The underlying basis for the trade-off between leaf size and leafing intensity

Recent studies have reported a consistent trade-off between leaf size (mass) and leafing intensity (the number of leaves produced per unit of supporting stem tissue volume); however, a theoretical basis for this trade-off has not been described. We explore the mechanistic basis for this trade-off and assess the relationship in the light of other prominent theories for allometric biomass partitioning. We show algebraically how the allocation of mass to leaves versus stems and the density of stem tissue can potentially influence this trade-off. To assess these possible effects, we compared the relationship between leaf size and leafing intensity, expressed on both mass and volume basis, at the level of a single branch as well as the entire above-ground plant in 61 forbs over a 3-year period. Our results support the idea that the trade-off between leaf size and volume-based leafing intensity depends on both biomass investment (leaves vs. stems) and stem bulk density (mass vs. volume), whereas the trade-off between leaf size and mass-based leafing intensity only depends on the biomass investment. Similar exponents in the scaling of leaf mass vs. stem mass and stem mass vs. stem volume at branch and whole plant levels lead to similar trade-offs. An isometric trade-off between leaf size and volume-based leafing intensity is consistent with a constant biomass partitioning between leaves and stems as well as constant stem tissue density. Conversely, an allometric trade-off between leaf size and volume-based leafing intensity arises when biomass allocation is allometric and stem bulk density varies with plant size.