*Author to whom correspondence should be addressed. Present address: UCLA-EEB Department, Box 951786, Los Angeles, CA 90095-1786, USA. E-mail: Andrea.Pluess@unibas.ch
1Reproductive behaviour of plants may change in contrasting habitats. In two separate glasshouse experiments, we studied effects of population origin (early vs. late successional and low vs. high altitudinal habitats) and environmental effects (competition and temperature) on plant size and sexual vs. clonal reproduction in Geum reptans L.
2Plant size and reproduction differed significantly among populations, but only plant size differed between contrasting habitats.
3If plants grew with competition or at warm temperature, plant size and reproduction were reduced and more plants reproduced only with stolons. Individuals with flowers were larger than those that reproduced only with stolons, indicating a smaller minimum plant size for clonal than for sexual reproduction.
4Populations of different origin changed little in their response to environmental treatments. Plants from early successional habitats tended to produce more flowers in the competition-free treatment, whereas in plants from late successional habitats the opposite was true.
5The results indicate limited adaptation in reproductive behaviour to contrasting habitats. Nevertheless, great size-dependent plasticity in the proportion of sexual vs. clonal reproduction ensures population persistence and reproduction in a large range of habitat conditions.
Steep environmental gradients are ubiquitous in alpine ecosystems. Climatic conditions change with altitude, exposure and slope, resulting in a patchy distribution of microhabitats. Changes in abiotic and biotic conditions can lead to major modifications in selection pressure on plant life-history traits (Stearns 1992; Cody & Overton 1996). Clonal growth is among the most noticeable adaptations to severe climatic conditions and nutrient shortage in cold environments (Callaghan 1988; Klimes et al. 1997). Recruitment by seeds is commonly assumed to be restricted in the cold and by short seasons (Jelinski & Cheliak 1992; Eriksson 1997). Indeed, many alpine plants reproduce clonally even in pioneer communities, and particularly in late successional grasslands (Gray 1993; Stöcklin & Bäumler 1996; Klimes et al. 1997).
Clonal plants tend to allocate more biomass to vegetative propagation than to sexual reproduction (Abrahamson 1980; Cook 1985; Eriksson 1997). As resources are usually limited (Cody 1966), a pronounced allocation to vegetative reproduction will reduce the investment for seeds, resulting in a trade-off between the two reproductive modes (Harper 1977; Watson 1984; Piquot et al. 1998; Prati & Schmid 2000; Ronsheim & Bever 2000; but see Cain & Damman 1997). In plants, meristem allocation may be even more important than resource partitioning, because meristems available for reproduction may be more limited than are carbon and other resources (Watson 1984; Geber 1990; Bonser & Aarssen 1996). The future availability of reproductive meristems is increased by clonal reproduction (Eriksson 1989). Extensive vegetative propagation may result in rapid, but spatially limited, spread of genotypes and can improve population persistence during phases of lacking sexual reproduction. Long-distance dispersed seeds, however, connect fragmented populations in the patchy alpine landscape or found new populations in unoccupied habitats. But seed production is more nutrient-demanding than vegetative reproduction (Harper 1977; Watson 1984). The onset of clonal and sexual reproduction can be determined by different minimum plant size requirements (Schmid, Bazzaz & Weiner 1995). In most cases, sexual reproduction increases with increasing plant size (Weiner 1988), whereas the relative allocation to clonal propagation is constant over a large range of plant sizes (Schmid et al. 1995).
Selection pressures affecting trade-offs differ in response to environmental heterogeneity (Bazzaz et al. 1987; Sultan 1987; Hutchings 1988) and can lead to local adaptation (Bradshaw 1984; Galen, Shore & Deyoe 1991). We expect variation in the partitioning of resources and meristems to reproductive strategies in contrasting habitats. The spatial isolation of alpine habitats can even increase the degree of local differentiation. However, phenotypic plasticity rather than genetic differentiation could be an alternative way of matching genotypes to environment, with increasing environmental heterogeneity favouring greater plasticity (Schlichting 1986; Sultan 1987). How the highly structured alpine landscape affects trait differentiation among sites, and the extent to which such effects are genetically based, is largely unknown.
Here we ask if genetic population differentiation in growth and reproductive behaviour is affected by two prominent alpine environmental gradients: succession and elevation. The most important factor changing along successional alpine gradients after the retreat of glaciers is increasing competition pressure as soil development proceeds and vegetation cover increases. With increasing altitude, the temperature and duration of the growing season decrease. Higher altitude is generally more stressful for plant life, particularly seedling establishment. Metapopulation models predict that individuals with high dispersal ability will be favoured in early successional habitats, because new populations are more likely to have been founded by high-dispersal genotypes (Olivieri, Michalakis & Gouyon 1995; Olivieri & Gouyon 1997). During succession, genotypes with an affinity to persistence (e.g. clonal propagation) could be more successful. Opposing selection for sexual and clonal reproduction in early and late successional habitats was reported by Piquot et al. (1998), whereas Van Kleunen, Fischer & Schmid (2001) found an increase in sexual reproduction at higher density. Vegetative reproduction may increase at higher altitudes within species (Young et al. 2002), or among species (Klimes et al. 1997). Other studies report greater allocation to clonal than to sexual reproduction in harsh environments (Bostock 1980); a maximum relative investment in clonal reproduction at medium altitude (Douglas 1981); or no altitudinal effect of reproductive allocation at all (Williams, Mack & Black 1995). Large differences in reproductive behaviour among populations of Geum reptans L. have been observed (Weppler & Stöcklin 2005), but the extent to which these differences have a genetic background is unknown.
In two separate experiments we studied the effects of population origin (genetic effects) and environment on the growth and relative importance of sexual and clonal reproduction in G. reptans. In a competition experiment, plants from early and late successional populations grew with or without competition with Poa alpina L. In the second experiment, plants from low- or high-altitude populations were exposed to different temperatures (cold and warm). Geum reptans is especially suited for studying the trade-off between reproductive modes because meristems in leaf axils are used for either sexual or clonal reproduction. We addressed the following hypotheses. (i) Reproductive behaviour of plants from early and late successional habitats differs. (ii) The relative importance of clonal growth is increased in populations from high altitude compared with populations from lower elevations. (iii) Plants from late successional populations will be favoured under competition, and plants from higher populations will be favoured at low temperature, indicating adaptation. Additionally, these two experiments tested whether sexual and clonal reproduction were affected by plant size, and if there was a trade-off between the two reproductive modes.
Materials and methods
The clonal Geum reptans (Rosaceae) is a perennial outcrossing rosette plant, and occurs preferentially on glacier forelands and moraines on siliceous rocks between 1950 and 3800 m a.s.l. (Hegi 1995). The plant is one of the first pioneers on virgin soils, and persists during succession. In the competition experiment, early successional habitats refer to recently (several years to a few decades) deglaciated areas; late successional habitats have been free from ice for at least 100 years.
Geum reptans reproduces clonally by forming new rosettes (ramets) with adventitious roots at the end of above-ground stolons, and sexually by producing flowering stems with a single terminal flower head. Reproductive meristems are located in the axils of leaves and are preformed in the season prior to emerging. At the end of summer, stolons, daughter rosettes that do not establish, and leaves of adult plants die back.
In the competition experiment, a total of 192 plants from four early and four late successional populations were grown either with or without P. alpina: 12 individuals per population and treatment (Table 1). Plants grew in two air-conditioned greenhouse compartments with an ecologically relevant air-temperature regime of 10 °C at night and 20 °C during daytime. In the temperature experiment, a total of 208 plants from five low- and four high-altitude populations were exposed to two temperature treatments. From each population, 12 individuals grew at cold temperature (7·5 °C at night and 17·5 °C during daytime) and 12 individuals at warm temperature (12 °C at night and 22 °C during daytime) in two air-conditioned greenhouse compartments per treatment (in one population only eight individuals per treatment were available; Table 1). The temperature difference of 4·5 K equals a difference in altitude of 750 m under the estimate of a lapse rate of 0·60 K per 100 m increase in altitude (Körner 1999). Plants within greenhouse compartments were randomized every second month. Within each compartment, a temperature sensor measured air temperature continuously and regulated the connected air conditioner (Airwell Type R-407C, ACE Klimatechnik GmbH, Frankfurt, Germany) to cool the compartment to the required temperature. Temperature increased during daytime due to incident solar radiation. All plants were kept inside the greenhouse from April to October, but overwintered outside in the garden of the Institute of Botany, University of Basel, Basel (270 m a.s.l.), Switzerland, where plants experienced frost.
Table 1. Location, population abbreviation, elevation, habitat type, and number of individuals per population and treatment of 17 populations of Geum reptans in the Swiss Alps used in two greenhouse experiments with a competition treatment and a temperature treatment, respectively
Longitude and latitude according to Swiss topographical maps (Bundesamt für Landestopographie, Wabern, Switzerland).
Early and late successional (ss) habitats; low and high altitudinal habitats.
Val Roseg, GR
Val Fex, GR
Flüela Schwarzhorn, GR
Piz Languard, GR
Plant material was collected in early autumn 2000 and 2001 for the competition and temperature experiment, respectively. We collected c. 30 daughter rosettes with their stolons, each from a different mother plant from each population. The randomly chosen mother plants were located along a transect, and were at least 4 m distant from each other, except in the populations at Eggishorn and Flüela Schwarzhorn where plants were aggregated along crevices. Because there were few reproducing adults in the populations at Val Roseg, Muttbach and Flüela Schwarzhorn, more than one clonal offspring from three, four and seven mother plants, respectively, were collected and allocated equally to treatments. Plants collected were kept in moist bags in the dark at 4 °C until planting. By the end of October, 24 equally sized plants per population were planted individually into single rectangular pots of 9 × 9 cm and 10 cm in height. Pots were filled with 2 cm pumice-gravel as drainage, followed by a 1 : 1 mixture of sand and pot-soil. Plants were randomly allocated to the two treatments in each experiment. After 3 weeks, in half the pots of the competition experiment four randomly chosen bulbils of P. alpina were planted as competitors in each pot, one at each edge of the pot. Poa alpina bulbils were collected at four sites at different altitudes and kept in a moist bag in the dark at 4 °C until planting. After the first winter, the number of P. alpina individuals per pot was reduced to one, and every second month individuals of P. alpina were reduced to two tillers. In each experiment plants within populations were randomly and equally partitioned to greenhouse compartments. Statistically, greenhouse compartments were treated as blocks. In the temperature experiment blocks and temperature treatments fall together.
The remaining ramets were potted the same way. During the first winter, 14 and 21 individuals died in the competition and temperature experiment, respectively. Whenever possible dead plants were replaced by individuals of the same population or, in the temperature experiment, of the same elevation. Plants that died later were not replaced. On 15 July 2002, the 161 remaining plants were brought to 2000 m a.s.l. near Davos, Graubünden, Switzerland, to compare their reproductive behaviour with the plants in the greenhouse.
In the greenhouse, plants received additional light from 1 kW lamps for 3–5 h during daytime, according to day-length. All plants were watered twice a week and received the same amount of full fertilizer (in 2002, 9 kg N ha−1; in 2003, 9 kg N ha−1 plus 23 kg N ha−1 with a higher K content, applied in equal portions during the vegetative period). Potassium promotes the growth of reproductive meristems.
Numbers of leaves per plant were recorded at the beginning of the experiment and analysed to test for equal plant size between treatments and blocks. This measure was also used as a covariate in statistical analyses.
The final harvest for both experiments was in July and August 2003, 3 and 2 years after the beginning of the competition experiment and the temperature experiment, respectively. Numbers of rosettes and of green and dead leaves produced in 2003 were counted. Above- and below-ground plant biomass was measured after drying at 80 °C for 48 h. Axils of leaves formed in the year of the final harvest were checked for reproductive meristems and, if present, they were cut and stored in 50% alcohol to determine if they were future flowers or stolons. In the greenhouse, few individuals reproduced in 2003 (8·7%), whereas 73% of the remaining plants kept at 2000 m a.s.l. reproduced, despite a similar number of preformed reproductive meristems in both groups. Therefore we judged preformed meristems as a more precise measurement of the reproductive potential of populations in the greenhouse than the actual number of flowers and stolons. In the common garden, vernalization was probably too short to initiate enough preformed meristems to grow because of the milder climate during winter compared with the conditions at 2000 m a.s.l.. Reproduction was constantly low in the earlier years and should not have influenced reproductive allocation in the year of the final harvest.
Vegetative and reproductive measurements were used to test for effects of origin (habitat type and population) and environment (treatment) by hierarchical analysis of covariance (ancovas). In the competition experiment, habitat type (early and late succession) was tested against population, treatment against the interaction of treatment and population, and population, as well as the interaction of habitat type and treatment against the remaining residual. In the temperature experiment, treatment was tested against block, habitat type (low and high altitude) against population, and population, the interaction of treatment and habitat type, as well as the interaction of treatment and population against the remaining residual. Plant measurements were ln-transformed to test for relative differences, and the proportion of stolons on all reproductive organs was arcsin-transformed. Because several individuals produced neither stolons nor flowers, the requirement for the ancova could not be fulfilled with individual data points. Instead, we used means of reproductive meristems per population within treatment and block for the ancova. Correlations were also calculated with population means with a non-parametric Spearman's rho (rs) procedure instead of parametric Pearson's product-moment (R) tests.
Phenotypic measurements of experimental plants included a genetic and a maternal environmental component. The maternal environment can influence the nutrition and growth of clonal offspring. Therefore we used the number of leaves at the beginning of the experiment as a measure of maternal effects. In both experiments, this measure did not affect the number of reproductive meristems (P > 0·1), and did not influence plant size at the final harvest in the competition experiment, but did influence size at final harvest in the temperature experiment (R = 0·22, P < 0·01 for leaves and above-ground biomass). To minimize any maternal effects we used the number of leaves at the beginning of the experiments as a covariate in all the analyses.
Plants were classified according to their reproductive meristems in individuals with stolons, with flowers, with stolons and flowers, or non-reproducing individuals. A Poisson log-linear model was used to test whether reproductive behaviour differed between habitat types or treatments.
All statistical analyses were carried out with R (Ihaka & Gentleman 1996), a shareware package similar to s-plus.
effects of population origin on plant size and reproduction
All measured traits of plant size and reproduction in the temperature experiment, and four of eight traits in the competition experiment, differed significantly among populations, indicating population differentiation. There were few effects on plant size and reproduction due to population origin in both experiments. Plants from early successional habitats produced 14% more above-ground biomass and 15% more below-ground biomass (P < 0·05 for both terms, Tables 2a and 3a), and plants from high altitude produced 18% more leaves (P = 0·06, Tables 2b and 3b). Furthermore, in two cases we found marginally significant interaction terms for origin × treatment, indicating that the response to environmental treatments was different among habitats or populations. In the competition experiment, plants from early successional populations produced more flowers in the treatment without competition, whereas plants from late successional populations produced more flowers in the treatment with competition (P = 0·07 for the interaction of habitat × treatment, Table 3a; Fig. 1). In the temperature experiment, the difference in the number of reproductive meristems in the warm and cold temperature treatments depended on population (P = 0·07 for the interaction of population × treatment, Table 3b).
Table 2. Plant size and reproduction (mean ± SE) of Geum reptans from different origins in two experiments: (a) competition experiment with plants from early and late successional habitats with and without competition from Poa alpina; (b) temperature experiment with plants from low and high altitude grown under cold or warm temperature treatment
Table 3. ancova summary of the effect of habitat type, population, treatment, and interaction of treatment with habitat type or population on plant size (above- and below-ground biomass, number of rosettes, or number of leaves) and reproductive meristems (total, flowers, stolons, or percentage of stolons on total reproductive meristems) in two experiments:(a) competition experiment; (b) temperature experiment : (a) Competition experiment
effects of environment on plant size and reproduction
In the competition experiment, the size of G. reptans plants was significantly reduced by competition with P. alpina: above-ground biomass was reduced by 61%, below-ground biomass by 72%, number of rosettes by 33%, and number of leaves by 40% (P < 0·001 for all terms, Tables 2a and 3a). The frequency of plants reproducing either by flowers, stolons, or with both reproductive modes shifted between the competition treatments. More individuals reproduced clonally or not at all in the treatment with competition, at the expense of individuals with flowers or both reproductive modes (P < 0·001, Fig. 2a). Plants grown with competition produced 50% fewer reproductive meristems in total, 38% fewer stolons, and 63% fewer flowers (P < 0·01 for all terms, Tables 2a and 3a). In general, plants produced relatively more stolons than flowers (by 60 ± 2·4%, Table 2a). The proportion of stolons, however, did not change significantly between treatments (P = 0·293, Tables 2a and 3a).
The temperature treatment had only a weak influence on above-ground biomass and leaf number per plant, and none on below-ground biomass and number of rosettes (Tables 2b and 3b). But temperature influenced the reproductive behaviour of plants. When warm, more individuals produced only stolons, only flowers, or were non-reproducing at the expense of individuals with both reproductive modes (P < 0·001, Fig. 2b). In total, the cold-treated plants produced 55% more reproductive meristems and 69% more stolons, whereas the number of flowers was unaffected by temperature (Tables 2b and 3b). Again, plants produced relatively more stolons than flowers (59 ± 3%), and the proportion of stolons did not differ between treatments (P = 0·791, Tables 2b and 3b).
size dependence of reproduction
In general, larger plants reproduced more than smaller plants. In both experiments mean above-ground biomass was largest in plants reproducing simultaneously with flowers and stolons. The biomass of plants reproducing only clonally, or not at all, was significantly smaller than that of sexual plants (paired t-tests: P < 0·001 in both experiments, Table 4). Biomass did not differ between plants reproducing only sexually or with both reproductive modes (P > 0·1 for both experiments).
Table 4. Mean above-ground biomass (± SE) of Geum reptans individuals with only stolons, only flowers, both reproductive modes, or no reproduction at all in a competition and a temperature experiment
Mean above-ground biomass (g) of individuals with:
Stolons and flowers
Different superscripts indicate significant differences among means, P < 0·05.
1·13 ± 0·07b
1·53 ± 0·13a
1·57 ± 0·07a
0·63 ± 0·14c
1·47 ± 0·05b
1·63 ± 0·14a
1·72 ± 0·04a
1·34 ± 0·13b
In the competition experiment, the total number of reproductive meristems increased with increasing above-ground plant weight (rs = 0·66, P = 0·007; rs = 0·51, P = 0·04; rs = 0·81, P = 0·02 for total reproductive meristems, stolons and flowers, respectively; N = 32) or increasing number of leaves. The percentage of stolons decreased with increasing above-ground biomass and increasing leaf numbers (rs = −0·49, P = 0·05; rs =−0·55, P = 0·03, respectively; N = 32).
In the temperature experiment, plants with more leaves produced more reproductive meristems and more stolons, whereas the number of flowers was not affected (rs = 0·60, P = 0·01; rs = 0·54, P = 0·02; rs = 0·23, P = 0·35, respectively; N = 36). The number of reproductive meristems did not correlate significantly with above-ground biomass, and the percentage of stolons did not change in relation to plant size.
trade-off between stolons and flowers
In both experiments the number of stolons decreased with increasing number of flowers, indicating the expected trade-off between reproductive modes [R = –0·18, P = 0·02; R = −0·16, P = 0·02 with all reproductive individuals in the competition experiment (N = 153) and temperature (N = 194) experiment, respectively].
In both greenhouse experiments we found significant differences in growth and reproduction among populations of G. reptans, and some evidence for adaptive population differentiation in contrasting habitats. If plants grew under environmental stress (competition, warmer temperature), plant size and reproduction were reduced and fewer plants reproduced sexually or simultaneously with both reproductive modes. In general, larger plants reproduced more, but the minimum plant size for clonal reproduction was smaller than for sexual reproduction. This shift in reproduction with plant size was mostly responsible for the different reproductive behaviour of plants in different experimental environments. Only some of the observed genetic differences among populations were explained by habitat contrasts, and were probably mainly a result of drift, as populations of G. reptans are spatially isolated and gene flow is limited (Pluess & Stöcklin 2004).
plant size and reproductive investment
Reproductive behaviour of G. reptans was dependent on plant size, larger plants producing more offspring as in other species, e.g. Viola sororia (Solbrig 1981), Scabiosa columbaria (Van Treuren et al. 1993), Epilobium dodonaei and Epilobium fleischeri (Stöcklin & Favre 1994), Pennisetum setaceum (Williams et al. 1995), and Asarum canadense (Cain & Damman 1997), but not Ranunculus acris (Hemborg & Karlsson 1998). The threshold size in G. reptans for clonal reproduction was significantly smaller than for sexual reproduction (Table 4), as predicted by life-history theory and observed in several species (Schmid et al. 1995). The preference of smaller plants for clonal reproduction could have a physiological and evolutionary basis. Clonal propagation in many plants is strongly self-sustainable because of the photosynthesis of the stolon itself, the leaflets at stolon nodes, and the leaves of the new rosette at the end of a stolon. Seed production is usually more costly (Harper 1977; Watson 1984). Secondly, from an evolutionary point of view it may be favourable to invest first in clonal persistence, and to postpone the more uncertain production of seeds. Such a strategy is particularly suited to harsh alpine habitats where the establishment of seeds is more risky (Stöcklin & Bäumler 1996). As a consequence of a smaller threshold size for clonal reproduction, the relative allocation to sexual reproduction increases in larger plants (Weiner 1988; Schmid & Weiner 1993). In the competition experiment, the percentage of flowers was positively related to plant size. In the temperature experiment, size was less variable and the percentage of flowers was independent of plant size. Differences in size may largely explain treatment effects on reproduction in the competition experiment, but not in the temperature experiment. As all plants in each experiment were of the same age, size variation was smaller than commonly observed in field populations. Nevertheless, large differences in reproductive behaviour, similar size effects, and an increase of flower production in larger plants were found in 20 field populations of G. reptans (Weppler & Stöcklin 2005). Thus the effects of age and environmental conditions on plant size may partly explain the different reproductive behaviour of field populations.
effects of environment
Competition reduced plant size and reproduction drastically, indicating weak competitive ability. In the competition treatment, more plants reproduced by stolons only, probably because of the generally smaller plants rather than as a direct consequence of competition (see above); whereas in the treatment without competition, more plants reproduced with both reproductive modes. Interestingly, this shift in reproductive behaviour of individuals did not change the population mean percentage stolon production, because in the treatment without competition many larger plants increased not only flower but also stolon production. We conclude that large variability in the extent of stolon and flower production among individuals can buffer the effects of contrasting habitats. Similarly, in a greenhouse experiment with Uvularia perfoliata, treatment effects on reproduction were weaker than the effect of ramet size (Wijesinghe & Whigham 1997).
The temperature treatment had marginally significant effects on plant size, but particularly on the number of reproductive meristems: notably, fewer stolons were produced at warmer temperature. Alpine plants may not be well adapted to warmer temperature. Plants from high altitudes grown at lowland temperatures often respire more quickly than lowland plants due to insufficient acclimation, and can therefore have reduced growth and reproduction, or even die (Körner 1999). Metabolic limitations may explain the reduced plant performance at the warmer temperature in our experiment. Stolon production was more negatively affected than flower production (Tables 2b and 3b), indicating that the fate of reproductive meristems is influenced directly by environmental conditions. A shift towards greater stolon production with lower temperature is an advantage for a plant such as G. reptans, because at higher altitude and lower temperature, population persistence from clonal reproduction is more important (Abrahamson 1980; Bostock 1980). Our results also suggest that unfavourable temperature regimes at low altitudes could constrain the lower distributional limits of G. reptans.
effects of origin
Significant differences in size and reproduction among populations in both experiments indicate genetic differentiation among those populations, as experimental conditions were common for all plants. There were some effects of contrasting habitats, but they were weak. Population differentiation is therefore mainly random and based on drift. That the differences result from adaptations to unknown habitat conditions cannot be fully excluded, but is unlikely, as unknown differences among habitats are probably much smaller than the known differences between contrasting successional and altitudinal habitats.
In addition to unspecific population differentiation, we found indications of differentiation in the competition experiment (Table 3), giving some support to the hypothesis of adaptations in G. reptans to contrasting environments. In particular, early successional plants produced more flowers in the absence of competition compared with plants from late succession (Fig. 1) and, at the same time, plants (irrespective of their smaller size) from late successional populations had twice as many flowers as those from early succession when they competed with Poa (P = 0·07, Tables 2a and 3a), suggesting a specialization to contrasting successional habitats. Plants from late succession invested more in stolons without competition, and plants from early succession invested more in stolons with competition (P < 0·1, Tables 2a and 3a). These interactions were only marginally significant, but as (for statistical reasons) we had to use population means instead of individual data, statistical power was reduced and we consider these results as an indication of the described effects. The results suggest that G. reptans from late successional populations is better adapted to produce flowers in a competitive environment than are plants from early successional populations, probably because they compete better for scarce nutrients. In the temperature experiment, the tendency to increase the number of leaves in populations from high altitude (P = 0·06, Table 3b) and the similar above-ground biomasses in low and high altitudinal habitats could indicate that smaller leaf size at high altitude is an adaptation to a colder and harsher climate.
As we found a trade-off between clonal and sexual reproduction in both experiments, opposing selection in contrasting habitats on reproductive behaviour in G. reptans is possible, as predicted from general metapopulation models that include evolutionary constraints via a trade-off between life-history traits (Olivieri et al. 1995). A trade-off between the two reproductive modes in contrasting habitats was found in Rumex acetosella (Houssard & Escarre 1995) and in Sparganium erectum (Piquot et al. 1998). These species allocated more resources to sexual reproduction in young populations, while in Ranunculus repens sexual reproduction increased with density (Van Kleunen et al. 2001). In G. reptans, however, the percentage of stolons changed only among populations, but not between contrasting habitats, indicating no particular selection pressure on the relationship between the two reproductive modes.
Despite large differences among alpine habitats, our results indicate that the selective forces shaping growth and reproduction of G. reptans in contrasting habitats are probably not strong enough for pronounced adaptive population differentiation. There are only weak indications that plants from late successional populations are better adapted to competition. Results suggest that population genetic differences in G. reptans are mostly an effect of drift, supporting the results of a molecular study (Pluess & Stöcklin 2004). Size-dependent plasticity in reproductive behaviour may explain why differences in the relative contributions of sexual vs. clonal reproduction in contrasting habitats were not stronger. Environmental effects on reproductive behaviour were pronounced, related to plant size, and could shift the reproduction of plants towards stolon production under stressful conditions, as exemplified by competition and warmer temperature.
The authors thank Tina Weppler who helped collecting clonal offspring, Daniela Dürig, Nadine Kueni and Petra Zibulski for help in the greenhouse and during the final harvest, and Frank Schurr for statistical advice. This study was supported by the Swiss National Science Foundation project no. 31-59271·99 and the Janggen-Poehn Foundation in St Gallen, Switzerland.