A novel index of leaf RGR predicts tree shade tolerance

Summary 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.