About 70% of the plant species in the temperate zone are characterised by clonal growth, clonal species are also in majority in the Arctic and Subarctic where they affect the structure and composition of the vegetation. It is therefore of great importance to increase our knowledge about clonal plants and their growth and life histories. I have investigated how ramets of the stoloniferous plant Linnaea borealis are affected by the naturally occurring variation in environmental factors, such as: light, nutrient and water availability. Moreover, I examined the seed set and how supplemental hand pollination affects seed set in L. borealis, and also investigated the significance of the apical meristem for shoot population fitness. All studies were performed under field conditions in northern Sweden in a Subarctic environment and most are experimental. The results show that nutrient resorption from senescing leaves is not significantly affecting the growth and nutrient pools of the ramet. This implies that the growth of L. borealis ramets is not governed by micro-site resource availability. However, removal of light competition resulted in increased branching and number of lateral meristems produced, reduced growth, and decreased root:shoot ratio on a per ramet basis. Thus, ramets of L. borealis can efficiently exploit favourable light patches through plastic growth. Apical dominance exerts a significant effect on shoot population fitness and can be lost through rodent grazing. However, loss of apical dominance is dependent on the timing of grazing, if the apical meristem is removed early in the autumn the ramet can repair the loss until the next summer. If grazing occur during spring the dry weight and leaf area production is affected negatively. Seed production in L. borealis in the Abisko area varies between years and sites, and was unaffected by supplemental hand pollination treatment, implying that there is no lack of pollinator activity.

<p>About 70% of the plant species in the temperate zone are characterised by clonal growth, clonal species are also in majority in the Arctic and Subarctic where they affect the structure and composition of the vegetation. It is therefore of great importance to increase our knowledge about clonal plants and their growth and life histories. I have investigated how ramets of the stoloniferous plant <i>Linnaea borealis</i> are affected by the naturally occurring variation in environmental factors, such as: light, nutrient and water availability. Moreover, I examined the seed set and how supplemental hand pollination affects seed set in <i>L. borealis</i>, and also investigated the significance of the apical meristem for shoot population fitness. All studies were performed under field conditions in northern Sweden in a Subarctic environment and most are experimental.</p><p>The results show that nutrient resorption from senescing leaves is not significantly affecting the growth and nutrient pools of the ramet. This implies that the growth of<i> L. borealis </i>ramets is not governed by micro-site resource availability. However, removal of light competition resulted in increased branching and number of lateral meristems produced, reduced growth, and decreased root:shoot ratio on a per ramet basis. Thus, ramets of <i>L. borealis </i>can efficiently exploit favourable light patches through plastic growth. Apical dominance exerts a significant effect on shoot population fitness and can be lost through rodent grazing. However, loss of apical dominance is dependent on the timing of grazing, if the apical meristem is removed early in the autumn the ramet can repair the loss until the next summer. If grazing occur during spring the dry weight and leaf area production is affected negatively. Seed production in <i>L. borealis</i> in the Abisko area varies between years and sites, and was unaffected by supplemental hand pollination treatment, implying that there is no lack of pollinator activity.</p>

About 70% of the plant species in the temperate zone are characterised by clonal growth, clonal species are also in majority in the Arctic and Subarctic where they affect the structure and composition of the vegetation. It is therefore of great importance to increase our knowledge about clonal plants and their growth and life histories. I have investigated how ramets of the stoloniferous plant Linnaea borealis are affected by the naturally occurring variation in environmental factors, such as: light, nutrient and water availability. Moreover, I examined the seed set and how supplemental hand pollination affects seed set in L. borealis, and also investigated the significance of the apical meristem for shoot population fitness. All studies were performed under field conditions in northern Sweden in a Subarctic environment and most are experimental. The results show that nutrient resorption from senescing leaves is not significantly affecting the growth and nutrient pools of the ramet. This implies that the growth of L. borealis ramets is not governed by micro-site resource availability. However, removal of light competition resulted in increased branching and number of lateral meristems produced, reduced growth, and decreased root:shoot ratio on a per ramet basis. Thus, ramets of L. borealis can efficiently exploit favourable light patches through plastic growth. Apical dominance exerts a significant effect on shoot population fitness and can be lost through rodent grazing. However, loss of apical dominance is dependent on the timing of grazing, if the apical meristem is removed early in the autumn the ramet can repair the loss until the next summer. If grazing occur during spring the dry weight and leaf area production is affected negatively. Seed production in L. borealis in the Abisko area varies between years and sites, and was unaffected by supplemental hand pollination treatment, implying that there is no lack of pollinator activity.

Altitudinal gradients offer attractive opportunities for studies of population differentiation in response to environmental heterogeneity. In this thesis, I examined population differentiation along altitudinal gradients by combining common-garden experiments with field studies and experiments in alpine, subalpine and boreal populations of the perennial herb Solidago virgaurea. More specifically, I determined whether leaf physiology in terms of nitrogen concentration and resorption, flowering phenology, flower production and reproductive effort vary along altitudinal gradients. Nitrogen concentration in green leaves were higher in alpine than in subalpine and boreal populations. These differences persisted when plants were grown from seeds in a common-garden experiment at two sites, suggesting that the differences have a genetic component. There was mixed support for a trade-off between maximized carbon gain through the maintenance of high nitrogen concentration, and minimized nitrogen loss through high resorption. In their natural habitats alpine populations began flowering later than subalpine populations, but this difference was reversed when plants were grown in a common environment. This suggests that genetic differences among populations counteract environmental effects and reduce phenotypic variation in flowering time among populations. Flowering time thus shows countergradient genetic variation in S. virgaurea. In a common-garden experiment, boreal populations produced more flowers and had a higher reproductive effort than subalpine and alpine populations indicating habitat-specific genetic differences in reproductive allocation. In a field study, which included three populations, seed set was close to zero in the alpine population, intermediate in the subalpine population, and high in the boreal population. Experimental flower removal showed that seed production was associated with a considerable cost in terms of reduced flowering propensity the following year, but did not support the hypothesis that a large floral display is important for pollination success.