Forest Mortality in Lake Tahoe Basin from 1985-2010: Influences of Forest Type, Stand Density, Topography and Climate
AuthorVan Gunst, Kristin Jane
AdvisorWeisberg, Peter J
Environmental and Natural Resource Sciences
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Widespread and synchronous outbreaks of forest mortality during the 1990s and continuing to present day have drastically altered millions of hectares of forestlands from Mexico to Alaska (Bentz 2009). Tree mortality is a complex process with a variety of both exogenous and endogenous factors influencing the extent, pattern, and severity of tree mortality (Franklin et al. 1987; Holdenreider et al. 2004; Powers et al; Raffa et al. 2008; Simard et al 2012). Although the vast majority of mortality is mediated by native bark beetles (Coleoptera: Scolytinae), more proximal exogenous factors such as drought, and endogenous factors such as stand density and environmental setting have also been implicated (Allen et al. 2010). This study utilizes remote sensing imagery from the 1985-2010 Landsat Thematic Mapper archive to explore relationships among forest types, climate, stand density, and environmental gradients and mortality in the Lake Tahoe Basin in the central Sierra Nevada. In coniferous forests of the western U.S. and Europe, increased stand density resulting from forest management and land use practices is widely hypothesized to increase forest-wide mortality levels (Dobbertin et al. 2007; Guarín and Taylor 2005; Millar et al. 2012; Maloney and Rizzo 2002; Maloney et al. 2011). In my first chapter, I examine how the relationship between mortality and stand density has differed among five forest types and wet and dry climatic periods. This part of the project uses annual maps of forest mortality (Plates 1-2) and an index of stocking level, a proxy for stand density, derived from classification of remotely sensed imagery. Logistic regression is then used to evaluate how a 20% increase in stocking level affects probability of mortality. Our analysis showed that the strength of density dependence for influencing mortality is variable: all forest types exhibited periods when density-dependent mortality was positive (increased density increases risk of mortality), negative (increased density decreases risk of mortality) or when density was not associated with mortality. However, positive density-dependent mortality was found to a greater degree in Jeffrey pine and lodgepole pine-dominated forests and during climatic periods characterized by drought. In middle- and upper-elevation forests, density-dependent mortality was more variable by climate with negative density-dependent mortality apparent over much of the time period. In my second chapter, I explore 1) Variations in mortality levels by forest type and climatic period, and 2) How the probability of mortality is associated with elevation and incident solar radiation. These two topographical variables are important in governing productivity, resource availability, and temperature and climate regimes in the topographically-complex LTB (Urban et al. 2000). We used generalized linear models and regression tree analysis to elucidate relationships between mortality levels, forest type and climatic period, and logistic regression to quantify association between mortality risk and environmental gradients. We discovered that mortality is not consistently associated with drought. Mortality levels were higher for all forest types during an early cold and dry period, but not during a later warm and dry period. Although forest mortality was episodic for upper-elevation forests, lower-and mid-elevation forests exhibited more stable mortality levels. Over the majority of the time period, we found that mortality risk was greater on more north-facing slopes. The relationship between mortality and elevation differed among forest types. In lower-elevation forests and during the first dry period, lower elevations were associated with decreased mortality risk. For middle and upper-elevation forests, higher elevations were associated with greater mortality risk. Results highlight the need for further study into the relative role of species traits, stand characteristics, insect herbivory, and climate for influencing forest mortality at both stand and landscape scales. Variable response to both climate and stand density means that scientists should be cautious about generalizing these effects to all western forests. Thinning treatments should not be expected to improve forest health in all forest types. Consistent relationships of forest mortality with forest type and environmental setting can be used to guide landscape-level management of forest health.