Testing the generality of the 'leafing intensity premium' hypothesis in temperate broad-leaved forests: a survey of variation in leaf size within and between habitats

Because leaf size scales negatively and isometrically with leaf number per shoot size (leafing intensity) in woody species, and because most tree and shrub species have small leaves, Kleiman and Aarssen (J Ecol 95:376-382, 2007) recently proposed that natural selection favors high leafing intensity resulting in small leaves, i.e., the leafing-intensity-premium hypothesis. However, empirical evidence for or against this hypothesis is still lacking. In addition, this hypothesis has not been examined in the context of how leaf size varies among habitats. To fill this void, we investigated leaf size frequency distributions of woody species from temperate China and explored the relationships among leaf mass, leaf number, and stem mass of current-year shoots of 133 woody species at low and high altitudes of three mountain ranges. The scaling relationships between leaf size and leafing intensity (leaf number per stem mass) were determined using both standardized major axis regression analyses and phylogenetically independent comparative techniques. In light of the leafing-intensity-premium hypothesis, we made three predictions: (1) leaf size frequency distributions should be right-skewed for each local study area and for the entire study region, (2) leafing intensities at different altitudes at different sites should differ while leafing intensities at comparable altitudes should be similar baring large taxonomic differences among sites, and (3) that leafing intensity should be higher for any given leaf size in habitats with small-leaved species. Significant negative and isometric scaling relationships between leaf size and leafing intensity were found to be consistently conserved independent of habitat type, both across species and across correlated evolutionary divergences. Within each mountain range or across the entire study region, leaf size frequency distributions were right-skewed, in accordance with our prediction. However, leafing intensity was smaller for any given leaf size at the altitude with smaller leafed species than for altitudes characterized by large leafed species, i.e., altitudes characterized by species with small leaves did not have consistently higher leafing intensities than other altitudes on each mountain range. Our analyses therefore indicate the direct adaptive value of leaf size but not the selective advantage in high leafing intensity as posited by the leafing-intensity-premium hypothesis. We suggest that this hypothesis explains less about the variation of leaf size among different habitats as it does about variation within habitats, i.e., the relative importance of the adaptive significance of leafing intensity and leaf size can and does vary with habitats.