Forest restoration treatments are prescribed across the West to increase resiliency of dry forests. Forest composition and structure vary dramatically with biophysical setting and yet often this information is lacking, particularly from studies of historical reference forest conditions. This information is needed to help guide project planning and stand level prescriptions that account for variability in forest conditions and fire effects in space and time.
Our research investigates the species composition and tree size distribution of presettlement Mediterranean mixed conifer hardwood forests of southwestern Oregon and how these tree communities varied across the landscape. All living and dead trees were identified on 0.1 ha plots and tree status and size was reconstructed to the date of last wildfire (1914). Fifty plots were placed across a 22,000 ac study area centered on the 14,000 acre Ashland watershed stratified among 10 biophysical settings that incorporated Pseudotsuga menziesii and Abies concolor plant series, topographic position, and solar insolation. Three forest density plots were also placed in biophysical settings characterizing the warm insolation, ridgeline settings in the Pseudotsuga plant series at three sites in the neighboring Applegate and Illinois watersheds for a total of 74 forest density plots. This sampling was designed to allow analysis of fine-scale topographic variability sampled in the Ashland watershed to be scaled up to similar biophysical settings at a regional scale.
Results/Conclusions
Species conversion was apparent in 2011 with a greater proportion of the forest trees of shade tolerant species. On warmer insolation sites Pinus ponderosa and scattered large Pseudotsuga menziesii, Quercus kelloggii, and Arbutus menziesii were historically dominant while in 2011 we found hyper-abundant, small Pseudotsuga menziesii and Arbutus menziesii. On cooler insolation sites stands dominated by Pinus ponderosa and Pseudotsuga menziesii were invaded by Abies concolor. In general, we observed a dramatic shift from fewer, larger trees to many more, smaller trees per hectare. This increase in tree number, combined with mortality of larger trees, resulted in a dramatic decrease in forest quadratic mean diameter. Interestingly, the quadratic mean diameter of Pinus ponderosa did not change, likely due to the absence of regeneration for this shade intolerant species. Promoting resilience of these forests into the future will likely entail promoting regeneration of the more shade intolerant species adapted to specific sites, and the development of large, fire resistant trees on a proportion of the landscape.