Stand Reconstruction and 200 Years of Forest Development on Selected Sites in the Upper South Umpqua Watershed
Dubrasich, Mike. 2010. Stand Reconstruction and 200 Years of Forest Development on Selected Sites in the Upper South Umpqua Watershed. W.I.S.E. White Paper 2010-5. Western Institute for Study of the Environment.
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Abstract
Stand development of ten structurally complex forest stands in the Upper South Umpqua Watershed was studied by backdating (reconstructing) stand conditions circa 1825. Mixed conifer and conifer/hardwood stands across a selected range of “Areas of Special Interest” sites were sampled for tree ages, tree characteristics, and fire history. Logistic regression analysis was used to create age/diameter models and the stands were backdated using increment core data and tree positions to create stand statistics for 185 years prior to measurement. The changes in number of trees and basal area over the last 185 years were calculated by species for each stand. Density of trees greater than 8 inches DBH increased from an average of 20 trees per acre to 90 (from 10 to 35 trees per acre to 60 to 115 trees per acre). Basal area increased 5-fold on average, from 65 square feet per acre to 225 (from 25 to 150 sqft/ac to 150 to 300 sqft/ac). In 1825 the ten stands were open and park-like with widely spaced trees. By 2010 the ten stands had accumulated from 10 to 20 times the tree biomass they had held 185 years earlier. In 1825 pines and oaks were dominant in stands below 3,800 feet in elevation. Today those same stands are dominated by Douglas-fir, grand fir, and incense-cedar, especially in younger age classes. In higher elevation stands the most abundant species has changed from Shasta red fir to Pacific silver fir.
Several lines of evidence suggest that the prairies, savannas, and open forests have been persistent vegetation types in the Upper South Umpqua Watershed for the last few thousand years, at least. Precontact forest development pathways were mediated by frequent, purposeful, anthropogenic fires deliberately set by skilled practitioners, informed by long cultural experience and traditional ecological knowledge in order to achieve specific land management objectives. At a landscape scale the result was maintenance of an (ancient) anthropogenic mosaic of agro-ecological patches. In the absence, over the last 150 years, of purposeful anthropogenic fires, the anthropogenic mosaic has been invaded and obscured by (principally) Douglas-fir. As a result, the Upper South Umpqua Watershed is now at risk from a-historical, catastrophic stand-replacing fires. …
Conclusions
1. In 1825 vegetation types in the Upper South Umpqua watershed consisted of prairie, oak savanna, sugar and ponderosa pine open woodlands, and high elevation shrublands.
2. Since 1825 the changes in stand structures have been dramatic. Density of trees greater than 8 inches DBH increased an average of 450 percent and basal area increased 5-fold. By 2010 the ten stands had accumulated from 10 to 20 times the tree biomass they had held 185 years earlier. In most of the stands the species relative proportions also changed significantly. In 1825 pines and oaks were dominant in stands below 3,800 feet in elevation. Today those same stands are dominated by Douglas-fir, grand fir, and incense-cedar, especially in younger age classes.
3. By implication the forest development pathways have changed since 1825. Tree recruitment and biomass accumulation rates have increased, and tree species relative proportions have changed (from dominance by pine and oak to dominance by Douglas-fir).
4. Human-set fire has played an important role in the development of these stands. Frequent anthropogenic fires maintained uneven-aged, sparsely stocked, open and park-like stands for thousands of years. The elimination of anthropogenic fire over the last 150 years is the key factor that has altered development pathways and forest structure and composition.
5. The anthropogenic fire regime was typified by frequent, low-severity fires of limited individual extent, which cumulatively burned over the entire watershed every 1 to 10 years. At a landscape scale the result was maintenance of an (ancient) anthropogenic mosaic of agro-ecological patches. In the absence of the purposeful fires set by skilled practitioners, the anthropogenic mosaic has been invaded and obscured by (principally) Douglas-fir. Infrequent, a-historical, catastrophic stand-replacing wildfires have replaced low severity fires due to the massive build-up of biomass (fuels).
These findings should be useful in:
* Advancing understanding of forest dynamics, historical human influences, and historical landscape geography,
* Informing the maintenance and preservation of historic cultural landscape features — the anthropogenic landscape patterns are cultural legacies by themselves (Lake 2005),
* Evaluating and mitigating catastrophic fire hazards and risks, and
* Informing restoration efforts, where restoration means active management to recover historical cultural landscapes, historical forest development pathways, and traditional ecological stewardship to achieve resiliency to fire and insects, provide sustainable resource products and services, and to preclude and prevent a-historical catastrophic fires that degrade and destroy myriad resource values (Charnley et al. 2008, Dubrasich 2010b). …
