No slope exposure effect on alpine treeline position in the Three Parallel Rivers Region, SW China

Abstract

The mean temperature of the growing season has been found to correlate with treeline position across the globe, but regional variation still needs to be explained. In the present study, we explored how the elevation of the alpine treeline varies with slope exposure and the distinct influence of the summer monsoon in the Three Parallel Rivers region of SW China (Yunnan). We hypothesized that the treeline elevation will not vary distinctly with slope exposure, because tree canopies are coupled to atmospheric conditions rather than ground temperature. We extracted treeline positions from Landsat TM images, DEM (digital elevation model) data and climatic data from Worldclim to analyze the relationship between treeline position and climate. The results revealed no significant exposure effect. Treeline elevations were almost the same at different slope exposure in each of these north–south stretching mountain ranges. Counter to expectation, south-facing slopes showed a slightly lower position than the others, which may be caused either by greater monsoon exposure (cloud cover) or human disturbance. The mean growing season temperature at treeline showed the same pattern as was obtained globally in earlier assessments (6.7 °C). However, the treeline elevation (as well as the isotherm position) increased from west (wet) to east (drier) along the summer monsoon route. Our results confirmed the theory that the treeline is strongly associated with atmospheric circulation and does not benefit from greater radiative warming on south-facing slopes during the growing season. The common treeline isotherm hypothesis matches with treeline position in this monsoon-driven climate regime.