The Californian hot drought of 2012 to 2015 created favorable conditions for unprecedented ponderosa pine (Pinus ponderosa) mortality in the Sierra Nevada mountain range, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Climate conditions and forest density may interact to affect tree mortality, but density is a coarse gauge of forest structure that can affect WPB behavior in a number of ways. Measuring broad-scale climate conditions simultaneously with local forest composition and structure-- the spatial distribution and size of trees-- will refine our understanding of how these variables interact, but is generally expensive and/or labor-intensive. We use drone surveys over a network of 160 field plots along a 350-km latitudinal and 1000-m elevational gradient in western slope Sierra Nevada ponderosa pine/mixed-conifer forests and structure from motion (SfM) processing to segment and classify more than 450,000 trees over 9 km2 of forest with WPB-induced tree mortality. We modeled the probability of ponderosa pine mortality as a function of forest structure and composition and their interaction with site-level climatic water deficit (CWD), accounting for spatial covariance using exact Gaussian processes.
Results/Conclusions
A greater local proportion of host trees strongly increased the probability of host mortality, with greater host density amplifying this effect. Further, we found a strong interaction between host size and CWD such that larger trees increased the probability of host mortality at hot/dry sites, but smaller trees tended to drive mortality in cool/wet sites. Our results demonstrate a variable response of WPB to local forest structure and composition across an environmental gradient, which may help reconcile differences between observed ecosystem-wide tree mortality patterns and predictions from models based on coarser-scale forest structure. Climate change adaptation strategies should consider that future disturbance outcomes may depend on interactions between local forest structure and broad-scale environmental gradients, with the potential for cross-scale interactions that challenge our current understanding of forest insect dynamics.