Elevation-specific responses of phenology in evergreen oaks from their low-dry to their extreme high-cold range limits in the SE Himalaya

While the high elevation limit of trees is commonly related to low temperature, the rear edge of their distribution is often associated with drought. Here we explore phenology traits that contribute to a mechanistic explanation of both these edges of the fundamental niche in the broad leaved evergreen Quercus pannosa s.l. Populations of this species reach a drought limit (DL) at 2510 m in the semi-arid upper Yangtze valley, and a cold limit (CL) at 4270 m, very close to the conifer treeline, within a short geographical distance. Trees reach a height of only 4–7 m at both climatic limits, and > 30 m height at optimum site (OS) at 3440 m. At OS, flushing starts in mid-May and at the summer solstice at CL (after late frosts end), suggesting a photoperiod control. At DL, oak phenology tracks the (irregular) arrival of the monsoon. Shoots and leaves grew fastest and for the shortest period at DL, and slowest at CL, in both cases forming 4–7 cm long new shoots per year, contrasted by 12–13 cm a⁻¹ at OS. Maturation of leaves (length and specific leaf area, SLA) was again fastest at DL, followed by CL and slowest at OS, with a much longer shoot growth duration per year and bigger leaves. We conclude that the period favorable for growth and maturation was more than halved at both range limits (by frost or drought) compared to the optimum site, pointing at a common range restriction by the duration of the growing season.