Tree allometry responses to competition and complementarity in mixed-species plantations of Betula alnoides

Tree allometry plays a crucial role in tree survival, stability, and timber quantity and quality of mixed-species plantations. However, the responses of tree allometry to resource utilisation within the framework of interspecific competition and complementarity remain poorly understood. Taking into consideration strong- and weak-space competition (SC and WC), as well as N₂-fixing and non-N₂-fixing tree species (FN and nFN), a mixed-species planting trial was conducted for Betula alnoides, a pioneer tree species, which was separately mixed with Acacia melanoxylon (SC ​+ ​FN), Erythrophleum fordii (WC ​+ ​FN), Eucalyptus cloeziana (SC ​+ ​nFN) and Pinus kesiya var. langbianensis (WC ​+ ​nFN) in southern China. Six years after planting, tree growth, total nitrogen (N) and carbon (C) contents, and the natural abundances of ¹⁵N and ¹³C in the leaves were measured for each species, and the mycorrhizal colonisation rates of B. alnoides were investigated under each treatment. Allometric variations and their relationships with space competition and nutrient-related factors were analyzed. The results showed a consistent effect of space competition on the height-diameter relationship of B. alnoides in mixtures with FN or nFN. The tree height growth of B. alnoides was significantly promoted under high space competition, and growth in diameter at breast height (DBH), tree height and crown size were all expedited in mixtures with FN. The symbiotic relationship between ectomycorrhizal fungi and B. alnoides was significantly influenced by both space competition and N₂ fixation by the accompanying tree species, whereas such significant effects were absent for arbuscular mycorrhizal fungi. Furthermore, high space competition significantly decreased the water use efficiency (WUE) of B. alnoides, and its N use efficiency (NUE) was much lower in the FN mixtures. Structural equation modeling further demonstrated that the stem allometry of B. alnoides was affected by its NUE and WUE via changes in its height growth, and crown allometry was influenced by the mycorrhizal symbiotic relationship. Our findings provide new insights into the mechanisms driving tree allometric responses to above- and below-ground resource competition and complementarity in mixed-species plantations, which are instructive for the establishment of mixed-species plantations.