Graduate Thesis Or Dissertation
 

Tree Vigor Response and Competitive Zone Density in Mature Ponderosa Pine

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mc87ps84d

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  • This study explored the relationship of mature ponderosa pine (Pinus ponderosa Dougl. ex Laws) tree vigor and competitive zone density following thinning and fuel reduction treatments. Competitive zone density (CZD) was defined as the total tree basal area (m²/ha) surrounding a given ponderosa pine tree within a 10m fixed radius plot. The vigor of 60 trees were observed at two separate mature ponderosa pine stands in south-central Oregon, Lookout Mountain (LM), and Sycan Marsh (SY). Tree vigor response was measured using three physiological metrics: xylem sap flow (kg/day), bole water potential (MPa), and resin exudation (ml/day). Dominant and co-dominant pines were chosen across the available range of CZD at each site. Data were collected over the course of one growing season in 2015, from late June to early October. Sap flow was recorded at four 6-day periods over the season to incorporate the influence of seasonal moisture decline on tree physiological response. Water potential was observed twice, at mid- and late-season. Resin exudation was observed in August to correspond generally with flight periods of various bark beetle species. To account for environmental influences on tree physiology, soil volumetric water content (SWC, m3/m3) was measured approximately every 3-4 weeks and daily potential evapotranspiration (PET) data were obtained from the Western Regional Climate Center’s Remote Access Weather Stations (WRCC RAWS) archives. Data were analyzed using a linear mixed model including both environmental and competitive variables. A quadratic relationship between post-thinning CZD and sap flow was evident, indicating a positive but highly variable association up to a threshold of roughly 40 m²/ha. Above this density threshold sap flow declined markedly, suggesting that tree competition becomes an important factor in constraining water resource access in these sites at higher densities. Bole water potential in mature ponderosa pine was also related to CZD at the LM site. Resin exudation showed disparate relationships to CZD by site and may be more directly influenced by factors other than CZD. Overall tree vigor was higher at LM despite comparatively lower SWC, potentially due to the greater size or age of the study trees. The positive correlation of vigor and CZD up to the middle density range implies that there may be facilitative effects of adjacent trees to be considered for mature ponderosa pine, especially when reducing drought vulnerability is a management goal.
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