Subcanopy leaf functional trait response to disturbance at different severities and implications for ecosystem production stability

Forest disturbances vary in spatial extent, severity, and frequency, with different consequences for the stability of net primary production (NPP) following disturbance. At higher severities in which a majority of canopy-dominant trees senesce, the ability of subcanopy trees to compensate for lost canopy leaf area becomes increasingly important to stabilizing NPP. Within the subcanopy stratum, variation in functional traits over space and time may allow for sustained optimization of whole-canopy C fixation, and therefore NPP, in the dynamic post-disturbance resource environment. At high disturbance severities, biological legacies in the form of subcanopy functional trait diversity and plasticity (i.e. leaf morphology, biochemistry, and physiology) may relate to NPP stability. Wider pre-disturbance functional trait diversity may predict greater stability in NPP following disturbance as a more functionally diverse subcanopy is primed for efficiently adjusting to a shifting light and resource environment. Here, we examine subcanopy functional response to disturbance across a gradient of disturbance severity, as well as across four distinct landscape ecosystem types, within the Forest Resilience Threshold Experiment (FoRTE) in northern Michigan. Experimental disturbance was initiated in 2019 via stem girdling (i.e. phloem flux disruption) targeting 0 %, 45 %, 65 %, and 85 % gross defoliation across four experimental replicates located on unique landforms. Through repeated measures of leaf physiology, morphology, and biochemistry at fixed points in the subcanopy stratum, we track pre- to post-disturbance shifts in leaf functional traits. We then analyze whether these shifts are correlated with differential ecosystem-scale production response to disturbance at differing severities. In the immediate post-disturbance period (2019), subcanopy leaf functional traits did not shift in response to rising disturbance severity, although they were found to vary by landscape ecosystem type. However, changes are expected in the year following disturbance (2020) as rapid crown death in some experimental replicates has impacted subcanopy resource environments, as well as ecosystem-scale NPP.