Stand structural dynamics of North American boreal forests

Stand structure, the arrangement and interrelationships of live and dead trees, has been linked to forest regeneration, nutrient cycling, wildlife habitat, and climate regulation. The objective of this review was to synthesize literature on stand structural dynamics of North American boreal forests, addressing both live tree and coarse woody debris (CWD) characteristics under different disturbance mechanisms (fire, clearcut, wind, and spruce budworm), while identifying regional differences based on climate and surficial deposit variability. In fire origin stands, both live tree and CWD attributes are influenced initially largely by the characteristics of the stand replacing fire and later increasingly by autogenic processes. Differences in stand structure have also been observed between various stand cover types. Blowdown and insect outbreaks are two significant non-stand replacing disturbances that can alter forest stand structure through removing canopy trees, freeing up available growing space, and creating microsites for new trees to establish. Climate and surficial deposits are highly variable in the boreal forest due to its extensive geographic range, influencing stand and landscape structure by affecting tree colonization, stand composition, successional trajectories, CWD dynamics, and disturbance regimes including regional fire cycles. Further, predicted climate change scenarios are likely to cause regional-specific alterations in stand and landscape structure, with the implications on ecosystem components including wildlife, biodiversity, and carbon balance still unclear. Some stand structural attributes are found to be similar between clearcut and fire origin stands, but others appear to be quite different. Future research shall focus on examining structural variability under both disturbance regimes and management alternatives emulating both stand replacing and nonstand replacing natural disturbances.