In natural environments, low nutrient availability often results in tradeoffs in growth, reproduction and defense in plants. We conducted a field experiment to quantify potential tradeoffs among these three components in Stryphnodendron adstringens (Mimosaceae) (barbatimão) a widespread native tree of Brazilian Cerrado. Cerrado soils are infamously low in nutrient availability, and cerrado plants often suffer moderate levels of herbivore attack. Also, S. adstringens is an important medicinal species beacuse it as high tannin concentration in its bark, but if it is exploited excessively, plants can die. In a completley crossed design (ambient nutrients vs. NPK fertilizer crossed with no herbivory vs. all leaves removed, 20 plants per treatment), we tested the hypothesis that tradeoffs would be most strongly manifested under the greatest stress (herbivory (‘cut’) and ambient nutrient levels) and least manifested under the lowest stress (plus nutrients but no herbivory). Leaf biomass measured 14 months after the start of the experiment represented plant growth. Anti-herbivore defense was measured as the concentration of total phenolics, and hydrolyzable and condensed tannins in leaves produced subsequent to the experimente start. Finally, reproduction was measured as the difference in dry fruit biomass before and after the beginning of the experiment.
Leaf biomass production was lowest in cut+fertilized plants, intermediate in cut and control plants, and highest in fertilized plants. Total phenolic concentration was highest in cut+fertilized plants, and equal in cut, control and fertilized plants. Hydrolyzable and condensed tannin concentrations did not vary by treatment. Fruit biomass was affected negatively by leaf removal, as fruit production was lowest in cut and cut+fertilized plants, and signficantly higher in control and fertilized plants. Our hypothesis that tradeoffs would be more severe under higher stress was mostly confirmed: cut plants invested equally in growth and defense, but less on reproduction; cut+fertilized plants invested less in growth and reproduction, but produced more total phenols than the other plants; control plants invested at intermediate levels for all three components; fertilized plants invested more than the other plants in growth, but had intermediate levels of reproduction and defense. In sum, our results demonstrate that plants can change their investment strategies according to nutrient availability but how they do so is constrained by resource availability manifested as tradeoffs. In addition, barbatimão leaves had high tannin concentrations, so we suggest leaves from fertilized plants as an alternative way to exploit this species as a source for medicine.