Drought Impacts and Compounding Mortality on Forest Trees in the Southern Sierra Nevada

Lauren Pile,Marc Meyer,Ramiro Rojas,Mark Smith,Olivia Roe

doi:10.3390/f10030237

Abstract

The increase in compounding disturbances, such as “hotter droughts” coupled with insect outbreaks, has significant impacts on the integrity of forested ecosystems and their subsequent management for important ecosystem services and multiple-use objectives. In the Southern Sierra Nevada, years of severe drought have resulted in unprecedented tree mortality across this mountainous landscape. Additionally, past land management practices, including fire suppression, have led to overly stocked, homogenous forest stand structures, dominated by small diameter, shade-tolerant and fire-intolerant tree species. Thus, the current condition of the landscape has further increased the susceptibility of forest trees to multiple stressors. We sought to determine the effects of extreme drought and insect outbreaks on tree mortality and their influence on forest stand structure and composition. To characterize mortality patterns, we monitored the condition of mature forest trees (>25.4 cm diameter at breast height) across 255 monitoring plots with four repeated measurements from 2015 through 2017. Tree mortality varied by species and through time. Reductions in pine species (Pinus lambertiana Douglas and P. ponderosa Lawson & C. Lawson) occurred earlier in the study period than Abies concolor (Gord. & Glend.) Lindl. Ex Hildebr. or Calocedrus decurrens (Torr.) Florin. Across species, larger tree size, most often associated with tree height, was consistently related to increased survival in mature, overstory trees. As expected, sites with greater pine stocking and subsequently more bark beetle (Curculionidae: Scolytinae) host availability had increased pine mortality, especially for P. ponderosa. For Abies concolor, lower overstory basal area increased tree survival for this species. This study highlights the importance of effective forest monitoring, especially during a period of unprecedented ecological change as the compounding disturbance had a disproportional effect on pine species in smaller diameter classes. Proactive forest management may be necessary to maintain and promote these ecologically important species in heterogeneous mixtures across the landscape.

Highlights

The increasing frequency, severity, and duration of drought due to climate change has a significant impact on forested communities [1,2,3]
Given the design, we were unable to elucidate whether changes in tree status were the result of morphological or physiological changes in response to severe drought, insect attack that likely contributed to the high levels of pine mortality, increased water stress associated with regional climate warming, or some combination of these mortality agents
The surviving P. ponderosa overstory trees may have drought-tolerance traits that could be important for recruitment

Summary

Introduction

The increasing frequency, severity, and duration of drought due to climate change has a significant impact on forested communities [1,2,3]. The effects can range from stressed trees that are more susceptible to secondary mortality to wholescale shifts in forest structure, composition, and function [2]. Extensive tree mortality on public lands may impact recreational activities, threaten human safety, change ecological conditions, diminish sensitive wildlife habitats, and increase fire hazard and risk. Drought is a natural disturbance in many forested ecosystems, exceptional drought events are becoming more frequent and severe with increasing global temperatures and the interaction of stressors [9]. Determining the role of factors contributing to tree mortality may aid forest management prioritization as droughts become more recurrent and severe

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