Tropical montane forest in South Asia: Composition, structure, and dieback in relation to soils and topography.

We evaluated the composition, structure, and dieback of a montane forest in relation to soils and physiography of an important biogeographic region that has been sparsely studied. Our objectives were to (1) describe the forest composition and structure; (2) assess the current extent of dieback; and (3) relate tree composition, structure, and dieback proneness to edaphic and physiographic measures. We enumerated all live and dead standing plants ≥3 cm dbh, in thirty 20 × 15 m2 overstory plots. We measured all regeneration ≤1 m height in subplots, sampled soils and measures of physiography, and visually rated the proportion of crown dieback, and recorded standing dead trees. Woody plants belonged to 58 species and 25 families, of which 5% were dead. The plant families Lauraceae, Myrtaceae, and Symplocaceae represented over 50% of the basal area; Rubiaceae alone represented 25% of the stems. Multiple regressions revealed measures of edaphic and physiographic factors are associated with tree species richness and diversity, structure (e.g., basal area and stem density), and tree dieback and standing dead trees. Canopy openness and measures of soil toxicity from metals (Al and Cu) on clay‐based soils showed a predictably negative relationship with tree species richness and diversity. Basal area and stem density are positively associated with aspect and slope, and predictably negatively associated with canopy openness. Lastly, dieback and standing dead trees of the dominant canopy tree species are both positively associated with south and southwest aspects. The forest is now in a period of regeneration recovery with high numbers of regenerating secondary successional species. We propose the primary underlying factors that caused the dieback of this forest was a drought during 1970–1980 compounded by accumulations of salts and metals in the soil conveyed by winds from the sea (sodium) and from an ancient period of metallurgy downwind of the forest. However, our results are only suggestive of these causative factors and further study is necessary to confirm or refute these speculative causes for dieback. [ABSTRACT FROM AUTHOR]