Northern latitude ecosystems are keystone contributors to global carbon (C) reservoirs. Soils in northern permafrost regions comprise less than 20% of the global soil area, yet they store approximately 50% of the belowground C pool. The role of northern ecosystems in global C dynamics is particularly relevant in the context of feedbacks from ecosystems to climate change. While northern latitude soils have historically served as a C sink, both climate predictions and recent climate trends portend a potential release of this large C stock to the atmosphere as a result of permafrost thaw and decomposition of previously frozen organic matter. However, warming may also enhance plant growth so that photosynthetic CO₂ uptake may balance or exceed these respiratory losses. To determine the effects of warming on tundra CO₂ exchange, we established an ecosystem warming experiment— the Carbon in Permafrost Experimental Heating Research (CiPEHR) project— near Healy, AK, in the northern foothills of the Alaska Range. We used snow fences coupled with spring snow removal to increase deep-soil temperatures and thaw depth (winter warming), and open top chambers to increase summer air temperatures (summer warming).

Results/Conclusions

Winter warming increased soil temperatures down to 40 cm by 1.5°C (3.6°C at maximum snowpack), which resulted in a 15% increase in thaw depth throughout the growing season. Surprisingly, the additional thawed soil organic matter in the winter-warming plots did not result in significant changes in cumulative growing season respiration, which may have been inhibited by soil saturation at the base of the active layer. In contrast to the limited effect on growing-season C dynamics, winter warming resulted in drastic changes in winter respiration and altered the annual C balance of this upland tundra ecosystem by doubling the net loss of CO₂ to the atmosphere. While most of the changes in the abiotic environment at CiPEHR were driven by winter-warming treatment, summer-warming effects on individual plant and soil processes resulted in 20% increases in both gross primary productivity and growing-season ecosystem respiration and caused significant changes in the age and sources of CO₂ respired to the atmosphere. These results demonstrate the vulnerability of organic C stored in permafrost to increasing air and deep-soil temperatures and the strong potential for warming upland tundra to serve as a positive feedback to global climate change.