PARENT SESSION
Oral Session #20: Invasions: Effects of invaders -- terrestrial systems. Presiding: R. Callaway.
Tuesday, August 7, 2001. 8:00 AM to 12:00 PM. Madison Ballroom C.
Effects of herbivory and resources on Chinese tallow (Sapium sebiferum) invasion of native coastal prairie.
Rogers, William1, Siemann, Evan1, 1
ABSTRACT- Chinese tallow tree (Sapium sebiferum) is a serious invader throughout the southeastern United States. In Texas coastal prairies it aggressively displaces native plants and forms monospecific stands. A leading hypothesis explaining Sapium invasions is that it is highly resistant to herbivores and thereby gains a competitive advantage over native vegetation. Because Sapium seedlings appear to invest resources in defense against herbivores, we predicted they would be negatively affected by defoliation when competing with coastal prairie vegetation. We conducted paired greenhouse and field studies designed to assess the effects of simulated herbivory and resources on the invasion success of Sapium. We grew Sapium seedlings in competition with coastal prairie vegetation and manipulated light and nitrogen levels while simulating insect herbivory on Sapium leaves. Contrary to our predictions, simulated leaf herbivory did not negatively affect Sapium seedling growth in any resource combinations. Sapium root:shoot mass was significantly affected by an interaction between nitrogen and herbivory suggesting that Sapium allocates more carbon to roots in the absence of herbivory, but defoliation shifts allocation toward rebuilding lost leaf mass when sufficient nitrogen is available. Nitrogen additions also favored Sapium in competition with native vegetation. Our results suggest that Sapium is an herbivore tolerant plant that is currently benefitting from herbivore resistance without the associated trade-offs. Escaping the "to grow or defend" dilemma of plants potentially explains the success of Sapium as an alien invader.
KEY WORDS: alien tree invasions, endangered coastal prairie, herbivory tolerance, plant competition