Detection of Alpine Tree Line Change with High Spatial Resolution Remotely Sensed Data

Forest distribution in mountainous areas has a bottom line and a top line, also known as the alpine tree line. A bottom line is defined as the boundary between forest and grassland or agricultural lands. Forest distribution and change at a bottom line is often influenced by both climate change and human activities, and is sensitive to external interruption. Conversely, forest distribution at the top line is, relative to the bottom line, less influenced by human activities, thus forest cover change at top lines can be largely attributed to climate change. Therefore, research on tree line change in mountainous regions has become an important research topic related to global climate change, biodiversity, and forest ecosystems.

Different methods such as field measurement, tree ring analysis, model simulation, remote sensing, and geostatistics have been explored to detect the tree line change due to its close relationship with climate change. Remote sensing techniques are used not only to distinguish tree line distribution, but also to identify vegetation types at the tree line, to select study areas at the project planning stage and to obtain the tree line features in a large area.

This study examined vegetation distribution and tree line change in the Sangong River watershed in the Tianshan Mountains, Xinjiang, China, using aerial photographs from 1962 and 1981 and a QuickBird image from 2006. An object-oriented classification method was used to map vegetation distribution, and the postclassification comparison approach was used to detect vegetation change.

The results were linked to digital elevation model data for exploring the relationships between vegetation distribution/expansion and topographic factors. It shows that Schrenkiana is mainly distributed in the region with elevations less than 2,600 m, but its tree line can reach 2,880 m. Sabina is mainly distributed in the region with elevations within 2,600 to 2,900 m, and its highest elevation can be 3,270 m. Approximately 73% of Schrenkiana areas are distributed in aspects of 0 to 45 deg and 270 to 360 deg and 52% of Sabina areas are in aspects of 45 to 90 deg and 225 to 270 deg. The Schrenkiana expansion is mainly distributed in slopes of 20 to 40 deg (accounting for 82%) and aspects of 0 to 90 deg and 270 to 315 deg (accounting for 77%). The elevation of Schrenkiana tree lines increased by 2 to 3 m from 1962 to 2006 and of Sabina increased by 7 to 12 m from 1981 to 2006. Both Schrenkiana and Sabina expansion is mainly located in vegetation frontiers. The result was published in Journal of Applied Remote Sensing on 19 August 2013.