Many land managers are interested in maintaining or restoring plant communities that contain Oregon white oak (OWO, Quercus garryana), yet there is relatively little information available about the species' growth rates and survival to guide management decisions. We used two studies to characterize growth (over multi-year periods and within individual years) and to evaluate the main factors that affect growth and survival. The objective of the first study was to revise the OWO components of the Forest Vegetation Simulator (FVS), a widely-used growth model. We first compiled a large database on growth and survival to develop equations to revise FVS. Diameter growth and survival over multi-year periods were strongly affected by stand density, the competitive position of the tree, tree size, and site productivity. The height growth potential of OWO was predicted from site productivity, stand density and tree size. In the second study, intra-annual patterns of OWO growth were evaluated by precisely measuring stem diameters with band dendrometers. OWO experienced two periods of stem expansion, with the first period likely representing growth (the production of new wood and bark) and the second representing stem rehydration in the fall and winter. As in the first study, growth was strongly affected by the level of competition around each tree. Our results show the sensitivity of Oregon white oak to competition and highlight the need to restore low stand densities in many cases to improve growth and the likelihood of survival.
Oregon white oak (OWO; Quercus garryana Dougl. ex Hook, also known as Garry oak) is valued in the Pacific Northwest for its contribution to biodiversity and for its cultural significance (Chappell and Crawford 1997, Wilson and Carey 2001). OWO and its associated plant communities were historically maintained in stands with relatively low tree densities (savannas and woodlands) through frequent burning by Native Americans (Thilenius 1968). Many areas where OWO historically occurred have been impacted by urban and agricultural development. Much of what remains is shifting in the absence of fire from savannas or open woodlands to closed conifer forests dominated by Douglas-fir (Pseudotsuga menziesii [Mirb] Franco) (Crawford and Hall 1997, Thysell and Carey 2001, Foster and Shaff 2003). Managers have recognized that OWO communities in many places are threatened and management intervention is necessary to maintain or restore them.
The success of treatments to promote OWO communities can be evaluated in many ways, but tree growth and survival are often of primary interest. Tree growth is a good indicator of a tree's health, and in some respects, its capacity to contribute to the ecosystem. Growth and survival are both related to a tree's ability to capture limited site resources that are needed for photosynthesis and other physiological processes. Slow diameter growth is indicative of long-term stresses, such as those caused by excessive competition, that can make trees vulnerable to other agents that ultimately cause tree death (Pedersen 1998). Some minimum level of diameter growth is needed to maintain enough functional xylem to transport water to the foliage and other tissues. This may be a particular problem for oaks which have ring-porous wood structure as water transport only occurs in the outer one or two rings of the sapwood (Huber and Schmidt 1937, as cited in Rogers and Hinckley 1979). Diameter growth beyond the minimum necessary for survival is considered to be a low priority in the allocation of photosynthate (Oliver and Larson 1996). A high level of growth suggests that the tree has adequate access to site resources to meet its higher priority needs and still allocate photosynthate to diameter growth. Diameter growth has been associated with acorn production in other oak species (Goodrum et al. 1971) and stand conditions that promote high diameter growth (i.e., low tree densities) also have been found to promote acorn production in OWO (Peter and Harrington 2002).
Many forest growth models predict tree growth and survival over time scales of decades to centuries. These models are useful tools for testing management alternatives before they are implemented. The Forest Vegetation Simulator (FVS) is a collection of forest growth models that is widely used by forest managers (Crookston and Dixon 2005). Different variants of FVS have been developed for specific regions in North America. All variants are individual-tree models that predict height and diameter growth and the probability of survival of each tree in a stand, typically using a 10-yr time interval. OWO is not a major species in the variants of FVS developed for the Pacific Northwest (i.e., the west Cascade [WC] and Pacific Northwest [PN] variants). Large datasets were used to develop equations for major species, but only a small number of observations (12 trees) had been used to develop equations to predict the growth and survival of OWO (Donnelly and Johnson 1997). Prior to the revisions described in this paper, FVS could not be expected to accurately project the development of stands that contain OWO (Gould et al. 2008). FVS is an important tool that forest managers use to evaluate the effects of proposed treatments (or the decision not to do any treatments). With better algorithms to predict its growth and survival, FVS can be useful for evaluating treatments to promote OWO.
The purpose of this paper is to evaluate factors affecting the growth and survival of OWO. We use results from two studies. The aim of the first study was to revise the PN and WC variants of FVS to improve their usefulness for evaluating treatment alternatives and for projecting the development of stands that contain OWO. We assembled a large database of remeasured inventory and research plots for the model revision. Analysis of these data allowed us to draw some general conclusions about the factors affecting OWO growth and survival over multi-year periods. In the second study, we examined intra-annual patterns of OWO diameter growth to evaluate the timing of growth and how growth is affected by competition. Examining seasonal patterns of diameter growth can provide insight into how competition and seasonal changes in the availability of resources affect tree growth (Hinckley et al. 1976).
Oregon White Oak in FVS
The Forest Vegetation Simulator (FVS) predicts growth and survival of individual trees for a 10-yr period based on the condition of the tree and that of the surrounding stand at the beginning of the growth period. A modeling database was assembled to relate tree and stand characteristics to growth and survival. The database was assembled from measurements on inventory and research plots in Washington, Oregon, and California (Figure 1). The sources of plot data were: 1) the USDA Forest Service Forest Inventory and Analysis plot network (on private lands in Washington, Oregon, and California), 2) USDA Forest Service Current Vegetation Survey plots (on National Forest lands in Washington and Oregon), 3) Oregon State University McDonald-Dunn Forest Inventory plots (near Corvallis, Oregon) and 4) research plots established by the USDA Forest Service, Pacific Northwest Research Station (various ownerships near Olympia, WA). All trees had been remeasured, with the exception of 40 open-grown trees where diameter growth for the most recent 10 years was estimated from increment cores.
Tree and plot data from the first measurements were summarized to produce a set of variables to predict growth and survival (Table 1). The predictor variables measure tree attribute