A Bite is a Bite is a Bite? Constraints on Response to Folivory in Piper Arieianum (Piperaceae)

Because resource movement is restricted within and among branches of woody plants, different patterns of folivory are likely to differentially affect plant fitness. I examined experimentally the effects of folivory pattern on growth and seed output in the understory tropical wet forest shrub Piper arieianum (Piperaceae) in Costa Rica. I removed 10% of a plant's leaf area in two constrating manners, either from a single reproductive branch or throughout the canopy. I also conducted the experiment at two times of year, 3 mo before flowering and during flowering, to determine the effects of damage relative to the phenology of reproduction. Leaf area was removed to simulate damage by natural folivores. I followed the effects of the removal for 1 yr, both for experimental plants and for control plants from which no leaf area was removed. Response to experimental folivory varied by treatment both at the branch and whole plant level. When 10% of a plant's leaf area was removed from single reproductive branches 3 mo before the main flowering season, those damaged branches grew less and produced 80% fewer viable seeds than did similar branches on control plants and plants that lost 10% of their leaf area, but with the losses scattered throughout the plant. These differences among treatments for individual branches translated into differences over the entire plant: plants with damage concentrated on one reproductive branch grew less overall and produced fewer seeds than plants of the other two treatments. Thus, restricted movement of resources in P. arieianum following leaf damage to single reproductive branches resulted in decreased whole plant fitness. When this same experiment was conducted at the time of flowering, there were no differences among treatments in seed production or growth, either at the branch or plant level. Results of these two experiments demonstrate that the effects of folivory are dependent on the timing of the loss relative to the reproductive phenology of the plant; resources necessary for fruit production are accumulated before flowering and not subsequent to it in P. arieianum. In 32% of plants in which damage was concentrated, seed production was reduced due to either abortion of the damaged branch (17%) or aborption of its bud inflorescences (15%). In the remaining 68% of branch—defoliated plants, neither level nor timing of flowering was affected by the experimental folivory, suggesting that reduction in seed production was due to a decrease in resource availability and not to a lack of pollinator visitation. Together these results add to the evidence that resource movement is restricted in woody plants, even in the face of a stress such as herbivory. The results further suggest that relatively small amounts of folivory can have major detrimental effects in woody plants and that damage pattern as well as damage level must be measured.