Van Gunst, K.J. 2012. Forest Mortality in Lake Tahoe Basin from ...

5Across the western United States, increased stand density resulting from over acentury of fire exclusion is often considered a significant cause of heightened forestmortality. The strength of density-dependent mortality can be positive, where increases instand density are associated with increased probability of mortality or negative, wheredecreases in density area associated with increased probability of mortality. The linkbetween mortality and density is based on the Optimal Defense Theory in whichsustained inter-tree competition for shared critical resources limits the ability of trees togenerate the energy-intensive chemical and physical structures necessary to survive barkbeetle attacks (Raffa and Berryman 1983; Showalter and Filip 1993; Stamp 2003).However, a competing theory of plant defense, the Growth-Differentiation-Balancehypothesis, states that when growth is slowed by environmental factors, the resource poolavailable to allocate to secondary defense increases (Coley et al. 1985; Loomis 1932,1953; Herms and Mattson 1992; Stamp 2003). Although a comprehensive understandingabout the impacts of plant stress on mortality may be lacking (Stamp 2003), many studieshave shown that decreased tree vigor increases risk of successful bark beetle attack.Although some have shown that declining growth rate separates surviving trees fromdead trees (Bigler et al. 2007; Das et al. 2007; Linares et al. 2009, 2010; Pedersen 1998;Suarez et al. 2004), others have not found that distinction (Kane and Kolb 2010; Powerset al. 1999; Reid and Robb 1999). Whether vigorous or weakened trees are preferred notonly depends on the bark beetle species, with some such as MPB preferentially attackinghealthy trees, but also on mortality induced during non-epidemic or epidemic outbreakswhen mass attacks can overwhelm tree defenses of even healthy trees (Logan et al. 1998;Powers et al. 1999; Raffa and Berryman 1983; Wallner 1987). In addition, manufacture