Defoliation and growth in an understory palm: quantifying the contributions of compensatory responses

We analyzed to what extent and by what mechanisms plants of the tropical understory palm Chamaedorea elegans are able to mitigate the negative effects of defoliation on performance (i.e., plant size, total growth, leaf lamina growth, and reproduction) and how this is related to light availability. For this purpose we developed a new approach that allowed us to quantify the performance of defoliated plants relative not only to the performance of undamaged plants, but also relative to the estimated performance of hypothetical defoliated plants that do not exhibit any mechanisms of compensatory growth. The latter provides a way to quantify the adaptive value of compensation with reference to a hypothetical noncompensating alternative state. C. elegans plants were grown in a greenhouse at two light levels (5% and 16% of natural daylight) and subjected to five defoliation treatments (a control and four levels of defoliation). Defoliation was repeated every three months. Growth analysis revealed that defoliated plants allocated considerably more mass to the production of leaf laminas (fₗₐₘ) than control plants, at the expense of allocation to other organs, particularly reproductive structures. Average growth rates per unit leaf area (NAR) and per unit plant mass (RGR), both measured on the basis of aboveground mass, increased with the level of defoliation at high light but not at low light. We estimated that the increases in fₗₐₘ and NAR enabled C. elegans to compensate for part of the potential loss in performance caused by defoliation, even in cases where their RGR values were lower than those of control plants. Sensitivity analysis indicated that changes in NAR contributed more to this compensation than fₗₐₘ, but the importance of fₗₐₘ increased with defoliation level and with decreasing light availability. The degree of compensation was higher in the high‐ than in the low‐light treatment, suggesting that the possession of traits associated with compensatory growth may be more important in sunny than in shaded environments. The degree of compensation differed depending on the measure of performance. Defoliated plants fully compensated for the potential reduction in lamina growth but compensated for <20% of estimated loss in reproductive output. Since survival of C. elegans plants appears to be strongly associated with their total leaf area, the greater compensation for lamina growth is important in relation to population dynamics.