Abstract
Abstract. Satellite images and digital elevation models provide an excellent database to analyze forest distribution patterns and forest limits in the mountain regions of semiarid central Asia on the regional scale. For the investigation area in the northern Tien Shan, a strong relationship between forest distribution and climate conditions could be found. Additionally areas of potential human impact on forested areas are identified at lower elevations near the edge of the mountains based on an analysis of the differences in climatic preconditions and the present occurrence of forest stands. The distribution of spruce (Picea schrenkiana) forests is hydrologically limited by a minimum annual precipitation of 250 mm and thermally by a minimum monthly mean temperature of 5 °C during the growing season. While the actual lower forest limit increases from 1600 m a.s.l. (above sea level) in the northwest to 2600 m a.s.l. in the southeast, the upper forest limit rises in the same direction from 1800 m a.s.l. to 2900 m a.s.l.. In accordance with the main wind directions, the steepest gradient of both forest lines and the greatest local vertical extent of the forest belt of 500 to 600 m to a maximum of 900 m occur at the northern and western mountain fronts. The forests in the investigation area are strongly restricted to north-facing slopes, which is a common feature in semiarid central Asia. Based on the presumption that variations in local climate conditions are a function of topography, the potential forest extent was analyzed with regard to the parameters slope, aspect, solar radiation input and elevation. All four parameters showed a strong relationship to forest distribution, yielding a total potential forest area that is 3.5 times larger than the present forest remains of 502 km2.
Highlights
The latitudinal and elevational variation in distinct plant associations and geomorphologic landscape units has been used for a long time to deduce regional environmental and climatical conditions in geosciences
The actual situation can strongly be influenced by human activities such as logging, fire clearing and animal grazing, which decreases the potential natural forest area (PFA)
This underlines the fact that the strong relation of forest distribution to slope aspect is caused by natural environmental conditions
Summary
The latitudinal and elevational variation in distinct plant associations and geomorphologic landscape units has been used for a long time to deduce regional environmental and climatical conditions in geosciences (e.g., von Humboldt, 1845–1862; Troll, 1973a, b; Hövermann, 1985). Satellite analysis based on automated image processing offers a quick and useful alternative to field mapping or manual digitalization from aerial images (Mayer and Bussemer, 2001) While satellite images such as Landsat data provide excellent information to delineate the spatial forest distribution (Hansen et al, 2013), SRTM (Shuttle Radar Topography Mission) data can be used to examine relief-dependent distribution patterns with a digital terrain model (DTM). The combination of these two data sets enables high-resolution mapping on a regional to local scale. The actual situation can strongly be influenced by human activities such as logging, fire clearing and animal grazing, which decreases the potential natural forest area (PFA). Human activity has reduced the forest area since prehistorical times so that the actual forest area (AFA) pattern mostly represents the minimum of the potential environmental dis-
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