Defence, growth and nutrient allocation in the tropical shrub Bauhinia brevipes (Leguminosae)

Herbivory in natural communities can be high, reducing the growth and reproduction of individual plants. To diminish the impact of herbivores, plants use a variety of defensive mechanisms. It is now recognized that the plant world is characterized by a proliferation of secondary metabolites, but models of the evolution of defence strategies assume that plants are constrained by finite resources, and that defences are costly. In the present study, the relationships between defence, growth and nutrient concentration in Bauhinia brevipes Vog. (Leguminosae) in south-eastern Brazil were investigated. Patterns of herbivory and defence were examined for 170 B. brevipes individuals in 1997 and 1998. Leaf age influenced tannin concentration and herbivore attack on B. brevipes. Mature leaves had higher concentrations of tannins than young leaves, and young leaves were approximately 60% more likely to be attacked than mature leaves. Carbon and nitrogen content explained 36% of the variation in the percentage of attacked leaves in B. brevipes (y = 16.29 - 0.31 C + 0.25 N; r(2) = 0.36; F = 9.56; P < 0.0001). Tannins acted as defensive compounds on B. brevipes leaves against free-feeding herbivores, but no clear pattern was observed against galling herbivores. The amounts of nitrogen, carbon, potassium, and aluminium explained approximately 69% of the variation in tannin concentration on B. brevipes leaves (y = 0.62 - 0.16 N - 0.36 K - 0.26 Al + 0.74 C; r(2) = 0.69; F = 21.18; P < 0.0001). We found an inverse relationship between shoot growth rates and tannin concentration in B. brevipes, indicating that the production of tannins may be costly, therefore offering some evidence for the trade-off hypothesis. Nevertheless, the weak relationship observed between herbivore damage and tannin concentration indicates that other factors are also involved with herbivore pressure.