Abstract: Aims Our objectives are to explore the relationship of leaf area and stand density and determine a convenient way to measure stand leaf area.
Methods Using direct and indirect methods during the growing season (from May to October) of 2004, we measured seasonal variation of the leaf area of tree and shrub species: Robinia pseudoacacia stands with four densities (3 333,1 666, 1 111 and 833 plants per hm²), Platycladus orientalis stands with three densities (3 333, 1 666 and 1 111 plants per hm²), Caragana korshinskii, Hippophae rhamnoidesand Amorpha fruticosa . We developed formulas for leaf area by correlating leaf fresh weight, diameter of base branch and leaf area.
Important findings Maximum leaf area and leaf area index (LAI) occurred in September for trees and August for shrubs. There is a significant power relation between leaf area and leaf fresh weight for Robinia pseudoacacia, significant linear relation between leaf area and leaf fresh weight for P. orientalis, C. korshinskii, H. rhamnoides and A. fruticosa, significant power relation between leaf area and diameter of base branch for C. korshinskii, and significant linear relation between leaf area and diameter of base branch for H. rhamnoides and A. fruticosa. After the planted vegetation in Loess hilly-gully region enters into a rapid growth stage, the LAI of R. pseudoacacia stands with different densities converge and LAI is not affected by initial or current density. This was also observed for P. orientalis stands. However, the leaf area of individual trees is negatively linearly related with stand density. Robinia pseudoacacia stands and P. orientalis stands have reached their maximum bearing capacities in the Loess hilly-gully region as a result of limited soil water. Therefore, to improve the quality of individual trees, we recommend stand densities not exceed 833 and 1 111 plants per hm² for R. pseudoacaciaand P. orientalis, respectively. To improve the quality of entire stands, we suggest reducing the density a little.