The Forest Resilience Threshold Experiment (FoRTE): An ecosystem experiment examining disturbance severity-production interactions

Temperate forests are a large component of the North American terrestrial carbon sink, and the future magnitude and stability of this sink is in question as forests undergo novel disturbance regimes as part of global change. Disturbances exhibit a wide range of severities within forested landscapes, and adequate model representation of variable disturbance impacts across this severity gradient is questionable. The Forest Resilience Threshold Experiment (FoRTE) is a new, multifaceted study at the University of Michigan Biological Station (UMBS) using field measurements, model simulations and a model-data feedback loop to examine the mechanisms underlying experimental disturbance's effects on forest carbon cycling. The disturbance treatment, implemented via the stem girdling of trees along a fully replicated severity gradient of 0-85% tree mortality, will take place in spring 2019.

Disturbance-mediated changes in vegetation structure at our experimental site may have important functional implications, including shifts in the mean and range of physiological functioning, and could in turn affect the degree of forest primary production resilience. In order to predict and track these ecosystem changes, we conducted pre-treatment data collection in summer 2018 to determine photosynthetic capacity and functional plasticity of canopy and subcanopy tree species. We sampled 240 upper canopy leaves from dominant deciduous species and 450 leaves from major subcanopy species for maximum rate of photosynthesis, stomatal conductance, leaf mass per area, and leaf reflectance. Leaf physiological data will be used to parameterize the Ecosystem Demography 2 (ED2) model and evaluate the model's capacity to represent hypothesized physiological plasticity.

Merging these leaf functional data with remotely sensed pre-treatment structural measures will enable a robust analysis of coupled and interacting shifts in forest structure, species composition, and functional diversity following the moderate severity disturbance implementation. We will use findings from a prior experimental disturbance at this site (the Forest Accelerated Succession Experiment) in addition to our data to formulate hypothetical trajectories for change in forest structure and function precipitated by disturbance.