Flowering phenology of tree rhododendron along an elevation gradient in two sites in the Eastern Himalayas

Flowering phenology of tree rhododendron (Rhododendron arboreum Sm.) was monitored in situ along elevation gradients in two distinct ecological settings. Observations were carried out in Gaoligong Nature Reserve (GNR) in China and in the Kanchenjunga Conservation Area (KCA) in Nepal. Using the crown density method, flowering events of the selected species were recorded. Flowering duration and synchrony were determined within each site and along the elevation gradient in each study area. Our observations showed high synchrony throughout the elevation gradient, especially for peak flowering. Mean 15-day soil temperature, soil parameters (soil moisture, nitrogen, organic matter and pH), age of the observed trees, and site characteristics (litter cover, canopy cover, inclination) were related to mean initial and peak flowering dates using partial least squares regression (PLS). Results differed between the two sites, but winter temperature was the most important variable affecting the regression model for both initial flowering and peak flowering at both sites. After temperature, soil moisture was the most important variable for explaining initial flowering dates. The distribution of tree rhododendron indicates that it is able to grow in a wide range of habitats with different environmental conditions. The recent trend of rising winter-spring temperature and the detected bloom-advancing effect of high temperatures during this period suggest that tree rhododendron might expand its distributional range in response to global warming.