Long-term effects of insect herbivory on responses by Salix cordata to sand accretion

This study examined the effects of long-term insect herbivory on the respon se of sand dune willow, Salix cordata, to sand accretion. Plants were eithe r exposed to insect herbivory (by a specialist flea beetle, Altica subplica ta) or protected from insect herbivory with cages for three years. Plant gr owth and mortality, as well as sand levels, were then assessed for the next four years, during which time natural sand accretion occurred. The goal wa s to examine how growth and mortality were affected by past herbivory and s and accretion, and most importantly, whether these two factors interacted t o affect plant performance. There was a highly significant positive correla tion between mortality rate and amount of sand accretion. Plants with past herbivory had significantly greater mortality rates than plants with no pas t herbivory, but this difference resulted from a plant size effect. Shorter plants were more susceptible to mortality from sand burial, and plants wit h past herbivory were significantly shorter than plants without past herbiv ory. Past herbivory stimulated both stem diameter and height growth over the ent ire study. The stem diameter growth response to sand accretion varied stron gly for plants with and without past herbivory, and the nature of the relat ionships also varied among dunes. On the west dune, where sand accumulated at a much faster rate, there was a negative relationship between sand accum ulation and stem diameter growth, but this relationship was only significan t for plants with past herbivory. On the east dune, with a slower sand accu mulation rate, stem diameter growth was positively related to sand level fo r plants with past herbivory, but negatively related to sand level for plan ts with no past herbivory. Past herbivory and sand accretion also affected future susceptibility to he rbivory. Beetle densities were significantly greater on plants with no past herbivory than on plants experiencing herbivory three years earlier. For p lants with no past herbivory, plants experiencing high amounts of sand had greater beetle densities than plants experiencing low amounts of sand. Thus , this study demonstrated long-term effects of herbivory on increasing plan t mortality (indirectly via a decrease in plant size), increasing both stem diameter and height growth of surviving plants, and decreasing future susc eptibility to herbivory. Past herbivory also strongly altered the stem diam eter growth response of plants to another stress, sand accretion.