Does overshoot in leaf development of ponderosa pine in wet years leads to bark beetle outbreaks on fine-textured soils in drier years?

Abstract

Background: Frequent outbreaks of insects and diseases have been recorded in the native forests of western North America during the last few decades, but the distribution of these outbreaks has been far from uniform. In some cases, recent climatic variations may explain some of this spatial variation along with the presence of expansive forests composed of dense, older trees. Forest managers and policy makers would benefit if areas especially prone to disturbance could be recognized so that mitigating actions could be taken. Methods: We use two ponderosa pine-dominated sites in western Montana, U.S.A. to apply a modeling approach that couples information acquired via remote sensing, soil surveys, and local weather stations to assess where bark beetle outbreaks might first occur and why. Although there was a general downward trend in precipitation for both sites over the period between 1998 and 2010 (slope = −1.3, R 2 = 0.08), interannual variability was high. Some years showed large increases followed by sharp decreases. Both sites had similar topography and fire histories, but bark beetle activity occurred earlier (circa 2000 to 2001) and more severely on one site than on the other. The initial canopy density of the two sites was also similar, with leaf area indices ranging between 1.7-2.0 m

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

“…Buildup of above-ground biomass from growth during favorable climate conditions can increase tree vulnerability under subsequent drought and heat stress, because these morphological commitments to leaf and stem tissues require substantial water and carbohydrate maintenance costs during later drought conditions, driving mortality risks from both carbon starvation and embolism , Peterman and Waring 2014, McDowell and Allen 2015. Similarly, trees at warmer temperatures generally grow more stem tissue, leaf biomass, and leaf area, with unchanged or even lower growth of roots, resulting in reduced root-to-leaf-area ratios; these allometric changes increase the difficulty of water transport and can predispose trees to greater vulnerability to episodic drought (Way and Oren 2010).…”

“…In places where global warming is better buffered by favorable geographic circumstances (e.g., coastal regions adjoining and downwind of cold oceans), forests may experience less warming, lower drought stress, and thus less tree mortality from hotter drought. At landscape scales the diversity of topographic (Adams et al 2014), soil (Peterman and Waring 2014, Twidwell et al 2014, Dorman et al 2015a, and hydrological Goulden 2013, Silvertown et al 2015) settings and microsites provides relatively buffered refugia where trees have cooler-moister conditions to survive hot drought stresses, as well as favorable sites to recover more readily after mortality events. Such landscape diversity can allow ''climate relict'' populations of trees to persist as climate conditions become less favorable (Hampe and Jump 2011).…”

“…In some ecosystems, climate change may actually hurt the insect pest agents that attack trees either through direct climate impacts on population growth or through increases in the abundance of predators which control those agents (Hicke et al 2006, Raffa et al 2008, Jamieson et al 2012. At landscape scales the diversity of topographic (Adams et al 2014), soil (Peterman and Waring 2014, Twidwell et al 2014, Dorman et al 2015a, and hydrological (Silvertown et al 2015) settings and microsites provides relatively buffered refuge locales where trees have cooler-moister conditions to survive hot drought stresses (e.g., Allen and Breshears 1998), as well as favorable sites to recover more readily after mortality events. Such landscape diversity can allow ''climate relict'' populations of trees to persist as climate conditions become less favorable (Hampe and Jump 2011).…”

Accounting for landscape heterogeneity improves spatial predictions of tree vulnerability to drought

“…Spatial patterns of tree mortality are associated with changes in soil texture and depth (Bowker et al, 2012;Peterman & Waring, 2014;Twidwell et al, 2014), topographic position (Adams et al, 2014;Hawthorne & Miniat, 2017) and local water stress gradients (Gitlin et al, 2006). Soil moisture variability across a landscape is driven by spatial variability in soil texture and depth to bedrock, surface runoff and subsurface lateral flow of water (Dunne et al, 1975;Beven & Kirkby, 1979), and differences in radiation and resulting evaporation due to aspect and slope (Moore et al, 1991;McCune & Keon, 2002).…”