DRYLANDS AND GLOBAL CHANGE: RAINFALL VARIABILITY AND SUSTAINABLE

RANGELAND PRODUCTION

James F. Reynolds,1 Roberto J. Fernández, 1,2 & Paul R. Kemp 3 11 Department of Botany, Duke University, Durham, NC 27708, USA Department of Botany, Duke University, Durham, NC 27708, USA

22 Permanent address: IFEVA-Facultad de Agronomía, Universidad de Buenos Aires, Buenos Aires 1417, Permanent address: IFEVA-Facultad de Agronomía, Universidad de Buenos Aires, Buenos Aires 1417, ArgentinaArgentina

33 Department of Biology, University of San Diego, San Diego, CA 92110, USA Department of Biology, University of San Diego, San Diego, CA 92110, USA

ABSTRACTOver a third of the land surface of the Earth is composed of drylands and much of this (about 65-70%) is seriously degraded or desertified. The two major desertification drivers—climate change and human activities—have ecological and social impacts at various temporal and spatial scales. These impacts include potential alterations of carbon, water, and trace gas budgets, loss of vegetation cover, and increased wind-borne dust, all of which may affect global biogeochemistry, radiation balance, and climate. The societal consequences of land degradation are also serious since the fate of rural people in drylands is dependent on the effective use of natural resources, e.g., water, soils, plants, livestock and wildlife. In this poster, we discuss how ecosystem-level predictions may be used to address issues relevant to sustainable development of these semi-arid regions. We argue that the next step in assessing sustainability is to incorporate ecological impacts into higher level models that consider direct and other human impacts on these systems. This will require further testing and evaluation of ecosystem-level models in the context of different management and land-use alternatives. We propose the incorporation of both "natural" and human factors into a spatially explicit model of landscape elements and human land-use patterns in order to develop predictive tools capable of dynamic, integrated assessments of impacts of global climate change on human-dominated ecosystems.

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Irrigated Cropland(145.5 Million ha)

Rainfed Cropland(457.9 Million ha)

Rangelands(4,556.4 Million ha)

TotalDegraded

47% degraded 73% degraded30% degraded

70% degraded

Global Drylands (5,159.8 Million ha)Global Drylands (5,159.8 Million ha)Global Drylands (5,159.8 Million ha)Global Drylands (5,159.8 Million ha)

S. AmericaS. America

N. AmericaN. America

EuropeEurope

AustraliaAustralia

AsiaAsia

AfricaAfrica

S. AmericaS. America

N. AmericaN. America

EuropeEurope

AustraliaAustralia

AsiaAsia

AfricaAfrica

Million ha

BACKGROUND: Land degradation and desertification. Arid and semiarid drylands compose 1/3 of the land surface of the world and are home to about 20% of the human population. The vast majority of these drylands consist of rangelands (ca. 88%), whereas the rest are classified as rainfed (3%) and irrigated croplands (9%). The rapid growth of populations in many of these regions, often in conjunction with imprudent land management, has led to increased social vulnerability and to rapid land degradation. Soils in drylands are especially vulnerable to wind and water erosion, loss of organic matter, decline in fertility, salinization, and compaction. This is particularly evident when the natural vegetative cover is reduced through human activities such as intensive livestock grazing, excessive cultivation, urbanization, and other land uses.

While global rates of land degradation and the total areas affected are difficult to estimate, there is ample evidence that extensive areas of the world’s drylands have experienced some form of chronic degradation during the last century. It is estimated that ca. 80% of the world’s rangelands, 60% of rainfed croplands, and 30% of irrigated croplands are threatened by various types of degradation, generally referred to as "desertification." Thus, desertified drylands make up about 65-70% of the total dryland area of the globe (arid, semiarid, and dry subhumid regions, but excluding hyperarid regions). All three of the developing regions of the world—Africa, Asia, and Latin America—have similar percentages of land degradation.

Two major desertification drivers—climate change and human activities—have ecological and social impacts at various temporal and spatial scales, ranging from local (and short-term) to global (and long-term) (see figure). Carbon, water, and trace gas budgets may be significantly altered; losses of vegetation may modify regional albedo, raise air temperatures, and increase wind-borne dust; and all of these changes have the potential to act in concert to affect global biogeochemistry, radiation balance, and climate. The societal consequences of land degradation are equally serious. The fate of rural people in drylands is dependent on the effective use of natural resources, e.g., water, soils, plants, livestock and wildlife. In spite of this, over large areas, natural vegetation continues to be degraded, soils are eroding, and the capacity of the land to support livestock and wild herbivores is being reduced. Combined with complex political, social, and economic factors, which often tend to have equally important roles, adverse human impacts are inevitable.

Climatic factors

Drought

Loss of soil &vegetation

Fires

MismanagementOverexploitation of resources

Human Factors - Population Pressures

Desertification(land

degradation)

short-termdynamics

long-termdynamics

overgrazing

elevated CO2

rodentcontrol

GRASSES

SHRUBS

shifts in vegetation

landusesuppression

Simplified model of various factors (natural, human-caused) that play a role in dryland desertification. Note that drought and desertification operate at different time scales (short- vs. long-term, respectively). Because nearly all drylands are characterized by extreme year-to-year weather fluctuations that may result in changes in ecosystem appearance, it is often difficult to distinguish between short- and long-term trends, as well as between temporary and permanent changes. Shifts in vegetation (grass -shrub) may or may not be reversible, depending on the interactions of numerous climatic, physical, and biological factors.

Economic sustainability ("EcS")

"Maintenance of capital," or keeping capital intact. Of the four forms

of capital (human- made, natural, social, and human), economists have

scarcely at all been concerned with natural capital (e.g., intact forests,

health air) because until relatively recently it had not been scarce.

Economics also prefers to value things in monetary terms so it is having

major problems valuing natural capital-intangible, intergenerational,

and especially common- access resources, such as air, etc. in addition,

environmental costs used to be "externalized," but are now starting to be

internalized through sound environmental policies and valuation

techniques. Because people and irreversible impacts are at stake,

economics has to use anticipation and the precautionary principle

routinely, and should err on the side of caution in the face of uncertainty

and risk.

Economic sustainability ("EcS")

"Maintenance of capital," or keeping capital intact. Of the four forms

of capital (human- made, natural, social, and human), economists have

scarcely at all been concerned with natural capital (e.g., intact forests,

health air) because until relatively recently it had not been scarce.

Economics also prefers to value things in monetary terms so it is having

major problems valuing natural capital-intangible, intergenerational,

and especially common- access resources, such as air, etc. in addition,

environmental costs used to be "externalized," but are now starting to be

internalized through sound environmental policies and valuation

techniques. Because people and irreversible impacts are at stake,

economics has to use anticipation and the precautionary principle

routinely, and should err on the side of caution in the face of uncertainty

and risk.

Sustainability. Natural, semi-natural, and intensively managed dryland ecosystems of the world offer a wide range of different goods and services vital to human populations: food production (humans and livestock), construction materials, climate regulation, soil maintenance, nutrient recycling, wildlife habitat, erosion control, tourism/recreation, and aesthetic enjoyment. When weighing the advantages and disadvantages of some particular course of action that affects these ecosystems—and hence their "sustainability"—decision-makers need quantitative assessments in order to consider these goods and services. Understanding the interplay between ecosystem services and ecosystem functioning and structure has urgent application to land use planning and management in dryland areas. Goodland & Daly (Environmental sustainability: universal and non-negotiable. Ecol Appl 1996;6(4):1002-1017) argue that the true meanings ofsustainability—social, economic and environmental—are obvious only when considered separately. In the context of In the context of global change and its potential impacts on drylands, it is inconceivable that one type of sustainability global change and its potential impacts on drylands, it is inconceivable that one type of sustainability could be realized in the absence of the others since the inter-dependencies are so strong. could be realized in the absence of the others since the inter-dependencies are so strong.

Social sustainability ("SS") Social cohesion, cultural identity, diversity, sodality, comity, sense of community, tolerance, humility, love, compassion, patience, forbearance, fellowship, fraternity, institutions, pluralism, commonly accepted standards of honesty, laws, discipline, etc., constitute the part of social capital that is east subject to rigorous measurement, but probably most important for SS. This "moral capital", as some have called it requires maintenance and replenishment by shared values and equal rights, and by community, religious, and cultural interaction. Without this care it will depreciate just as surely as will physical capital. Will be achieved only by systematic community participation and strong civil society.

Social sustainability ("SS") Social cohesion, cultural identity, diversity, sodality, comity, sense of community, tolerance, humility, love, compassion, patience, forbearance, fellowship, fraternity, institutions, pluralism, commonly accepted standards of honesty, laws, discipline, etc., constitute the part of social capital that is east subject to rigorous measurement, but probably most important for SS. This "moral capital", as some have called it requires maintenance and replenishment by shared values and equal rights, and by community, religious, and cultural interaction. Without this care it will depreciate just as surely as will physical capital. Will be achieved only by systematic community participation and strong civil society.

Environmental sustainability ("ES") ES means maintaining natural capital, akin to the definition of EcS. Although ES is needed by humans and originated because of social

concerns, ES itself seeks to improve human welfare and SS by protecting the sources of raw materials used for human needs and ensuring that the sinks for human wastes are not exceeded, in order to prevent harm to humans. Humanity must learn to live within the limitations of

the biological and physical environment ("sources") and as a "sink" for wastes. This translates into holding waste emissions within the assimilative capacity of the environment without impairing it. It also means keeping harvest rates of renewables to within regeneration rates.

Quasi-ES can be approached by holding depletion rates equal to the rate at which renewable substitutes can be created.

Environmental sustainability ("ES") ES means maintaining natural capital, akin to the definition of EcS. Although ES is needed by humans and originated because of social

concerns, ES itself seeks to improve human welfare and SS by protecting the sources of raw materials used for human needs and ensuring that the sinks for human wastes are not exceeded, in order to prevent harm to humans. Humanity must learn to live within the limitations of

the biological and physical environment ("sources") and as a "sink" for wastes. This translates into holding waste emissions within the assimilative capacity of the environment without impairing it. It also means keeping harvest rates of renewables to within regeneration rates.

Quasi-ES can be approached by holding depletion rates equal to the rate at which renewable substitutes can be created.

CLIMATE VARIABILITY: Long- vs. short-term impacts. Drylands are particularly vulnerable to climate variability, of which precipitation is the most important component. For example, a slight shift in seasonal precipitation and/or frequency of extreme rain events could potentially lead to the over-exploitation of the meager resources of drylands and contribute to the further degradation of the very resource base on which human populations are so dependant.

It is difficult to distinguish between short-term variability and long-term changes in ecosystem appearance, as well as between temporary and permanent changes. Short-term variability tends to affect the range and frequency of "shocks," where long-term change alters the resource base. Shifts in vegetation may or may not be reversible, depending on the interactions of numerous climatic, edaphic, and biological factors. In the long run, however, global climate change may further exacerbate the already high natural variability of drylands, leading to permanent degradation of their productive potentials, particularly since there is a lack of "buffering" by large reserves of organic matter in the soils or in woody vegetation.

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Long term precipitation patterns from the Jornada Experimental Range in southern New Mexico and the Sahel in West Africa. Variability is the norm, NOT the average.

Sahel data from Sharon E. Nicholson, Florida St.Univ.

CASE STUDY: USE OF SIMULATION MODELING

We are conducting a series of field, laboratory, and modeling studies in rangelands to elucidate the

relative importance of different climate scenarios and management practices on ecosystem processes in drylands. We now present a brief case study that illustrates the importance of the question—how do

the different plant functional types respond to variation in seasonal and annual precipitation?

Rainfall variability and plant production

Ranchers and farmers in arid and semi-arid regions of the world have long recognized the importance of short-term rainfall variability on farm and livestock production, and rainfall variability continues to be the principal source of fluctuations in global food production, particularly in developing countries. Given the extreme variability of rainfall in drylands and the low primary production, we might ask: is there a relationship between rainfall variability and above-ground primary production? Le Houérou et al.examined this relationship for a variety of vegetation types around the world and concluded that dryland ecosystems are highly variable in response to water inputs. They reported that variability in annual production was 50% greater than the corresponding variability in annual rainfall on sites receiving less than 600 mm).

However, the great diversity of sites evaluated (e.g., shrublands, grasslands, etc.) make it difficult to understand what causes the high variability in plant production in relation to rainfall. For example, we might expect that arid shrub communities should show less variation in production than semi-arid grasslands because the former are deeper rooted and, therefore, less dependent on current year’s precipitation than shallow-rooted grasslands.

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1.2CVP = 0.16913 + 0.92707 *CVR

R2 = 0.247 (P=0.0001)

Coe

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P)

Coefficient of variation in rainfall (CVR)

Le Houérou HN, et al. Relationship between the variability of primary production and the variability of annual precipitation in world arid lands. J Arid Environ 1988;15:1-18

CASE STUDY: Our case study is based on work in the Jornada Basin of south-central New Mexico. The Jornada Basin was once dominated by warm-season perennial grasses (e.g. Bouteloua eriopoda), but much of the area is now dominated by shrubs (e.g., Larrea tridentata and Prosopis glandulosa). This transition—which was initiated in the late 1800s or early 1900s— is believed to have been driven by overgrazing and global change. The Jornada Basin now contains remnant grassland communities, and most areas are dominated by shrubs or co-dominated by shrubs, subshrubs, forbs, succulents, and grasses. The perennial grasses provide the most valuable forage for livestock in this region.

To address the importance of rainfall variability and rangeland production, with particular emphasis on the relative effect on different plant functional types, we used the Patch Arid Land Simulator (PALS) developed for the Jornada. PALS is a physiologically-based ecosystem model that contains the principal components of ecosystem carbon, water, and nutrient cycles. The results presented here are from our phenology-based version (PALS-FT), which includes the key plant functional types (FT) of the northern Chihuahuan Desert—shrubs, cool season annuals, summer annuals, forbs, and perennial grasses.

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Jornada Experimental

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RESULTS:In the first series of PALS simulations, we examined the potential impacts of historical variations in precipitation on productivity, on both a year-to-year basis and over periods associated with decade-length climate shifts. We selected three periods from the long term records at the Jornada Basin (1914-1997, average annual rainfall = 247 mm): a "normal" decade (1968-1977, average=250 mm); a "dry" decade (1947-1956, 33% below normal or 166 mm); and a "wet" decade (1984-1993, 32% above average rainfall or 325 mm) (see previous figure).

Grouping all of these years together, our model simulations show that rainfall has large impacts on simulated annual net primary production (ANPP), although the absolute magnitude varies with plant functional type. The scatter for these simulated results is substantial, which illustrates that while there is a general increase in productivity with increasing annual precipitation, there is also considerable variation associated with the timing of that rainfall within individual years and with the utilization of this moisture by different functional types.

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Annual Precipitation (mm y-1)

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RESULTS: 10 series of 10-year model runs using rainfall of normal decade (1968-1977), but in which each year's rainfall modified by a random amount varying from -30 to +30% (daily distributions fixed). Thus each 10-year rainfall series had a coefficient of variation (CV) varying from a low of 0.25 to a high of about 0.45, and for the 100-year period, the CV was 0.351 (identical to that of the natural rainfall for the period 1915-1997).

(1) Both shrubs & forbs in had low CVs (ca. 0.4) that were statistically invariant with increasing variation of rainfall (p > .05). This CV is close to the long term average CV of rainfall in the Jornada Basin (0.35). These functional types best exemplify plants capable of utilizing moisture that may occur during any season (note that he dominant shrub is a drought-tolerant evergreen and the forbs are short-lived, herbaceous plants that can grow in any season).

(2) both winter and summer annuals had production CVs much higher than CV rainfall (ca. 1.45 for winter annuals and 1.25 for summer annuals) and, again, not dependent on rainfall. This high variation reflects the fact that productivity in these species depends on germination responses that are highly seasonal and rainfall specific. Small rainfall events within a season or large rainfall outside of their strict seasons result in almost no productivity because of lack of germination. Furthermore, their roots are located in the upper soil profile, which is subject to rapid drying. Thus, there are a number of seasons where substantial rain may not translate into any production; at other times, a small amount of rainfall that happens to be precisely timed for use by a seasonal annual species, may result in substantial productivity. (3) Grasses only functional type with significant increase (p < .05, r2 = 0.63) in CV of production with increasing CV of precipitation (Fig. B). Because these perennial grasses grow mainly during the summer, they appear to be utilizing the most reliable moisture resource. However, there is competition for this moisture from other plant functional types, and high evaporative demand can quickly remove moisture from small rainfall events. Many small events are not equivalent to the same amount of rain in a single event that percolates deeper within the profile. Thus, timing and amount of individual events becomes more important in summer, when grasses are growing.

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CONCLUDING REMARKS

The case study we present in this poster illustrates how fundamental knowledge of environmental factors (rainfall variability) and ecosystem structure and function (plant functional type physiology) can be integrated within a modeling synthesis. Our results underscore the impact of short-term climate shifts and year-to-year variability upon plant production, especially perennial grasses. Year-to-year variability in total plant productivity was found to be roughly 50% greater than the variability in "normal" rainfall and variability in grass productivity was 3x greater. From a management standpoint, perennial grass is one of the most important functional types because of its contribution to forage production and ground cover. Maintaining a stable grass cover is desirable in order to be able to project long-term land use strategies and economic return. However, our modeling results suggest that maintaining stable grass cover may be very difficult in this semi-arid region, given the potential impacts of natural climate variability upon vegetation production. Furthermore, our results support the point of view that dryland agriculturists and pastoralists must be adaptive in terms of their response to climate variability.

While these ecosystem-level predictions may be used independently to address issues relevant to sustainable development of these semi-arid regions, we suggest that the next step in assessing sustainability is the incorporation of the ecological impacts into higher level models that incorporate direct and other human impacts on these systems. This step will require further testing and evaluation of ecosystem-level models in the context of different management and land-use alternatives. Finally, at the highest level of integration, we propose the incorporation of both "natural" and human factors into a spatially explicit model of landscape elements and human land-use patterns. Only at this level, will we have predictive tools capable of dynamic, integrated assessments of impacts of global climate change on human-dominated ecosystems. Although we are far from being able to do this, a number of programs around the world, such as the urban LTERs in the U.S., are striving to develop integrated approaches to link human and natural systems. We believe this task is essential in preparing for—and planning corrective measures to—the complex interactions between climate, human activities, and the well-being of the Earth