Abstract: This paper introduces the concept and count method of Kira's warmth index (WI) and coldncss index (CI). Based on WI, humidity index (HI=P/WI) is proposed by the author, where p is the mean annual precipitation (mm). Correlations between the distribution of 43 tree species and thermal status in northeastern China are studied. The WI of distribution limit of trees in mountainous regions of northeastern China is 15℃. month. The border line between forest and grassland has the HI value of 7.5 mm/℃. month, and that between forest grassland and steppe, 5.2mm/℃. month.

Key words: Elevated CO₂ concentration, Physiological and biochemical response, Pine broadleaf forest, Changbai Mountain