We investigated the effects of defoliation by herbivores on leaf elongation and foliar nutrient dynamics in six tropical seagrasses (Thalassia hemprichii, Thalassodendron ciliatum, Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis and Syringodium isoetifolium). Defoliation (clipping) treatment resulted in decreased leaf elongation in three of the six seagrasses studied. Mean shoot re-growth of clipped plant plots was 45%, 51%, and 53% lower than in controls in Thalassodendron ciliatum, Syringodium isoetifolium, and Cymodocea serrulata, respectively. Clipping had little effect on the nitrogen content of young, fully expanded leaves, except in one species, Thalassodendron ciliatum (20% reduction in nitrogen). However, there was significantly less nitrogen in senesced leaves of treated plants than in the leaves of control plants of Thalassodendron ciliatum, Halodule uninervis, Cymodocea serrulata and Cymodocea rotundata. Nitrogen resorption efficiencies from leaves averaged 35% higher in defoliated plants than in controls in five of the six species, supporting the hypothesis that resorption in seagrasses may be plastic in response to changes in the magnitude of internal nutrient pools. Seagrasses in this study seem to be variably affected by herbivore pressure. Except for one species (Thalassia hemprichii), where growth and nitrogen pools did not respond to the treatment, clipping increased the nitrogen resorption efficiency of seagrasses, partially compensating for the reduction in growth that occurred following defoliation. The magnitude of the fall in leaf elongation rate in response to defoliation was correlated with the species' resorption proficiency, indicating that the levels to which nitrogen can be reduced in senescing leaves is probably not constant in all seagrass species, and this plasticity will determine the success of resorption as a nitrogen conservation mechanism when nitrogen losses occur as a consequence of herbivory.

Peer reviewed

8 páginas.