Increased plant productivity in Alaskan tundra with experimental warming of deep soil and permafrost

Background/Question/Methods

Northern tundra play a critical role in global carbon cycling because of the vast pool of thermally-protected carbon stored in these ecosystems and the strong potential for changes in carbon storage in a warmer climate.  While warming may enhance microbial decomposition of soil organic matter, these respiratory losses may be offset by warming-mediated increases in plant growth.  The potential for plant C uptake to serve as a negative feedback to warming is not well quantified, and changes in uptake may ultimately determine whether tundra remain C sinks or become a source of C to the atmosphere in a warmer world.  This study, which focuses on the response of plant growth to warming, was conducted at the Carbon in Permafrost Experimental Heating Research (CiPEHR) 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).  We measured aboveground plant productivity (ANPP) using a point-intercept method and examined changes in ecosystem N dynamics and phenology to determine the drivers of warming-mediated shifts in ANPP.

Results/Conclusions

Winter warming increased wintertime soil temperatures (5-40 cm) by 2.3^o C, resulting in a 10% increase in growing season thaw depth.  Summer warming significantly increased air temperatures, with peak differences around midday when summer warming plots were approximately 1.0°C warmer than ambient.  Changes in the soil environment with winter warming resulted in a 20% increase in ANPP, which was likely driven by increased N availability and growing season length.  Both summer and winter warming caused earlier leaf out and delayed senescence, significantly extending the growing season despite equivalent snow-free days across treatments.  As with ANPP, winter warming increased total foliar N mass by 20%.  Surprisingly, summer warming significantly decreased resin-available soil N, while there was no change in soil N in the winter warming plots.  The increase in ecosystem N availability (plant + soil) with winter warming may have driven higher rates of ANPP, while the decline in soil N availability with summer warming may explain the lack of summer warming effect on ANPP.  These results demonstrate the importance warming-mediated changes in N cycling for regulating plant and ecosystem responses to climate change and highlight the potential for increased plant uptake to offset respiratory CO₂ losses in warming tundra ecosystems.