Plant Species Composition Patterns with Differences in Tree Dominance on a southwestern Utah Pinon-Juniper Site

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USDA Forest Service, General Technical Report

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.-Fourteen plots covering the range from scattered small trees to full tree dominance were sampled from a single site in southwestern Utah. The rate of reduction in understory cover with increasing tree cover was less than the rate of reduction in total understory leaf biomass with increasing tree leaf biomass. Differences were primarily due to changes that occurred in the crowns of the shrubs. Shrub leaf biomass was rapidly reduced as tree leaf biomass increased, but this initially coincided with minimal loss in shrub crown area or height. Forb leaf biomass initially declined but then increased with increasing tree leaf biomass. A tradeoff occurred between shrubs and forbs at later stages of succession on this site. The grasses were not so affected. Rank by natural logarithm of abundance curves for the plots had several straight lines, reflecting strong dominance by the growing trees. Species interactions within the understory can affect the patterns of understory composition as tree dominance increases.

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