Graduate Thesis Or Dissertation
 

Spatial and temporal patterns of "super-old" Douglas-fir trees of the central western Cascades, Oregon

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  • Old-growth in the Pacific Northwest is generally defined as trees that are more than 200 years old. A great deal of analysis and discussion about old-growth forests in western Oregon, however, has focused on the relatively widespread 400 to 500 year age class of primarily Douglas-fir (Pseudotsuga menziesii) trees. The existence of older trees, with establishment dates prior to 1450, though rare, is documented in treering based fire history studies conducted in the central western Cascades of Oregon. These trees, referred to in this study as "super-old growth," embody significant information about forest and climate history extending nearly a millennium into the past. The primary objectives of this study were to assay where and why super-old trees persists on the landscape, and to develop a predictive model for their occurrence. I synthesized spatial and temporal data related to super-old trees from previous treering based fire history studies conducted in the central western Cascades of Oregon. I then created a composite, geographically referenced database, which was used to analyze the synthesized data. Super-old Douglas-fir trees were found at 12% of 874 sample sites, mostly on north-facing aspects and on gentle slope gradients. Even-aged stands of super-old trees were more likely to exist in areas that are prone to long fire intervals and highseverity fires. Single, remnant super-old trees were more likely to exist in areas that are prone to more frequent, lower-severity fires. This study identified landscape positions that provide a refuge where Douglasfir trees can attain maximum longevity. This information may be useful in determining optimal locations for late-successional reserves, as directed by the Northwest Forest Plan. Over 350 hectares of unsampled terrain within the study area were identified where more super-old trees could likely be found. Efforts to locate more super-old trees using these data could facilitate future paleoclimate research and other tree-ring based studies.
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