Elevational decline of vascular plant richness and distinct species turnover driven by soil carbon and precipitation in the Eastern Himalayan hotspot

Abstract

The scarcity of comprehensive floristic data limits understanding of how species richness (SR) changes along altitudinal gradients. This study provides the first systematic floristic inventory and ecological assessment of altitudinal SR in Phek District, Nagaland, an under-documented region within a global biodiversity hotspot. Four Community Reserved Forests (CRFs), representing temperate, montane, sub-tropical, and tropical forest types, were surveyed with a total of 237 vascular plant species recorded, including 102 trees, 53 shrubs, and 82 herbs, accounting for approximately 9.71% of Nagaland’s known flora. Rather than a consistent monotonic decline, SR followed a unimodal (hump-shaped) pattern along the altitudinal gradient, with the highest richness documented in the temperate zone (1800–2200 m). SR showed a significant positive relationship with soil organic carbon and increased markedly when mean annual precipitation exceeded 122.9 mm p < 0.001). Vegetation distribution exhibited strong elevational structuring, with mean elevations of tree, shrub, and herb assemblages declining from montane (2301–2379 m) through temperate (2035–2063 m) and sub-tropical forests (1632–1669 m) to tropical forests (1033–1087 m). Dominant species varied distinctly along the gradient, with Saurauia montana and Euonymus attenuatus characterizing montane forests, Quercus griffithii and Q. lamellosa temperate forests, Croton persimilis and Adenanthera pavonina sub-tropical forests, and Dysoxylum cauliflorum and Micromelum integerrimum tropical forests. β-diversity partitioning indicated that species turnover overwhelmingly governed community dissimilarity (≈ 73–75%), with minimal contribution from nestedness, highlighting species replacement rather than progressive species loss along elevation. Overall, while species composition shifted linearly via high turnover, total richness reached its peak at mid-elevations before declining toward the high montane summits, highlighting the temperate zone as a critical diversity “sweet spot.” These findings establish critical ecological baselines and provide data-driven insights essential for targeted conservation and sustainable management of community-protected forests facing escalating anthropogenic pressures.