Herbivory, disease, recruitment limitation, and success of alien and native tree species

The Enemies Hypothesis predicts that alien plants have a competitive advantage over native plants because they are often introduced with few herbivores or diseases. To investigate this hypothesis, we transplanted seedlings of the invasive alien tree, Sapitan sebiferum (Chinese tallow tree) and an ecologically similar native tree, Celtis laevigata (hackberry), into mesic forest, floodplain forest, and coastal prairie sites in east Texas and manipulated foliar fungal diseases and insect herbivores with fungicidal and insecticidal sprays. As predicted by the Enemies Hypothesis, insect herbivores caused significantly greater damage to untreated Celtis seedlings than to untreated Sapium seedlings. However, contrary to predictions, suppression of insect herbivores caused significantly greater increases in survivorship and growth of Sapium seedlings compared to Celtis seedlings. Regressions suggested that Sapium seedlings compensate for damage in the first year but that this greatly increases the risk of mortality in subsequent years. Fungal diseases had no effects on seedling survival or growth. The Recruitment Limitation Hypothesis predicts that the local abundance of a species will depend more on local seed input than on competitive ability at that location. To investigate this hypothesis, we added seeds of Celtis and Sapitan on and off of artificial soil disturbances at all three sites. Adding seeds increased the density of Celtis seedlings and sometimes Sapitan seedlings, with soil disturbance only affecting density of Celtis. Together the results of these experiments suggest that the success of Sapium may depend on high rates of seed input into these ecosystems and high growth potential. as well as performance advantages of seedlings caused by low rates of herbivory.