Patterns of Structural Response to Simulated Partial Harvesting of Boreal Mixedwood Stands

Partial harvesting has been proposed as an approach for maintaining late-successional structure within managed boreal mixedwood stands. Although little long-term data is available to evaluate its effects in this stand type, recent advances in individual tree-based stand modeling provide an opportunity to simulate post-harvest stand development following different retention harvests. Using the stand dynamics model SORTIE-ND, we examined 40-year patterns of structural change in response to different intensities (30%, 50%, and 70% removal) and spatial patterns (uniform, small patch, large patch) of harvesting in aspen-dominated mixedwood stands. We assessed structural dynamics through a suite of variables representing the distribution of tree sizes, understory development, regeneration, standing and fallen dead wood characteristics, and within-stand heterogeneity. Partial harvesting induced a reciprocal increase in understory and downed woody debris development and decrease in overstory structure over the first 20 years after harvest, with this effect reversing after 25 years as harvest-induced regeneration reached the canopy. Densities of large trees and snags were reduced by harvesting, and did not recover to pre-harvest levels within 40 years. Harvesting promoted within-stand heterogeneity in the short and long term, and also produced transient increases in early-decay downed woody debris and ground exposure. These effects largely increased in proportion to harvest intensity. Although spatial pattern was of lesser importance than intensity, aggregated harvests induced somewhat less pronounced impacts on structure (with the exception of heterogeneity) than dispersed harvesting. These simulation results can form a basis for more detailed hypotheses regarding maintenance of late-successional stand structure and function through partial harvesting. Such hypotheses may in turn be translated into real-world silvicultural experiments to be evaluated, refined, and either accepted or rejected within an adaptive management framework