Components of variation in seedling potential relative growth rate: phylogenetically independent contrasts.

Variation between species in seedling potential relative growth rate (RGR) is among the most important spectra of plant adaptation. Investigations are reported into the components responsible for this variation, using phylogenetically independent contrasts (PICs). The two species for each PIC were selected to diverge in seed mass at least four-fold, seed mass being a known correlate of RGR. Consistent with previous reports, the main influence on RGR differences between species was leaf area per unit leaf mass (SLA), rather than net assimilation rate per leaf area (NARa). The PIC design showed that SLA differences both underpinned old RGR divergences between orders and families, and also were repeatedly responsible for more recent RGR divergences between genera and species.