Surface soil phytolith assemblages in different vegetational zones of the Nilgiri Biogeographical Region, Western Ghats, India: implications for palaeoclimatic interpretation

Abstract

In the present study, we analyzed surface soil phytolith assemblages from 70 sites across seven vegetation types spanning an altitudinal range of 300–2400 m in the Nilgiri Biogeographic Region (NBR), Western Ghats, India. A total of 29 morphotypes were identified, dominated by BILOBATE, SADDLE, RONDEL, ELONGATE, BLOCKY, and POLYHEDRAL forms. Lowland seasonally-dry tropical forests (STDFs) were characterized by high proportions of C4 grass indicators, mid-elevation moist and wet forests exhibited mixed C3-C4 signatures, while upper montane forests and grasslands contained phytolith assemblages indicating relative grass dominance, although absolute grass cover was underestimated and woody signals overrepresented. Discriminant analysis achieved a moderate classification accuracy of ∼55 %, indicating a possible overlap between vegetation types and transitional communities. Principal Component Analysis indicated that elevation and mean annual precipitation as primary controls on phytolith composition. Morphometric analysis of BILOBATE phytoliths revealed decreasing dimensions (length and shank length) with elevation and strong positive correlations with mean annual temperature, highlighting temperature as a key driver of silica deposition. Overall, this first phytolith-based vegetation study in the NBR establishes a modern reference framework for interpreting the fossil record. Our findings demonstrate both the potential and limitations of phytolith assemblages and morphometrics for reconstructing past vegetation and climate in NBR and similar tropical montane ecosystems.