I have only found the following few old references that indicate an increase in biodiversity when the soil pH decreases with little numerical data.
HUSTON Michael, 1980, "Soil nutrients and tree species richness in Costa Rican forests", J. Biogeogr., 7: 147-157
PROCTOR J., ANDERSON J.M., CHAI P. & H.W. VALLACK, 1983, "Ecological studies in four contrasting rain forests in Gunung Mulu National Park, Sarawak. I. Forest environment: structure and floristics", J. Ecol.,71: 237-260
FABER-LANGENDOEN Don & GENTRY Alwyn H., 1991, "Structure and diversity of rain forests at Bajo Calima, Choco region, western Colombia", Biotropica, 23: 2-11
I cant offer up more papers, but would suggest that differences in soil acidity are not an important driver/filter of the biodiversity in tropical forests referred to in the papers. Soils in tropical environments (unless they are young and are developed on calcareous materials) are generally naturally highly leached, acidic with low base saturation. Because this is the norm (pH 4.5 really not that unusual) - it would be reasonable to expect a large number of species to be well adapted to such conditions in this environment. Changes is hydrology / nutrient cycling differences associated with different soils types (that may in some cases also result in a a different pH) are likely more important. I would imagine that diversity at the extremes of pH in the same environment (such as peat soils or soils in limestone) would actually have lower diversity (ie increasing constraint results in decreasing diversity) - so strict linear relationship of increasing acidity resulting in increasing biodiversity would not hold.
Thank you Anoop but I need data relating soil pH and Aluminium saturation to plant diversity
Andrew, tropical soils can be very hetereogenous, depending on topographic position, parent material, vegetation history, drainage, and so on. Beteween pH 5.5 and 4.5 exchangeable aluminium raise from 0 to near 100% of the ECEC!
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