Characterizing variability in the effects of tree mortality in the Pacific Northwest
Tree mortality is a primary driver of structure and function in forested ecosystems worldwide. It has recently emerged as a major focus of ecological research following the occurrence of large scale “die-off” events associated with increased occurrence of wildfire and drought related epidemics of disease and pathogens. However, few studies have actually investigated regional scale patterns and consequences of mortality in an ecological context and we lack a framework on which to base an understanding of the associated ecological effects across large geographic extents. We use a regional inventory to characterize variation in the effects of different levels of stand level tree mortality on forest structure and compare across a range of stand ages among major vegetation zones of the Pacific Northwest. We first compare the frequency distribution of mortality rates among vegetation zones, then examine the effects of different levels of mortality on changes in basal area and directional changes in stand density and mean tree size.
Our results demonstrate a wide range of potential ecological effects associated with various levels of mortality in forests of differing in age and productivity. We found significant differences in the distribution of mortality rates between wet and dry forests. Mortality rates were primarily less than 2.5%/yr in wet forests, while dry forests had higher proportions of very low mortality rates (<0.5%/yr) and much higher proportions of rates >5%/yr. We found that young forests were able to maintain positive increases in basal at much higher rates of mortality than older forests, but that wet forests were able to maintain positive increases in basal area at higher rates than in dry forests of similar age. Structural changes related to density and mean tree size varied with mortality rate and stand age, enabling the interpretation of various levels of mortality in the context of structural development. Our results suggest that most forests in the Pacific Northwest are experiencing low levels of mortality and positive growth, especially in wet forests. In dry forests where high mortality rates were much more common but still relatively rare, decreases in density and increases in mean tree size suggest that recent mortality is potentially increasing resilience of these forests to future drought and wildfire.