Defoliation in Perennial Plants: Predictable and Surprising Results in Senna spp.

When some plants are defoliated, they may suffer by reaching a smaller final size than if they had not been damaged. Other plants may compensate for damage, ending up the same size as if they had not been damaged. Still, others may overcompensate, ending up larger after defoliation than if they had been spared from damage. We investigated the response of Senna species (Fabaceae) to defoliation, comparing two native and several ornamental congeners, all of which grow locally in southern Florida. Many Senna spp. bear foliar nectaries as nutritional resources for beneficial insects that may, in exchange, protect them from herbivores. We grew five species from seed and subjected them to three levels of defoliation for a period of several months to measure effects of leaf area removal on plant height, number of leaves, and number of extrafloral nectaries. Only three of five species displayed shorter plant heights with greater levels of damage. Two species produced fewer new leaves with moderate to severe defoliation. In only one species, the number of extrafloral nectaries decreased with defoliation, suggesting that while extrafloral nectar production may be an inducible defense in some species, producing more nectaries in response to damage does not occur in these Senna species.

Automated summary

When plants are defoliated, their response varies. Some may be stunted in growth, while others may compensate and reach the same size as undamaged plants. A few may even overcompensate and end up larger than before. In this study, we examined how different Senna species reacted to defoliation and found that only three out of five displayed shorter height with more damage. Additionally, only one species showed a decrease in the number of extrafloral nectaries, suggesting that extrafloral nectar production is not inducible in these Senna species.