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Lateral root distribution of trees in an old-growth Douglas-fir forest inferred from uptake of tracer 15N.
McKane, Robert*,1, Rygiewicz, Paul1, Beedlow, Peter1, Andersen, Christian1, Brooks, Renee1, Hogsett, William1, Hynes, Meagan2, Laurence, John3, 1 U.S. Environmental Protection Agency, Corvallis, OR, USA2 Dept. of Land, Air and Water Resources, Davis, CA, USA3 U.S. Forest Service, Corvallis, OR, USA
ABSTRACT- Belowground competition for nutrients and water is considered a key factor affecting spatial organization and productivity of individual stems within forest stands, yet there are few data describing the lateral extent and overlap of competing root systems. We quantified the functional root distribution of different tree size classes in an old-growth forest in the Oregon Cascades by measuring aboveground uptake of 15N (99 atom % as NH4Cl) injected at different treatment radii around target Douglas-fir trees. Tree size classes included sapling, intermediate and dominant trees. Each treatment included 50 evenly-spaced soil injections of 15N at 0.5, 1 or 1.5 times the mean crown radius of each tree-size class. Preliminary analyses of the 15N content of aboveground tissues 4 months after injection show a monotonic decrease in root function with distance, with uptake beyond 1 crown radius accounting for over 1/4 of total uptake. These results suggest significant extension of root systems beyond the dripline of trees and direct belowground competition among neighboring stems. We illustrate applying the tracer data in combination with spatial data on stem location and size to calculate the functional belowground overlap among neighbors. We also examine how the intensity of belowground overlap is distributed throughout this stand and how it is correlated with growth rates of individual stems. Our approach establishes a means for improving the representation of belowground competition in stand models of forest growth, a feature that existing models either lack or treat theoretically.
Key words: forests, 15N, roots, competition