ReviewAgroecosystem responses to combinations of elevated CO2, ozone, and global climate change
Introduction
Anthropogenic emissions of carbon dioxide (CO2), and of other greenhouse gases, have serious implications for the global climate system. In its recent third assessment report, the Intergovernmental Panel on Climate Change (IPCC) has concluded that a change in global mean temperature of 1.4–5.8 °C, in combination with changes in precipitation and an increased frequency of extreme weather events, is likely to occur until 2100 (IPCC, 2001). This shift in climate will affect terrestrial ecosystems at different temporal and spatial scales, reaching from effects at the global biosphere level, which may take years to millennia, to localized effects at the level of individual organisms or even organelles, which may take only minutes to a few years. Given the implications for ecosystems with their goods and services, sensitivity analyses are needed to identify areas at risk, and critical conditions and thresholds, and to elaborate mitigation and adaptation options at the scales of interest to decision makers and practitioners. At the smaller spatial scales, effects of climate change may differ from those projected at the larger scales. In fact, high resolution mechanistic ecosystem modeling suggests that the response of a particular system depends on the fine-scale spatial pattern of soil characteristics, and on the nutrient cycling properties of the soil/vegetation system (Pan et al., 1996, Riedo et al., 1999).
Agricultural ecosystems—or agroecosystems—comprise polycultures, monocultures, and mixed systems, including crop–livestock systems, agroforestry, agro–silvo–pastoral systems, aquaculture, as well as rangelands, pastures and fallow lands. They are found all over the world from wetlands and lowlands to drylands and mountains, and their interactions with human activities are determinant. Today, the agricultural share of the total land area is about 30% in the US, 45% in Europe, and 38% worldwide (FAOSTAT, 1999). Effects of climate change on agroecosystems have received considerable research attention (see Parry, 1990, Rosenzweig and Hillel, 1998), but hitherto the sensitivity of many of the types of ecosystems listed above have not been studied in detail. Most studies focused on cropping systems, and projected effects include changes in yield and spatial shifts of production potentials (Reilly and Schimmelpfennig, 1999), or altered insect pest occurrence (Porter et al., 1991). Changing temperature or rainfall alters the local suitability for specific crops (Rötter and van de Geijn, 1999) and grasslands (Rounsevell et al., 1996), and the need for irrigation and fertilization (Adams et al., 1990). In the case of temperate grasslands, model simulations suggest increased productivity (Riedo et al., 1999, Parton et al., 1995), which may stimulate animal production (Baker et al., 1993). Secondary effects include altered farm profitability and regional costs (Rosenzweig and Parry, 1994).
However, many predictions of climate change effects are confounded by the interactions with direct or indirect effects of elevated atmospheric CO2, and with other global change components. An increase in CO2 adds additional carbon to the ecosystems via photosynthesis, leading to changes in the cycling of water and nutrients, and the energy balance. Moreover, the changing chemistry of the atmosphere affects ecosystem processes. In many industrialized regions, the increase in tropospheric ozone (O3) is a key factor of atmospheric change, and the widespread occurrence of visible plant damage across Europe is well documented (Benton et al., 2000). Effects of O3 pollution on crops received considerable research attention (Heagle, 1989). Direct negative effects of O3 on photosynthetic C fixation, leading to productivity losses, are well established (e.g. Lehnherr et al., 1997). It is also known that the thinning of the protective stratospheric ozone layer, leading to increased levels of ultraviolet (UV-B) radiation at the earth surface, may have implications for the productivity and quality of sensitive crop species and cultivars (Krupa et al., 1998), but this aspect is not considered in this review.
Following the concept cited in Goudriaan and Zadoks (1995), the potential yield, which is determined by climate, CO2 and crop characteristics, is almost never achieved because of yield limiting factors such as low soil moisture and nutrient availability, and yield reducing factors such as pests and pathogens, weeds, and air pollutants (Fig. 1). To understand agroecosystem responses, the role these factors may play under altered climatic conditions and elevated CO2 must be known. The aim of this review is to summarize recent findings with regard to the interactions between various factors, with a focus on agroecosystems productivity. Other relevant aspects such as biodiversity or carbon sequestration are not considered.
Section snippets
Influence of elevated CO2 and temperature
Atmospheric CO2 is the sole carbon source for plants. Current levels of CO2 limit CO2 assimilation in C3 crops, and increasing CO2 concentrations up to about 800–1000 ppm stimulate photosynthesis (Amthor, 2001). However, stimulation of photosynthesis does not directly translate in increased biomass, or yield. In determinate crops such as cereals, grain yield not only depends on photosynthesis but also on the length of the active phase of leaf photosynthesis, and the sink capacity of the grains.
Nutrients
The aim of modern agricultural practices is to achieve an optimum efficiency in the use of resources (i.e. minimum amount of a resource used per unit of yield) such as nutrients and water, thereby preserving resources and minimizing environmental impacts due to resource losses to the environment. Excessive application or low uptake efficiency of nutrients from concentrated inorganic fertilizers or from renewable nutrient sources (e.g. animal manure, cover crops, or green manures) can result in
Weeds, insect pests and diseases
The occurrence of plant pests (weeds, insects or microbial pathogens) is an important constraint with global average yield losses estimated at about 40% (Oerke et al., 1994), and production costs significantly dependent on the extent of measures necessary for plant protection. Consequently, changes in the occurrence of pests due to changes in the atmospheric conditions are of both ecological and economic importance.
Implications for management
Factors which limit productivity today are also subject to future changes, including biotic and abiotic stresses, and the direct effects of increased CO2, O3 or temperature on the plants may be of less importance than effects on the plants’ ability to cope with the change in factors limiting or reducing yields. This raises the question of how management may need to adapt in order to mitigate the change in these factors under future atmospheric conditions. Major management adjustments could be
Conclusions
Global climate change, increasing CO2, and regional O3 pollution are three important aspects of the changing atmosphere with pronounced effects on all agricultural ecosystems, but the exact outcome of the interactive effects cannot be predicted in any generalized way. Information gained from experiments with a single factor have little predictive value because in reality, variable combinations of limiting factors, differing in their temporal and spatial variability, are interacting, and
Acknowledgements
This review was prepared in the framework of the Swiss National Science NCCR Climate project ‘GRASS—Climate Change and Food Production.’ Constructive comments from two anonymous reviewers and from the Editor-in-Chief were greatly appreciated.
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