Effects of density and herbivory on leaf traits of common milkweed (Asclepias syriaca)
The effect of insect herbivores may vary across a plant density gradient resulting in consequences that are either positively or negatively density dependent. This study was designed to investigate simultaneously the effects of density and artificial herbivory on host plant traits after the herbivory treatment. Common milkweed (Asclepias syriaca) provides an exceptional study system to investigate the effects of density on trait-herbivore interactions because numerous studies have documented individual milkweed response to herbivory; we utilize this prior work and apply it to a new ecological context. We hypothesized that density would have a negative effect on leaf traits (plants at high density would be less well-defended or have lower quality) and that herbivory may also affect leaf traits. We grew common milkweed plants in the greenhouse with 1, 3, 6 or 12 plants per pot (= 4, 12, 23 and 46 plants m-2, respectively). After six weeks of growth, artificial herbivory treatments were imposed: no damage and low, medium, and high damage levels. At 0, 24, 48 and 72 hours after herbivory, we collected leaf samples.
Density and herbivory had significant, but not interacting, effects on leaf traits. Leaf nutrient content (total carbon and nitrogen) decreased with increasing density. Density did not affect latex production directly. Instead, there were indirect effects via leaf area—plants in high density plots produced smaller leaves which produced less latex (leaf area and latex production were significantly correlated). We also found significant effects of herbivory on lignin, fiber and total C—all three were higher after artificial herbivory. Latex exudation also increased with increasing intensity of damage. Our results demonstrate effects of population density on leaf traits which may interact with insect herbivores to influence plant population dynamics.