Determinants of growth rate in Ficus benjamina L. compared to related faster-growing woody and herbaceous species

Abstract

Woody plants are believed to have inherently lower growth rates than herbaceous plants. For economically important species like the ornamental plant Ficus benjamina (Moraceae) it would be advantageous if such a limitation could be partly overcome in order to reduce growing periods. To identify the physiological causes for the low relative growth rate of this species, we compared it with three species of taxonomically related families: Musanga cecropioides (Moraceae), a fast-growing tropical pioneer tree; Cannabis sativa (Cannabaceae), a ruderal herb; and Pilea nummularifolia (Urticaceae), a tropical forest understorey herb. Relative growth rate at similar plant size was about half for F. benjamina (65 mg −1 day −1) as compared to the other tree species and the two herbaceous species (113–137 mg −1 day −1), due mainly to differences in leaf area ratio (LAR; 15.9 vs. 25.9–34.1 m 2 kg −1). Net assimilation rates were neither different, nor were measured rates of leaf photosynthesis. The differences in LAR were brought about by differences in leaf morphology/anatomy rather than differences in biomass allocation to leaves: specific leaf area (SLA) of F. benjamina was much lower (28.9 vs. 46.6–52.7 m 2 kg −1), whereas leaf mass ratio (LMR) was not. Both leaf thickness and dry matter fraction varied among species, but F. benjamina was the only species with high values for both variables, and hence had the lowest SLA. The two trees did not show higher investment in woody stems. We conclude that at similar size woody plants do not necessarily have lower growth rates than herbaceous plants. SLA is a key trait determining growth rate. Selection for higher SLA may enhance productivity, but caution is warranted due to likely reductions in leaf life-span and tolerance to abiotic and biotic stress.