Prairie and savanna dominated by Oregon White Oak (Quercus garryana) were widespread in western Oregon and Washington prior to Euro-American settlement but currently <2% remains.  Although a principal mechanism of loss of former savanna and prairie has been succession to forest due to fire suppression, there is limited knowledge about factors that have controlled successional dynamics, including why certain sites retain savanna and prairie structure despite lack of fire or recent grazing.  Thus, our objective was to determine the factors that control successional pathways in former oak savanna and prairie in Oregon and growth rates of the dominant tree species.

We established 320 plots in seven sites in the southern Willamette Valley, Oregon that encompass a wide variability in the environmental conditions and plant community composition that typify historic savanna and prairie.  In each plot, we measured the species and diameter at breast height (DBH) of each tree, percent overhead canopy, and a variety of soil and topographic variables.  We seasonally measured moisture through the soil profile in a subset of plots.  We also took increment cores for a subset of trees to determine age/DBH relationships to reconstruct successional histories and to determine growth rates.

Results/Conclusions

Soil and topographic factors are related to current community composition, but the specific of these relationships differ strongly among sites.  Although we attempted to choose sites that had not been grazed for several decades, we have limited knowledge of site disturbance history, and past grazing may have obscured environmental controls over succession.  In general, shallow soil depth appears to have slowed invasion of trees into prairie and savanna.  Soil moisture through the rooting profile was seasonally lower in more open sites, but it too was strongly dependent on soil depth.  Preliminary analyses indicate that heat load, percent clay, and soil pH are the most important controls on tree radial growth rates. 

After Euro-American settlement (c. 1850), new cohorts of all major tree species established rapidly, reflecting lower fire frequencies, leaving remnant prairie and savanna in only the harshest areas as determined by soil depth, heat load, and soil moisture availability.  There is intense interest in restoring prairie and savanna in the Pacific Northwest, but the degree of management necessary to maintain an open condition will strongly depend on soil and topographic conditions.