Leaf Specific Mass Confounds Leaf Density and Thickness

We explored the relationship between leaf specific mass (LSM) and its two components, leaf density and thickness. These were assessed on the leaves of (a) the moderately sclerophyllous tree Arbutus menziesii distributed along a natural nutrient/moisture gradient in California, (b) eight sclerophyllous shrub species on four substrates in south-western Australia, and (c) seedlings of two morphologically contrasting Hakea species grown under varying soil nutrient, moisture and light regimes in a glasshouse experiment. Leaf area, mass, LSM, density and thickness varied greatly between leaves on the same plant, different species, and with different nutrient, moisture and light regimes. In some cases, variations in LSM were due to changes in leaf density in particular or thickness or both, while in others, density and thickness varied without a net effect on LSM. At lower nutrient or moisture availabilities or at higher light irradiances, leaves tended to be smaller, with higher LSM, density and thickness. Under increased stress, the thickness (diameter) of needle leaves decreased despite an increase in LSM. We concluded that, while LSM is a useful measure of sclerophylly, its separation into leaf density and thickness may be more appropriate as they often vary independently and appear to be more responsive to environmental gradients than LSM.