Relative growth rate for trees at the growth stage is coordinated with leaf bulk modulus of elasticity and osmotic potential in a subtropical forest of China

Current theoretical models for tree relative growth rate (RGR) have considered physiological processes based on carbon economy and nutrient-productivity relations, but have not taken water relations into account. According to the ecological energetics and Lockhart equation theories, if more energy is needed to maintain water balance in plants, less energy should be invested towards growth, resulting in a trade-off between growth and drought tolerance. Leaf pressure–volume (PV) curve parameters are the best recognised classical indicators of plant drought tolerance, however, the relationships between tree growth and leaf PV parameters have rarely been investigated in forest communities. In the study, we selected two evergreen (Lithocarpus glaber, Cyclobalanopsis oxyodon) and two deciduous species (Quercus serrata, Platycarya strobilacea) that dominate northern subtropical forests on Mt. Shennongjia, central China. For each species, we selected individuals at different developmental stages (growth vs. mature stage), and measured the relative growth rate of diameter at breast height (RGRDBH) and leaf PV curve parameters, including leaf saturated osmotic potential (Ψsat), maximum bulk elastic modulus (εmax), turgor loss osmotic potential (Ψtlp) and leaf water capacity (Cleaf). Correlation analysis and structural equation models (SEM) were applied to explore the relationships between RGRDBH and PV parameters. The results showed that: 1) at both stages, RGRDBH was positively correlated with Ψsat, but not with Ψtlp and Cleaf; 2) εmax was negatively related to RGRDBH at the growth stage; 3) the relationship between PV parameters and RGRDBH was determined mainly by inter-specific difference; 4) SEM showed that both Ψtlp and Ψsat were affected by εmax; and 5) due to the interaction between the PV parameters, Ψsat and εmax had a direct effect on RGRDBH only during the growth stage. Our results showed that the functional/structural coordination between PV parameters adjusted the RGRDBH of trees, and among different species, there exists a tradeoff between energy allocation for drought tolerance and growth. These findings can be applied to improve RGR models, and show how forests respond to warm-induced drought, which is a major threat to global forests in a future warmer climate