How do mixed forests always increase community productivity? The contribution of leaf area, photosynthesis and temporal niche differentiation

Mixed forests often exhibit higher community productivity; however, opposite results have also been reported, and the underlying mechanisms remain unclear. We hypothesized that, under the condition of stable water and nutrient niches and consistent source–sink relationships across species, enhanced productivity in diversified plant communities primarily depends on community leaf area index (LAI), community-weighted mean photosynthetic capacity (CMW-P n ), and the degree of temporal niche differentiation. To test this, we studied Pinus tabuliformis Carr. (P) and Quercus variabilis Blume (Q) mixed forests with varying mixing ratios (P9Q1: 90 % P, 10 % Q; P6Q4: 60 % P, 40 % Q; P2Q8: 20 % P, 80 % Q). Basal area increment (BAI) served as a proxy for the mean productivity in the last five years, while microcore technique was applied to capture monthly and daily dynamics of growth. Monthly changes in LAI and CMW-P n were monitored, and stable isotope tracers, hydraulic traits, and soil nutrients were used to evaluate water and nutrient niches. Results showed neither changes in moisture or nutrient niches, nor changes in non-structural carbohydrate patterns. The P6Q4 community had the highest 5-years mean annual productivity. Mean annual productivity showed a significant positive correlation with community LAI but not with CMW-P n . Monthly scale analysis revealed that CMW-P n contributed little to monthly productivity (3.95 %), whereas monthly ΔLAI was the main driver (59.37 %). Further, under simplified assumptions, we found that growth temporal niche differentiation increased community annual productivity by about 15 % per hectare, compared to no temporal niche differentiation. Our results suggest that regulation of community LAI, as well as the degree of interspecific differentiation in growth temporal niches, is a key ecological mechanism determining productivity in mixed forests. • The mixing ratio significantly affects community productivity and the individual growth performance of both species. • Mixed-forest productivity is primarily driven by community LAI rather than photosynthetic capacity. • Temporal niche differentiation is an important mechanism enhancing mixed forest productivity. • Community LAI and temporal niche differentiation are key ecological mechanisms driving mixed forest productivity.