The Changing Vegetation of Europe What Is the Role of Elevated Carbon Dioxide

I. Introduction

Although there is no doubt that atmospheric carbon dioxide concentrations are rising rapidly (Keeling et al., 1982) and will continue to do so (IPCC, 1992), there is considerable uncertainty with regard to the consequences for plant communities and ecosystems (Korner, 1993). The difficulties in predicting the impact of elevated C02 first become evident in the laboratory or growth chamber and multiply as we move outdoors and begin to consider large-scale processes operating over extended periods of time.

From laboratory studies we know that plant species differ in responsiveness to elevated C02 (Hunt et al., 1991; Poorter, 1993) and we may be certain that patterns of response detected under controlled conditions will be subject, in more natural habitats, to the modifying effects of other environmental factors, some of which (temperature, rainfall, UV-B) are themselves implicated in global environmental change. At this point in the analysis it is tempting to conclude that the task of predicting the ecological impacts of rising C02 falls almost exclusively in the domain of plant physiology (e.g., Schulze and Mooney, 1993). Little doubt remains that physiological insights are needed for a mechanistic and predictive understanding of vegetation responses to elevated C02. However, in this chapter I shall argue that the most urgent requirement is to place C02 research in the context of other global and regional changes in vegetation driven by more powerful forces.

Carbon Dioxide, Populations, and Communities

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At the present time, the most potent forces for change acting on vegetation are the effects of land use. These arise from the direct effects of human activity (habitat modification by agriculture, forestry, industry, human settlements, overgrazing) and indirect effects (eutrophication through groundwater and atmospheric pollutants, and phytotoxicity resulting from aerial and soil contamination). Reviewed on a world scale, the most consistent effect of these phenomena is the inexorable replacement of mature, often species-rich ecosystems by early successional states in which the vegetation is composed of recently established, fast-growing clonal herbs and ephemeral species. This process has two important implications for studies which seek to predict the impacts of rising C02. The first is the notion that vegetation is already experiencing such radical processes of change that impacts of C02 are perhaps most appropriately analyzed as a fine-tuning of the rates and trajectories of changes which are already well advanced and are driven by land use. The second implication arises from the faster growth rates and reduction in the average life span in the constituent species of vegetation arising from modern forms of intensive or disruptive land use (Grime et al., 1988). Later in this chapter we will examine evidence that these expanding species are more responsive to elevated C02. Moreover, the higher rates of population turnover characteristic of the vegetation of disturbed and intensively exploited landscapes create conditions in which the plant cover is likely to respond more quickly to selection driven by elevated C02 either by permitting more invasions and extinctions or by allowing rapid genetic changes within component populations. Hence land use is likely to be an essential factor in any calculations of the direction and rate of vegetation responses to elevated C02.

In order to explore further the interaction between changing land use and C02 responses, let us now look at recent evidence of floristic change in Western Europe.

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