Density management strategy for overstory and understory of urban woodland based on ecological size-density allometry

Urban woodland is a dominant landscape element in supporting sustainability of urban systems. The aesthetic, ecological and psychological benefits provided by urban woodland are closely related to characteristics of the overstory and understory. The distribution and dynamics of tree canopy cover has been well elucidated, however, few studies focused on functioning and structure of urban understory and the interaction between different vegetation layers. In this study, ecological size-density allometry obtained through linear, quadratic and piecewise linear models was employed to quantify the resource competition of understory vegetation and effects of overstory canopy cover on understory in Shanghai metropolitan area of China. We found that canopy cover shapes the size-density pattern rather than affecting biomass and density directly. Size-density allometric relationships from piecewise linear models provided the optimal planting density under different canopy cover levels. Considering the linkage between vegetation layers and resource competition, we proposed that middle canopy cover of overstory (60%–80%) and relative low density of understory herb (175 m−2) is an optimal management strategy for maintaining high ecosystem services and functioning of urban woodlands.