Drought and the Environment

Drought deeply affects the land, water, and people of the Southwest. It occurs when precipitation averages fall below the norm. A drought can persist for many years, punctuated by particularly severe dry stretches and sometimes a relatively rainy year. The cloudless skies associated with drought not only imply below-average rainfall, but also an increase in the amount of direct sunlight hitting the ground, which leads to higher evaporation rates.

The projected temperature increases and possible precipitation decreases for the Southwest will likely intensify the frequency of drought events and the severity of their impacts on the landscape. During the recent regional drought in the early 2000s, rainfall tallies were comparable to those during the regional drought in the 1950s. However, the effects on Arizona and New Mexico’s forests were much more severe in the early 2000s; researchers link this difference to warmer temperatures during the recent drought.1

During a drought, the combined effect of reduced rainfall and increased sunlight creates a number of environmental effects, including:

• Soil moisture depletion
• Vegetation stress and die-off
• More intense wildfires
• Degraded wildlife habitat

Soil moisture depletion

In the Southwest, the winter rains are most important for replenishing soil moisture and recharging groundwater aquifers. Decreased levels of precipitation during winter months, less cloud cover, increased sunlight, and warmer temperatures cause moisture to evaporate from the ground. The combination of increased soil aridity and associated plant mortality makes the soil more prone to wind erosion. Wind erosion can cause dust storms and increased sand deposition, which often kills even more vegetation.

Vegetation stress and die-off

Climate change will likely affect regional vegetation patterns in the Southwest, particularly along ecotones, or the boundaries between ecosystems.2 In northern New Mexico in the 1950s, the fastest recorded ecotone shift occurred bewteen semi-arid ponderosa pine forest and piñon-juniper woodland. In a study site spanning almost 2,400 hectares, the landscape shifted more than two kilometers in less than five years caused by the lack of precipitation during the early 1950s. Parts of this forest became fragmented and soil erosion increased. Some of this landscape has never shifted back.2

In 2002–03, Southwest lands reacted to severe drought when thousands of acres of trees began to die. Lack of water instigated a chain of events causing forest die-offs in ponderosa pine and piñon-juniper ecosystems across Arizona and New Mexico. As the soil became dry, so did the trees; more intense heat and direct sunlight caused faster evapotranspiration rates (when plants lose moisture to the atmosphere). Without water, the trees were unable to defend themselves against predators — in this case, bark beetles. Normally, the trees used sap to push beetles and other pests out of their bark. But under these dry conditions, the trees had to conserve water and could not produce the sap.

David Breshears and his colleagues investigated the extent of die-off from the particularly severe 2002–03 drought event and its related bark beetle outbreak (Figure 1).1 After 15 months of depleted soil water content, study sites in Arizona, Colorado, and New Mexico lost more than 90 percent of their piñon trees.1 These numbers indicate that this drought had more deeply-rooted impacts than the 1950s drought, even though the earlier drought was somewhat drier. Temperatures however, were significantly warmer during the drought in the early 2000s. Research from the National Data Climatic Center shows that one-fifth less land would have experienced severe drought during the recent drought if average temperatures had not increased since 1976.3 With temperatures projected to rise in the Southwest, the convergence of hotter temperatures and drier conditions could lead to more regional vegetation die-offs (Figure 2).

Wildfires

Drought impacts the frequency and the severity of forest fires by setting up ideal fire conditions. Warmer average temperatures during spring and summer also correlate to higher frequency wildfires and intensify the effects of drought.4 Large-scale forest die-offs create prime conditions for high intensity wildfires. The recent drought and related beetle outbreaks mostly affected tall, overstory trees, but as these trees died-off, other changes to the ecosystem occurred. Shorter trees, shrubs, grasses, and other vegetation beneath the overstory dried out too.

Historical tree-ring records show that the largest fires before the 1900s all correspond to years of severe drought, preceded by wet years.5 This cycle of wet and dry years appears to promote wildfires. Wet years encourage plant growth, while dry years cause plant mortality, thereby creating the fine fuels wildfires need to burn.

During 2002, drought conditions facilitated the burning of over 500,000 acres in northern Arizona, a wildfire known as the Rodeo-Chediski fire. These fires are extremely costly—the combined cost of wildfires that occurred between 2002 and 2004 was estimated at $196.8 million.

Degraded wildlife habitat

With drought bringing so many changes to the Southwest ecosystems, wildlife is sure to feel the impacts. Animals will face a reduction in available drinking water, habitat, and food (both vegetation and prey). Mortality rates could increase for the most vulnerable animal species, especially regional endangered species. This may cause increased competition between livestock and wild animals for grazing and water.

Lack of water, food, and habitat protection may also cause decreased reproductive success and survivorship. During the 2002 drought event, over 80 percent of the existing pronghorn population died, leaving an estimated 21 pronghorn in Arizona.6 Research shows that annual pronghorn reproduction is significantly tied to winter rainfall (Figure 3).

Lack of water, food, and habitat protection may also cause decreased reproductive success and survivorship. For example, the Arizona Game and Fish Department estimates the endangered Sonoran pronghorn population fell by 4,000 between 1987 and 2000. The pronghorns normally get enough water by eating forbs and other vegetation throughout the year. But when the forage dies due to drought, they congregate in areas near surface water, increasing competition for resources and the threat of predation.

Drought may also cause existing animal habitats to become patchy, isolating some populations. Other populations will be forced to migrate in search of new resources, increasing the potential for human-wildlife contact.

References

• Breshears, D. et al. 2005. Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences, Vol 102, 15144–15148 pp.
• Allen, C., and D. Breshears. 1998. Drought induced shift of a forest woodland ecotone: Rapid landscape response to climate variation. Proceedings of the National Academy of Sciences, Vol. 95, 14839–14842 pp.
• Lenart, M. 2007. Southwestern drought regimes might worsen with climate change. In Lenart, M. (ed.) Global Warming in the Southwest: Projection, Observations, and Impacts. Climate Assessment of the Southwest, Tucson, AZ.
• Westerling, A.L., H.G. Hidalgo, D.R. Cayan, and T.W. Swetnam. 2006. Warming and earlier spring increase western U.S. forest wildfire activity. Science, 313:940–943 pp.
• Swetnam, T.W., and J.L. Betancourt. 1998. Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate, 11:3128–3147
• Bright, J., and J. Hervert. 2005. Adult and fawn mortality of Sonoran pronghorn. Wildlife Society Bulletin, 33:43-50 pp.

• Figure 1. Areas in Arizona and New Mexico with bark beetle-caused pinon and ponderosa pine mortality (2003). - Credit: U.S. Forest Service
• Figure 2. The forest around Los Alamos before and after drought stress and a bark beetle outbreak. Credit: Craig Allen, U.S. Geological Survey
• Figure 3. Endangered Sonoran pronghorn fawn survival is linked to winter precipitation. Many fawns, as seen in the graph, survived during years of higher winter precipitation (when there was sufficient forage to last until the next rains), while fawn mortality was greatest during very dry years.Credit: James R Bright, Harvard University.