CORVALLIS, Ore.—The magnificent white oak trees in the Willamette Valley that stand alone in farmers' fields may provide critical resources for birds living in and around agricultural fields.
Craig DeMars, an Oregon State University graduate student in fisheries and wildlife, discovered in his research this spring that isolated, "legacy" white oak trees, hundreds of years old, have the potential to contribute to conservation of a wide range of bird species in the valley.
DeMars compared bird use of the long-lived white oaks in crop and pasture lands to their use in reserve areas, and discovered usage to be similar. He found that 47 species of birds use the isolated white oaks to perch, feed, sing and nest, both on and off reserves.
"The most important factors for predicting avian use are tree size and forest density in the surrounding landscape," DeMars said. In DeMars' study, bird use of these individual trees increased with increasing tree size and decreasing tree density in the surrounding landscape.
"A large, isolated oak tree acts as a 'habitat magnet,' concentrating tree-dependent bird species around this focal habitat structure on the landscape," DeMars said.
"In otherwise treeless landscapes, the gigantic white oaks may act as keystone structures," DeMars said. "Their influence on wildlife may be disproportionately large, relative to their actual physical footprint on the landscape."
Oak savanna areas are defined by trees spaced at least 100 feet apart, with grasses growing below them, and less than a 30 percent canopy cover, DeMars said. His research focuses on oak savannas because they are "one of the most imperiled systems in the state," he said. Parts of the Willamette Valley oak savanna have been preserved at the Finley National Wildlife Preserve and the Mount Pisgah Arboretum.
The Willamette Valley's white oak savanna habitat is only about one percent of what it was 200 years ago, said DeMars, who bases the number on old photos and journals. "For birds associated with oak savanna habitats, a single isolated tree in an agricultural field may be a critical resource for nesting, safe refuge, and foraging as well as providing a high perch for singing," he said.
"Native Americans who lived in the valley before settlers arrived maintained oak savannas by seasonal burning of the understory grasses, which prevented encroachment of conifer trees into savanna habitats," DeMars said. That allowed the large oaks, which cannot tolerate shade, to survive. Today, farmers contribute to the trees' longevity by not plowing and planting underneath them.
DeMars plans to turn his research into educational materials that will help farmers conserve and replace the oaks. He expects a positive response. As he drove back roads to find legacy white oaks to study, DeMars was worried about how farmers would react to a biologist asking to come onto their land. He found 20, however, who agreed and were "quite interested," he said. Only one said no.
White oaks are slow growing, can't be crowded or shaded and need fencing as saplings. The trees are treasured for their beauty and longevity and have become part of life on many farms. "I grew up with that tree," one farmer remarked. DeMars encourages thinking about replacing the old oaks as "planting a tree for your grandchildren to enjoy."
Further research continues in wildlife use of the Willamette Valley's white oak habitats in a collaborative research program initiated by DeMars' major adviser Dan Rosenberg, an associate professor of wildlife biology.