The leaf economics spectrum describes relationships among leaf mass per area (LMA), leaf lifespan (LL), and traits related to photosynthesis and metabolism. Strong correlations among these traits have been interpreted as evidence for a single dominant axis of leaf functional diversity. However, mass- vs. area-dependence of photosynthetic traits (i.e., the degree to which whole-leaf trait values depend on leaf mass vs. leaf area) shows divergent patterns across different leaf assemblages, suggesting the presence of multiple dimensions of functional variation. A proposed explanation for these divergent patterns is that photosynthetic traits should be strongly area-dependent in assemblages in which most LMA variation is due to structural (non-photosynthetic) leaf mass per area. In such assemblages, whole-leaf values of photosynthetic traits should depend primarily on leaf area (rather than leaf mass). To better understand how different LMA components affect photosynthetic traits, we developed an analysis framework to partition LMA into photosynthetic and structural components (LMAp and LMAs, respectively), assuming LMA = LMAp + LMAs. We applied this framework to data on interspecific trait variation from the GLOPNET global database and data on intra- and interspecific variation from wet and dry forests in Panama.
Two-dimensional models, in which LMA was partitioned into LMAp and LMAs, fit observations of area-normalized photosynthetic capacity (Amax/area), area-normalized leaf dark respiration (Rdark/area), and leaf lifespan (LL) significantly better than one-dimensional models based on total LMA. Estimates of LMAp and LMAs were only weakly correlated with each other, suggesting the presence of two largely independent axes of variation. Most LMA variation among deciduous species was due to LMAp variation, whereas most LMA variation among evergreen species was due to LMAs variation. In Panama, sun leaves had both higher LMAp and higher LMAs than shade leaves. Wet and dry Panama sites had similar LMAp, but the wet site had higher LMAs. Area-dependence of Amax depended on the variances of LMAp and LMAs, as well as the sign and magnitude of their effects. For example, in GLOPNET, Amax/area increased weakly with LMAp and decreased with LMAs, leading to strong area-dependence despite LMAs accounting for only 50% of LMA variance. In contrast, for Panama sun leaves, Amax/area increased strongly with LMAp, leading to weaker area-dependence than observed in GLOPNET despite LMAs accounting for a larger fraction (70%) of LMA variance.