Landscape Topoedaphic Features Create Refugia from Drought and Insect Disturbance in a Lodgepole and Whitebark Pine Forest

Jennifer Cartwright

doi:10.3390/f9110715

Abstract

Droughts and insect outbreaks are primary disturbance processes linking climate change to tree mortality in western North America. Refugia from these disturbances—locations where impacts are less severe relative to the surrounding landscape—may be priorities for conservation, restoration, and monitoring. In this study, hypotheses concerning physical and biological processes supporting refugia were investigated by modelling the landscape controls on disturbance refugia that were identified using remotely sensed vegetation indicators. Refugia were identified at 30-m resolution using anomalies of Landsat-derived Normalized Difference Moisture Index in lodgepole and whitebark pine forests in southern Oregon, USA, in 2001 (a single-year drought with no insect outbreak) and 2009 (during a multi-year drought and severe outbreak of mountain pine beetle). Landscape controls on refugia (topographic, soil, and forest characteristics) were modeled using boosted regression trees. Landscape characteristics better explained and predicted refugia locations in 2009, when forest impacts were greater, than in 2001. Refugia in lodgepole and whitebark pine forests were generally associated with topographically shaded slopes, convergent environments such as valleys, areas of relatively low soil bulk density, and in thinner forest stands. In whitebark pine forest, refugia were associated with riparian areas along headwater streams. Spatial patterns in evapotranspiration, snowmelt dynamics, soil water storage, and drought-tolerance and insect-resistance abilities may help create refugia from drought and mountain pine beetle. Identification of the landscape characteristics supporting refugia can help forest managers target conservation resources in an era of climate-change exacerbation of droughts and insect outbreaks.

Highlights

Droughts and insect outbreaks are primary disturbance processes by which climate change has been implicated in tree mortality and forest change globally [1,2]
In lodgepole pine (LP) and whitebark pine (WP) forest in a mountainous study area that experienced a single year drought without insect disturbance (2001) and a multi-year drought combined with a severe MPB outbreak (2007–2010), refugia from drought and insect disturbance were identified using a simple anomaly-based approach and time-series of remotely sensed Normalized Differenced Moisture Index (NDMI)
Refugia from drought and MPB impacts were found on topographically shaded slopes, in convergent areas such as valleys, in areas of low soil bulk density and in thinner forest stands

Summary

Introduction

Droughts and insect outbreaks are primary disturbance processes by which climate change has been implicated in tree mortality and forest change globally [1,2]. Many forest ecosystems are adapted to episodic droughts and insect outbreaks, intensification of these disturbances (i.e., increased frequency or severity) is a growing conservation concern in the context of projected warmer and drier climates in much of western North America [6,7,8]. U.S.itsEndangered and successional due toWP fire was suppression [9,10].

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