Age structure of Picea schrenkiana forest along an altitudinal gradient in the central Tianshan Mountains, northwestern China

Size distribution and age structure of Schrenk spruce (Picea schrenkiana Fisch. et Mey.) forest along the altitudinal gradient on the central Tianshan Mountains was studied in this paper. The forest was divided into three transects, i.e., lower limit (1500–1700m), mid-altitude (1800–2400m) and upper limit (2500–2700m). Age of each individual was estimated by the age–height relationships of seedlings or saplings and the age–DBH (diameter at breast height) relationships of big trees. A reverse-J shape age structure was found in the mid-altitude transect; the age structure of the low altitude transect was characterised by the dominance of young trees, whereas the transect of high altitude lack of young seedlings and saplings. It is implied that different limiting factors play important roles in shaping the age structure and regeneration of forest at different altitudes. In the mid-altitudinal transect, density dependent competition between trees is probably the most important influencing factor. We propose that limiting environmental factors, e.g. temperature and precipitation, play important roles in determining the age structure of P. schrenkiana population in the high or low limit transacts.