Effects of experimental partial harvesting regimes on forest canopy structure and complexity

The physical structure of forest canopies regulates numerous ecological processes and functions, and management strategies to promote specific structural features, such as high structural complexity, are increasingly desired. Partial disturbances - including partial harvests - can modify forest canopy structure, but their specific effects may be influenced by differences in characteristics such as intensity and frequency. Understanding the variable effects of silvicultural treatments on canopy structure will be essential to designing silvicultural regimes that can promote canopy structural complexity. We evaluated the effect of partial harvesting regimes on canopy structure (i.e., density, cover, height, arrangement, and heterogeneity) and complexity (as rugosity), quantified using a suite of metrics derived from portable canopy lidar, across three levels of harvesting intensity in three multi-decadal silvicultural experiments. We specifically assessed: 1) how repeated partial harvesting disturbance affected canopy structure and complexity relative to unmanipulated controls, 2) whether effects of partial harvesting differed among harvesting intensities or 3) between old- and second-growth stands, and 4) if canopy light interception differed among stands with different histories of partial harvesting disturbance. Findings indicated that canopy structure development under partial harvesting regimes differed from that of control stands, reducing vertical and canopy structural complexity at a fine (i.e., 1-6 ha treatment unit) scale, but increasing among-unit variability in canopy structure relative to controls. Variation in intensity among treatments produced relatively little distinction in fine-scale canopy structural outcomes, but the higher intensity selection harvesting regime in particular differed significantly from controls across all metrics of canopy structure. Partial harvesting affected the developmental trajectories of both old-growth and maturing, secondary forest stands, where selection-based management seemed to decrease structural features associated with old forests. Our findings suggest management regimes developed for timber goals could accommodate alternative goals that promote multi-scale canopy structural complexity across managed northern hardwood forest landscapes by adapting the timing (e.g., extending cutting intervals) and intensity (e.g., low intensity harvests) of partial harvesting to retain or develop elements of canopy complexity.