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The effects of area, environmental heterogeneity, and site favorability on plant species diversity and slope of the species—area curves were evaluated at thirty study sites in shrub—dominated communities within the pinyon—juniper zones of central Utah and northern New Mexico. Species diversity, as measured by total number of species per site, varies widely (24—87 species per hectare). Plant species diversity is highly correlated with increasing size of the area at each site (average r2 = .92). Environmental heterogeneity is also strongly positively heterogeneity account for > 98% of the variation in species number. Overall site heterogeneity and favorability combine to account for 74% of the observed variation in species number at Utah sites and 84% among New Mexico sites. Species diversity within these communities is largely controlled by two abiotic variables: (1) the amount of available moisture and (2) the number of different soil types within the site. Slope of the species—area curve (z—value) varies widely among the several study sites (.09—0.28). The observed values overlap expected z—values for continental communities. Environmental heterogeneity is shown to be positively associated with z—value. At Utah sites, heterogeneity of site solar irradiation, potential soil moisture, and soil depth explain 66% of the variation in z—values. At New Mexico sites, heterogeneity of solar irradiation, soil depth, and surface rock cover account for 90% of the variation in observed z—values. |
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