Wednesday, August 5, 2009 - 3:40 PM

COS 81-7: Stand structure of Quercus garryana-dominated woodlands in relation to environment and disturbance history in southwestern Oregon 

Laurie A. Gilligan and Patricia S. Muir. Oregon State University


We lack information about historical fire regimes in southwestern Oregon’s interior valleys, but it is generally assumed that fire suppression has resulted in fuel loads that are higher today than they were pre-European settlement. Many ecosystems in this area are being treated for fuels reduction to reduce potential fire severity and accomplish ecosystem restoration; however restoration goals are based on an untested assumption that wildfire historically burned frequently and with low severity across the range of non-coniferous ecosystems that occur in these valleys. While fuels treatments may diminish fire hazard in the short term, they also have potential to alter structure or composition of plant communities to states that lack historic precedents. Quercus garryana (Oregon white oak) dominated woodlands are a characteristic feature of these valleys and are sometimes targeted for fuels reduction treatments, yet their structure and relationships to environment and disturbance history are poorly understood. To advance understanding of these relationships, we inventoried tree characteristics in 40 untreated Quercus garryana (Oregon white oak) dominated stands to describe their current condition and assess whether age or structural attributes varied systematically with site conditions or inferred disturbance history.


Multiple-stemmed oaks, which indicate past disturbance such as fire, tended to be associated with relatively even-aged stands, and occurred more often in relatively high elevation and steeply-sloped sites. Multiple-stemmed oaks did, however, occur across a range of environmental conditions; analyses of their occurrence in relation to known or inferred disturbance history are on-going. Multi-aged stands were common and occurred across a range of tree densities, suggesting various disturbance histories. In general, tree size-structures (frequency distributions of diameters at breast height) formed reverse J-shaped curves, which often indicate continuous recruitment. However, 13% of stands did not support a sapling layer, indicating that successful regeneration was not occurring in these stands. Given that stand structures and inferred responses to disturbance varied widely across stands, site-by-site management prescriptions may be more effective than uniform-prescription treatments for accomplishing restoration goals.