Increased plant productivity in Alaskan tundra as a result of experimental warming of soil and permafrost

Summary 1. The response of northern tundra plant communities to warming temperatures is of critical concern because permafrost ecosystems play a key role in global carbon (C) storage, and climate‐induced ecological shifts in the plant community will affect the transfer of carbon‐dioxide between biological and atmospheric pools. 2. This study, which focuses on the response of tundra plant growth and phenology to experimental warming, was conducted at the Carbon in Permafrost Experimental Heating Research project, located 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). 3. Winter warming increased wintertime soil temperature (5–40 cm) by 2.3 °C, resulting in a 10% increase in growing season thaw depth. Summer warming significantly increased growing season air temperature; peak temperature differences occurred near midday when summer warming plots were approximately 1.0 °C warmer than ambient plots. 4. Changes in the soil environment as a result of winter warming treatment resulted in a 20% increase in above‐ground biomass and net primary productivity (ANPP), while there was no detected summer warming effect on ecosystem‐level ANPP or biomass. Both summer and winter warming extended the growing season through earlier bud break and delayed senescence, despite equivalent snow‐free days across treatments. As with ANPP, winter warming increased canopy N mass by 20%, while there was no summer warming effect on canopy N. 5. The warming‐mediated increase in N availability, coupled with phenological shifts, may have driven higher rates of ANPP in the winter warming plots, and the lack of ecosystem‐level N and ANPP response to summer warming suggest continued N limitation in the summer warming plots. 6. Synthesis: These results highlight the role of soil and permafrost dynamics in regulating plant response to climate change and provide evidence that warming may promote greater C accumulation in tundra plant biomass. While warming temperatures are expected to enhance microbial decomposition of the large pool of organic matter stored in tundra soils and permafrost, these respiratory losses may be offset, at least in part, by warming‐mediated increases in plant growth.