Vegetation responses to summer- and winter warming : flower power in the Alaskan tussock tundra?

Sammanfattning: Plants have an important role in the tundra carbon (C) cycle by storing C in primary production and thus potentially counteract the C released from thawing permafrost. Tundra vegetation is limited by nitrogen (N), which is predicted to increase with rising temperatures and increased snow depth. In permafrost systems, rooting depth will determine whether plants can access N in the deep soil which, with increasing snow depth, has the potential to turn into a significant N source. Increased plant-available N is thus expected to affect both plant productivity and vegetation composition. This study aims to investigate vegetation responses to increased temperature and snow depth in a permafrost system of moist tussock tundra by combining open-top chambers with a realistic snow manipulation (snowfences). The shallow-rooted shrubs, Betula nana and Rhododendron tomentosum, and the deep-rooted sedge Eriophorum vaginatum were analyzed for responses in growth and reproduction effort. Also, vegetation responses in terms of normalized difference vegetation index (NDVI) were investigated. Winter warming increased flower density of E. vaginatum while B. nana showed an increased shoot growth in response to winter warming, but only during mid-growing season. Although winter warming increased winter soil temperature and generated a trend of increased thaw depth, there were no responses in NDVI or further species-specific responses in reproduction effort, leaf and shoot growth, leaf production or leaf dry weight to warming treatments. These results indicate that E. vaginatum respond in reproduction effort while B. nana respond in (mid-season) growth to winter warming. In total, the warming treatments generated a weak response in tundra plants which indicate that tussock tundra might not be very responsive to short-term warming. These results suggest that tundra plants have a low ability to counteract increased releases of soil C in response to short-term warming.