Microtopographical Characteristics of Forest Dieback in a Semi-Arid Region Retrieved from Ground and Satellite Data.

Various factors have been cited for forest dieback including global warming and dryness, overgrazing damage by wild animals, forest fires, and damage by insects. Hustai National Park (HNP) of Mongolia, which is 506 square kilometers (km ² ) in size, encompasses high mountains, hills and valleys. The land cover of HNP, includes stepe land, mountain steppe and forest steppe. In recent years, HNP forests started to dry out significantly and the entire landscape has changed from forest steppe to mountain steppe. Currently, in HNP forests cover only 13.1 km ² of the park. The dominate tree species are Betula platyphylla Sukaczev (Siberian silver birch) and Populus tremula (Eurasian aspen). Siberian silver birch is drought sensitive, frost resistant and able to grow well under different environmental conditions. Siberian silver birch is distributed only on the north-facing slopes of the mountains because evapotranspiration is much higher than precipitation in Mongolia. In this study, we conducted field surveys at three sites in the park over a five-year period and sampled 1130 points. We apply a tree measurement (TM) survey to examine living and dead (dieback) trees, as well as obtain a Diameter at Breast Height (DBH) measurement. Using the DBH and height of the trees we clarify the factors that influence white birch forest. Extraction of fallen and dieback tree pixels using time series Landsat satellite data, including an analysis of microgeomorphology characteristics (slope and solar radiation) of dieback trees using 30m SRTM DEM data were employed. Results show the DBH of the birch tree with dieback was around 10–15 cm and the tree height was 2–3 m. It was found that many of the dead trees have a negative correlation with the curvature of microtopography and a positive correlation with the amount of solar radiation.