Variation in the maximum stand density index and its linkage to climate in mixed species forests of the North American Acadian Region

Maximum stand density index (SDImax) is an important factor controlling stand dynamics that varies by species and region, but less is understood how it differs within a region for a given species. In this analysis, linear quantile mixed modeling (LQMM) and an extensive network of permanent plots were utilized to examine regional variation in the SDImax of 15 species (7 softwoods and 8 hardwoods) across the complex forests of the Acadian Region in North America. Observed plot-level SDImax was then linked to various stand, plant trait, site, and climatic factors and the spatial patterns throughout the region examined. Results indicated high variability of SDImax for a given species with northern hardwood species like yellow birch (Betula alleghaniensis Britt.), sugar maple (Acer saccharum Marsh.), and American beech (Fagus grandifolia Ehrh.) having the lowest values (555 - 627), while softwood species such as northern white cedar (Thuja occidentalis L.; 1014), eastern hemlock (Tsuga canadensis (L.) Carr.; 1026), and white pine (Pinus strobus L.; 967) had the highest values. Compared to the other stand and site factors examined, climate showed the strongest relationship with SDImax with R2 ranging from 82 to 98%. Of the climatic variables examined, those representing growing season length and the timing of precipitation were most influential. For the majority of the species examined, reductions in SDImax were forecasted due to changing climatic conditions. Across species, mean SDImax was found to linearly decline with wood specific gravity and increase with leaf longevity, but showed limited relationships with other species-level functional traits. Overall, the analysis highlights the strong variability of SDImax within and between species as well as the important role that climate has on this attribute within a region.