Effects of simulated herbivory and resource availability on native and invasive exotic tree seedlings
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The introduced tree, Sapium sebiferum, is a serious invader throughout the southeastern United States. Low herbivore loads, commonly attributed to high herbivore resistance, are assumed to give Sapium a competitive advantage over native vegetation. Because herbivory resistance is considered energetically expensive, we predicted that if Sapium experienced higher levels of damage its growth would significantly decrease. We examined the effects of different levels of simulated leaf herbivory on the growth of Sapium and Celtis laevigata, a native tree, at three levels of nitrogen and three levels of light availability. Potted seedlings were grown in a complete factorial design. Stem height growth rate, diameter growth rate and plant mass of both Sapium and Celtis were significantly affected by shade and nitrogen treatments. Nitrogen increased the growth of both species and seedlings in shaded conditions grew larger than those in ambient sunlight. Simulated leaf herbivory had a significantly negative impact on diameter growth rate and plant mass of Celtis. Surprisingly, neither Sapium height growth rate, diameter growth rate, nor any measure of plant mass were significantly affected by simulated leaf herbivory. Rather, Sapium exhibited considerable phenotypic plasticity and was able to compensate for leaf damage in all resource conditions. Our findings suggest herbivory tolerance is a previously unappreciated trait of Sapium that, in conjunction with phenotypic plasticity and low levels of herbivory in its introduced range, likely contributes to its invasiveness.
