Linking a belowground climate change driver with aboveground processes: Species-specific foliar responses to chronic soil warming in a temperate forest

^{In an even-aged mixed hardwood stand in the Harvard Forest LTER, we characterized plant and soil community responses to chronic soil warming.  The soil in a 30 x 30 m experimental area has been heated to 5°C above ambient soil temperature since 2003.  We observed two primary responses:  (1) an increased rate of CO₂ loss from the heated soils due to increased microbial decomposition and potentially, increased root respiration; (2) an increased rate of C storage in woody tissues of certain species due to an increased soil N availability and lengthened growing season. After 4 years, this ecosystem acts as a net C source. Based on our previous small-scale warming experiment, we hypothesize that over time the elevated soil respiration rate will decline to control levels, while N mineralization will remain elevated.  The decoupling of C and N cycling observed in the previous study may result in elevated N availability and drive increased vegetative C storage while soil respiration declines.  We measured leaf-level photosynthetic potential and related traits (foliar chemistry, specific leaf area, stomatal density) in juvenile and adult warming study trees. We found a species-specific effect of soil warming on leaf-level C gain.  For example, photosynthetic response to warming differed for two primary hardwood species represented in the study.  Acer rubra showed a positive leaf-level response to warming, including a 33% greater relative photosynthetic potential for canopy leaves, an effect not seen in Quercus rubrum.  These results suggest that species-specific physiological differences may affect vegetative response to soil warming.}