The Distribution of Evergreen and Deciduous Trees Relative to Soil Type: An Example from the Sierra Madre, Mexico, and a General Model

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Abstract
An example of the association of evergreen plants with nutrient—poor soils is described from the Sierra Occidental of northwestern Mexico. At elevations from 600 to 1100 m in this mountain range, distinct patches of evergreen—oak woodland are surrounded by subtropical deciduous forest in a mosaic pattern. To quantify the vegetation pattern and examine its environmental basis, vegetation and soil samples were taken throughout the elevational range of the mosaic. Analysis of the samples indicated that the patches of evergreen vegetation occur on extremely acid, infertile soils (derived from hydrothermally altered rock), while deciduous vegetation dominates on less acid, more fertile soil (derived from unaltered volcanic rock). The two community types have very little floristic overlap, and the deciduous forest has much higher density of woody plants and greater species richness. The actual location of boundaries between patches of the two community types can be more abrupt than can be accounted for by soil change. At higher elevations (above 1100 m) evergreen woodland is not restricted to acid soils and occurs on all soil types. These data are discussed in terms of a general model to account for the association of evergreen plants with nutrient—poor soils in the Sierra Madre as well as in other areas with similar growth form soil fertility patterns. I propose that evergreen plants would grow and survive at least as well on fertile soils as on infertile soils in isolation, but are unable to establish in dense, productive vegetation because of low growth rates associated with high carbon costs and low photosynthetic rates of evergreen leaves.