Abstract: Nonthermal factors, such as terrain, may influence treeline position. Such a fact would explain the uncoupling relationship between recent treeline dynamics and climate warming. Here, we tested this idea by analyzing juniper and sprucefir alpine treelines across the Tibetan Plateau which were located on sunny and shady slopes, respectively. We tested if slope aspect modulated the relationships between treeline elevation and climate on the Tibetan Plateau. Herein, treeline variables (elevation, topography) were collected from published investigations performed on the Tibetan Plateau to assess the impacts of slope aspect on treeline elevation. Temperature conditions at treeline sites were estimated using global climate database. We evaluated the relative contribution of different variables on treeline position including thermal (growingseason air temperatures) and nonthermal (aspect, precipitation) factors. The minimum temperature in July was the most important predictor of Tibetan Plateau treeline elevation, explaining 40% of the variation of treeline elevation, whereas slope aspect accounted for 7% of the variation. We concluded that treeline position across the Tibetan Plateau was mainly driven by the minimum temperature in July, whilst slope aspect played a second role. Our results provide insight into the mechanistic explanations for alpine treeline formation.

Key words: competition, biodiversity conservation, subtro-pical forest, endangered plant, plant growth form