Tree death is a fundamental ecological process, but many forest ecosystems lack serviceable estimates of basic mortality rates and patterns. Recent warm droughts, bark beetle irruptions, and legacies of ecosystem degradation have contributed to massive and recurring mortality events, particularly throughout the North American West. To put these events in context, anticipate future mortality, and mitigate potential damage, current rates and trends are essential tools. This motivates our study of the environmental conditions and forest characteristics that influence mortality in California forests. In this work, we use a comprehensive ground-based inventory (USFS Forest Inventory and Analysis Program data) to quantify contemporary patterns and drivers of mortality across California forests, 2001-2015. Our goals are to 1) estimate mortality rates of tree species, sizes, and communities in detail, and 2) quantify the contributions of the tree, stand, and forest characteristics that influence elevated mortality. We particularly emphasize yellow pine and mixed conifer forests, which were severely impacted by the 2012-2015 drought, an event unparalleled in hundreds of years. We also investigate how the contributions of structure, canopy position, and species vary with drought severity and between ecoregions.
Results/Conclusions
We found that annual mortality rates for all trees more than doubled, from 0.54 ± 0.09% of basal area lost per year in 2001-2003 to 1.23 ± 0.10% in 2012-2015. In 2012-2015, mean annual mortality for trees exclusive of fire effects was 0.71 ± 0.04% of basal area per year. Species with highest mean annual mortality 2008-2015, and largest increases through time, were red fir (Abies magnifica), sugar pine (Pinus lambertiana) and white fir (A. concolor). Mortality patterns were dependent on canopy position, diameter, and crown condition. Local precipitation and maximum temperature anomalies also shaped mortality patterns. Measures of tree mortality derived from widespread plot-based inventory provide an essential estimate of forest vital rates, patterns rarely captured in other approaches. Inventory driven quantification, both alone and in combination with other estimates of vital rates at many scales, can serve as a valuable reference in assessing long-term trends and discerning the condition and resilience of forest ecosystems.