Herbivory tolerance and compensatory differences in native and invasive ecotypes of Chinese tallow tree (Sapium sebiferum)

The Evolution of Increased Competitive Ability (EICA) hypothesis proposes that exotic plants may become superior competitors in their introduced ranges because they have evolved reduced defense allocation and increased growth in response to low herbivory. Long-term common garden studies with Chinese tallow tree (Sapium sebiferum), a problematic invader in the southeastern United States, support EICA predictions of invasive ecotypes displaying rapid growth with poorly defended leaves, whereas native ecotypes are slower growing and have nitrogen rich, highly defended leaves. We conducted two full-factorial experiments designed to assess the effects of leaf herbivory and soil fertility on growth of Sapium seedlings derived from seeds obtained in the species' native Chinese range and introduced range along the Texas Gulf Coast. The first was a field study employing simulated defoliation to examine herbivory tolerance, while the second was a pot study using grasshopper bioassays to measure the compensatory regrowth of native and introduced Sapium ecotypes. In accordance with EICA predictions, introduced Texas ecotypes compensated for mechanical and natural defoliation regardless of resource conditions while growth of native Chinese ecotypes was significantly reduced as a result of both simulated and natural herbivory damage. These results demonstrate that invasive Sapium ecotypes have a greater capacity to compensate for herbivory damage than native Sapium ecotypes. The differences between Sapium ecotypes are consistent with the mechanisms of EICA and other findings demonstrating that invasive ecotypes have become herbivory tolerant due to post-introduction changes in tissue quality.