Alpine vegetation and species-richness patterns along two altitudinal gradients in the Gyama Valley, south-central Tibet, China

Background: The study of the variation in species composition and richness along gradients has a long history in ecology. Despite this, knowledge about how composition and richness varies at high altitudes has been relatively little studied at fine spatial scales. Aims: We investigated variation in species composition and altitude along two altitudinal transects in the Gyama Valley of south-central Tibet, China: one north-facing transect on Mount Lamu (4154-4504 m), and one west-facing transect on Mount Kaya (4225-4800 m). Methods: For each band of 25 m of altitude we recorded all vascular plants and estimated the total vegetation cover in six 1 m x 1 m plots. Results: Species composition in the plots varied more with altitude and vegetation cover than between transects and hence aspects. Two-way indicator species analysis delimited eight major vegetation types in the total dataset. For species richness in the plots we found a monotonic increasing pattern of richness with altitude on Mount Lamu and a slightly inverted U-shaped pattern on Mount Kaya. For total species richness in each altitudinal band (combining the six plots in each band), a monotonic decreasing species richness with altitude on Mount Lamu and a hump-shaped richness pattern on Mount Kaya were found. Conclusions: Species-richness patterns differed between transects and with the grain size used. At coarse scale, the richness pattern is comparable to what has been found in other studies, whereas the unexpected richness patterns at plot scale may be due to the negative influence of high cover of shrubs on species richness.