Spatial patterns of tree species in chronosequences of Douglas-fir forest on Vancouver Island.
Getzin, Stephan*,1, He, Fangliang2, Wiegand, Kerstin3, Dean, Charmaine4, 1 University of Jena, Jena, Germany2 University of Alberta, Edmonton, Alberta, Canada3 University of Jena, Jena, Germany4 Simon Fraser University, Burnaby/Vancouver, British Columbia, Canada
ABSTRACT- Douglas-fir forest is a major forest in the Pacific Northwest region. While the stand structure and dynamics of the forest are well studied, the fine-scale spatial characteristics are still poorly understood. Here we investigated spatial distributions of dominant tree species in the forest. Six plots along two Douglas-fir chronosequences were mapped on Vancouver Island between 1997 and 2000. Each chronosequence comprised of three plots from immature (35 and 42 years old, respectively), mature (94 and 102 years old) to old-growth (247 and 318 years old). The plot sizes vary from 0.5 ha to 1.2 ha. All trees were stem-mapped and each individual was identified to species. There are three dominant species in the plots, including Douglas-fir, western hemlock and western red cedar. We used Ripley's L-function and the O-ring statistic to quantify the spatial distributions of the species and compared the changes in the spatial pattern along the sequence. We also measured intra- and interspecific competition from correlations between nearest neighbor distances and tree size (DBH). We found an increase in mean DBH with stand age, from 5.1 cm for the immature stand to 19.5 cm for old-growth, and a corresponding increase in the mean nearest neighbor distance from 0.63 to 0.98 m. In immature plots, tree survival was highly dependent on DBH, having least survival chances in the smallest DBH class below 5 cm. However, tree death was a random process in older plots. Competition decreased strongly from immature to mature stands. Interspecific competition was higher than intraspecific competition, reflecting the competitive replacement of Douglas-fir by western hemlock and western redcedar in the late successional stage of the forest. Regularity in the distribution pattern of Douglas-fir doubled in scale from the immature stand to the old-growth northern stand due to intraspecific competition. In contrast, western hemlock and western red cedar were at all ages significantly more clustered than Douglas-fir. Our results emphasize the importance of spatial structures for the coexistence of species in Douglas-fir forests.
Key words: Douglas-fir, chronosequence, spatial patterns, L- and g-function
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