Geographical Differences in Alpine Timberline and Its Climatic Interpretation in China

The variations of alpine timberline altitude with latitude and longitude in China were investigated and their relationships with climatic indices were analyzed in this study. The results are summarized as follows: (1) The altitude of alpine timberline in China changes significantly along latitudinal and longitudinal gradients. North of 30oN, the timberline altitude decreases with the increase of latitude at the rate of 112 m per latitude degree. South of 30oN, the latitudinal change shows a different pattern: the altitude does not vary significantly with latitude in the eastern part and increases with latitude in the western part. The timberline altitude drops with the increasing of longitude at similar latitudes, resulting from increased temperature conditions from east to west due to large-scale topographic and geographic factors. The highest timberline (4600 m) was observed in southeast Tibet (29o-32oN, 94o-96oN), the highest timberline in the world. (2) The timberline is limited by growing season temperature, i.e. Annual Biotemperature (ABT) of 3.5oC, Warmth Index (WI) of 14.2 oC·month, and Mean Temperature for Growing Season (MTGS) of 8.2 oC. The corresponding altitudes of these temperature thresholds change with geographic factors, and this change has led to the changes in the latitudinal and longitudinal timberline patterns and timberline variations from the oceanic to continental climate. (3) Precipitation affects obviously timberline altitude at the middle-high latitudes in China; timberline is higher in arid regions than in humid areas at the similar latitudes, caused by increase of temperature at the low humid conditions.