::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), Accepted 7 December 1998 cJipyright © ECOGRAPHY 1999 IS-IN 0906-7590 Pr nted in Ireland - all rights reserved 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 form- ing 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.