Aim This study explores the relationship between orchid species richness along elevational gradients in the central and eastern Himalaya. Specifically, we aimed to test predictions of maximum richness at different elevational levels for orchid species geographically restricted to central and eastern Himalaya (endemics), and total orchid richness, including different life forms. We then compared and assessed climatic factors together with area as causal explanations for the observed patterns.
Location Nepal (100–5200 m a.s.l.), Bhutan (100–5200 m a.s.l.), and adjacent regions of India, i.e. Sikkim and Darjeeling.
Methods We used published data on the distribution of orchids to interpolate presence between maximum and minimum recorded elevations in 100-m elevational zones. These patterns were compared with previously published patterns for other plant groups, which were derived in the same way. Descriptive regressions with metres above sea level (m a.s.l.) as the independent variable were performed using nonparametric generalized additive models (GAMs), whereas tests of the diversity models were carried out using parametric generalized linear models (GLMs).
Results A hump-shaped relationship between orchid species richness and elevation was observed in Nepal and Bhutan, with maximum richness at 1600 m a.s.l. corresponding to c. 16 °C mean annual temperature. The richness of orchid species that are endemic to central and eastern Himalaya showed a bimodal pattern. Richness of endemic epiphytes peaked at elevation zones similar to those for total richness, but the peak in numbers of endemic terrestrial species occurred at a higher elevation.
Main conclusions This study shows that climatic variables (temperature and precipitation) provide a good potential explanation for orchid species richness along the Himalayan elevational gradient, even when the effect of area is considered. The principal elevational peak in richness of central and eastern Himalayan endemics was consistent with the peak in total orchid richness, but maximum terrestrial endemic richness occurred at higher elevations.