Biomass and toxicity responses of poison ivy (Toxicodendron radicans) to elevated atmospheric CO2: comment.

As the earth’s environment changes with the alteration of important putative environmental drivers (e.g., CO2, temperature, nitrogen deposition, biotic invasions, and the frequency and severity of extreme weather events), ecologists and environmental biologists are scrambling to predict what the world may look like under these new conditions. To accomplish this goal, these scientists often use experimental manipulations of the most likely drivers of community and ecosystem change, particularly CO2, temperature, and nitrogen (e.g., Bergner et al. 2004, Mohan et al. 2006, Reich et al. 2006a). But are these relatively small-scale experiments enough to give us an accurate picture of the structure and function of a future world and is there a way to test their predictions? In a six-year study on the impact of elevated CO2 on the growth and toxicity of poison ivy (Toxicodendron radicans) at the Duke Forest FACE site in North Carolina, USA, Mohan et al. (2006) reported that mean plant biomass of poison ivy increased significantly more in elevated CO2 than in ambient CO2 (Fig. 1), and that this increase was far more than in other woody species. They found that elevated atmospheric CO2 substantially increased poison ivy photosynthesis, water use efficiency, urushiol (the ‘‘toxic’’ compound found in poison ivy), growth, and population biomass. Consequently, Mohan et al. (2006) concluded that poison ivy is likely to become far more abundant (and toxic) with increasing concentrations of atmospheric CO2. Lianas (woody vines), such as poison ivy, are hypothesized to benefit more from elevated CO2 than other woody growth forms because lianas allocate a relatively large proportion of their biomass to leaves (a result of low structural tissue requirements), and thus they should gain more leaf area per total plant biomass with increasing CO2 and carbon fixation than other growth forms.More leaf area, in turn, would allow lianas to fix even more carbon and thus increase faster in total biomass (size and abundance) with elevated CO2 (e.g., Körner 2006, Mohan et al. 2006, Zotz et al. 2006). The study by Mohan and colleagues was methodologically sound and their findings and conclusions were consistent with other experimental studies on the effects of elevated CO2 on liana performance (e.g., Sasek and Strain 1990, 1991, Condon et al. 1992, Granados and Körner 2002, Hättenschwiler and Körner 2003, Zotz et al. 2006). In most cases, however, we have little or no ability to test the validity of predictions based on these types of studies because we lack long-term data on the change in natural plant communities. In a separate study, Londré and Schnitzer (2006) used empirical observations of temperate liana abundance in the interiors of 14 forests in southern Wisconsin, USA, over a 45-year period (1959–2005) to test whether these long-term data were consistent with the hypothesis that lianas, including poison ivy, have proliferated more than co-occurring trees with global change, which includes the increases in CO2 (reviewed by Schnitzer and Bongers 2002, Körner 2006). Globally, atmospheric CO2 levels have risen ;40% over the last 150 years, with nearly 60% of this increase occurring after 1958 (Keeling and Whorf 2005). Thus, if the predictions of experimental studies previously cited are accurate and broadly applicable, both poison ivy and total liana density and biomass should have increased in Wisconsin forests over the past 45 years. Curiously, the empirical observations of Londré and Schnitzer did not match the predictions of experimental studies. Not only did mean liana abundance and basal area remain constant over the past 45 years in Wisconsin forests (Londré and Schnitzer 2006), but poison ivy was the only liana species to decrease significantly over this period (Fig. 2). In a separate study in an old-growth maritime forest on Fire Island in New York, USA, Forrester et al. (2006) reported that over a 19-year period (1967–1986) poison ivy ground cover decreased from nearly 10% to ,1%. There are a number of potential and non-mutually exclusive explanations to reconcile the disparate findings between the predictions generated from experimental studies and the long-term observational studies. One likely explanation is that plant growth and proliferation rates are controlled by multiple factors, not just by CO2 (e.g., Körner 2006, Reich et al. 2006b). For example, although both mean and low winter Manuscript received 25 September 2006; revised 2 March 2007; accepted 5 March 2007. Corresponding Editor: B. A. Hungate. 1 Department of Biological Sciences, P.O. Box 413, University of Wisconsin, Milwaukee, Wisconsin 53201 USA. 2 Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Republic of Panama. 3 Department of Integrative Biology, University of Guelph, Guelph, Ontario Canada. 4 Department of Forest Resources, University of Minnesota, St. Paul, Minnesota USA. 5 E-mail: [email protected]