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°C from 2007–2009. The mean air temperature for the warmest month (July) was 7.9 ± 0.5°C, while mean air temperatures during the growing season were 6.8 ± 0.3°C (Index 1) and 6.2 ± 0.2°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°C and soil temperature at 10 cm depth >3.2°C. However, the mean soil temperature at a depth of 10 cm during the growing season (8.0 ± 0.2°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.
The upper altitudinal limit of forest on high mountains, commonly referred to as treeline or timberline (see the schematic representation in figure 1 in Körner and Paulsen 2004), is likely to be one of the first forested ecosystems to register climate impacts in terms of changes in structure and position (Grace et al 2002; Holtmeier 2003; Körner 2003; Broll and Keplin 2005; Wieser and Tausz 2007; Harsch et al 2009; Smith et al 2009). One of the world's highest natural treelines is found on the southeastern Tibetan Plateau (Miehe et al 2007). There is a considerable need in research focusing on treelines or timberlines, such as the recent dendroecological studies in the Sygera Mountains on the southeastern Tibetan (Liang et al 2009, 2010, 2011), for better understanding of the microclimate of local treelines. By comparison with the European Alps, Subarctic North America, and northern Europe (Holtmeier 2003; Körner 2003), little is known about micrometeorological conditions at the treelines or timberlines on the Tibetan Plateau (Li BS 1993; Li WH 1993; Cui et al 2005; Schickhoff 2005; Li et al 2008; He et al 2009; Liu and Luo 2011).
As for the treeline/timberline environmental studies, Holtmeier (2003) emphasized that one of the problems is the use of climate records from valley floors or lowlands to predict climatic variables such as precipitation at higher elevations. In particular, difficulties increase when applying local results to the alpine treelines of distant mountain ranges on the Tibetan Plateau (Miehe and Miehe 2000). Unfortunately, few uninterrupted in situ micrometeorological field measurements are available for the high-elevation treelines, primarily due to poor access and bad weather conditions throughout the year on the southeastern Tibetan Plateau, where almost all the meteorological stations are located at lower altitudes on the floor of the river valley. Limited knowledge of the treeline microclimate on the southeastern Tibetan Plateau to date has been based on short-term observations of soil temperature and moisture (Shi et al 2008; He et al 2009; Liu and Luo 2011).
To better understand the microclimatic conditions of the treeline on the southeastern Tibetan Plateau, we set up an automatic weather station (AWS) on topography exposed to the sun on the eastern side of the Sygera (Sergyemla) Mountains. The objective of this study was to characterize micrometeorological conditions of Blackseed juniper (Juniperus saltuaria [Rehd & Wils], syn: Sabina saltuaria) treeline based on AWS measurements from 1 December 2006 to 28 February 2010.
Material and methods
The study site is located in Nyingchi area on the eastern side of the Sygera Mountains (29°10′–30°15′N, 93°12′–95°35′E), a transition zone between semihumid and humid climates in southern Tibet, China (Figure 1A). The South Asian monsoon reaches the Sygera Mountains along the Yarlung Zangbo River, resulting in abundant summer monsoon rainfall. Funded by several agencies in China, a Tibetan Observation and Research Platform (TORP) is now focusing on land-surface processes and environment over the Tibetan Plateau (Ma et al 2008). The Southeast Tibet Station for Alpine Environmental Observation and Research of the Institute of Tibetan Plateau Research of the Chinese Academy of Sciences, established in August 2005, is one of the stations focusing research on the alpine environment on the southeastern Tibetan Plateau.