The relationship between leaf and stem biomass as well as the relationship between leaf biomass and stem length and diameter are important to our understanding of a broad range of important plant scaling relationship because of their relationship to photosynthesis and thus growth. To understand how twig architecture (i.e., current year leaves, and stem diameter and length) affects stem diameter and length, and leaf number and biomass, we examined the twigs of 64 woody species collected from three forest types along an elevational gradient in the Wuyi Mountains, Jiangxi Province, China. We also compared the scaling relationships we observed with biomass allocation patterns reported at the whole tree level. Our results revealed isometric relationship between leaf and stem biomass on twigs despite differences in forest communities and despite changes in environmental factors along an elevational gradient. Across the 64 species, from twigs to individual trees, leaf biomass scaled approximately as the 2.0-power of stem diameter (but not for stem length or leaf number). These results help to identify a general rule that operates at two different levels of biological organization (twigs and whole trees). The scaling relationship between leaf biomass and stem diameter in twigs is insensitive to differences in species composition, elevation, or forest type. We speculate that this rule emerges because stem diameter serves as a proxy for the amount of resources supplied per unit cross section to developing leaves and for the flow of photosynthates from mature leaves to the rest of the plant body.

The leaf economics spectrum (LES) characterizes multivariate correlations that confine the global diversity of leaf functional traits onto a single axis of variation. Although LES is well established for traits of sun leaves, it is unclear how well LES characterizes the diversity of traits for shade leaves. Here, we evaluate LES using the sun and shade leaves of 75 woody species sampled at the extremes of a within-canopy light gradient in a subtropical forest. Shading significantly decreased the mean values of LMA and the rates of photosynthesis and dark respiration, but had no discernable effect on nitrogen and phosphorus content. Sun and shade leaves manifested the same relationships among N_mass, P_mass, A_mass, and R_mass (i.e., the slopes of log–log scaling relations of LES traits did not differ between sun and shade leaves). However, the difference between the normalization constants of shade and sun leaves was correlated with functional trait plasticity. Although the generality of this finding should be evaluated further using larger datasets comprising more phylogenetically diverse taxa and biomes, these findings support a unified LES across shade as well as sun leaves.

Evergreen and deciduous species co-exist in the subtropical forests in southeastern China. It has been suggested that phosphorus is the main limiting nutrient in subtropical forests, and that evergreen and deciduous species adopt different carbon capture strategies to deal with this limitation. However, these hypotheses have not been examined empirically to a sufficient degree. In order to address this gap in our knowledge, we measured leaf photosynthetic and respiration rates, and nutrient traits related to phosphorus (P), nitrogen (N), and carbon (C) use efficiencies and resorption using 75 woody species (44 evergreen and 31 deciduous species) sampled in a subtropical forest. The photosynthetic N-use efficiency (PNUE), respiration rate per unit N and P (R_d,N and R_d,P, respectively) of the deciduous species were all significantly higher than those of evergreen species, but not in the case of photosynthetic P-use efficiency (PPUE).These results indicated that, for any given leaf P, evergreen species manifest higher carbon use efficiency (CUE) than deciduous species, a speculation that was empirically confirmed. In addition, no significant differences were observed between deciduous and evergreen species for nitrogen resorption efficiency (NRE), phosphorus resorption efficiency (PRE), or N:P ratios. The data indicate that evergreen species coexist with deciduous species and maintain dominance in P limited subtropical forests by maintaining CUE. The data also indicate that it is important to compare the PNUE of deciduous species with evergreen species in other biomes. These observations provide insights into modeling community dynamics in subtropical forests, particularly in light of future climate change.