Severe drought can cause lagged effects on tree physiology that negatively impact forest functioning for years. These “drought legacy effects” have been widely documented in tree ring records and could have important implications for our understanding of broader-scale forest carbon cycling. However, legacy effects in tree ring increments may be decoupled from ecosystem fluxes due to 1) post-drought alterations in carbon allocation patterns, 2) temporal asynchrony between radial growth and carbon uptake, and 3) dendrochronological sampling biases. In order to link legacy effects from tree rings to whole forests, we leveraged a rich dataset from a Midwestern US forest that was severely impacted by a drought in 2012. At this site, we compiled tree ring records, leaf-level gas exchange, eddy flux measurements, dendrometer band data, and satellite remote-sensing estimates of greenness and leaf area before, during, and after the 2012 drought.
Results/Conclusions
After accounting for the relative abundance of tree species in the stand, we estimate that legacy effects led to ~10% reductions in tree ring width increments. Despite this stand-scale reduction in radial growth, we found that leaf-level photosynthesis, gross primary productivity, and vegetation greenness were not suppressed following the 2012 drought, and that dendrochronological sampling biases and temporal asynchrony were not mechanistically linked to tree ring legacy effects. Instead, elevated leaf-level photosynthesis co-occurred with reduced leaf area in early 2013, indicating that resources may have been allocated away from radial growth in conjunction with post-drought upregulation of photosynthesis and repair of canopy damage. Collectively, our results indicate that legacy effects inferred from tree rings may have minor impacts on ecosystem carbon uptake, and that lagged canopy allocation could be an important mechanism that decouples tree ring signals from gross primary productivity.