Broad-scale forest mortality has been observed over the past decade in association with increased climate change-induced drought, particularly in western U.S. forests. While predictions generally indicate continued increases in regional forest mortality, forest vulnerability to drought will likely vary with species composition, forest structure, and variation in local demography. In particular, establishment and survival of seedlings—processes necessary for long-term forest persistence—will likely be key determinants of the effects of drought on forests. Seedling vulnerability to drought may be attenuated by the buffered below-canopy microclimate created by dense forest canopies, particularly in the characteristically productive conifer forests of the Pacific Northwest. This study evaluated whether forest canopies can attenuate drought-induced seedling mortality in western Oregon forests by quantifying seedling mortality rates over an extreme drought year in relation to range position, seedling age, and canopy density. Seedling survival was monitored across a network of six long-term monitoring plots spanning a large climatic gradient extending from the Oregon coast to the eastern slope of the Cascade Range. Hierarchical Bayesian logistic regression models were constructed for each species to quantify annual mortality rates and to determine the effects of canopy density, range position, and seedling age on mortality.
Results/Conclusions
Preliminary results show that seedling mortality over the drought year was high across all sites. Much of the variation in seedling mortality was explained by canopy density, range position, and seedling age, although variation was minimal due to the high mortality rates observed across all sites. Range position explained a greater proportion of the variance in mortality than canopy density, although relationships with canopy density were significant across both species and age classes. Lower mortality rates corresponded with higher canopy density, suggesting that buffering may increase the availability of climatically suitable microsites for seedlings during otherwise severe drought conditions. Rates of mortality were highest in first year seedlings and declined with age. Mortality rates also varied with range position, with the lowest mortality in the coastal forest, slightly higher mortality in high-elevation stands in the Cascade mountains, and the highest mortality rates observed in low-elevation forests in the Cascade foothills. This likely reflects the warmer, drier conditions that characterize these forests. Collectively, these findings indicate that dense forest canopies can moderate the effects of drought on seedlings by reducing mortality, but these effects are likely too weak to substantially enhance forest resilience under conditions of increased drought frequency and severity.