Forest succession and inhibition on a former oak-pine-fir savanna: The influence of spatial heterogeneity on temporal change

Oak savanna, once abundant in Oregon’s Willamette Valley, is now an imperiled ecosystem. Agriculture, urbanization and fire suppression have contributed to its loss; less than 1% remains. We analyzed historical savanna structure and forest succession at Jim’s Creek, Willamette National Forest using tree distribution, tree-age and environmental data. Detailed data on current vegetation, soils and site physiography were collected in 60 30-m x 30-m plots along a series of 30-m wide belt transects. Data on potential presettlement trees, including live trees, snags and logs were collected along more than 3,000-m of the belt transects. Monte Carlo methods were used to simulate community-wide tree age-class and species distributions. Results indicate that the site was once savanna with 17 trees/ha composed of Oregon white oak, ponderosa pine and Douglas-fir. Beginning 125-150 years ago, dramatic changes took place and the area is now principally Douglas-fir forest with up to 566 trees/ha. Oak comprised 35% of former savanna trees, but only 2% of current trees. Edaphic features have strongly influenced the spatial distribution of succession. Current tree distribution across the site is associated with soil clay and silt content, slope, pH and soil and organic layer nitrogen. The persistence of remnant savanna appears to be edaphically controlled by shallow soil depth and high clay content. The understanding that edaphic features have led to differential rates of succession has important implications for savanna restoration and long-term site management. Oaks have been excluded from much of their historic habitat because of reductions in fire frequencies. They now persist in more extreme edaphic conditions. Consequently, restoration that focuses only on areas with extant oaks will not conserve their full range of historic habitat variability. Reintroduction of fire or other disturbance regimes in more mesic areas will be necessary to inhibit succession to Douglas-fir forest.