Impacts of stand density on tree crown structure and biomass: A global meta-analysis

Stand density is a critical factor in the variation of crown structure and biomass, and can significantly change the growing space and resource acquisition of trees. However, an understanding of the interactive mechanisms of this effect with stand characteristics and environmental conditions is still lacking. In this study, our goal was to quantify the responses of crown structure and biomass to stand density and explore how shade tolerance, tree species functional group, tree age and climate affect these responses. We synthesized the outcomes of 356 observations from 60 studies published since 2000 using a meta-analysis approach. The results showed that stand density did not change the ratio of branch plus leaf biomass to aboveground biomass (CB:TB) but did have a significant effect on the crown structure and the branch and leaf biomass. This indicates that in the face of density stress, trees are generally less able to adjust biomass allocation among different organs than they are to alter the morphology of organs. The effect of stand density on crowns varied greatly among different shade-tolerant species, tree species functional groups and tree age, while climatic gradients had no significant effect on the response of crowns to stand density, which reflected the change in tree growth strategies in response to the combined effect of many factors. Overall, our meta-analysis indicates that crown growth can be regulated by changing stand density but that the interaction of stand characteristics and environmental conditions with stand density results in different tree growth strategies. This meta-analysis helps to deepen the understanding of the adaptation strategies of trees in response to competition and environmental stress and serves to promote sustainable management of forest ecosystems.