Interactions of predominant insects and diseases with climate change in Douglas-fir forests of western Oregon and Washington, U.S.A.
Introduction
Disturbance regimes in forests of western North America are showing evidence of climate-mediated shifts associated with global climate change in the form of historically unprecedented tree mortality (Anderegg et al., 2012, van Mantgem et al., 2009). Instigating factors for these mortality events include extreme drought (Allen et al., 2015, Asner et al., 2015), increased fire severity and extent (Abatzoglou and Williams, 2016), and expansion of bark beetles into previously climatically unsuitable habitat (Bentz et al., 2010, Björkman and Niemelä, 2015). The frequency and severity of forest disturbances will likely continue to increase given predicted climate-related changes in environmental conditions over the 21 st century (Allen et al., 2015), which will influence a range of characteristics of these forests including the ecosystem services that they provide (Johnstone et al., 2016, Seidl et al., 2016).
Projected changes in climate will make forests more vulnerable to tree mortality resulting from physiological stress interacting with other climate-influenced events, such as insect and disease outbreaks, droughts and fires (Beedlow et al., 2013, Kolb et al., 2016, Weed et al., 2013). Current predictions for major climate-related trends affecting forests in western North America include increased fire season length and burned area (Abatzoglou and Williams, 2016, Flannigan et al., 2013), increased occurrence of severe drought (Allen et al., 2015), reduced mountain snowpack (Kapnick and Hall, 2012), and generally increasing temperature, with seasonal trends including warmer wetter winters, and hotter drier growing seasons (Rupp et al., 2016).
There is a growing interest in understanding the interactions of multiple disturbance factors (Anderegg et al., 2015, Johnstone et al., 2016, Law and Waring, 2015) in forest ecosystems because their combined effects can differ from that of any single agent acting alone (Seidl et al., 2016). However, interactions for any given forest type vary by landscape character, forest structure, specific insect herbivores and forest pathogens, as well as seasonal climatic factors, storms, and fire patterns. Consequently, to inform management decisions, disturbance regimes must be characterized specifically by forest type and region in order to accurately capture these interactions and allow for prediction of future climate-mediated changes. If conducted at a scale at which management actions are implemented, such as forests with the same dominant tree species and climatic conditions, assessments of potential climate-related changes to disturbance regimes can greatly improve our ability to adaptively manage our forests and the ecosystem services they provide.
Here, we examine the primary insects and diseases (referred to throughout as biotic disturbance agents) of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco). Although western hemlock is commonly found as a co-dominant in Douglas-fir forests, especially in old growth forests, Douglas-fir is the dominant species on most of the forested land from near sea-level to roughly 1200 m elevation in western Oregon and Washington, U.S.A. Further, Douglas-fir is the principle timber species, and it is ecologically important for carbon sequestration, and wildlife habitat, as well as being vital for the production of hydropower, irrigation, and drinking water (Curtis and Carey, 1996, Harmon et al., 2004, Kline et al., 2016, Ruggiero et al., 1991). Ecological effects of the major disturbance regimes, especially fire and logging, have been studied extensively in this forest type (Cohen et al., 2002, Creutzburg et al., 2016, Healey et al., 2008, Tepley et al., 2013, Wimberly and Spies, 2001). However, the role of insects and diseases within the disturbance regime has not been adequately addressed, even though there is currently a major foliage disease (Swiss needle cast) epidemic occurring in the region (Ritóková et al., 2016).
Here, we: (1) identify the impacts of the major insects and diseases affecting Douglas-fir and their interactions, (2) integrate our understanding of temperature and moisture stress in trees with future climate projections to hypothesize changes in disturbance agent behavior under climate change, and (3) highlight important knowledge gaps in the understanding of the current and projected disturbance regimes in coastal Douglas-fir forests.
Section snippets
Ecological setting
Coastal Douglas-fir forests extend from British Columbia through northwestern California. However, we constrain the geographical extent to Washington and Oregon, west of the crest of the Cascade Mountain Range where Douglas-fir is the dominant tree species (Fig. 1), which has traditionally been called the “Douglas-fir region” (Franklin, 1979, Jensen, 1955). This is a moist temperate forest region is dominated by conifers and has relatively long time periods between high severity natural
Physiological response to water and temperature stress
The proximate effects of climate change on Douglas-fir forests are increased temperature and water stress, particularly during the summer drought. However, they interact with insect pests and diseases to affect tree mortality. Here, we present the basic physiological effects of water and temperature stress in trees to better understand the interactions of biotic disturbance agents within a forest context. Water stress typically appears first in trees growing on sites with features such as
Biotic disturbance agents
Disturbance dynamics vary widely among Douglas-fir forests by structure and age. We focus on the insects and pathogens, which interacting with temperature and moisture stress commonly result in climate-related tree mortality within the region. These include laminated root rot (caused by Phellinus sulphurascens [Pilat]), black stain root disease (caused by Leptographium wagneri [Kendrick] Wingfield), Swiss needle cast (caused by Phaeocryptopus gaeumannii [Rohde] Petrak), and Douglas-fir beetle (
Predicted climate change impacts
There are three components of changing climate likely to influence the ecology of the Douglas-fir region: hotter drier summers, warmer wetter winters, and elevated atmospheric CO2 levels. Climate model simulations suggest that by mid-century the Douglas-fir region will experience hotter drier summers and warmer wetter winters with substantial decreases in snowpack (Mote and Salathé, 2010). Averaged across a number of regionally downscaled climate models, it is predicted that, compared to the
Knowledge gaps
Regardless of climate change scenarios, the biotic disturbance agents discussed above are likely to remain prominent or increase on the landscape. However, significant questions remain regarding interactions of trees, climate, and disturbance factors. We outline key areas in which further study is needed below.
Conclusions
The disturbance regime in Douglas-fir region consists of generally long-term fire return intervals interacting with biotic and abiotic factors, which then interact with the anthropogenic disturbance of forest management. Generalizations beyond the region are limited due to the unique aspects of abiotic and biotic disturbance agents in these Douglas-fir forests. Because climate-disturbance interactions vary depending on topographic and edaphic conditions, management actions should be tailored to
Acknowledgements
The collaborative research described in this article has been funded by the U.S. Environmental Protection Agency (EPA), U.S. Department of Agriculture Forest Service (USFS) and the Oregon State University Swiss Needle Cast Cooperative. It has been subjected to review by the USFS and the EPA National Health and Environmental Effects Research Laboratory’s Western Ecology Division and approved for publication. Approval does not signify that the contents reflect the views of the EPA or the USFS,
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