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ABOVE- AND BELOWGROUND CONTROLS ON FOREST TREE GROWTH, MORTALITY AND SPATIAL PATTERN
Citation:
McKane, R B., P T. Rygiewicz, D. MacFarlane, P A. Beedlow, C P. Andersen, J R. Brooks, W E. Hogsett, AND M. Hynes. ABOVE- AND BELOWGROUND CONTROLS ON FOREST TREE GROWTH, MORTALITY AND SPATIAL PATTERN. Presented at Ecological Society of America meeting, Portland, OR, August 1-6, 2004.
Description:
We investigated the relative importance of above- and belowground competition in controlling growth, mortality and spatial patterns of trees in a nitrogen-limited, old-growth forest in western Oregon. To assess the effects of competition for light, we applied a spatially-explicit, light capture model (tRAYci) to crown data (height, depth, width) for each tree >10 cm in diameter within the fully-mapped 3-hectare study plot. The model calculated the percentage of above canopy light reaching the surface of individual crowns. To assess effects of competition for soil N, we quantified the lateral root distribution of different tree sizes and species (Douglas-fir, western hemlock and Pacific silver fir) by measuring aboveground uptake of 15N-labeled ammonium chloride injected at different distances from target trees. The 15N data were combined with spatial data for individual stems to calculate functional belowground overlap (competition) among neighbors. Preliminary results suggest that the effects of above- and belowground competition on growth and mortality of individual stems depend on crown position within the canopy. Growth rates of dominant (emergent) trees were negatively correlated with belowground competition, with a weaker positive correlation with light availability. In contrast, growth and mortality of subordinate trees were controlled primarily by light availability. Comparing these observations to "null" stands in which stem locations were randomized suggests that actual stem locations are "optimized" to minimize above- and belowground competition. Our approach establishes a means for improving the representation of belowground competition in forest stand models, a feature that existing models either lack or treat theoretically.