Microclimatic Conditions for Juniperus saltuaria Treeline in the Sygera Mountains, Southeastern Tibetan Plateau

Although the southeastern Tibetan Plateau has one of the world's highest natural treelines, little is known about its microclimatic conditions. In order to characterize microclimatic conditions for natural Blackseed juniper (Juniperus saltuaria [Rehd & Wils], syn: Sabina saltuaria) treeline (4390 masl) in the Sygera (Sergyemla) Mountains, southeastern Tibetan Plateau, an in situ field measurement based on an automatic weather station (AWS) has been running since November 2006. The annual mean air temperature ranged from 0 to 0.8 degrees C from 2007-2009. The mean air temperature for the warmest month (July) was 7.9 +/- 0.5 degrees C, while mean air temperatures during the growing season were 6.8 +/- 0.3 degrees C (Index 1) and 6.2 +/- 0.2 degrees C (Index 2, corresponding to the global scale), based on a definition of the growing season according to a daily mean air temperature of >5 degrees C and soil temperature at 10 cm depth >3.2 degrees C. However, the mean soil temperature at a depth of 10 cm during the growing season (8.0 +/- 0.2 degrees C) was higher than that measured for the global treelines. The juniper treeline is characterized by a humid microclimate, as shown by the mean daily relative humidity of 76.4%, annual total precipitation of 871.3 mm, and mean soil volumetric moisture content of 35.5% during periods when the soil is not frozen. The annual mean wind speed was 0.9 +/- 0.1 m/s. Uninterrupted in situ micrometeorological field measurements for alpine treelines should be the next step to achieve a better understanding of treeline ecological conditions and treeline formation on the Tibetan Plateau.