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 si mulated by leaf clipping. The clipping procedure was intended to mimic the removal by herbivores which only consume the leaves of a single s hoot at every feeding attack and which do not feed over the same shoot s selectively (i.e., most poikilotherm vertebrate and invertebrate her bivores). We tested whether (1) this defoliation procedure does not no rmally depress shoot leaf growth rates (i.e., the occurrence of compen satory leaf growth), and (2) whether leaf nutrient content, relative l eaf growth rate, average distance between consecutive short shoots and rhizome diameter influence the response of the leaf growth rate per s hoot to a gradient of defoliation. The leaf growth rate per shoot vari ed among clipping treatments in nine of the 15 populations treated (AN OVA, p < 0.05) and meta-analyses techniques revealed a significant ove rall variation (chi(2) test, p <0001) when all the populations were co nsidered in concert. The leaf growth rate per shoot was persistently d epressed in all the clipping treatments only in one of the 15 populati ons treated, with only three more populations showing depressed leaf g rowth under some treatments (Tukey HSD test, p < 0.05). The response o f the leaf growth rate to clipping intensity, which was analysed on a per shoot basis (i.e. relationship between the leaf growth rate per sh oot and clipping intensity on the shoot) was significant only for four populations, although meta-analyses revealed a tendency towards a gen eral significance. None of the seagrass properties considered was rela ted to the response of leaf growth to clipping intensity. Our results stress the remarkable variability seagrass leaf growth may exhibit und er single events of defoliation on scattered shoots. Furthermore, beca use leaf growth rates are rarely depressed, these results suggest that most poikilotherm vertebrate and invertebrate herbivores, which typic ally remove < 30% of leaf production, have a modest impact on the depr ession of leaf growth rates through removal of photosynthetic tissue. (C) 1998 Elsevier Science B.V.