Abstract: Unlike the well understood cold limit of trees, the causes of the dry trailing edge of trees await explanations. Here we aimed at explaining the drought limit of an evergreen oak species (Quercus pannosa s.l.) in a typical dry valley of the upper Yangtze region, SE Himalaya, where rains (c. 250 mm a^-1) are largely confined to the typical monsoon season (July-August) with drought during the remaining 9-10 month. We capitalized on an unintentional year-round irrigation treatment with trees growing along the overflow of a water reservoir serving as moist controls. We measured shoot water potential (Ψ), leaf conductance (g), flushing phenology, leaf mass per area (LMA), foliar and stemδ¹³C, leaf nutrients, non-structural carbohydrates (NSC) across the transition from non-monsoon to monsoon season from April to August 2018. At the dry site, Ψ and g were high during the monsoon but declined to < -3 MPa as drought proceeded in the non-monsoon season. Irrigated oaks retained high values year-round. Oaks experiencing the natural drought, flushed at the full strength onset of the monsoon only, that is, 80 days later than irrigated oaks. The annual shoot increment in oaks under natural drought was c. 10 % of that in irrigated oaks. However, mature foliage showed no difference in LMA and δ¹³C between dry and moist site. We conclude that these oaks drastically reduce their activity in response to drought, with growth strictly confined to the monsoon season, the minimum duration of which, presumably is setting the range limit.

Key words: Quercus, Monsoon climate, Adaptation, Water stress, Himalaya, Species distribution