Forest ecosystems, disturbance, and climatic change in Washington State, USA

Abstract

C limatic change is likely to affect Pacific Northwest (PNW) forests in several important ways. In this paper, we address the role of climate in four forest ecosystem processes and project the effects of future climatic change on these processes. First, we analyze how climate affects Douglas-fir growth across the region to understand potential changes in future growth. In areas where Douglas-fir is not water-limited, future growth will continue to vary with interannual climate variability, but in places where Douglas-fir is water-limited, growth is likely to decline due to projected increase in summer potential evapotranspiration. Second, we use existing analyses of climatic controls on future potential tree species ranges to highlight areas where species turnover may be greatest. By the mid 21 st century, some areas of the interior Columbia Basin and eastern Cascades are likely to have climates poorly suited to pine species that are susceptible to mountain pine beetle, and if these pines are climatically stressed, they may be more vulnerable to pine beetle attack. Climatic suitability for Douglas-fir is also likely to change, with substantial decreases in climatically suitable area in the Puget Trough and the Okanogan Highlands. Third, using regression approaches, we examine the relationships between climate and the area burned by fire in the PNW and in eight Washington ecosystems and project future area burned in response to changing climate. Area burned is significantly related to both temperature and precipitation in summer, but more physiologically relevant variables, such as water balance deficit, perform as well or better in models. Regional area burned is likely to double or even triple by the end of the 2040s, although Washington ecosystems have different sensitivities to climate and thus different responses to climatic change. Fourth, we evaluate the influence of climatic change on mountain pine beetle (MPB) outbreaks by quantifying both host-tree vulnerability and pine beetle adaptive seasonality. Host-tree vulnerability is closely related to vapor pressure deficit (VPD), and future projections support the hypothesis that summer VPD will increase over a significant portion of the range of host tree species. Due to the increased host 255vulnerability, MPB populations are expected to become more viable at higher elevations leading to increased incidence of MPB outbreaks. The increased rates of disturbance by fire and mountain pine beetle are likely to be more significant agents of changes in forest structure and composition in the 21st century than species turnover or declines in productivity. This suggests that understanding future disturbance regimes is critical for successful adaptation to climate change.

Impact of anthropogenic climate change on wildfire across western US forests

“…Although numerous studies have projected changes in burned area over the twenty-first century due to ACC, we are unaware of other studies that have attempted to quantify the contribution of ACC to recent forested burned area over the western United States. The near doubling of forested burned area we attribute to ACC exceeds changes in burned area projected by some modeling efforts to occur by the mid-twentyfirst century (29,30), but is proportionally consistent with midtwenty-first century increases in burned area projected by other modeling efforts (17,(31)(32)(33).…”

A New Metric for Quantifying Burn Severity: The Relativized Burn Ratio

“…We demonstrated that both RBR and RdNBR are less correlated to pre-fire NBR than is dNBR, indicating that the relativized metrics are better at detecting high severity effects across the full range of pre-fire vegetation cover. Areas of relatively sparse or spatially discontinuous vegetation are fairly common throughout the western US and will potentially become more common as climate becomes more arid and fire becomes more frequent [24,25]. Accurately characterizing burn severity in such areas will become increasingly important.…”

Conifer growth and reproduction in urban forest fragments: Predictors of future responses to global change?

“…Climate envelope models and tree ring analyses imply suitable habitat is largely defined by climate variables (Littell et al 2010;Mckenzie et al 2003;Nitschke and Innes 2008). Additionally, growth and mortality of conifers are known to be sensitive to temperature, soil moisture (likely to decrease with warmer temperatures), and CO 2 (Fritts 1974;Gregg et al 2003;Handa et al 2006;Holman and Peterson 2006), particularly at seedling stages (Yin et al 2007).…”