How can we understand the limitations to plant growth at high altitudes? Our aim was to test the hypotheses that for alpine grasslands along a large altitudinal gradient in semi‐arid regions, plant growth is mainly limited by drought at low altitudes but by low temperature at high altitudes, resulting in a unimodal pattern of biomass and productivity associated with an optimal combination of temperature and precipitation. Such knowledge is important to understanding the response of alpine ecosystems to climate change.

It is unclear whether annual ring widths (ARW) are good predictors of changes in net primary productivity (NPP) of trees or shrubs in cold environments. We test if the simulated NPP with inputs of observed leaf nitrogen concentration (N mass) and carbon isotope ratio (δ13C) explains altitudinal variations of ARW, relative growth rate (RGR), and maximum photosynthetic rate (P max) within a widespread woody species at moist timberline ecotones. We measured plant-level ARW and RGR, and related leaf traits (P max, N mass, δ13C etc.) for an alpine Rhododendron shrub (R. aganniphum var. schizopeplum) across ten altitudes (4,190–4,500 m) in the Sergyemla Mountains, southeast Tibet. Based on climate data available from Nyingchi station at 3,000 m, non-age-related ARW chronologies (1960–2008) for each of ten altitudes were positively correlated with June mean temperature, but related little with precipitation and other monthly mean temperatures. With increasing altitude, N mass and P max de