Discussion

Results from these experiments suggest that declining diversity within an ecosystem can decrease biomass accumulation and, conversely, that elevated C02 can change the biomass accumulation of an ecosystem. Interpreted more broadly, our results suggest that ecosystem response to elevated C02 is a function of both diversity and C02 levels.

Though biomass accumulation may be trivially a function of the species found within an ecosystem, the response of biomass accumulation to random declines in diversity is not a trivial problem. For example, neither theory nor experiments in intercropping provide steadfast rules for how diversity and yield (biomass accumulation) might be associated in even simple agroecosystems (Vandermeer, 1989; Swift and Anderson, 1993). Our biodiversity experiment suggested that if a decline in plant diversity is associated with decreasing interception of light by the canopy, then C02 sequestration by an ecosystem may decline. Our elevated C02 experiment, however, suggests that changes in ecosystem biomass accumulation generated by elevated C02 could compensate for such a loss of carbon sequestration. Although we cannot readily extrapolate from these simple experimental systems to larger more complex naturally occurring systems, these experiments point to an often neglected possibility that understanding how ecosystems will respond to elevated C02 will be a function of how diversity changes over the next few decades.

Our results suggest that manipulating both C02 and community composition may improve our understanding of global change. Most research on the ecological consequences of elevated C02 has been conducted using, on average, 550-700 ppm C02, or levels likely to occur 50-60 years from now (Houghton et al., 1990) and this research has rarely manipulated community composition. By the time these 50-60 years pass, changing C02, in addition to many other globally changing factors (e.g., N fertilization and habitat fragmentation) (Vitousek, 1994), may have already changed community composition. Indeed, some authors (e.g., K├Ârner, Chapters 11 and 28; Polley et al., 1994, and Chapter 12) have argued that some of these effects have already occurred. Even without the effects of elevated C02, the community composition of most ecosystems is likely to be substantially altered in the near future (e.g., Wilson and Peter, 1988; Soule, 1991; Ehrlich and Wilson, 1991; Groombridge, 1992; Sisk et al., 1994; Lawton and May, 1995). Understanding the interactions and feedbacks between ecosystem processes and community composition and how human impacts contribute to these processes will prove useful for predicting and understanding the effects of elevated C02 on global change.

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