Leaf growth response to simulated herbivory: a comparison among seagrass species

We examined in seven seagrass species the response of the leaf growth rate per shoot (mg DW shoot−1 day−1) to a gradient of herbivory simulated by leaf clipping. The clipping procedure was intended to mimic the removal by herbivores which only consume the leaves of a single shoot at every feeding attack and which do not feed over the same shoots selectively (i.e., most poikilotherm vertebrate and invertebrate herbivores). We tested whether (1) this defoliation procedure does not normally depress shoot leaf growth rates (i.e., the occurrence of compensatory leaf growth), and (2) whether leaf nutrient content, relative leaf growth rate, average distance between consecutive short shoots and rhizome diameter influence the response of the leaf growth rate per shoot to a gradient of defoliation. The leaf growth rate per shoot varied among clipping treatments in nine of the 15 populations treated (ANOVA, p<0.05) and meta-analyses techniques revealed a significant overall variation (χ2 test, p<0.001) when all the populations were considered in concert. The leaf growth rate per shoot was persistently depressed in all the clipping treatments only in one of the 15 populations treated, with only three more populations showing depressed leaf growth under some treatments (Tukey HSD test, p<0.05). The response of the leaf growth rate to clipping intensity, which was analysed on a per shoot basis (i.e. relationship between the leaf growth rate per shoot and clipping intensity on the shoot) was significant only for four populations, although meta-analyses revealed a tendency towards a general significance. None of the seagrass properties considered was related to the response of leaf growth to clipping intensity. Our results stress the remarkable variability seagrass leaf growth may exhibit under single events of defoliation on scattered shoots. Furthermore, because leaf growth rates are rarely depressed, these results suggest that most poikilotherm vertebrate and invertebrate herbivores, which typically remove <30% of leaf production, have a modest impact on the depression of leaf growth rates through removal of photosynthetic tissue.