Spatial and temporal dynamics of leaf physiological response to rising disturbance severity

Forest disturbances alter core ecosystem processes, including production, with potentially profound impacts on the global carbon cycle. Disturbance-mediated changes in forest vegetation structure and community composition may have important functional implications, including shifts in the mean and range of canopy and subcanopy physiology, and could in turn affect forest primary production resilience. However, disturbance type and severity vary widely in forested landscapes, and adequate model representation of variable disturbance impacts across severity gradients is questionable.

The Forest Resilience Threshold Experiment (FoRTE) is a multifaceted study at the University of Michigan Biological Station (UMBS) examining the mechanisms underlying experimental disturbance's effects on forest carbon cycling along a fully replicated severity gradient of 0-85% tree mortality. To assess the impacts of rising disturbance severity on processes linked to production, a suite of key biochemical and physiological parameters was measured in two forest strata at two contrasting spatial scales, both before and immediately following disturbance treatment. At the landscape scale, subcanopy leaves were selected from deciduous saplings representing four distinct communities (experimental replicates) at our site. At the scale of individual crowns, leaves from 72 focal trees representing three canopy dominant species, one early successional and two mid- to late-successional, were selected within a single community type. Preliminary results suggest that functional trait plasticity across understory plants and within surviving crowns supports sustained leaf photosynthetic capacity up to a threshold of disturbance severity, beyond which processes underpinning primary production become unstable and decline.