Spatial arrangement of canopy trees following adaptation treatments in northern hardwood forests

Background/Question/Methods

As global change is expected to alter disturbance regimes and create novel growing conditions, there is an increased need for understanding how past management history, individual tree species ecology, and current forest management focused on climate change adaptation may influence ecosystem structure and associated feedbacks with changing environmental conditions. Spatial arrangement of forest canopy trees influences ecosystem function and processes and response to disturbances, and heterogeneity of overstory tree arrangements can be linked with greater adaptive capacity. Quantification of tree spatial arrangement can further provide valuable insight into historic regeneration and recruitment patterns of individual species. We stem-mapped canopy trees in 16 1-ha forest plots in silviculture treatments designed to examine resistance, resilience, transition, and no action as approaches to climate change adaptation in northern hardwood forests in New Hampshire, USA. We used replicate point pattern analysis with Ripley’s K and pairwise correlation functions to examine patterns in tree arrangement at various spatial scales and assessed whether trees are aggregated, uniformly, or randomly distributed in each adaptation treatment. Our analyses quantify how individual species (dominant species Acer saccharum, Fagus grandifolia, and Betula allegheniensis) are arranged in space, as well as how adaptation treatments affect arrangement of forest canopy trees.

Results/Conclusions

Each dominant species displayed unique spatial patterning at varying distance scales, with moderate differences based on silvicultural treatment. Patterns reflected species regeneration traits, with A. saccharum uniformly distributed in concert with its shade tolerance and ability to germinate through leaf litter and F. grandifolia uniformly distributed at short distances, likely as a result of root sprouting induced by beech bark disease, but more aggregated at greater distances. B. allegheniensis displayed aggregated, or clumped distribution, which can be explained by its relative shade-intolerance, and common pattern of germination in gaps or on patches of bare mineral soil or downed wood. At the treatment scale, in the no action and resistance, canopy trees were uniformly distributed across the plots, whereas in resilience and transition treatments, aggregation began to occur, concentrated at distances of 1-15 meters. Our results highlight that spatial patterning of individual tree species is influenced by both management and species regeneration traits. Additionally, our outcomes suggest that when tree aggregation is used as an indicator of structural heterogeneity influencing adaptive capacity, adaptive management designed to facilitate change leads to a greater diversity of tree arrangements.