Why do forests occur mainly on the northern slopes of the Tien Shan?

Forests in the Tien Shan occur primarily on the northern slopes because these areas receive significantly more moisture than other parts of the range. The region's climate is heavily influenced by moisture-bearing westerly winds from the Atlantic Ocean, which deposit the majority of their precipitation on the windward western and northwestern slopes.1

This increased humidity on the northern slopes supports the growth of forests between elevations of approximately 5,000 and 10,000 feet (1,500 and 3,000 metres). In contrast, the southern slopes are typically drier, often resulting in the replacement of forest-supporting conditions with mountain steppes or other vegetation types.2