Rainfall Variability and Plant Production

Ranchers and farmers in arid and semiarid 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. For example, communally-owned Mexican

Table 6.2. Comparison of social, economic, and environmental sustainability.

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 least 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. SS will be achieved only by systematic community participation and strong civil society.

Economic sustainability ("-EcS"-)

EcS is concerned with "-maintenance of capital,"- or keeping capital intact. Of the four forms of capital (human made, natural, social, and human), economists have scarcely been concerned at all with natural capital (e.g., intact forests, healthy 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.

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.

From Goodland and Daly.9

rangelands were recently authorized to begin privatization in hopes of improving resource conditions and productivity. However, a recent study showed no differences between private and communal tenure systems in these ecosystems; instead, annual precipitation was still the most important factor related to rangeland conditions.14

Given the extreme variability of rainfall in drylands and the low primary production, we might ask: Is there a relationship between rainfall variability and aboveground primary production? Le Houerou et al15 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 (Fig. 6.3A).

Fig. 6.2. 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). Shifts in vegetation (grass-shrub) may or may not be reversible, depending on the interactions of numerous climatic, physical, and biological factors.

Rainfall Botswana Analysis

Fig. 6.2. 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). Shifts in vegetation (grass-shrub) may or may not be reversible, depending on the interactions of numerous climatic, physical, and biological factors.

However, the great diversity of sites evaluated (e.g., shrublands, grasslands, etc.) makes 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 the current year's precipitation than shallow-rooted grasslands. A number of factors could influence the relationship between rainfall and plant production, including:

1. Interactions between various aspects of water input, such as timing, frequency and intensity of precipitation events, and the particular requirements of different plant functional types (shrubs, grasses, forbs, etc.);

2. Topographic and edaphic characteristics of the landscape via their influence on the pattern of spatial redistribution and retention of water; and

3. Factors other than water availability, such as herbivory or nutrient limitations.

We are conducting a series of field, laboratory, and modeling studies in rangelands to elucidate the relative importance of each of these explanations under different climate scenarios and management practices. Next, we present a brief case study that illustrates the importance of the first explanation—quantitatively, how do the different plant functional types respond to variation in seasonal and annual precipitation?

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