Tillage agriculture and afforestation threaten tropical savanna plant communities across a broad rainfall gradient in India
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AbstractThe consequences of landuse change for savanna biodiversity remain undocumented in most regions of tropical Asia. One such region is western Maharashtra, India, where oldgrowth savannas occupy a broad rainfall gradient and are increasingly rare due to agricultural conversion and afforestation.To understand the consequences of landuse change, we sampled herbaceous plant communities of oldgrowth savannas and three alternative landuse types: tree plantations, tillage agriculture and agricultural fallows (n=15 sites per type). Study sites spanned 457 to 1954mm of mean annual precipitationcorresponding to the typical rainfall range of mesic savannas globally.Across the rainfall gradient, we found consistent declines in oldgrowth savanna plant communities due to landuse change. Localscale native species richness dropped from a mean of 12 species/m2 in oldgrowth savannas to 8, 6 and 3 species/m2 in tree plantations, fallows and tillage agriculture, respectively. Cover of native plants declined from a mean of 49% in oldgrowth savannas to 27% in both tree plantations and fallows, and 4% in tillage agriculture. Reduced native cover coincided with increased cover of invasive species in tree plantations (18%), fallows (18%) and tillage agriculture (3%).In analyses of community composition, tillage agriculture was most dissimilar to oldgrowth savannas, while tree plantations and fallows showed intermediate dissimilarity. These compositional changes were driven partly by the loss of characteristic savanna species: 65 species recorded in oldgrowth savannas were absent in other land uses. Indicator analysis revealed 21 oldgrowth species, comprised mostly of native savanna specialists. Indicators of tree plantations (nine species) and fallows (13 species) were both invasive and native species, while the two indicators of tillage agriculture were invasive. As reflective of declines in savanna communities, mean native perennial graminoid cover of 27% in oldgrowth savannas dropped to 9%, 7%, and 0.1% in tree plantations, fallows and tillage agriculture, respectively.Synthesis. Agricultural conversion and afforestation of oldgrowth savannas in India destroys and degrades herbaceous plant communities that do not spontaneously recover on fallowed land. Efforts to conserve India's native biodiversity should encompass the country's widespread savanna biome and seek to limit conversion of irreplaceable oldgrowth savannas.
