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FUNCTIONAL OVERLAP OF ROOT SYSTEMS IN AN OLD-GROWTH FOREST INFERRED FROM TRACER 15N UPTAKE
Citation:
McKane, R B., P T. Rygiewicz, P A. Beedlow, C P. Andersen, J R. Brooks, W E. Hogsett, M. Hynes, J. A. Laurence, T G. Pfleeger, AND R. S. Waschmann. FUNCTIONAL OVERLAP OF ROOT SYSTEMS IN AN OLD-GROWTH FOREST INFERRED FROM TRACER 15N UPTAKE. Presented at 4th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, Wellington, New Zealand, April 19-23, 2004.
Description:
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 Cascade Range by measuring aboveground uptake of 15N (99 atom % as NH4Cl) injected at different radii around target Douglas-fir trees. Tree size classes included sapling, intermediate and dominant trees. Each target tree received 50 evenly-spaced soil injections of 15N at 0.5, 1 or 1.5 times the mean crown radius of each tree-size class. Analysis of the 15N content of aboveground tissues collected 4 months after injection show a monotonic decrease in root function with distance; however, uptake beyond 1 crown radius accounts for over 1/4 of total uptake. These results indicate significant extension of root systems beyond the "dripline" of trees and direct belowground competition among neighboring stems.
We applied the tracer data in combination with spatial data on stem location and size to calculate the functional belowground overlap among neighbors within our 3-ha study plot. Results show that the intensity of belowground overlap (competition) is unevenly distributed within this stand. Growth rates of individual stems are negatively correlated with belowground competition, but only for dominant (emergent) trees (P<0.03). Subordinate trees show no correlation between growth and belowground competition, suggesting that these trees are limited primarily by light. 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.