Historical structure and composition of dry forests in south central Oregon

Background/Question/Methods

Remnants of dry forest systems that existed in the past operate now in a novel context of confounding and compounding stressors including increases in: human-generated fragmentation; loss of redundancy; pollution; connectivity for competitors and pathogens; and rapid, unpredictable climate change. Adding to the complexity of the current management and policy context is explicit recognition of a broad social range of variability ‒ “the ecological condition acceptable to a society at a given time”. In this context, reference conditions reflecting processes that shaped forests for millennia are consistent with management objectives to reduce risk of loss given current and projected stressors. Spatially explicit, landscape-level timber inventories conducted from 1914-22 provide detailed records of the coniferous forests of the former Klamath Reservation, now the Fremont-Winema National Forest.  Using this record of conifers at least 15 cm in diameter at breast height (dbh), we describe variation across > 50,000 hectares in ponderosa pine and dry and moist mixed-conifer forests. Cruisers oriented transects to surveyed positions of Bureau of Land Management (BLM) Public Land Survey System (PLSS) land divisions and inventoried trees in a strip two chains (40 m) wide twice across a quarter-quarter or quarter section. Cruisers tallied trees by species and diameter class. 

Results/Conclusions

Tree densities were low on all three plant association groups (PAGs): ponderosa pine and dry and moist mixed conifer; mean values and ranges were greater on mixed conifer than on ponderosa pine sites. Basal areas were overwhelmingly dominated by large diameter trees (>53.3 cm dbh) and by ponderosa pine on all PAGs. Ponderosa pine dominated species composition in the ponderosa pine PAGs but shared dominance on mixed-conifer PAGs. In this landscape the structure of ponderosa pine and dry and moist mixed-conifer forests was essentially the same. We infer from the consistently low tree densities and dominance by large diameter trees across the moisture gradient represented by three different plant association groups (ponderosa pine and dry and moist mixed-conifer) that the disturbance regime (frequent wildfire) trumped productivity in structuring the historic forests in at least these areas of the Klamath Reservation.