Abstract

Concentrations of the greenhouse gases CO₂, CH₄ and N₂O are increasing in the atmosphere, and evidence suggests that this is already causing significant changes in the Earth’s climate. Results from numerous investigations indicate that increased atmospheric CO₂ often leads to increased plant production, greater water use efficiency and higher soil water content, but may also result in reduced forage quality, with consequent lowered digestibility, particularly in nutrient-limited systems. Predicted future increases in temperature will have varied effects on plants and animals through alterations in growing season length, metabolism, energy dynamics and system water relations. The potential for increased plant productivity due to CO₂ fertilization as temperatures increase will often be greater in present-day mesic, temperate and cold ecosystems, but may be offset to the extent temperatures increase and conditions become more desiccating in world regions that are predicted to become more drought prone. Rainfall is predicted to increase in some areas, but may not result in more production in cases where increased storm intensity will lead to more runoff and erosion. These basic responses will be modulated by changes in plant and animal species composition that result from competitive species interactions in native ecosystem responses to global climate change, or are the consequence of management strategies to select better adapted plant and animal species or genotypes.  Management options for adapting to global climate change are discussed for intensively managed improved pastures as well as for native rangelands.

Introduction

We live in a time when the imprint of human activities can be detected in virtually every corner of the globe. A rapidly expanding human population is the main driver behind this, causing dramatic changes in the Earth’s ecosystems. Much of this transformation can be viewed as a success story, like the green revolution that combined advances in plant breeding, irrigation and fertilizer technology to significantly increase agricultural productivity in developing nations. Yet these new technologies have often created or been accompanied by new problems that have required attention and new innovation.

Most such development-related environmental problems (e.g. surface and groundwater pollution by N fertilization, salinization, erosion) were observed primarily in the local neighbourhood. In more recent times, the environmental consequences of human development have been more far-reaching. Such is the nature of the increasing concentrations of trace gases in Earth’s atmosphere, and associated global climate change. We cannot see these gases that are being emitted by human activities in greater quantities now than in any recent time, yet there is increasing evidence that they may have already caused significant changes in Earth’s climate and ecosystems, and are likely to induce even greater changes in the twenty-first century. The invisibility of greenhouse gas emissions and their global reach create unique problems: the issue is unappreciated by many, who, not seeing the problem, are sceptical; trace gas emissions from one country mix in the atmosphere and affect the entire world, thereby complicating control strategies; and the complex interactions among greenhouse gases and their varied effects on earth’s climate and ecosystems confound simple predictions of future consequences.

In this chapter, some of the evidence and explanations of trace greenhouse gas emissions and consequences for climate change and grassland responses are reviewed. The ecological and management implications for grazing lands will also be discussed, focusing primarily on altered CO₂ concentrations, increased temperature and altered precipitation patterns. The discussion includes both native rangeland systems and more productive grasslands. Rangelands are natural or semi-natural areas of typically low productivity and are grazed by wild and domesticated animals (Polley et al., 2000). In contrast, productive grasslands are largely created by human activities (Nösberger, Blum and Fuhrer, 2000), and so are more intensely managed than rangelands. They range from intensively managed monocultures to species-rich natural or semi-natural grasslands. In general they are considerably less limited by precipitation than rangelands, accounting for their greater productivity.

The case for global climate change

The anthropogenic release of significant quantities of greenhouse gases (CO₂, CH₄ and N₂O) into the atmosphere began in the Industrial Age, and continues today at increasingly higher rates (Albritton et al., 2001). Greenhouse gases reduce the efficiency with which the earth radiates its energy into space, and their increased atmospheric concentrations warm the lower atmosphere and surface of the planet. The warming potential of these gases can be expressed as radiative forcing. Figure 10.1 illustrates how increases in atmospheric CO₂, CH₄ and N₂O concentrations over the past 200 years have increased the respective radiative forcings of these greenhouse gases. Increases in atmospheric SO₂ (Figure 10.1), also a result of industrialization, cause the opposite effect, since they are precursors to aerosols, and result in cooling through cloud formation and reflectance of solar radiation back into space. Aerosols have a much shorter lifetime (days to weeks) compared with most greenhouse gases (decades to centuries), but can have important effects on radiative transfer. Despite the cooling due to aerosols, the net effect of gas emissions is one of warming.

Greenhouse gas emissions resulting from human activities occur against a background of variable solar radiation cycles, natural emissions of greenhouse gases (volcanic activity), and other natural changes in climate forcing unrelated to greenhouse gases, like the El Niño-Southern Oscillation (ENSO) phenomenon (ocean-atmospheric interactions in the Pacific tropics, which affect world climates). Evaluating the influence of anthropogenic gas emissions on past and projected future climates must account for these other natural anomalies. A modelling exercise by Mitchell et al. (2001) included both anthropogenic and natural forcings to evaluate the performance of a coupled ocean-atmosphere climate model; the model performed well when compared with measured temperature anomalies from the late 1800s until 2000 (Figure 10.2). The results suggest we may have a good degree of confidence in our ability to predict future scenarios climate change induced by greenhouse gases.

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Figure 10.1 Records of changes in atmospheric gases. (a) Atmospheric concentrations of CO2, CH4 and N2O over the past 1 000 years. Ice core data for several sites in Antarctica and Greenland (shown by different symbols) are supplemented with data from atmospheric samples taken over the past few decades (shown by line for CO2 or incorporated into the curve for CH4). The estimated radiative forcing is indicated on right-hand scale. (b) Sulphate concentration in several Greenland ice cores and total SO2 emissions from sources in the USA and Europe (crosses). Source: Figure reprinted with permission from the Intergovernmental Panel on Climate Change (Albritton et al., 2001; Figure 8).

Figure 10.2 Global mean surface temperature anomalies relative to the 1880 and 1920 mean from the instrumental record (red line) compared with ensembles of four simulations with a coupled ocean-atmosphere climate model (black line). Simulations include both anthropogenic and natural forcings.  Source: Figure reprinted with permission from the Intergovernmental Panel on Climate Change (Albritton et al., 2001; Figure 15c).

Table 10.1  Estimates of confidence in observed and projected changes in extreme weather and climate events. The table depicts an assessment of confidence in observed changes in extremes of weather and climate during the latter half of the twentieth century (left column) and in projected changes during the twenty-first century (right column). This assessment relies on observational and modelling studies, as well as physical plausibility of future projections across all commonly used scenarios and is based on expert judgment. Table reprinted with permission from the Intergovernmental Panel on Climate Change.

Notes:  (1) Heat index: A combination of temperature and humidity that measures effects on human comfort.
(2) For other areas there are either insufficient data or conflicting analyses.
(3) Past and future changes in tropical cyclone location and frequency are uncertain.

Source: Albritton et al., 2001; Table 4.

The construction of future scenarios of climate change relies on projected patterns in demographic, economic and technical forces that affect greenhouse gas and sulphur emissions. Since it is impossible to develop one or even several scenarios that capture the many possibilities for Earth’s continued human development, the Intergovernmental Panel on Climate Change utilizes up to 40 different scenarios as evaluation tools to predict a range of possible future climates. In their latest report, Albritton et al. (2001) present results from numerous models, which were run for 35 different scenarios, and concluded that globally averaged surface temperatures would increase by between 1.4° and 5.8°C from 1990 to 2100, with a wider range and warmer temperatures for terrestrial surfaces, especially in winter and in the Northern Hemisphere, where warming is predicted to be maximal. A similar exercise was done for precipitation patterns, and the results suggest increased summer and winter precipitation at higher latitudes. Increased winter precipitation is also predicted over northern mid-latitudes, tropical Africa and Antarctica, and in the summer for southern and eastern Asia. Decreases in winter precipitation are predicted for Australia, Central America and southern Africa.

A summary of some of the major predictions and confidences in them are given in Table 10.1. The models predict warmer global temperatures, more hot days over most land areas, increased heat index, and more intense precipitation events. Since more intense storms will lead to increased surface runoff and erosion (Campbell, Stafford Smith and McKeon, 1997), increased rainfall in some areas may not translate into more mesic conditions. In fact, with warming added to the picture, there will probably be increased continental drying and associated risk of drought (Table 10.1).

Primary Production Responses

Carbon dioxide

Carbon dioxide (CO₂) is a substrate for photosynthesis. Increases in its concentration above present atmospheric levels result in higher photosynthesis rates in plants and plant communities, and to a lesser extent, lead to greater aboveground primary production (Drake, Gonzàlez-Meler and Long, 1997; Jackson et al., 1994; Lawlor and Mitchell, 1991; Mielnick et al., 2001; Morgan et al., 2001, 2004; Owensby et al., 1996). However, the degree of plant response to CO₂ can vary between species or environmental conditions (Porter, 1993; Wand et al., 1999), so information on species or functional group responses is important in predicting responses of plant communities and ecosystems to rising atmospheric CO₂ concentrations.

Differences in photosynthetic pathway may be a useful indicator of a plant’s potential to respond to CO₂. Plants with the C₃ photosynthetic pathway are the most abundant and widely distributed class of plants, and have a photosynthetic metabolism that is unsaturated at present atmospheric CO₂ concentrations. In contrast, photosynthesis of the second most abundant class of plants, those with the C₄ pathway, is saturated near present atmospheric CO₂ concentrations of near 370 mlitre litre^-1 air. Because of these differing photosynthetic sensitivities to CO₂, early predictions were that increasing CO₂ would tend to give C₃ plants a photosynthetic advantage over C₄ plants in future CO₂-enriched atmospheres. (Bazzaz, 1990; Pearcy and Ehleringer, 1984). However, subsequent experimentation revealed that, while the relative photosynthetic and growth responses are on average less in C₄ than C₃ species, the differences are often not as great as expected based on their underlying biochemistry (Bowes, 1993; Porter, 1993; Wand et al., 1999).

There are a couple of explanations of why elevated CO₂ does not induce large differences in photosynthetic and growth responses between C₃ and C₄ plants. First and foremost, raising CO₂ induces stomatal closure in most herbaceous species, regardless of photosynthetic class (Field, Jackson and Mooney, 1995; Kimball and Idso, 1983; Wand et al., 1999). This reduces transpiration, resulting in higher plant and soil water potentials, and results in enhanced plant biomass yields through improved water use efficiency (Drake, Gonzàlez-Meler and Long, 1997; Knapp, Hamerlynck and Owensby, 1993; Morgan et al., 2001; Niklaus, Spinnler and Körner, 1998; Owensby et al., 1997; Sindhøj et al., 2000; Volk, Niklaus and Körner, 2000). Such water relations responses may be most important in native grassland and rangelands, where water scarcity is an important limiting factor in production. Second, photosynthetic acclimation, a downward adjustment of photosynthetic capacity in C₃ plants exposed long-term to elevated CO₂, can reduce or even eliminate the photosynthetic enhancement achieved under elevated CO₂ (Lee et al., 2001; Sage, Sharkey and Seeman, 1989; Sage, 1994). Acclimation lessens CO₂ response differences between C₃ and C₄ species.

Some plants may respond more to CO₂ due to different strategies in balancing C assimilation and its utilization for growth and maintenance. Porter (1993) noted that herbaceous C₃ crop plants were more responsive than wild C₃ species to CO₂ enrichment, and also that CO₂-induced growth responses were greater in fast-growing compared with slow-growing wild species. He suggested that plants with stronger C sinks may transport reduced C compounds more effectively away from leaves and reduce metabolic feedbacks that lower photosynthetic activity, and, ultimately, reduce growth response to CO₂.

Legumes are one of the most responsive functional groups to CO₂, in part because of their ability to fix atmospheric N (Hebeisen et al., 1997; Jablonski, Wang and Curtis, 2002; Lüscher et al., 2000; Porter, 1993; Reich et al., 2001; Wand et al., 1999). Insufficient N appears to be one of the major causes behind photosynthetic acclimation and reduced growth responses of plants to elevated CO₂ (Lee et al., 2001; Morgan et al., 2001; Sage, Sharkey and Seeman, 1989), so legumes may have an advantage over non-legumes by virtue of their ability to fix atmospheric N into organic compounds. Not only does the N fixation benefit the plant by providing a strong sink for carbohydrates, but it also provides the N needed for biosynthesis of proteins, nucleic acids and other vital plant compounds. However, legumes may not be as responsive to CO₂ enrichment in soils with low available phosphorus (Stöcklin, Schweizer and Körner, 1998; Warwick, Taylor and Blum, 1998), or when soil N is in abundant supply (Geeske et al., 2001).

As a group, forbs tend to be more responsive than grasses (Leadley et al., 1999; Morgan et al., 2001; Owensby et al., 1999; Porter, 1993; Potvin and Vasseur, 1997; Reich et al., 2001; Teyssonneyre et al., 2002). However, no satisfactory explanation has yet been proposed to explain apparent high CO₂ sensitivity in this diverse group of plants.

Growth of plants at elevated CO₂ may also affect their respiration. This has important consequences for the atmosphere as well as for plants, since as much as 50 percent of C fixed in C₃ photosynthesis may be respired back into the atmosphere (Farrar, 1985). There is some evidence that plant respiration may be reduced in CO₂-enriched environments (Bunce, 1990; Drake, Gonzàlez-Meler and Long, 1997; Rogers et al., 1997). However, since CO₂ often increases the root : shoot ratio in plants, and root systems have higher specific respiration rates than shoots, whole plant respiration may tend to increase as CO₂ rises (Rogers et al., 1997). Unfortunately, much plant respiration research has not been conducted in a manner to evaluate the particular mechanism behind the plant responses (Rogers et al., 1997), or to evaluate yet whether interspecific differences may be an important aspect of variable species responses to CO₂.

Temperature and CO₂

Temperature is a significant factor in setting the boundaries on growing seasons and plant communities, as well as determining growth rates, plant developmental responses and the availability and utilization of resources like water, radiation and nutrients. Warmer climates will lengthen growing seasons and will accelerate plant development; growth rate will increase up to a point, and then decline as super-optimal temperatures are reached. A meta-analysis by Rustad et al. (2001) suggests that global warming may enhance primary production in this century by an average of 19 percent in tundra, grassland and forested biomes, and the response is likely to be greater in the colder ecosystems, particularly in the tundra.

There is considerable evidence that CO₂ and temperature responses are not additive, so it is useful to understand warming in the context of rising CO₂. For instance, the stimulatory effect of CO₂ enrichment on plant growth is often enhanced at warmer temperatures (Cure, 1985; Greer et al., 2000). For C₃ plants this stimulation may be related to CO₂-induced changes in photorespiration (Long, 1991) and activity of RuBisco, the enzyme responsible for fixation of CO₂ into organic compounds (Bowes, 1993); changes in both tend to increase photosynthesis more at high than low temperatures (Greer et al., 2000; Sage, Sharkey and Seeman, 1989). For many plants, warmer temperatures enhance carbohydrate sink demand (Arp, 1991; Farrar and Williams, 1991), which may stimulate the CO₂ growth response (Greer et al., 2000; Newton et al., 1994). For these reasons, the effect of elevated CO₂ on primary production in temperate pastures is likely to be highly seasonal (Newton et al., 1994; Nösberger, Blum and Fuhrer, 2000).

Many of the present predictions for combined CO₂ and temperature responses of rangelands, mostly based on simulation models, suggest increasing CO₂ and warming will lead to greater primary production of most grasslands (Baker et al., 1993; Coughenour and Chen, 1997; Neilson et al., 1998; Parton et al., 1995; Riedo, Gyalistras and Fuhrer, 2000, 2001), although the degree of response varies considerably depending on the particular ecosystem and other environment conditions. Warmer temperatures may be especially important at enhancing relative CO₂ production responses at high altitudes and at high- and mid-latitude regions where low temperatures currently limit CO₂ production responses (Körner, 1996; Riedo, Gyalistras and Fuhrer, 2001; Rounsevell, Brignall and Siddons, 1996).

Precipitation, temperature and CO₂

Ninety percent of the variability in rangeland primary production is accounted for by variation in annual precipitation (Campbell, Stafford Smith and McKeon, 1997). Production is therefore likely to follow any changes in precipitation patterns to the extent they impact upon seasonal soil water dynamics. In more mesic and productive grasslands, the consequences of altered rainfall will probably be less, but will still be important since seasonal productivity is commonly limited by distribution and amounts of precipitation (Knapp, Briggs and Koelliker, 2001).

While increased precipitation in many parts of the world may lead to more production, these changes must be considered in the context of global warming, which will tend to offset the precipitation responses by desiccation (Polley et al., 2000; Rounsevell, Brignall and Siddons, 1996). Changes in the seasonal distribution and intensity of rainfall will affect seasonal soil water dynamics and the efficiency of plant use, and may have a larger impact on rangelands than changes in precipitation amounts (Giorgi et al., 1998). More intense storms are likely to lead to increased surface runoff and erosion and concentration of water in smaller portions of the landscape (Campbell, Stafford Smith and McKeon, 1997). The negative impact of increased global temperatures through desiccation may be especially important in present arid and semi-arid regions of the world, especially in Central America, Australia, the Near East, Southeast Asia and southern Africa, where precipitation may be little affected or may decline (Albritton et al., 2001). However, as mentioned previously, rising atmospheric CO₂ will lead to increased water use efficiency and improved water relations. Thus, the overall hydraulic and plant productivity response to changes in precipitation will depend on complicated interactions with temperature and CO₂.

Plant reproductive and recruitment responses

In addition to growth responses, reproductive and plant recruitment response to CO₂ enrichment and associated climate change can have important impacts on productivity, species composition and, ultimately, biodiversity (Edwards, Clark and Newton, 2001; Grünzweig and Körner, 2001).

There are a number of ways in which CO₂ may effect recruitment of new individuals. In terms of direct plant responses, elevated CO₂ is often, but not always (Farnsworth and Bazzaz, 1995; Smith et al., 2000), associated with increases in seed number (Edwards, Clark and Newton, 2001; Grünzweig and Körner 2001; Jablonski, Wang and Curtis, 2002; Lawlor and Mitchell, 1991; Smith et al., 2000), size or weight (Edwards, Clark and Newton, 2001; Jablonski, Wang and Curtis, 2002; Lawlor and Mitchell, 1991; Newton, 1991; Steinger, Gall and Schmid, 2000). These responses, however, vary with both species and environmental conditions (Jablonski, Wang and Curtis, 2002; Grünzweig and Körner, 2001); often responses in one reproductive trait (seed number or size) offset or cancel out another. Further, CO₂-induced changes in seed number, size and dispersal become important only to the extent that seedling recruitment is limited by seed number, which may often be the case for arid and semi-arid grasslands, but less so for mesic grasslands (Turnbull, Crawley and Rees, 2000). Seed size may not necessarily be a good indicator of reproductive success, especially in CO₂-enriched atmospheres where larger seeds produced may sometimes be depleted in nutrients (Jablonski, Wang and Curtis, 2002; Steinger, Gall and Schmid, 2000). Other factors, such as competitive plant interactions (Bazzaz and McConnaughay, 1992) and seed predation (Arnone et al., 1995), can prevent the establishment of simple relationships between seed number, seed size and recruitment. Nevertheless, CO₂-induced changes and species differences in seed dispersal and seed size can be important in affecting recruitment of new individuals (Edwards, Clark and Newton, 2001). Ultimately, the outcome will depend on interactions between reproduction and the microsite conditions that determine germination and establishment of the seedling.

Proper soil temperature and moisture for germination and establishment of seedlings is an important factor in the establishment of grassland species (Defosse, Bertiller and Robberecht, 1997; Minnick and Coffin, 1999; Wester, 1995). Thus, any changes in temperature or precipitation patterns that affect soil water dynamics have the potential to affect germination and recruitment processes. Higher CO₂ also has the potential to enhance recruitment indirectly through its improvement of soil water content resulting from reduced plant transpiration.

Below-ground responses

In native grasslands, the majority of plant production occurs below ground, and root biomass pools often exceed above-ground pools by a factor of two to five (Arnone et al., 2000; Stanton, 1988). In more productive grassland systems, growth responses to CO₂ are often greater and more consistent in below-ground than above-ground plant organs (Newton, 1991; Rogers, Runion and Krupa, 1994; Rogers et al., 1997). Further, the long-term cycling of nutrients, their availability for plant growth, and the dependence of soil processes on dynamic root responses may control, in large part, the long-term response of ecosystems to CO₂ enrichment and climate change (Arnone et al., 2000; Curtis et al., 1994; Norby and Jackson, 2000; Thornley and Cannell, 2000). An understanding of these complex but important interactions is required in considering ecosystem responses and possible management options for grazing lands.

Root responses

Increasing global air temperatures will lead to higher soil temperatures (Schlesinger and Andrews, 2000). As a result, root production and mortality should increase, assuming soil water and nutrients remain at sufficient levels (Norby and Jackson, 2000). In natural systems, conditions of sufficient water and nutrients are often not met, making the prediction of root dynamic responses to increased temperature difficult. The seasonality of root activity will also be affected by climate change (Reeder, Franks and Milchunas, 2001), with root activity commencing earlier in the growing season, and continuing later as growing seasons extend.

In rangelands where plant production is limited mostly by water, any change in soil water dynamics, whether resulting directly from altered precipitation patterns or indirectly from warmer temperatures or increased CO₂, will alter root activity. In arid and semi-arid rangelands, increased water will enhance root activity. In more mesic grasslands, increased precipitation may have both positive and negative consequences for root activities. Under dry conditions, increased nutrient mineralization and root activity will be important features of belowground system responses to increased water (Mosier, 1998; Rogers et al., 1999) leading to enhanced plant growth. Under wetter conditions, negative consequences for roots may occur with water additions, such as leaching of mobile nutrients, loss of N through de-nitrification, or soil erosion (Rounsevell, Evans and Bullock, 1999).

Although elevated CO₂ can significantly enhance production of belowground organs in productive grasslands (Newton, 1991; Nösberger, Blum and Fuhrer, 2000; Rogers, Runion and Krupa, 1994), the response of less productive native grasslands is less certain. About half of native grassland studies exhibit little or no response in root biomass to growth at elevated CO₂, while the other half exhibit substantial increases in root biomass under elevated CO₂ (Arnone et al., 2000). These variable responses will probably result from the differing soil and environmental conditions of the experiments, and the consequences of those conditions on plant growth and on root-shoot partitioning (Larigauderie, Hilbert and Oechel, 1988).

Nutrient cycling and soil feedbacks

Soil feedbacks may ultimately determine ecosystem responses to CO₂ and climate change. Perhaps the strongest evidence for this has been the almost universal decline in shoot N concentration that occurs in CO₂-enrichment studies, especially those in which CO₂-induced production changes are evident (Drake, Gonzàlez-Meler and Long, 1997). While part of this response may be due to the enhanced resource-use efficiency that plants experience under elevated CO₂, there is good evidence that soils cannot release N quickly enough to keep pace with CO₂-induced production increases (Zak et al., 2000). Most research has focused on the N cycle, but nutrient immobilization and release processes also involve other essential nutrients.

Briefly, CO₂-enhanced production will lead to increased carbon substrates entering the soil, which will fuel microbial growth and initially immobilize soil nutrients (Figure 10.3). However, the additional microbial biomass will eventually cycle and release more nutrients back into the soil through mineralization (Morgan, 2002). Thus, the balance between these and other competing processes can result in increased, decreased or no change in plant available nutrients (Zak et al., 2000). In the long term, soil nutrient cycles may adjust to changes in organic inputs, and nutrient availability will increase to meet new demands in CO₂-enriched atmospheres (Thornley and Cannell, 2000); the time this takes to occur will differ among ecosystems due to differences in species composition and soil type (Reeder, Franks and Milchunas, 2001), as well as to differences in climate.

The degree to which the N, or any other nutrient, cycle is altered due to CO₂-enhanced plant production changes may account for much of the variability among studies and ecosystems in their responses to CO₂. For instance, the increase in root-shoot ratios in many CO₂-enrichment studies has been interpreted as an indirect response to CO₂-induced plant N deficits, which causes plants to shift more substrates below ground to acquire more N (Larigauderie, Hilbert and Oechel, 1988; Newton, 1991; Rogers, Runion and Krupa, 1994; Rogers et al., 1997, 1999; Stulen and den Hertog, 1993). The acclimation of plants that often occurs under long-term CO₂ enrichment, whereby photosynthetic and growth responses become less responsive over time, appears to be related to the build up of carbohydrates that occurs more readily under limited soil N (Arp, 1991; Sage, 1994).

Warmer temperatures may lead to increased organic matter decomposition rates in mesic grasslands, but the same may not hold for dry rangelands. Globally, warmer regions are known to exhibit higher rates of soil respiration and organic matter decomposition compared with colder regions (Epstein, Burke and Lauenroth, 2002; Rustad et al., 2001). However, decomposition is typically modelled by an interaction of temperature and soil water (Esser, 1992; Parton et al., 1987, 1995; Rastetter et al., 1991). As water becomes limiting, decomposition and respiration are predicated less by temperature and more by soil water content (Epstein, Burke and Lauenroth, 2002), with lower soil water resulting in reduced decomposition rates. To the extent it affects soil water dynamics, rising atmospheric CO₂ will also affect decomposition processes (Mosier, 1998; Rogers et al., 1999).

Figure 10.3 Nutrient Cycling Feedbacks. While CO2 enrichment may lead to increased photosynthesis and enhanced plant growth, the long-term response will depend on nutrient cycling feedbacks.  Litter from decaying plants and root exudates enters a large soil nutrient pool that is unavailable to plants until they are broken down and released by microbial activity. Soil microbes may also fix available nutrients into new microbial biomass, thereby temporarily immobilizing them. The balance between these and other nutrient release and immobilization processes determines available nutrients and ultimate plant response.  Source: Figure reprinted with permission from Science (Morgan, 2002).

Plant Community Changes

While responses of plant community productivity to CO₂ and climate change are predicted to be important in many ecosystems, long-term changes in plant community composition may be even more important with regard to ecological impacts and effects on ecosystem goods and services (Körner, Bazzaz and Field, 1996; Leadley and Körner, 1996; Polley et al., 2000; VEMAP Members, 1995).

Climatic changes

The distribution of plant types is affected strongly by spatial and temporal patterns of precipitation and temperature (Epstein et al., 1997; Paruelo and Lauenroth, 1996). Water is the most important determinant of geographical plant distribution (Stephenson, 1990) and productivity (Sala et al., 1988; Webb et al., 1978). Any changes in climate that affect soil water dynamics will have an equally important effect on plant communities. Although precipitation is predicted to increase over much of the globe, the concentration of more rainfall into fewer rainfall events will lead to more uneven spatial distribution of water across the landscape, and more drought in some regions (Campbell, Stafford Smith and McKeon, 1997). In rangelands, where plant community responses are commonly event-driven, changes in timing and amount of precipitation will have especially important consequences for plant community responses (Polley et al., 2000). Patterns of precipitation that significantly alter the vertical distribution of water will correspondingly alter the proportion of shallow-rooted versus deep-rooted species (Ehleringer et al., 1991).

Temperature is also important in the distribution and functioning of plant communities (Epstein et al., 1997; Field and Forbe, 1990; Pyankov et al., 2000; Terri and Stowe, 1976). The increased presence of C₄ grasses toward tropical latitudes is one of the simplest and yet most successful models of plant distribution (Pyankov et al., 2000: Terri and Stowe, 1976). Modelling exercises that consider both temperature and precipitation aspects of climate change predict an increase in relative abundance of C₄ grasses and a decrease in C₃ grasses in North and South American grasslands and shrublands (Coffin and Lauenroth, 1996; Epstein et al., 1997; Epstein, Burke and Lauenroth, 2002; VEMAP Members, 1995). In Europe, latitudinal vegetation gradients in calcareous grasslands exhibit good correlations between species composition and summer temperature (Duckworth, Bunce and Malloch, 2000), suggesting that present-day transitions in grassland communities from north to south may be good indicators of future plant community changes to warming.

Predictions of plant community changes for shrublands are less certain than for grasslands. There is some evidence that warming or precipitation increases, or a combination, may lead to an increase in subtropical shrublands, but a decrease in temperate shrublands (Epstein, Burke and Lauenroth, 2002; VEMAP, 1995). However, these predictions are uncertain given our incomplete understanding of shrubland ecology. The encroachment of shrubs into former semi-arid and arid grasslands has been attributed to livestock grazing, fire suppression, increased atmospheric CO₂, and climate change (Archer, Schimel and Holland, 1995; Bond and Midgley, 2000; Moore et al., 2001; Polley et al., 1997; Roques, O’Connor and Watkinson, 2001). Unfortunately, sorting out the relative importance of these competing forces is difficult at best, and predictions based on them may be unreliable.

The long-term responses of grazing land plant communities to warming and precipitation changes will be determined to a large extent by soil nutrient cycling and water dynamic feedbacks, which affect plant productive, reproductive and recruitment responses, and alter the competitive balance in ecosystems. The rate at which these climate changes occur will be an important factor in determining the ability of plant species to expand their distribution range into new regions.

Rising atmospheric CO₂

Knowledge of individual plant traits like photosynthetic pathway, domesticated compared with wild species, ability to fix atmospheric N, reproductive traits, and morphological considerations such as rooting morphology and depth, have been examined to determine adaptability and responses to future CO₂-enriched atmospheres (Bazzaz, 1990; Bowes, 1993; Cure, 1985; Gitay et al., 2001; Körner, 1996; Long, 1991; Porter, 1993; Wand et al., 1999). However, studies involving field experiments with complex plant communities have revealed that single species traits and responses are often not good predictors of field performance and competitive interactions (Ackerly and Bazzaz, 1995; He, Bazzaz and Schmid, 2002; Joel et al., 2001; Leadley et al., 1999; Morgan et al., 2001; Owensby et al., 1996, 1999; Reich et al., 2001). For instance, while legumes may be particularly responsive to CO₂, He, Bazzaz and Schmid (2002) observed reduced growth of N-fixers under elevated CO₂ in communities with high species richness. Competitive interactions in plant communities can modify the dynamics of soil resources and affect the ability of species to respond to CO₂ (Geeske et al., 2001). There is some evidence that more diverse ecosystems may exhibit increased capacity to adapt and respond to rising CO₂ (He, Bazzaz and Schmid, 2002; Niklaus et al., 2001). Like climatic changes, the long-term consequences of CO₂ enrichment on water and nutrient cycling will probably determine major species and ecosystem responses (Joel et al., 2001; Thornley and Cannell, 2000).

Multiple global changes

Present knowledge would suggest that plants with the C₃ photosynthetic pathway, forbs and legumes might be among the more responsive plant types to rising atmospheric CO₂ concentrations, and thus may expand their distribution in future CO₂-enriched environments. However, significant increases in temperature and altered precipitation patterns will tend to offset these functional group responses. Warmer, drier temperatures will tend to favour C₄ metabolisms; so significant increases in temperature may cancel out some of the competitive advantage afforded C₃ plants at elevated CO₂ levels. Further, other global changes, such as increased ozone and atmospheric N deposition, may be important modifiers of these responses, especially near urbanized centres. Increased atmospheric ozone, a pollutant in rural areas of industrial countries, can reduce the contribution of legumes in grass-legume mixtures (Nösberger, Blum and Fuhrer, 2000). Increased N deposition, a function of animal production as well as industrial activity, will add N to grazing lands (Mosier, 1998; Vitousek et al., 1997), thereby countering the tendency of rising CO₂ to result in reduced available soil N. Both of these will tend to reduce the importance of legumes in maintaining sufficient N in systems to promote CO₂-induced growth responses.

Livestock responses

Since animals attempt to maintain an optimal temperature (Rötter and Van De Geijn, 1999), an understanding of the changing climate and how that affects the thermal balance of animals is critical to understanding how livestock production systems respond to climate change (Parsons et al., 2001).

In general, heat stress reduces feed intake, feed efficiency, animal gain, milk production, and reproduction (Adams et al., 1998; Baker et al., 1993; Bianca, 1965; Hanson, Baker and Bourdon, 1993; Klinedinst et al., 1993), and extreme conditions ultimately lead to animal disease and death (Gitay et al., 2001; Rötter and Van De Geijn, 1999). These negative impacts of global warming will be especially pertinent in regions where summer temperatures are already warm and further increases will significantly affect animal performance (Gitay et al., 2001; Rötter and Van De Geijn, 1999). However, livestock operations in cooler regions of the earth may be little affected by increasing temperature, or may even benefit through reduced feed requirements (Thompson, 1973), increased growth, increased survival and decreased energy costs (Rötter and Van De Geijn, 1999). Further, the particular effect of any change in temperature will also depend on the breed.

In addition to the direct impacts of altered climates, livestock performance will also be influenced by changes in the amount and quality of forage produced. While most research indicates greater primary production under elevated CO₂, some research has also indicated a change in forage quality. Plant N concentrations tend to decline under elevated CO₂, especially when production responses are large (Körner, 2002). However, elevated CO₂ can increase total non-structural carbohydrates (TNC), which tends to enhance forage quality (Lilley et al., 2001). Where N is not limiting for animal performance (e.g., in improved pastures), elevated CO₂ may enhance forage quality (Lilley et al., 2001). Conversely, in native rangelands and semi-natural grasslands where N is typically limiting, the constraints of reduced N and increased fibre in CO₂-enriched forage may reduce digestibility and have a negative impact on ruminant performance (Körner, 2002; Morgan et al., 2004; Owensby, Cochran and Auen, 1996). A change in species composition due to CO₂ enrichment or to climatic change may also affect forage quality (Körner, 2002; Morgan et al., 2004), and the results can be either positive or negative for quality, depending on the particular species involved.

Finally, the range of many animal diseases and pests will also increase at warmer temperatures (Stem et al., 1988). Animal management problems resulting from these expansions will be experienced most acutely in regions just outside of present-day boundaries of particular diseases and pests.

Management of Grazing lands for the Future

The rate and character of future grazing land changes will incorporate responses to land use changes and management, as well as to climatic change and rising CO₂ concentrations. Management of these systems will ultimately determine their ecological and economic trajectories. Thus, an understanding of management options for future grazing lands is important not only as a practical matter, but also to predict the future character of grazing lands. In a general sense, land managers will need to be adaptive and proactive in considering how to match their management practices with the changing environment, and should not be satisfied with management that has worked in the past. Decision-support systems that can assess multiple environmental changes will become increasingly important as managers build risk assessment more explicitly into their enterprises.

A list of the more important grazing land factors and responses to increased atmospheric CO₂ and climate change discussed previously are presented in Table 10.2, followed by some management options. The following discussion will elaborate on most of the options listed, and will separately consider rangelands and productive pastures.

Table 10.2 CO₂ and climate change responses and management options for grazing land factors.

Rangelands

Stocking rate and stocking system decisions are based on seasonal variability in forage production, compositional changes in the plant community, forage quality, and the particular animal breed. Thus, rising CO₂ and climate change will lead to a variety of altered livestock stocking rates and systems to adapt to new environments of the future (Ojima and Lackett, 2002; Polley et al., 2000; Campbell et al., 2000).

Increases or decrease in stocking rates will follow changes in primary productivity, and may not be great in many ecosystems due to compensating changes in production resulting from multiple global changes (Campbell et al., 2000; Smith and Lazo, 2001). Significant changes in seasonality of production and plant community composition will alter the timing of animal operations like calving, lambing or weaning (Ojima et al., 2002), as well as grazing season duration, and may affect the necessity and timing of supplemental livestock feeding. In warmer, drier regions, even small reductions in stocking rates, resulting from lowered forage production, may drive some producers out of business or subsistence people into famine (Campbell et al., 2000).

Changes in plant community composition will be among the most important changes for rangelands. One particular community change that has been underway for the past 100 years, and that might involve both rising atmospheric CO₂ concentrations and climate change, is the incursion of woody plants into former semi-arid and arid grasslands (Polley et al., 1997; Scholes and Archer, 1997). Unfortunately, management considerations for converting some shrublands back to native grasslands are constrained by the creation of new, stable states that resist further transition (Schlesinger et al., 1990; Whitford, Martinez-Turanzas and Martinez-Meza, 1995). Nevertheless, in systems where change is still feasible, the re-institution of fire and light grazing may be used to reverse brush encroachment (Roques, O’Connor and Watkinson, 2001). These types of management will be even more important in regions where increased precipitation will tend to push systems further towards woody plant invasion. At the same time, maintaining shrublands of low economic value may be a cost-effective carbon offset for reducing greenhouse gas emissions (Moore et al., 2001), in which case continued fire suppression may be desired.

In contrast to the above story, invasion of thousands of hectares of sagebrush steppe rangelands in western North America by the exotic Bromus tectorum may be linked to increased atmospheric CO₂ concentrations (Smith et al., 2000). This is converting native sagebrush steppe vegetation into a fire-controlled annual grassland. Numerous other weed invasions are underway around the world, and while much of these are the result of human transportation and movement of exotics around the globe, and are often linked to domestic livestock grazing, the changing climate and increased atmospheric CO₂ are also implicated in some of these invasions (Watkinson and Ormerod, 2001). Management research that traditionally has focused on livestock performance must increasingly evaluate the ecological responses of rangelands to grazing systems, and in particular, to alterations in nutrient and water cycles that may be important in the growth and recruitment of exotics. Differentiating between responses due to CO₂ or climatic changes and responses due to management practices will be fundamental in developing management practices that can cope with the rapidly changing environment.

Some potentially useful plant community changes for livestock production may occur in response to rising CO₂ concentrations, like increasing the competitiveness or presence of legumes, thereby providing additional N to support CO₂-enhanced plant growth. An increase in legumes may be especially important for enhancing forage quality that otherwise may decline due to lowered plant N concentrations (Owensby, Cochran and Auen, 1996). It may be useful to consider interseeding legumes into rangelands as a means to facilitate this process, as well as a means to sequester C in the soil, thereby abating greenhouse gas emissions (Mortenson, Schuman and Ingram, 2003). However, research needs to determine whether such increases in legumes will have effects on the exchange of other trace gases like N₂O or methane, which contribute to greenhouse gas warming. Selection of appropriate legume species and the development of improved germplasm and management practices may be required to maximize the benefits of such legume usage for rangelands.

Climate change will no doubt lead to altered animal usage. Camels, sheep and goats are better adapted to dry regions, and will probably become more important in regions that experience increased drought (Squires and Sidahmed, 1997). Goats may become economically more practical in subtropical rangelands experiencing more climatic variability (Souza Neto et al., 1998). A high degree of genetic diversity already exists within livestock species, so warming in temperate regions may be accommodated by selecting livestock from more tropical climates (Squires and Sidahmed, 1997). Attention has been given in the past to adapting the genetics of livestock to targeted environments, like the tropics (Bonsma, 1949; Rötter and Van De Geijn, 1999). However, there is little information on how these adapted animals would perform with the further warming predicted for this century, and virtually no information on how such selected or altered livestock will perform in future rangelands that will undoubtedly undergo significant plant compositional changes (Squires and Sidahmed, 1997).

Implications for rangeland livestock management systems

Two types of grazing operations prevail on rangelands: pastoralism and ranching (Squires and Sidahmed, 1997). Pastoralism is a practice dating to ancient times, and traditionally centred on herding domestic livestock, but may include some cropping where feasible. Pastoralists typically maintain herds as insurance against extreme drought or hardship. It is a system that in modern times has tended to exist on marginal lands, and is currently in crisis due to a complex interaction of ecological, socio-economic and population pressure problems (Squires and Sidahmed, 1997). Commercial ranching, a more recently developed enterprise, tends to be more economically motivated (Squires and Sidahmed, 1997). Private ownership of land is common, operational scale is large, and improvements in the way of fencing, improved pastures and access to water and other resources are common.

The impacts of climate change are likely to differ between pastoralists and commercial ranchers. For instance, Smith and Lazo (2001) concluded that climate change will have less impact on livestock yields than on forage yields, in part because there currently exists excess capacity for livestock, or that some expansion of rangelands is possible. However, this may not apply to some pastoral groups that are already constrained by human population growth and decreasing grazing lands (Squires and Sidahmed, 1997). Further, pastoral grazing often involves complex social arrangements in the seasonal movement of animals across the landscape (Parish and Funnell, 1999). Although pastoralists are characterized by an ability to adapt to extreme climatic events (Mortimore and Adams, 2001), their complex socio-economic structures and dwindling land resources in some regions may make them more susceptible than ranchers to changes brought about by climate change (Squires and Sidahmed, 1997; Turner, 1993). Climate changes that affect production and plant community composition may challenge social arrangements that have developed over hundreds of years, and further complicate modern-day attempts to protect the landscape while preserving important cultural and societal arrangements as human populations expand.

Other uses for rangelands

In addition to livestock grazing, rangelands provide many other goods and services, including biodiversity, tourism, fuelwood (Gitay et al., 2001; Watkinson and Ormerod, 2001), and more recently, C stores (Meeting et al., 2001; Moore et al., 2001; Schuman, Herrick and Janzen, 2001). Undoubtedly, climate change, government policy and economics will alter the balance among these uses, and may shift management from one enterprise to another. Commercial ranching is already moving away from intensive livestock grazing and towards urbanization, game ranching and transfer of homelands back to indigenous peoples (Polley et al., 2000). Some rangelands will be taken out of production and converted to cropping, as human populations increase and crop production on marginal lands becomes economically more feasible. The contribution livestock make to climate change through methane production is an important problem that will no doubt be considered in future research and management options (Gitay et al., 2001; McCrabb and Hunter, 1999). However, less intensive livestock practices may be encouraged due to the lower net contribution of rangeland to methane production, and thus to greenhouse gas warming compared with more intensive livestock operations (Subak, 1999).

Productive grasslands

Production in many temperate humid grasslands is likely to remain unchanged (Rounsevell, Brignall and Siddons, 1996), or may increase due to increased CO₂ and warmer temperatures, which will extend growing seasons, and in some cases increase primary production. Because water limitations are generally less severe in productive grasslands, the negative impact of warmer temperatures on desiccation will be less of a problem than in semi-arid and arid rangelands. However, temperature increases in excess of 4°C are likely to have negative consequences because of increased drought stress and reduced growing season lengths (Baethgen, 1997; Rounsevell, Brignall and Siddons, 1996). Subtropical and tropical grasslands may be more vulnerable to climate change than cooler, temperate grasslands. A major risk in subtropics and tropics would be a decline in forage quality (Polley et al., 2000), increased heat stress in livestock, and – in dry zones or regions with periodic droughts –increased incidences of drought or more severe droughts (Baethgen, 1997). However, predicted increased precipitation in some regions may counter the negative response to temperature. As for rangelands, management options may have much to do with altered species composition, changed stocking rates and systems, and, in some situations, different grazing animals or breeds. The options in productive grasslands will tend to be greater since manipulations are economically more practicable than in rangelands.

In semi-natural grasslands with high species diversity, species composition is expected to gradually change due to rising CO₂ (Teyssonneyre et al., 2002) and changes in climate (Duckworth, Bunce and Malloch, 2000). Defoliation can affect the balance of functional group and species composition, so management practices developed to deal with rising CO₂ and climate change will have to balance animal productivity responses with management that maintains or enhances biodiversity (Teyssonneyre et al., 2002). As with rangelands, an understanding of rising CO₂ and climate change impacts on grassland invasions will be required to develop viable management practices to combat those invasions (Buckland et al., 2001; Davis, Grime and Thompson, 2000; Watkinson and Ormerod, 2001).

Contrasting rangelands and productive grasslands

While many of the same issues brought on by rising atmospheric CO₂ and climate change are likely to occur in rangeland and productive grasslands, differences between these two types of systems are likely to affect how they are manifested and addressed. For instance, livestock grazing practices for improved pastures rely on the introduction of adapted species, as well as on ecological considerations of maintaining the optimal balance of desired species that is important for both rangelands and productive grasslands. Sowing legumes into productive grasslands is already an accepted practice for improving N nutrition of pastures. Thus, the increased development and use of legumes to combat N limitations in a future CO₂-enriched world should be a relatively easy technology to adapt (Gitay et al., 2001; Schenk, Jäger and Weigel, 1997). The same may not be true for rangelands, where establishment and subsequent control of introduced species is more problematic. In general, much of the species changes in improved pastures will be due to conscious management decisions to sow or nurture particular adapted species. In rangelands, species issues will involve management of the ecological changes and states resulting from combined climate change and past management to ensure that future desired plant communities result.

Conclusions

Our present understanding of atmospheric CO₂ and climate change and their impact on grazing lands is modest. We know that atmospheric trace gas concentrations are rising and that significant changes in the Earth’s climate are already under way, and we are beginning to understand how these global changes will manifest in different world regions. They will have important consequences for grazing lands, leading ultimately to management modifications, and in some instances, changes in enterprise. In addition to the ecological effects, significant socio-economic consequences will surely follow. But our ability to predict and prepare for possible future scenarios of ecological and management responses is poor.

Our challenge as we look to the future of world grazing lands is to acknowledge that these complex ecosystem responses to CO₂, climate change and other aspects of global change – like increased N deposition or higher levels of ozone – are under way, and to conduct research to understand what has already occurred, as well as develop appropriate technologies and decision-support systems to deal with future changes. The success of global change research programmes will depend in large part on our ability to involve ranchers, pastoralists and other land managers in the design of new research projects to develop proactive strategies and technologies to prepare for the future (Campbell et al., 2000). Where past research has focused on the responses of ecosystems to changes in one or two environmental factors, future research must consider the multi-factorial nature of global changes. More work is needed to understand the complex below-ground processes that will ultimately govern long-term responses. Animal responses and their effects on grazing systems need more attention if we are to realistically simulate combined global change and management responses. And we must advance our capability to respond by identifying low-risk, realistic strategies that allow adaptation, but make sense in the face of uncertainty concerning the degree and rate of global changes and ecosystem responses. Grazing land agriculture has always been characterized by change and adaptability – despite our increasing knowledge about the Earth’s environment, there is little reason to indicate that it will be any different in the future.

Acknowledgements

The author would like to thank Dana Blumenthal, Dan LeCain, Jean Reeder, Robin Kelly and Justin Derner, who made many useful contributions that helped greatly in developing and clarifying this manuscript.

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