Warming responses of Arctic vegetation: Experiments and reality

Background/Question/Methods

The Arctic region has warmed significantly over the past 30 years and arctic plants and ecosystems are already changing in response, including a general “greening” of the arctic landscape, changes in species distributions and abundance, and changes in biogeochemistry at local and regional scales.   Many of these responses to warming are limited or regulated by low inputs, turnover, and availability of essential elements like N and P, and thus much recent research on arctic warming  has focused on interactions between warming and element cycling, particularly in soils.  A common experimental method for studying these interactions is the use of open-top or closed chambers or greenhouses, which warm the air by reducing wind and by retaining infrared energy.  Similar methods have been used in at least 25 arctic sites, with treatments sustained for up to 20 summer seasons.

Results/Conclusions

This talk will compare the results and predictions from artificial warming experiments with recent changes observed in unmanipulated tundras.  Experimental warming of arctic vegetation often results in increases in production, biomass, canopy height, and leaf area, as well as changes in species composition favoring species that can easily grow taller (especially deciduous shrubs and graminoids).  These changes in response to artificial warming occur only slowly, however, especially relative to much larger changes in fertilizer experiments.  Changes in unmanipulated tundra in response to regional climate warming also appear to be occurring very slowly relative to experimental manipulations.   The greatest changes in species composition and canopy characteristics are occurring in sites that have been physically disturbed by fire, thermokarst (loss of soil ice), grazing, and economic development.  The changes on disturbed sites are often in the same direction as suggested by warming and other experiments, but more rapid and larger in magnitude.  Overall, these observations suggest that a changing disturbance regime may be more important than direct effects of regional warming in driving Low Arctic vegetation to a greener, taller, more productive, and shrubbier state.