Phenological variation in height growth and needle unfolding of Smith fir along an altitudinal gradient on the southeastern Tibetan Plateau

Little is known about variations in tree phenology and their driving forces on the Tibetan Plateau. Herein, we monitored shoot growth and needle unfolding of Smith fir (Abies georgei var. smithii) between 3,800 and 4,360 m a.s.l. in the Sygera Mountains, southeastern Tibetan Plateau. The trees were 0.45–1.48 m high and 12–39 years old. Their phenology was observed every week between May 5 and August 26, 2011. With increasing elevation, shoot growth and needle unfolding started increasingly later, thus indicating a thermal driving force. Although the weekly shoot increment was decreasing with increasing elevation, height growth at various elevations ended in the same week, implying other factors than temperature being responsible for the end of height growth. The accumulated heat sum for the onset of shoot growth appeared to be lower between 4,200 and 4,360 m than between 3,800 and 4,000 m. The anticipated spring warming will likely induce an earlier onset of shoot growth, whereas shoot growth will apparently not benefit from autumn warming. However, the lack of long-term data records precluded a robust statistical test of the underlying cause-and-effect relationships involved in the phenological variations of height growth and needle unfolding.