A novel index of leaf RGR predicts tree shade tolerance

Plant traits responsible for shade tolerance are controversial. An important feature of shade‐tolerant trees is the ability to maintain a positive whole‐plant carbon balance [i.e. positive relative growth rate (RGRₚₗₐₙₜ)] in low‐light environments, but a positive RGRₚₗₐₙₜ does not always ensure continuous growth. To grow successfully in shaded environments, a plant must increase its leaf biomass with time. However, because RGRₚₗₐₙₜ is determined by whole‐plant biomass change, RGRₚₗₐₙₜ can be positive without any increment of leaf mass. Therefore, we developed a new index focussing on the leaf carbon balance to evaluate growth potential in shaded understoreys. We applied this to cool–temperate forest trees, such as Abies firma (evergreen conifer), Fagus crenata and F. japonica (deciduous broad‐leaved trees), and compared their shade tolerance. We termed the new index ‘RGRₗₑₐf’ expressed using net assimilation rate (NARₗₑₐf), leaf lifespan (LL), leaf mass per area (LMA) and leaf partitioning rate (LP), as [(NARₗₑₐf × LP)/LMA – (1/LL)]. RGRₗₑₐf >0 shows a positive leaf carbon balance, leading to continuous growth. RGRₗₑₐf allows a quantitative analysis of traits affecting the leaf carbon balance. We planted seedlings of each species in different light environments (open site, deciduous canopy site and evergreen canopy site) and measured their traits. RGRₗₑₐf and RGRₚₗₐₙₜ were strongly correlated, but RGRₚₗₐₙₜ was positive even when RGRₗₑₐf was negative (i.e. no potential of continuous growth). The negative RGRₗₑₐf values appeared correspond to low survival rates. We found that in the deciduous canopy site, A. firma showed higher RGRₗₑₐf, RGRₚₗₐₙₜ and survival rates compared with F. japonica and F. crenata. Analyses of RGRₗₑₐf and its traits explained that A. firma's relatively high NARₗₑₐf and long LL are effective in maintaining a positive leaf carbon balance. Especially in deep shade (i.e. very low NARₗₑₐf), a prolonged LL is essential for maintaining a positive leaf carbon balance. These results suggest that in cool–temperate forests, evergreen plants can be more shade‐tolerant than deciduous ones.