Woody plant diversity in relation to environmental factors in a seasonally dry tropical forest landscape

Questions: Water availability is known to be a first-order driver of plant diversity; yet water also affects fire regimes and soil fertility, which, in turn, affect plant diversity. We examined how precipitation, fire and soil properties jointly determine woody plant diversity. Specifically, we asked how woody plant diversity varies along a sharp precipitation gradient (about 600–1,800 mm mean annual precipitation [MAP]within a ∼45-km distance) exhibiting considerable variation in long-term fire burn frequency and soil fertility, in a southern Indian seasonally dry tropical forest (SDTF) landscape. Location: Mudumalai, Western Ghats, India. Methods: Woody plants ≥1-cm DBH were enumerated in 19 1-ha permanent plots spanning a range of tropical vegetation types from dry thorn forest, through dry and moist deciduous forest to semi-evergreen forest. Burn frequencies were derived from annual fire maps. Six measures of surface soil properties – total exchangeable bases (Ca + Mg + K), organic carbon (OC), total N, pH, plant available P and micronutrients (Fe + Cu + Zn + Mn) were used in the analyses. Five measures of diversity – species richness, Shannon diversity, the rarefied/extrapolated versions of these two measures, and Fisher's α – were modelled as functions of MAP, annual fire burn frequency and the principal components of soil properties. Results: Most soil nutrients and OC increased with MAP, except in the wettest sites. Woody productivity increased with MAP, while fire frequency was highest at intermediate values of MAP. Woody plant diversity increased with MAP but decreased with increasing fire frequency, resulting in two local diversity maxima along the MAP gradient – in the semi-evergreen and dry thorn forest – separated by a low-diversity central region in dry deciduous forest where fire frequency was highest. Soil variables were, on the whole, less strongly correlated with diversity than MAP. Conclusions: Although woody plant diversity in this landscape, representative of regional SDTFs, is primarily limited by water availability, our study emphasizes the role of fire as a potentially important second-order driver that acts to reduce diversity in this landscape.