Structure, trends and drivers for the sub-arctic northern treeline in face of climate change and pathogen outbreaks in Abisko National Park, Sweden.
Lead Author: Carl Beierkuhnlein (UBT)
Contributors: Beierkuhnlein, Jentsch, Hoffmann, Janeck, Schweiger (UBT)
The subarctic ecosystems in the tundra and the norther margins of the boreal forest are very sensitive ecosystems that are species-poor. The Abisko National Park represents these biomes in the ECOPOTENTIAL sites. There, only a few key species are driving major ecosystem processes and functions. Here, in the oceanic climate of Northern Sweden, deciduous trees (Betula pubescens) make up the boreal forest whereas conifers are rare and just mixed in sparsely (Pinus sylvestris). This is considered to be an exceptional case within the large boreal biome.
However, the monodominant stands of birch are regularly attacked by large outbreaks of herbivorous insects. Particularly, the autumnal moth (Epirrita autumnata) can cause defoliation at the landscape scale, but such events are rare in time and fluctuate in a quasi-decadal rhythm. The ecotone of the forest and the tundra ecosystems thus can be seen as a sensitive instrument to indicate ecosystem responses to climate change, especially in protected areas where human land use is restricted.
The coevolutional developed relationship between birches and their pathogens will be modified with increasing temperatures: either the trees profit with a northward shift of the minimum growth temperature and the number of growth days per year or the autumnal moth will attack birch forests with higher frequency and magnitude, causing in a decline of birches and maybe the establishment of further treeline species (e.g. Pinus sylvestris, Cairns & Moen, 2004).
Consequently, major changes will follow in conservation issues, tourism (nature values) and available resources for livestock. Reindeer herds are an important value for tourism and the Sami people and tend to decrease in numbers of individuals with climate change because of shifts of plant and insect phenology (Vors & Boyce, 2009). Furthermore, reindeers seem to play an important role in the nutrient cycles and support primary production (Olofsson et al., 2004). However, reindeer seedling herbivory and autumnal moth together seem to inhibit tree encroachment significantly higher than climate variables (Seppälä & Rastas 1980, Cairns & Moen, 2004; Van Bogaert et al., 2011). Consequently, the interaction of reindeer seedling herbivory, autumnal moth and birches under climate change is an ongoing serious issue.
Treeline shifts will modulate tree biomass and arctic soils play a major role as global carbon sinks. Ecosystem changes by global warming will further release greenhouse gases because of lower carbon storage capacities and a higher conversion rate and the release of methane from permafrost soils (positive feedback, Christensen et al., 2004). Furthermore, global warming is assumed to increase erosion due to the unfreezing of permafrost mountain slopes which has been already shown for Abisko (Jonasson & Nyberg, 1999).
The spatial extension of defoliation, the mid-term ecosystem responses and can be well traced by earth observation. Future local climate scenarios can help to estimate reindeer populations, shifts of the arctic treeline and erosion scenarios.
Figure 1: Total defoliation of Birch (Betula pubescens) during an outbreak of autumnal moth Epirrita autumnata). It is under debate whether the tree profits from warming and responds via northward shift or if the pest is profiting more and may cause southward shift of the treeline.
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