Bazzaz, F. A. 1990.

Bazzaz, F. A. 1990. The Response of Natural Ecosystems to the Rising Global CO₂ Levels. Annual Review of Ecology and Systematics, 21:167-196

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Introduction

Evidence from many sources shows that the concentration of atmospheric CO₂ is steadily rising (61, 17). This rise is strongly correlated with the increase in global consumption of fossil fuels (104). There are also significant contributions from the clearing of forests, especially in the tropics (136, 55). Controversy continues, however, as to whether the biosphere is presently a source or a sink for carbon (see 52, 54, 56).

Despite this controversy, most scientists agree that rising CO₂ levels will have substantial direct and indirect effects on the biosphere (80). Because CO₂ is a greenhouse gas, its increase in the atmosphere may influence the earth's energy budget. Several climatologists have used general circulation models to predict changes in mean annual global temperature (58, 108).

While these models differ in detail, they all predict increased global warming and substantial shifts in precipitation patterns. Recently, some scientists (60) have questioned the predictions of these models. But regardless of changes in global temperature and other climate variables, rising CO₂ can influence world ecosystems by direct effects on plant growth and development.

The large body of literature on the response of crops and intensively managed forests to elevated CO₂ is not treated in this review because there are several excellent and recent reviews of it (e.g. 2, 28, 62, 127, 132 for crops, and 37, 65, 111 for trees). Instead, this review concentrates on the response of natural vegetation to elevated CO₂ and some of the predicted climate change.

The review addresses the CO₂ response of individuals at the physiological level and the consequences of that response to population, community, and ecosystem levels. It must, however, be emphasized that most of the findings on the physiological and allocational response to CO₂ were first discovered in agricultural crops, and that much of the initial work on plants from natural ecosystems (69) tests the variation among species in these responses.

Conclusions

It is clear from this review that some general patterns of response of plants, especially at the physiological level, to the rising CO₂ and the associated climate change are beginning to emerge (Figure 1). Enhanced photosynthesis and growth, increased allocation to underground parts, and particularly water use efficiency have been strongly documented. However, photosynthesis and growth enhancement in some species can be of limited duration, perhaps because of shortages of sinks and the resulting simulation of photosynthates in leaves. It is also clear that CO₂ interacts strongly with other environmental factors, especially nutrients and temperature, to generate the response at the individual level.

Work at the community and ecosystem level has clearly shown that, in most situations, the response at the individual level may become highly modified and may not predict the response of communities. It is quite likely that the impact on productivity of ecosystems may result mainly from changes in species composition brought about by differential species response to elevated CO₂. The number and the identity of neighboring plants, the levels of environmental resources, the activities of herbivores, pathogens, and symbionts are crucial to the way plants respond to elevated CO₂. Because of the complexity of these interactions, and our limited knowledge of them, our predictions about the future impact of the rising CO₂ and associated climate change are very tenuous. In fact, for some ecosystems we cannot presently even predict the direction of the change that would result from increasing CO₂. Nevertheless, the work on a model system of annual plants, and with other assemblages, is giving us some insights into the mechanisms of the response to CO₂ at the community level. We are beginning to identify certain parameters that seem to explain significant amounts of the response to elevated CO₂. For example, initial relative plant growth rates and biomass allocation seem very important determinants of plant response to CO₂. Responses at the population level are essentially unknown, but that research in this area, particularly plant-animal interactions, will be of great importance in understanding the future of biological systems in a high CO₂ world.