Interactions of predominant insects and diseases with climate change in Douglas-fir forests of western Oregon and Washington, U.S.A.

Abstract

Forest disturbance regimes are beginning to show evidence of climate-mediated changes, such as increasing severity of droughts and insect outbreaks. We review the major insects and pathogens affecting the disturbance regime for coastal Douglas-fir forests in western Oregon and Washington State, USA, and ask how future climate changes may influence their role in disturbance ecology. Although the physiological constraints of light, temperature, and moisture largely control tree growth, episodic and chronic disturbances interacting with biological factors have substantial impacts on the structure and functioning of forest ecosystems in this region. Understanding insect and disease interactions is critical to predicting forest response to climate change and the consequences for ecosystem services, such as timber, clean water, fish and wildlife. We focused on future predictions for warmer wetter winters, hotter drier summers, and elevated atmospheric CO to hypothesize the response of Douglas-fir forests to the major insects and diseases influencing this forest type: Douglas-fir beetle, Swiss needle cast, black stain root disease, and laminated root rot. We hypothesize that 1) Douglas-fir beetle and black stain root disease could become more prevalent with increasing, fire, temperature stress, and moisture stress, 2) future impacts of Swiss needle cast are difficult to predict due to uncertainties in May-July leaf wetness, but warmer winters could contribute to intensification at higher elevations, and 3) laminated root rot will be influenced primarily by forest management, rather than climatic change. Furthermore, these biotic disturbance agents interact in complex ways that are poorly understood. Consequently, to inform management decisions, insect and disease influences on disturbance regimes must be characterized specifically by forest type and region in order to accurately capture these interactions in light of future climate-mediated changes.

How does tree regeneration respond to mixed‐severity fire in the western Oregon Cascades,USA?

“…We define forest resilience as the capacity of this forest type to absorb disturbance and reorganize while undergoing change, such that it retains essentially the same function, structure, identity, and feedbacks (Walker et al 2004). Forest resilience is enhanced by structural complexity and response diversity because species vary in their (1) (4) susceptibility to insects and disease (Agne et al 2018). While our observations may be limited to the highly productive Douglas-fir forests we sampled, similar regeneration responses have been reported in moister forests at the northern reach or north of our study area despite having a less diverse species pool Franklin 2005, Tepley et al 2013).…”

“…This successional model may explain the unexpected overstory communities (e.g., incense-cedar and western redcedar or sugar pine and western hemlock as co-dominant trees) observed regularly in our study area. Forest resilience is enhanced by structural complexity and response diversity because species vary in their (1) (4) susceptibility to insects and disease (Agne et al 2018). This increase in structural complexity and response diversity suggests moderate-severity fire increases forest resilience to disturbances like disease, insect, drought, and fire (Elmqvist et al 2003).…”

Changing wildfire, changing forests: the effects of climate change on fire regimes and vegetation in the Pacific Northwest, USA

“…For example, on the west side of the Cascade Range, laminated root rot (Phellinus weirii [Murrill] Gilb.) is widespread, causing small pockets of mortality in Douglas-fir (Agne et al 2018). However, no evidence exists that this pathogen has been or will be accelerated by a warmer climate.…”

“…However, no evidence exists that this pathogen has been or will be accelerated by a warmer climate. Other pathogens, such as Swiss needle cast (Phaeocryptopus gaeumannii [T. Rohde] Petrak), may be favored by warmer and wetter winters (Agne et al 2018). Fungal pathogens stress trees and may increase susceptibility to insect infestations.…”

Environmental variables associated withNothophaeocryptopus gaeumanniipopulation structure and Swiss needle cast severity in Western Oregon and Washington

“…Microbial pathogens and insects, because of their shorter generation times, respond more rapidly to environmental changes than forest communities and may amplify the effects of climate change on forest health (Kliejunas et al, 2009). Changes to forest disturbance regimes due to the influences of climate on the interactions between disease, fire, and insects are expected to impact the future ecological trajectories of forested landscapes (Agne et al, 2018;Sturrock et al, 2011).…”