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 response of sand dune willow, Salix cordata, to sand accretion. Plants were either exposed to insect herbivory (by a specialist flea beetle, Altica subplicata) or protected from insect herbivory with cages for three years. Plant growth and mortality, as well as sand levels, were then assessed for the next four years, during which time natural sand accretion occurred. The goal was to examine how growth and mortality were affected by past herbivory and sand accretion, and most importantly, whether these two factors interacted to affect plant performance. There was a highly significant positive correlation between mortality rate and amount of sand accretion. Plants with past herbivory had significantly greater mortality rates than plants with no past herbivory, but this difference resulted from a plant size effect. Shorter plants were more susceptible to mortality from sand burial, and plants with past herbivory were significantly shorter than plants without past herbivory. Past herbivory stimulated both stem diameter and height growth over the entire study. The stem diameter growth response to sand accretion varied strongly for plants with and without past herbivory, and the nature of the relationships also varied among dunes. On the west dune, where sand accumulated at a much faster rate, there was a negative relationship between sand accumulation and stem diameter growth, but this relationship was only significant for plants with past herbivory. On the east dune, with a slower sand accumulation rate, stem diameter growth was positively related to sand level for plants with past herbivory, but negatively related to sand level for plants with no past herbivory. Past herbivory and sand accretion also affected future susceptibility to herbivory. Beetle densities were significantly greater on plants with no past herbivory than on plants experiencing herbivory three years earlier. For plants 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 plant mortality (indirectly via a decrease in plant size), increasing both stem diameter and height growth of surviving plants, and decreasing future susceptibility to herbivory. Past herbivory also strongly altered the stem diameter growth response of plants to another stress, sand accretion.