Comparative growth analysis of tropical forest and savanna woody plants using phylogenetically independent contrasts

1 The savanna–forest boundary in the tropics is marked by a discontinuity not only in tree density, but also in species composition, with few tree species regularly occurring in both savanna and forest environments. 2 We performed comparative growth analysis for nine congeneric pairs, each containing one savanna species and one forest species, grown in a factorial design involving two light and two nutrient levels. 3 Contrary to predictions, there was no difference in relative growth rates (RGR) between savanna and forest species. However, there were clear differences in allocation patterns and in phenotypic responses to light intensity. Savanna species allocated more biomass to roots and maintained lower leaf area per unit plant mass (LAR), and lower leaf area per unit leaf mass (SLA). 4 Savanna species also exhibited greater phenotypic plasticity in specific leaf area, leaf area ratio and net assimilation rate in response to light intensity. An increase in LAR in response to shading of savanna species offset a decrease in net assimilation rate per unit leaf area (NAR), such that RGR was largely unaffected. 5 For most plant traits measured, more of the interspecific variation could be attributed to differences among genera than to differences between the two functional types, indicating that these seedling traits are highly conserved within the congeneric species pairs. Many of these traits were correlated to seed mass, which is itself highly conserved within genera.