FACTORS INFLUENCING SPECIES COMPOSITION IN TROPICAL LOWLAND RAIN-FOREST - DOES SOIL MATTER

The soil properties most likely to influence species composition in lo wland order of importance: P availability, Al toxicity, drainage, wate r-holding capacity, and availability of K, Ca, and Mg. A total of 18 s tudies were located in which species occurrence was studied in relatio n to such soil properties. Several of these report clear trends with s oil physical properties, mainly drainage as affected by topographic po sition. Only three offer evidence for correlations with soil chemical properties. In all three, the study area spanned soils of very widely differing age and thus soil fertility. Failure to find correlations wi th chemical properties may be due to: lack of range in soil fertility across the sites studied. failure of soil test methods to measure avai lability of nutrients to plants, or temporal and spatial variability i n soil properties.Existing soil classification systems do not provide enough information on any particular soil to help establish relations between its chemical properties and plant distribution. Nonetheless, s oils at ecological study sites must be classified if their nature and properties are to be made clear to others working worldwide. Tradition al, largely subjective, soil-mapping ;; techniques may reduce sampling needs by allowing stratification of soil and plant sampling by broad soil types. The mapping must be done, however, at a scale similar to t he patchiness of the plant community (usually finer than 1:5000), some thing done in only 3 of the 18 studies located. Correlative studies; a re ?only, the ;first step in understanding causal relations between so il properties and plant species distribution. Next, nutritional, drain age, and water requirements must be established for individual species . Then field experiments must be set up to establish cause and effect.