Plant functional traits can directly reflect plants performance under environmental change, and thus may contribute to a better understanding of the influence of global change on the demographic rates and species composition. Few studies have shown how trait-growth relationships change with tree size from saplings to large trees. To date, size-dependent variations of functional traits and their contributions to trait-demography relationships are still largely unexplored. 

In a study published in Functional Plant Biology, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) examined associations between stem and leaf functional traits and growth rates and their ontogenetic shifts in a tropical seasonal rainforest in Xishuangbanna, south-west China. 

The researchers measured stem and leaf traits for 181 individuals of 20 tree species across three size classes (small, diameter at breast height (DBH) 5–10 cm; middle, DBH 10–20 cm; big, DBH >20 cm) in a tropical seasonal forest in Xishuangbanna. They evaluated the associations between hydraulic traits, leaf economic traits and growth performance. 

They found that stem diameter growth rates of the big sized trees were related most strongly to hydraulic traits, while growth rates of middle sized trees were related to leaf economic traits. This suggests that different traits have different predict ability on tree growth rate across different size classes. 

Thus, the association between diameter growth rates and functional traits is size dependent, which may relate to the differences of environmental stresses shift from light limitation for small trees to water limitation for big trees. 

"Our results highlight that tropical larger trees may be particularly challenged to maintain functional hydraulic pathways under future climates which are projected to be more water limiting," said Dr. FAN Zexin, principal investigator of the study.