The Effects of Aboveground and Belowground Competition on Understory Species Composition in a Pinus ponderosa Forest
The objective of this research was to test the hypothesis that water and nutrients, and not light, control understory plant species composition in a Pinus ponderosa forest in northeastern Oregon. The experiment was conducted as a split-plot experimental design with a 2 x 2 factorial analysis. To assess the effects of root competition of overstory trees on understory species composition, 20 plots (4 x 4 m) were trenched approximately 1.0 m in depth, and compared to 24 nontrenched plots. To increase light levels to understory vegetation, trees were thinned from 345 to 148 trees ha-1 in half of each block (2.5 ha) during the winter and early spring of 1986. Canonical discriminant analysis indicated that light accounted for the greatest environmental resource response among the treatments. The number of species (8) that increased in cover and density was 60% greater when tree root competition was reduced in the root-reduction treatment, versus 5 when tree canopy influences were reduced in the canopy-reduction treatment. Simple correlation showed that changes in species composition were significantly (P ≤ 0.05) related to both changes in aboveground attributes (light, midday air temperature, and soil temperature) and belowground attributes (soil water potential, pH, and nitrogen). Competition for limited resources, light, water, and nutrients, does affect understory species composition as evidenced by the response of individual species to increasing availability of these resources. For. Sci. 41(4):864-889.
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