Individual-tree radial growth in a subtropical broad-leaved forest: The role of local neighbourhood competition

Abstract

Establishing the competitive relationships at the local neighbourhood level is essential for improving our understanding of tree growth dynamics in structurally heterogeneous and species-rich forests. We studied the competitive interactions influencing individual-tree five-year radial growth of the two species Castanopsis fargesii (Franch.) and Quercus fabri (Hance) in a diverse young secondary evergreen broad-leaved forest (EBLF) in eastern China. Different spatially explicit individual-based competition indices were examined for their effectiveness at predicting radial growth. These indices were based on one of the three tree size variables – diameter at breast height (dbh), total height, and crown projection area – and were combined with different approaches to identify potential competitors. Furthermore, we tested for competitive equivalence of conspecific and heterospecific neighbours and analysed the effects of local diversity, initial dbh (measured at the beginning of the five-year growth period) and abiotic environmental variables on individual-tree radial growth. Competition accounted for up to 78% and 75% of radial growth variation in C. fargesii and Q. fabri, respectively. The best results were provided by competition indices using crown projection area as the variable describing tree size and the angular height method as the approach to identify potential competitors (i.e. neighbours greater than the minimum angular height, measured from the base of the target trees, are selected as competitors). Competitive equivalence of conspecific and heterospecific neighbours was found in C. fargesii, whereas heterospecifics were stronger competitors than conspecifics in Q. fabri. We could not detect diversity effects on radial growth. The addition of initial dbh or abiotic environmental variables as further explanatory variables failed to improve the predictive ability of growth models. Our results indicate that diameter growth in this EBLF is largely a function of local neighbourhood competition and suggest that the mode of competition is primarily size-asymmetric. It appears that there may be high competitive equivalence among different species, but this remains to be experimentally tested.