Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality

Michael J Koontz,Christopher J Fettig,Andrew M Latimer,Malcolm P North,Leif A Mortenson

doi:10.1038/s41467-020-20455-y

Abstract

The recent Californian hot drought (2012–2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Broad-scale climate conditions can directly shape tree mortality patterns, but mortality rates respond non-linearly to climate when local-scale forest characteristics influence the behavior of tree-killing bark beetles (e.g., WPB). To test for these cross-scale interactions, we conduct aerial drone surveys at 32 sites along a gradient of climatic water deficit (CWD) spanning 350 km of latitude and 1000 m of elevation in WPB-impacted Sierra Nevada forests. We map, measure, and classify over 450,000 trees within 9 km2, validating measurements with coincident field plots. We find greater size, proportion, and density of ponderosa pine (the WPB host) increase host mortality rates, as does greater CWD. Critically, we find a CWD/host size interaction such that larger trees amplify host mortality rates in hot/dry sites. Management strategies for climate change adaptation should consider how bark beetle disturbances can depend on cross-scale interactions, which challenge our ability to predict and understand patterns of tree mortality.

Highlights

The recent Californian hot drought (2012–2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB)
Our work suggests that the WPB was cueing into different aspects of forest structure across an environmental gradient in a spatial context in a parallel manner to the temporal context noted by Stovall et al.[65] and Pile et al.[70], who observed that mortality was increasingly driven by larger trees as the hot drought proceeded and became more severe
A temporal signal of bark beetles attacking larger and larger host trees reflects the positive feedback between forest structure and bark beetle population dynamics as the population phase cycles from endemic to epidemic[13]. This positive feedback leading to eruptive population dynamics is well-documented as a temporal phenomenon, and here we show a similar pattern in a spatial context mediated through site-level climatic water deficit (CWD)

Summary

Introduction

The recent Californian hot drought (2012–2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Taking forest structure alone, high-density forests are more prone to bark beetle-induced tree mortality compared to thinned forests[6,9], which may arise as greater competition for water resources amongst crowded trees lowers average tree resistance[31], or because smaller gaps between trees protect pheromone plumes from dissipation by the wind and enhance intraspecific beetle communication[32] Tree size is another aspect of forest structure that affects bark beetle host selection behavior with smaller trees tending to have a lower capacity for resisting attack, but larger trees being more desirable targets on account of their thicker phloem providing greater nutritional content[13,33,34,35]. Colonization by primary bark beetles can depend on the local relative frequencies of tree species in forest stands, reflecting the more general pattern that specialist insect herbivory tends to be lower in taxonomically diverse forests compared to monocultures[40,41]

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Results

Discussion

Conclusion