Differential determinants of growth rates in subtropical evergreen and deciduous juvenile trees: Carbon gain, hydraulics and nutrient use efficiencies

Growth rate varies across plant species and represents an important ecological strategy for competition, resource use and fitness. However, empirical studies often show a low predictability of functional traits to tree growth. We measured stem diameter and height growth rates of 96 juvenile trees (2 to 5 m tall) of eight evergreen and eight deciduous broadleaf tree species over three consecutive years in a subtropical forest in southwestern China. We examined the relationships between tree growth rates and 20 leaf/stem traits that associated with carbon gain, stem hydraulics and nutrient use efficiency, as well as the difference between evergreen and deciduous trees. We found that cross-species variations of stem diameter/height growth rate can be predicted by leaf photosynthetic capacity, leaf mass per area, xylem theoretical specific hydraulic conductivity, wood density and photosynthetic nutrient use efficiencies. Higher leaf carbon assimilation and lower leaf/stem constructing costs facilitate deciduous species to be more resource acquisitive and consequently faster growth within a relatively shorter growing season, whereas evergreen species exhibit a more conservative strategies and thus slower growth. Further, stem growth rates of evergreen species showed were more dependence on leaf carbon gains, whereas stem hydraulic efficiency were more important for deciduous tree growth. Our results suggest that physiological traits (photosynthesis, hydraulics, nutrient use efficiency) can predict tree diameter and height growth of subtropical tree species. The differential resource acquisition and use strategies and their associations with tree growth between evergreen and deciduous trees provide insights in explaining the co-existence of evergreen and deciduous tree species in subtropical forests.