The Photosynthetic Response of Alaskan Tundra Plants to Increased Season Length and Soil Warming

ABSTRACT How the carbon balance of arctic ecosystems responds to climate warming will depend on the changes in carbon assimilation capacity of tundra plant species. Along with air and soil warming, one of the consequences of warming likely to be important for carbon assimilation of tundra plant species is an expected 40% increase in growing season length. We examined the effects of a lengthened growing season and soil warming on the photosynthetic capacity of seven tundra plant species from four growth forms that comprise >90% of the vascular cover of wet tussock tundra. Maximum photosynthetic capacity of these key species was relatively unchanged by the manipulation that significantly altered growing season length, active layer depth, and soil temperatures. Highest photosynthetic rates were found for the forb, Polygonum bistorta, and the lowest for dwarf evergreen shrubs. Seasonal patterns revealed that plants maintained relatively high light-saturated photosynthetic capacity (Amax) values throughout most of the growing season. Interannual variation was significant, but differences were small for most species. The study shows that tundra species operate within a relatively narrow range for maximum photosynthetic capacity with this maximum seldom being reached under ambient conditions. Thus, when evaluating the effects of climate change on tundra ecosystem carbon uptake, species composition and total photosynthetic leaf area should be considered first. These two factors will affect the system carbon exchange capacity during climate warming more so than species-level assimilation capacity.

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