Plant size effects on the relationships among specific leaf area, leaf nutrient content, and photosynthetic capacity in tropical woody species

To test whether leaf trait relationships vary with plant size, specific leaf area (SLA), area- and mass-based leaf nutrient contents (Narea and Nmass, Parea and Pmass), and area- and mass-based leaf photosynthetic capacities (Aarea and Amass) of 127 small individuals (woody plants shorter than 2 m) and 47 large trees (taller than 10 m) were measured in a tropical montane rain forest, China. A standardized major axis (SMA) method was used to scale the mass-based trait relationships. We found that Amass, Parea, Pmass, and SLA were higher in the small individuals than in the large trees, but the changes of Aarea, Narea and Nmass leveled off. Trait pairs of Nmass − Amass, SLA − Amass and Nmass − SLA were positively correlated with slopes in common, but the y-intercept of SLA − Nmass was higher in the large trees than in the small individuals, indicating large trees had a higher N investment at a given SLA. In the pairs of Nmass − Amass and SLA − Amass, small individuals were shifted upwards along a common slope than large trees, corresponding to their large Amass and SLA. At a given SLA, the upwards shift along a common slope of small individuals does not mean that they have higher photosynthetic nitrogen use efficiency (PNUE) than large trees, since a common slope and homogenous elevations of SLA-PNUE were found between them. Trait pairs of Pmass − Amass and Nmass − Pmass were positively correlated and the slopes differed between small and large trees. The slope of Pmass − Amass was steeper in the large than in the small individuals, suggesting large trees would have a disproportionally larger return with the same increase in Pmass. However, the pattern of slope of Nmass − Pmass was the reverse of that observed for Pmass − Amass, indicating a higher N/P ratio in the large than in the small individuals. At a given SLA, large trees had higher photosynthetic phosphorus use efficiency (PPUE) than small individuals, corresponding to their higher elevation of SLA-PPUE. In conclusion, leaf trait relationships can be modified by plant size but the positive correlations among SLA, Nmass, Pmass and Amass generally remain invariant.