Thermal ecology: The influence of tundra vegetation on local thermal environments and the fate of the arctic permafrost
Vast stores of carbon in substrates underlying vegetation represent a significant global carbon pool whose flux is controlled by a variety of processes such as gains through sequestration of autotrophic-derived carbon, loss through plant, microbial and root respiration, and limits imposed by nutrient and water availability. These processes, however, are all sensitive to changes in temperature which, at the canopy-substrate interface, is likely to be influenced by leaf-, plant-, and community-level control over local thermal conditions. This possibility, however, remains unexplored. To address this issue, we examined tundra vegetation influence of over local thermal conditions at Toolik Lake, Alaska, during the 2014 growing season between June and August. The continuous light and the importance of permafrost in retaining sub-canopy carbon stores made this an ideal study system. If vegetation’s influence over local thermal environments is significant, it may contribute to changes in permafrost and active layer depths which, in turn, would impact carbon flux. We conducted three investigations on these issues: (1) quantified the relative control of leaf temperature by plant architecture (e.g., placement and angle of leaf incidence to solar radiation) compared to abiotic factors (e.g., light, temperature, and wind) using a geodesic sensor array; (2) examined if vegetation composition could predict vegetation thermal properties as quantified by thermography; (3) tested whether the thermal environment created by vegetation, also quantified by thermography, could predict permafrost depth.
We found that (1) the percent maximum attainable leaf temperature as modeled by our sensor array was strongly influenced by leaf height and angle of incidence to solar radiation, but primarily under clear, windless conditions. (2) We did not find a clear relationship between vegetation composition and vegetation thermal properties as quantified by thermography. Finally, (3) we found that thermal properties of the vegetation were significantly, albeit weakly associated with permafrost depth. These findings reveal that vegetation can influence local thermal environments and, by this mechanism, may influence permafrost depth, but the thermodynamic complexity of the system attributable to interactions between biotic (i.e., plant control over local thermal environments) and abiotic factors (i.e., percent clear, windless days) makes precise predictions difficult. This study suggests that the influence of tundra vegetation on local thermal conditions, although complex and strongly interactive with abiotic conditions, is potentially an important factor in determining permafrost depth, with important implications for climate change.