Biophysical Indirect and Direct Drivers

Freshwater resources like lakes, rivers, and aquifers are essential to the transition of rangelands to croplands by providing fresh water for irrigation. The intensity of this driver decreases with distance of the dryland from the source. The proximity of fresh water generates interventions in water transportation infrastructure, which accelerate the use of the provisioning services. These interventions can cause a degradation of several dryland services.

Anthropogenically induced global warming has been detected in the last 50 years (IPCC 2001). Dryland-specific and comprehensive information and predictions for the dryland system are not readily available, but it can be inferred that global warming has driven climate changes that have also affected many drylands. These may include the 0.3% rainfall decrease per decade during the twentieth century between 10 N to 30 N; the 2–4% increase in the frequency of heavy precipitation events in midlatitudes of the Northern Hemisphere over the latter half of the last century; the more frequent, persistent, and intense warm episodes of the El Nin˜o-Southern Oscillation phenomenon since the mid-1970s; and the increased frequency and intensity of droughts in parts of Asia and Africa in recent decades. These trends are expected to continue, whereas precipitation will either decrease or increase in different regions (IPCC 2001). The combination of global climate change induced by anthropogenic emissions of greenhouse gases and the fact that carbon dioxide is both the most significant greenhouse gas and an important ingredient for primary production constitutes a potential driver of dryland services. The service of biological productivity, on which so many dryland peoples directly depend, is most sensitive to this combined driver. The water deficit by which dryland primary production is constrained is caused by low precipitation but also high evaporative demand of the dryland atmosphere, which makes plants lose water each time they open their stomata to let in carbon dioxide, a raw material for primary production. With increased CO2 concentration in the air, plants shorten the time of stomatal opening, thus reducing water losses, or they maintain transpiration rate but increase overall production, made possible by the increased CO2 concentration. Furthermore, rainfall may locally increase due to climate change, which too may promote primary production. However, increased temperatures may be above the optimum for dryland plants and may also increase evaporation from soil surfaces, hence reducing soil moisture and even negating possible increases in rainfall. Should plant cover decline, the service of water regulation and hence also primary production will be disrupted. Modeling projects decreases in grain and forage quality in the drylands (IPCC 2001). Climate change is also likely to drive changes in the water provision service through reduction of water quality and due to increased solubility of minerals with the temperature increase.  

Since global climate change is expected to increase the intensity of rainstorms, this together with the reduced plant cover will increase the incidence of flashfloods. These increased freshwater flows (Mirza et al. 1998) may offset the water quality degradation but increase soil erosion. Also, the projected higher frequency of dry spells might encourage dryland farmers to increase water withdrawals for irrigation. Since sea level rise induced by global warming will affect coastal drylands through salt-water intrusion into coastal groundwater, the reduced water quality in already overpumped aquifers will further impair primary production of irrigated croplands.

Rangelands will be affected too, by projected changes in grassland/ shrubland boundaries due to climate change driving changes in plant community composition (Sala et al. 2000). On the other hand, dryland scrublands and woodlands, used mostly for livestock grazing, will be affected by a greater frequency and extent of fire (Howden et al. 1999). Climate change will also increase habitat fragmentation and thus detrimentally affect dryland biodiversity (Neilson et al. 1998).

Overall, climate change is expected to exacerbate desertification (Schlesinger et al. 1990). Furthermore, it is conceivable that it might amplify the potential negative effects of an existing management regime on the services of interest, increase the risks of land degradation, and raise the cost of intervention and reversal (Fernandez et al. 2002). However, climate change is expected to have different effects on the various dryland subtypes; Canziani et al. (1998) suggested that since plants and animals of fluctuating environments are better adapted to environmental change, the adaptability of biodiversity to climate change will increase with aridity, since the drier dryland subtypes are also environmentally less stable than the less dry subtypes.

The expected annual rainfall typically occurs in a limited number of intensive, highly erosive storms. This produces overland flows that usually develop into violent floods. These floods can be a major driver of soil erosion and soil loss, and the dry spells between storms increase the risk of crop failure. However, these floodwaters can also replenish freshwater resources, deposit fertile minerals and organic debris, and recharge groundwater  or the soil profile. The prevalence of flash floods in drylands typically leads to a number of responses from farmers directed at storage of runoff and flood water, mainly for increasing crops and forage. These include using catchments of up to several hectares (Pacey and Cullis 1986) with or without mechanical or chemical treatment to reduce infiltrability (UNEP 1983); creating micro-catchments of several square meters around a single bush or tree; cultivating wadis that are naturally flooded following rainstorms; spreading the water over extensive tracts to reduce the kinetic energy and enhance infiltration; constructing diversion channels, stone or earth bunds, and even wood bunds to irrigate farmlands of hundreds or even a few thousands hectares; and combinations of several of these techniques (Reij et al. 1988; van Dijk 1995; Niemeijer 1999). Runoff farming is suitable in arid and semiarid areas where direct rainfall is too low for cropping, but in dry subhumid areas it would lead to extensive periods of waterlogging, causing yield reduction. In many of the drier areas floodwaters are also used to recharge wells and fill basins used for drinking water for livestock and humans or for some dry season gardening.

Natural and induced fires are drivers of land cover, soil condition, and biodiversity, especially in the dry subhumid and semiarid dryland subtype. With respect to soil condition, nitrogen and organic carbon are largely lost to the atmosphere or converted to inert forms (charcoal) by fire, while soil erodibility increases for a period after the event. Thus highly recurrent fires can lead to soil degradation. Historically, land use and management changes have modified the temporal and spatial patterns of fire occurrence and intensity, with strong consequences on soil fertility and the composition of the vegetation it supports. In general, traditional land users maintained a fine-grained spatial pattern of small fires with low on-site recurrence, such as the aboriginal fire management in Australia (Griffin and Friedel, 1984a, 1984b, 1985), which could be extrapolated to other semiarid and dry subhumid dryland. Dryland fires are often controlled by grazing and browsing of either wild herbivores or livestock. The twentieth-century commercial agricultural and stock breeding systems as well as wildlife management regulations led to widespread fire prevention together with overstocking of rangelands. The outcome has been a new pattern of larger patches of higher-intensity fires, which is claimed to be one of the triggers for grass-shrub transition (Scholes and Hall 1996). World carbon emissions from savanna burning are estimated at 0.87 billion tons of carbon per year (Scholes and Hall 1996). In the northern Mediterranean basin, the burned area has been increasing at an annual rate of 4.7% since 1960 due to vegetation regrowth after agricultural abandonment (Le Houerou 1992).