespeeiesth seed maturation,e.g.Leoniodonautumnalis(Totlan amount oftimeavailabltorflowering,pollinationand plants showthesampattern,atherilimited and fruitweighinC,bigelowii.Otheraretiealpin though fromahigherlevel. bigelowii ispollenlimited.Eveawarmyearlike pbllen availableandthfruitseforagiveamounof enced bytheamountofpollenthaaramereceives, fruit seandn=3forweight. pollen", buttherewasnoeffecfruiweight=17for giving lessandheavierfruitatThingvellithanLatnja- differed betweenthesites,withsamamountofpolle done in1997atbothLatnjajaurandThingvellir.Theffec (55000 +1640pollengrains/ramet).Theexperimentwas (380000 ±10230Upollengrains/ramet)and"littlepollen" SE) forCarexbigelowiithetreatments"muchpollen" Fig. 3(a)Fruilset(mean+SEand(b)IVuiweigh three eonseeutivyearsinLatnjajaurindieatingthatC. pollen. Thefruitseincreasedwithhanpollinationi have adoubleffect,bothaffeetingthamountof weather eonditions{temperatureandpreeipitation)thu strom andJonsdottir1997).Sincethfruitseisinflu- proportion oftnalespikesthatreleaspolle(Sten- 1997), Ranuncutmacris(Totlan d 1994)anRummculus 1997, handpollinationstillinereasethefruitset.al- (b) 0 (a) it

An earlieronsetoffloweringincreasedbothfruise Fruit Vveight {mg ::tlower ratio 0.1 0.2 0.3 0.2 0.4 0.6 0.8 1.0 0 iij little Latnjajaure T pollen . r, much pollen little pollen T Thingvellir cantly differenta l thesamlevewhentestedwithi n years. trols), n=10.excep t forfruiweighin1995wher e n=7for the CPandOT treatmentsandtoosmalltbeteste d inthe tion), CP{hand-pollinatedcontrols ) andC(unlreatecon- SF) forCarexbigelowiiinLatnjajaur e during1995-1997with similar tothoseobservedbyJonsdotti(1995)interms Thingvellir thetwostudyyearswersimilaandals difference infruitseamongcontrols{Fig2and4).A the mornorthernsite,therwasaeight-foldannual the differentyearsofstudy.ALatnjajaure,whichi both betweenthetwositesandwithionsingli seed set{e.g.Sorensen1941Kalela1962)Ithisstudy weight ontheonsefflowering. this couldexplainthedependencoffruitsean ature duringthistimdecreaslaterinthseason,and time leftforfruimaturationandthintegratetemper- untreated controls . Valueswithdifferentletterar c signifi- Ihe treatmentsOTC(increasedtemperatur e +hand-pollina- Fig. 4,(a)Fruitse(mean+SFan d {b)fruilweight(mean+ sites, withThingvellirhavingawarmebutwette This tnightbeduoclimaticdifferencebetweenth at Latnjajaureproducingtiianysmallfruitsandth between thesitesipresentstudy,withramet and Callaghan{1990).Thereisaeonsistentdifferene of fruitset.Theannualfluctuationsinsea there werlargdifferencesinfruitseandweigh pollen limitatiowasstrongestthen.However,boththe higher inthebeginningofseason,indicatinthat set betweenhand-pollinatedancontrolrametswas lighter fruitslateinthseason.Thdifferencfruit ramefs atThingvellirproducingfewbulargefruits. Latnjajaure agrewithpreviousstudiebyCarlsson Many aretieplantsshowlargannualfluctuationsin 1995 1996 1997 31 July both in time and space they can be looked upon as "windows of opportunity" (Jelinski and Cheliak 1992, • hand pollination Eriksson and Froborg 1996), i.e. when conditions for o control seedling establishment occur only rarely and under special circumstances. But since clones of many Carex species can reach considerable age (Steinger et al. 1996). these very infrequent events of establishment seem to be enough to maintain a high level of genetic variation in this species (Jonsson et al. 1996).

Acknowlt'dgemenl.'i I want to thank the staff at Abisko Scientific Station, Thingvellir National Park and Lars Lidstrom for all logistic and practical help, and Olga Khitun, Annika Jagerbrand, Ingibjorg S. Jonsdottir and Jon H. Bjornsson for help in the field, Ingibjorg S. Jonsdottir, LJlf Molau, A.sl6g Dahl and Mikael Stenstrom kindly gave valu- able comments during various stages of this paper. Finaneial support was given by The Swedish Environmental Protection Agency, Helge Axison Johnsons Stiftelse, Letterstedtska Foreningen, Nordic Academy for Advanced Study (NorFA), The Royal Swedish Aeademy of Scienees and Stiftelsen Lars Hiertas minne.

tb) 0.1 References 180 185 190 195 200 205 210 215 220 Agren, J. and Willson, M. F. 1992. Determinants of seed Cessation of female flowering (daynumber) production in Geranium macululuni. Oeeologia 92: 177- 182. Fig. 5. (a) Linear regressions of tVuit set and (b) Iriiit wcighi Billings, W. D. 1987. Constraints to plant growth, reproduc- on flowering time in untreated controls and hand-poliiiiated tion, and establishment in aretie environments. - Arct, ramets of Carex higelowii. Flowering time was measured as the Alp. Res, 19: 357-365, lime for eessation of female flowering. For fruit set in un- Callaghan, T. V. 1976. Growth and population dynamics of treated controls: r2 = 0.791, p = O.UUUl. DF^,., = 28. and in Carex higelowii in an alpine environment. - Oikos 27: hand-pollinated ramets: r^ = 0.722. p = 0.0001, DF,^, = 28. 402-413. The difference between hand-pollinated and control ramets Callaghan, T, V. and Emanuelsson, U. 1985. Populalion struc- deereased during the season, (t, j,, = 2.07. p < 0.05). For fruit ture and processes of tundra plants and vegetation. In: weight in untreated controls: r^ = 0.638, p = 0.0001, DFr,,, = White. J. (ed.), The population structure of vegetation. Dr 22 and in hand-pollinaied ramets: r- = 0.814, p = 0.0001, W Junk Publ., Dordrecht, pp. 399-439. DF^s = 25 and there was no difference between the two (slope Carlsson, B, A. and Callaghan, T. V. 1990. Effects of fiower- of regression: t2 51 =0.51, p > 0.679 and level of regression ing on the shoot dynamics of Cafe.x higeiowii along an t2.48 = 0.26, p> 0.680), altitudinal gradient in Swedish Lapland. - J. F,col, 78: 152-165. ^ Carlsson, B. A. and Callaghan, T. V. 1994. Impact of climate climate than Latnjajaure. The larger amount of rain at change factors on the clonal sedge Carex higelowii: impli- Thingvellir makes wind pollination less likely and might cations for population growth and vegetative spread, - Eeography 17: 321-330. explain the very low fruit set there. There may also be Carlsson, B, A, et al. 1990. Aspeet of elonality in the Arctic: a other ditTerences between the two populations such as comparison between Lycopodium amiotium and Carex genetic differences of biogeographical reasons e.g, more bigelowii. In: van Groenendael, J. and de Kroon, H, (eds), Clonal growth in plants: regulation and function, defect pollen grains at Thingvellir. This is indicated by SPB Acad. Publ., The Hague, pp. 131-151. the lower fruit set at Thingvellir from the same amount Cook, R, E. 1985. Growth and development in clonal plant of pollen compared to Latnjajaure. populations, - In: Jackson, J. B. C, Buss, L. W. and Cook R, E, (eds). Population biology and evolution of clonal No C. higelowii seedlings were found in tmdisturbed organisms. Yale Univ. Press. New Haven, pp. 259-296. areas in any of the years at either site, which agrees Corbert, S. A. 1990. Pollination and the weather. - Isr. J. Bot, with results by Callaghan (1976), Carlsson and 39: J3-3O. Dahl, A. and Strandhede, S. O. 1996. Predicting the intensity Callaghan (1990). and Jonsdottir (1991, 1995). How- of the birch pollen season. - Aerobiologia 12: 97-106. ever, both in the present study at Latnjajaure and a Einarsson, M. A. 1992. Climatic conditions of the Thing- previous study at Thingvelhr (Jonsdottir 1995) a low vallavatn area, - Oikos 64: 96- 104. Ellstrand, N. C. and Roose. M. L. 1987. Patterns of genotypic rate of emergence was observed in disturbed plots, diversity in clonal plant species, - Am. J. Bot. 74: 123- indicating that C. bigelowii depends on disturbance for 131. seedling recruitment. Seedling recruitment in popula- Eriksson. O. 1993. Dynamics of genets in clonal plants. - Trends Ecol. Evol. 8: 313 316. tions of long-lived clonal plants is often infrequent (e.g. Eriksson. O. and Froborg, H. 1996. "Windows oi opportu- Cook 1985. Eriksson 1993). When disturbances are rare nity" for recruitment in long-lived cional plants: experi-

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