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Growth patterns prior to mortality of mature subalpine fir in the Southern Interior
Antos, Joseph A.
Mortality of mature trees is a key process driving the dynamics of forest stands (Franklin et al. 1987, Pedersen and McCune 2002). The ascendancy of new trees into the canopy is typically contingent on the mortality of existing canopy trees. Thus mortality is critical to canopy turnover. Without an understanding of tree mortality, processes such as gap dynamics, succession, and stand development can not be adequately understood. In addition, dead trees are a critical habitat component for many animals and an important structural component of forests (Franklin et al. 1987). They are also important in nutrient cycling and other ecosystem processes. The amounts of snags and logs (coarse woody debris) in a forest and the dynamics of this material are critically linked to mortality processes. Thus managers need to consider mortality both from the perspective of removing growing stock or harvestable timber, and as the source of snags and logs. Predicting mortality and understanding the causes of mortality are essential to the effective management of forests both for timber production and ecosystem protection. Mortality can occur suddenly from agents such as fire or bark beetles, in which case the relationship to tree condition may be tenuous. In contrast, mortality can also be the end result of a gradual decline in tree vigour over a number of years, which may be attributed to multiple factors. For example, a stress such as drought may reduce growth of a tree and predispose it to a factor such as an insect attack. Increasing competition can also gradually reduce tree growth and eventually lead directly or indirectly to mortality. Although determining the progression of factors leading to mortality is important to predicting and understanding mortality, there is very little information on the growth patterns of trees prior to mortality. Tree-ring patterns give a good indication of tree health and have been used very successfully to reconstruct insect damage and responses to changing levels of competition. Thus dendroecology can be used to evaluate growth prior to mortality and provide indications of possible causal factors. For example, no change in growth prior to mortality would indicate a sudden development unrelated to tree condition, whereas a slow decline in growth down to a very low level would indicate gradually deteriorating conditions for a tree. A series of abrupt declines in growth would be indicative of multiple insults separated in time with inadequate recovery during intervening periods. The few studies that have examined ring-growth prior to mortality have found a variety of patterns. Declining growth prior to mortality appears common in many situations, including conifers in Europe (Bigler and Bugmann 2003, 2004, Bigler et al. 2004) and oaks in the Midwestern United States (Pedersen 1998, 1999). Low growth rates are often associated with mortality resulting from self-thinning or from some types of disturbance (e.g., Wyckoff and Clark 2000, 2002, Lussier et al. 2002). For example, mortality during drought is higher for slow-growing trees in such different systems as pinyon pine woodlands in western North America (Ogle et al. 2000) and Nothofagus forests in southern South America (Suarez et al. 2004). Patterns can be complex as one disturbance event can reduce growth predisposing a tree to mortality in a subsequent disturbance (e.g., Lussier et al. 2002). Both the overall rate of growth and the trend in growth appear to be important in predicting mortality (Bigler and Bugmann 2003, Bigler et al. 2004), although inadequate information exists for firm generalization. The pattern and magnitude of growth prior to mortality can in a sense be considered as a fingerprint that can help distinguish causal agents. For example, different pathogens can cause different patterns of growth prior to death of the host tree (Cherubini et al. 2002). Although the number of studies are rather limited, it is clear that ring patterns can pro
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Final Technical Report

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