COS 260-4 - Effects of disturbance severity and spatial patterning on near-term forest canopy structure and complexity

Background/Question/Methods

Canopy structural complexity (CSC) is an emergent property broadly associated with light absorption, resource-use efficiency, and productivity in eastern US forests. However, factors that promote CSC are not well understood. Intermediate-severity forest disturbances modify canopy structure without stand replacement, potentially providing a mechanism that promotes CSC. However, variation in disturbance severity, spatial patterning associated with causal agents, and other factors result in diverse canopy structural outcomes. We developed and applied an empirical modeling and simulation approach to evaluate how vertical and horizontal spatial patterns of disturbance, as well as their interactions, influence stand-level canopy structure and CSC across a range of intermediate disturbance severities (20 – 80% basal area removal). Three-dimensional, spatially explicit models of 2480 trees were constructed from terrestrial laser scans of twelve forested plots in Connecticut. Disturbances were simulated through automated and random removal of individual tree models. Vertical (top-down, bottom-up, neutral) and horizontal (clumped, ordered, and random) patterns of disturbance were assigned to outcomes post-hoc based on canopy strata most affected (lower, middle, upper), and aggregation indices (Clark-Evans Index; contagion), respectively. Canopy structure and complexity were evaluated pre- and post-disturbance with the forestr package in R. Treatment-level differences were assessed through Analysis of Variance and ordination.

Results/Conclusions

Analysis of 12,000 simulated disturbance outcomes indicated that post-disturbance canopy structural traits differed significantly among disturbance typologies. Disturbances with bottom-up directionality generally promoted CSC and mean canopy height, while top-down patterns tended to reduce mean height with more variable effects on CSC. These results suggest that specific disturbance characteristics may result in variable and, putatively predictable canopy structural outcomes that vary somewhat independently of disturbance severity. Interactions among some combinations of disturbance characteristics resulted in highly variable structural outcomes. For example, top-down patterns at lower-to-moderate severities produced outcomes with the greatest variation in CSC, while top-down patterns at higher severities tended to reduce CSC. This study identified specific disturbance traits associated with enhancement of CSC, which could be leveraged to inform and improve ecologically-focused forest management. The presented simulation approach also provides a novel means of evaluating interactions among disturbance factors such as horizontal and vertical patterning, and severity, which may facilitate future investigations into effects of contemporary and shifting disturbance regimes on forest structure and function.