Annual ring widths are good predictors of changes in net primary productivity of alpine Rhododendron shrubs in the Sergyemla Mountains, southeast Tibet

摘要

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 decreased and δ13C increased, resulting in decreases of observed RGR and simulated NPP. Current-year and recent 50-year-averaged ARWs were well correlated with observed RGR and P max and simulated NPP. June mean temperature explained 62 % of the altitudinal variations in observed RGR and ARW as well as simulated NPP. At moist high altitudes, ARWs can be used as predictors of changes in NPP of alpine shrubs because the low temperature in the early growing season is the primary factor limiting both ARW and NPP. This study suggests a methodology detecting the sensitivity of alpine woody species to varying climatic conditions.