Disentangling mechanisms of early succession following harvest: Implications for climate change adaptation in Canada's boreal-temperate forests

摘要

PrediCting forest succession is not a trivial pursuit and has remained a central challenge for scientists and foresters for well over a century. While thousands of papers have been published on the topic, contributing to a plethora of concepts and terminologies, there remains confusion over causal mechanisms, inhibiting our ability to adopt many lessons learned to sustainable forest management. Based on an emerging conceptual model of forest succession, presented here as the "Succession Triangle", we sought to explore the relative importance of mechanisms hypothesized to drive succession following commercial harvesting across a wide landscape in the Acadian Forest Region of eastern Canada. Using machine learning techniques and repeatedly measured forest stand inventory data, including 19,332 forest stands measured over a approximate to 30 year period, we uncovered dominant early succession pathways and investigated key mechanisms driving these pathways. Overall, our results show stands may follow multiple succession pathways following harvesting, including early dominance by so-called "late-succession" species (e.g., black spruce and sugar maple), depending on causal mechanisms. More specifically, mechanisms related to the state of the environment (primarily climate) and differential species availability (mainly pre- and post-harvest overstorey composition) had the strongest control over early succession. The strong influence of pre-harvest composition on early succession supports the 'direct regeneration' hypothesis; while the importance of post-harvest overstorey composition suggests the use of partial-cutting as a means of regenerating shade-tolerant species (e.g., sugar maple and red spruce). Site conditions (i.e., slope and drainage) influenced the direction of succession, but landscape-level variation in climate had the overall strongest effect, with colder, more moist climates promoting regeneration of balsam fir, while warmer, dryer climates encouraged shade-intolerant, broadleaf species (birches, red maple and poplars), especially following clear-cutting.