Arctic and alpine ecosystems are characterized by a cold and relatively short growing season. Within these landscapes topography and prevailing wind directions shape heterogeneous snow distribution patterns. The heterogeneous snow distribution leads to habitats differing in snow depth during winter and snow melt timing in spring. The alpine tundra represents a mosaic of early-melting habitats on wind-exposed ridges with shallow snow cover, and late-melting habitats in wind-sheltered depressions with deep snow cover. Also in sub-arctic birch forest, birch stems act as snow traps, leading to accumulation of snow. Consequently, the various habitats are characterized by plant species and communities, which avoid and prefer snow cover, respectively. However, some species occupy a wide range of habitats and intraspecific differences in responses to variation in snow depth and duration can affect growth habit, phenology and reproduction. The response of plant species along natural gradients might be similar to temporal changes of environmental conditions. Thus, studies along environmental gradients, encompassing the range of climate change predictions, is more likely to give a realistic picture concerning extent of intraspecific phenotypic trait variation, which may determine the long-term adaptive potential of plant species to climate change. Arctic ecosystems face strong changes in snow conditions due to global warming by an increase in temperature, most pronounced in winter and spring, causing an earlier onset of snowmelt and an earlier start of the growing season. One such species with a broad habitat range is Empetrum hermaphroditum, a prominent evergreen dwarf shrub in several subarctic heath and mountain birch forest communities. The present study investigated growth, flowering phenology, reproduction and clonal structure of Empetrum hermaphroditum along a natural snow cover gradient in four study areas. The study areas are located along a latitudinal gradient (northern Sweden vs. central Norway), and at each latitude along a local climatic gradient (sub-continental vs. sub-oceanic climate). Along the natural gradient, significant differences in snow depth during winter, snow melt timing in late spring and summer irradiation were observed. Snow depth increased from the exposed ridge habitat in the alpine tundra to the sheltered depression habitat in the alpine tundra to birch forest. Consequently, snow melt occurred first on ridges and later in depressions and birch forest. During growing season, the birch forest habitat experienced the lowest light availability and the ridge habitat the highest, light availability in the depression habitat is intermediate. The results show that Empetrum hermaphroditum shoots from shallow snow cover and high summer irradiation habitats had significant higher numbers of flowers and fruits, lower plant heights, shorter shoot segments, lower numbers of lateral shoots and total biomass but higher leaf density and higher relative leaf allocation than shoots from habitats with higher snow depth and lower summer irradiation. This pattern was observed across all study areas. Furthermore, the study demonstrates that Empetrum hermaphroditum flowers about the same time in early and late melting habitats, independent of snowmelt timing. Therefore, the plants in the exposed and sunny, early melting habitat have a longer time-lag. Flowering of Empetrum hermaphroditum does not seem to be related to snowmelt time across all habitats, but to temperature conditions during the lag-phase between snowmelt and flowering. The results show that clonal structure of Empetrum hermaphroditum is affected by the prevailing local habitat conditions. A decrease of clonal diversity from the exposed ridges over the sheltered depression habitat to birch forest was observed across all study areas. A low proportion of clonal reproduction implies a relatively high reproduction by seedlings. The study revealed that sexual reproduction in addition to vegetative spread is important in this clonal species. In general, this thesis revealed that Empetrum hermaphroditum has a broad ecological niche, which enables the potential to cope with changing snow conditions in the course of climate change. Furthermore, changes in snow cover and temperature are not likely to cause changes in flowering synchrony but general changes in flowering time. The high phenological overlap might indicate that reproductive isolation and genetic differentiation among the habitats is rather unlikely. However, while phenotypic plasticity will allow individuals to immediately adapt to changing conditions, locally adapted populations may locally go extinct. The latter will offer the possibility for seedling recruitment of adapted genotypes.
Giessen: Universitätsbibliothek , 2016.
Schneedeckungs Gradient, Empetrum hermaphroditum, Wachstum, Blühphänologie, klonale Struktur