Koontz, M. J., A. M. Latimer, L. A. Mortenson, C. J. Fettig, and M. P. North. 2021. Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality. Nature Communications 12:129–141. https://www.nature.com/articles/s41467-020-20455-y **Title**: Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality **Abstract**: The Californian hot drought of 2012 to 2016 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 can partially explain tree mortality patterns through their direct effect on tree vigor, but tree mortality rates can respond non-linearly to climate when bark beetles interact with local forest characteristics while they colonize drought-stressed trees. 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 conducted aerial surveys using a drone-mounted multispectral sensor over 32 sites along a broad gradient of climatic water deficit spanning 350 km of latitude and 1000 m of elevation in western slope Sierra Nevada ponderosa pine/mixed-conifer forests. With structure from motion (SfM) photogrammetry, we processed imagery into maps of reflectance and vegetation height, which we used to segment and classify more than 450,000 trees over 9 km² of forest with WPB-induced ponderosa pine mortality. We validated the segmentation and classification of aerial data using 160 coincident field plots, and assumed that dead trees were all ponderosa pine killed by WPB as this was the primary field-verified scenario. 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. A greater local proportion of ponderosa pine trees (the WPB host) strongly increased the probability of host mortality, with greater host density amplifying this effect. Further, larger host trees increased the probability of host mortality in accordance with well-known life history of WPB. Critically, we found a strong interaction between host size and CWD such that larger trees exacerbated host mortality rates in hot/dry 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. Management strategies for climate change adaptation 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. [1]: https://www.nature.com/articles/s41467-020-20455-yr current understanding of forest insect dynamics.