Additive and nonadditive effects of herbivory and competition on tree seedling mortality, growth, and allocation

The interaction between simulated cotyledon herbivory and interspecific com petition was studied in a greenhouse experiment using two species of trees, Acer rubrum and Quercus palustris is, which commonly invade abandoned agri cultural fields. Herbivory treatments were applied as a gradient of cotyled on removal for A. rubrum with 0, 25, 50, 75, and 100% of cotyledon tissue r emoved. Cotyledons from Q. palustris were clipped and removed (control, ear ly, and late removal) to create a gradient of seed reserve availability. Th e competition treatment consisted of plugs of old-field vegetation that fil led the pots with perennial cover. Mortality of seedlings was higher with c ompetition. There was a significant interaction between herbivory and compe tition with the highest mortality occurring with competition at the highest intensity of herbivory in both species. Herbivory reduced biomass for Q. p alustris only, while competition reduced biomass in both species. Neither s pecies showed an interaction between herbivory and competition for growth. There was a significant interaction between herbivory and competition on al location patterns for both species, with greater allocation to roots with c ompetition at the highest intensity of herbivory. This study demonstrates t he potential for cotyledon herbivory and competition to interact, altering the invasion of tree seedlings into abandoned agricultural land.