Response of white birch (Betula platyphylla ...

tThe mountain forest steppe and taiga in northern Mongolia have experienced a forest decline in area andquality since the end of the last century. Changes in land use, climate, fire frequency and pest occurrenceare considered to be the main drivers of this vegetation shift and desertification. Because this region isthe source for major rivers, is home to a unique flora and fauna and represents an important source oftimber for Mongolia, the ability of different tree species to respond to these changes and regenerate is ofincreasing interest. Our contribution focuses on the climate-growth relationship of old and young birchtrees from two valleys in the Mongolian province of Selenge Aimag.The research site Bugant, located in the Western Khentey Mountains, was the most important loggingcentre in Mongolia during socialist times. Today, the vegetation is dominated by succession forests oflight taiga. The research site Altansumber, on the border of the Sant and Khushat soum, is dominatedby light taiga and mountain forest steppe. Traditional nomads who depend on these forests for differentreasons inhabit this area.Wood cores were sampled and chronologies of young and old birch trees at Bugant and Altansumberwere created. Climate data were obtained from the Eroo station, which is known in the region for itslong and reliable climate record. We analysed the climate-growth relationships of the chronologies from1962 to 2009. At both sites and in both age classes, correlations with temperature were predominantlynegative, particularly in April (Bugant, south- and east-facing slopes) and May (Altansumber, north-facingslopes). Precipitation of the late summer of the previous year (August/September) positively correlatedwith the growth of birch at Altansumber. We assume that the significant negative correlation betweenwinter precipitation (December/January) and the growth of old birches at both sites is due to positiveeffects of snow cover on the survival rate of herbivorous insect populations. Our results indicate thatduring the early vegetation period, younger birch trees are more dependent on water availability thanolder ones. Negative pointer years were characterized by below-average precipitation during the currentsummer period and above-average spring temperatures. For the old trees, positive pointer years werecharacterized by above-average summer precipitation. We conclude that water availability is the mostcrucial factor for the growth of white birch in northern Mongolia.