Effects of elevated CO₂ on a forest understory community dominated by two invasive plants

Rising levels of atmospheric carbon dioxide (CO2) and invasions by nonnative organisms are predicted to change patterns and processes of native ecosystems in the near future. However, no studies to date investigated both (1) the effects of elevated CO2 on invasive species in natural settings and (2) how interactions between invaders may drive the response of an entire community to CO2 enrichment. In this thesis, I examine the effects of elevated CO2 on an understory plant community dominated by two invasive species (Chapter 2) and determine the nature of interactions and mechanisms mediating the interactions between two codominant invasive plants (Chapter 3). I examined species composition, aboveground production, and cover of the understory plant community in ambient and elevated CO2 treatments in an ongoing, free-air CO2 enrichment (FACE) facility on the Oak Ridge National Environmental Research Park, Tennessee. Five 25-m diameter plots have received either ambient [CO2] or elevated [CO2] since 1998. The understory plant community in these plots is dominated by several non-native plant species including Lonicera japonica and Microstegium vimineum, two highly invasive plants in the Southeastern U.S. L. japonica responded similarly between 2001 and 2002 with a three-fold increase in production in plots receiving elevated [CO2]. In contrast, production of M. vimineum was greater in plots receiving ambient [CO2] in 2001, a relatively wet year, but did not differ between [CO2] treatments in 2002, a drier year. In 2001, species diversity and evenness were greater in elevated CO2 plots, but total understory production did not differ between [CO2] treatments. In 2002, diversity and evenness did not differ between [CO2] treatments, but total understory production was greater in plots receiving elevated [CO2]. These results suggest that community responses to increases in atmospheric [CO2] - in terms of production and diversity - will depend on individual species responses, but will likely be mediated by other resources. Moreover, rising [CO2] may enhance the performance of some, but not all, invasive plants. The co-dominance of the two invasive species, L. japonica and M. vimineum, raised intriguing questions related to invasion biology: Is the pattern of co-dominance between these two species explained by facilitative interactions? Results of a 'press' removal experiment indicated that the relationship between the two species was mutually detrimental, though M. vimineum had a competitive advantage over L. japonica, because its growth form enabled it to acquire light to the detriment of the prostrate L. japonica (Chapter 3). In contrast to our prediction that greater water availability might release the two species from competition, increased soil water availability only benefited M. vimineum, which further reduced the light available to L. japonica. Water and light were important resources mediating the interactions between the two species. However, the growth forms and phenology of each plant may determine the extent and nature of their interactions and govern their response under elevated [CO2].