The relative importance of tree and stand properties in susceptibility to spruce beetle outbreak in the mid‐20th century

Abstract

Citation: Bakaj, F., N. Mietkiewicz, T. T. Veblen, and D. Kulakowski. 2016. The relative importance of tree and stand properties in susceptibility to spruce beetle outbreak in the mid-20th century. Ecosphere 7(10):e01485. 10.1002/ecs2.1485Abstract. Tree susceptibility to potentially lethal agents is determined not only by attributes of individual trees, but also by neighborhood effects at a range of scales. For example, effects of disturbances on individual trees are often contingent on the size, configuration, and other properties of neighboring trees. Wildfires can modify postfire properties of individual trees as well as of entire forest stands, both of which can affect subsequent ecological processes, including subsequent disturbances. In recent years, much has been learned about how disturbances interact, but numerous questions concerning underlying mechanisms remain unresolved. For example, the relative importance of forest properties at different spatial scales in determining how fires affect forest susceptibility to subsequent disturbances is not well understood. This study explicitly compares the relative importance of tree vs. fine-scale neighborhood effects (e.g., stand properties at <7 m radii), on susceptibility to a 1940s' spruce beetle outbreak. Attributes of individual trees and of stand structure were spatially reconstructed at five 250-m 2 sites that were partly burned in the late 19th century and then affected by spruce beetle outbreak in the 1940s. Random Forest models and classification trees were used to compare the relative importance of variables for susceptibility to spruce beetle attack. Individual tree properties (diameter at breast height and age) were the most important predictors of susceptibility to the outbreak across all sites combined and at each of the sites individually. In contrast, neighborhood effects were poor predictors of susceptibility. This study suggests wildfires reduce susceptibility to outbreaks primarily by reducing the size of postfire live trees and only secondarily by modifying stand structure. One implication of this is that management strategies that aim to modify stand structure over large areas in order to reduce susceptibility to spruce beetle outbreaks may be unnecessarily intensive.

Spruce beetle outbreak was not driven by drought stress: Evidence from a tree‐ring iso‐demographic approach indicates temperatures were more important

“…Although most evidence suggests that surface air temperatures are the predominate influence on spruce beetle dynamics, drought stress should not be dismissed entirely as a determinant in patterns of outbreak initiation and progression at certain scales, particularly when interacting with stand structure. Indeed, a number of other studies on spruce beetle mortality have investigated how certain aspects of stand structure have affected percentage mortality across landscapes (e.g., Bakaj, Mietkiewicz, Veblen, & Kulakowski, 2016; Doak, 2004; Hart, Veblen, & Kulakowski, 2014; Temperli et al., 2014). These studies have largely found tree size to be the best predictors of tree mortality, whereas stand structure has been of lesser importance (Bakaj et al., 2016; DeRose & Long, 2012b).…”

“…Indeed, a number of other studies on spruce beetle mortality have investigated how certain aspects of stand structure have affected percentage mortality across landscapes (e.g., Bakaj, Mietkiewicz, Veblen, & Kulakowski, 2016; Doak, 2004; Hart, Veblen, & Kulakowski, 2014; Temperli et al., 2014). These studies have largely found tree size to be the best predictors of tree mortality, whereas stand structure has been of lesser importance (Bakaj et al., 2016; DeRose & Long, 2012b). Our study confirms that large trees tended to die earlier in the outbreak, signaling beetles preferred these large trees (Appendix S1: Figure S3).…”

Neighborhood context mediates probability of host tree mortality in a severe bark beetle outbreak

“…Spatial attributes of the neighborhood surrounding a host tree, in addition to characteristics of the tree itself, can also influence susceptibility to bark beetle attack (Bakaj et al 2016). For example, while large diameter host trees are preferentially attacked during an outbreak, small diameter host trees may also be attacked when in close proximity to large diameter host trees (Preisler and Mitchell 1993, Kashian et al 2011).…”

“…Further, prevalence and basal area of beetle‐killed host trees may be greater in denser neighborhoods or clusters within a stand (Olsen et al 1996, Negrón et al 2001). Fine scale neighborhood effects are expected to be secondary to tree scale characteristics in determining host tree susceptibility to bark beetle outbreak (Bakaj et al 2016). However, it remains unclear how tree neighborhood characteristics might mediate individual host tree susceptibility, and whether these effects vary with tree size.…”

Climate change vulnerability assessment of aquatic and terrestrial ecosystems in the U.S. Forest Service Rocky Mountain Region

“…Variety across landscapes is also created in areas disturbed by snow avalanches, which can create fire breaks (Veblen et al 1994). Wildfires may reduce susceptibility to spruce beetle (Dendoctronus rufipennis) outbreaks by reducing large Engelmann spruce trees (Bakaj et al 2016).…”