Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of climate change on life and ecosystems

Monitoring Climate and Vegetation Changes at USGS GEOMET Sites

Paula J. Helm
U.S. Geological Survey


Overview

Arid and semiarid lands respond swiftly and dramatically to changes in climate. Eolian (wind-driven) processes are a major factor in the destruction of natural ecosystems and the evolution of new ecosystems on the rangelands of the West. Monitoring of surface meteorology and related sand movement at U.S. Geological Survey Geomet sites provide:

  1. a long-term data base for understanding the range of environmental conditions that can be expected to occur normally in arid and semiarid areas of the desert southwest,

  2. baseline data to assess changes in the desert such as changes in vegetation, migration of sand, and increased dust storms that may occur due to climate change in desert regions,

and

  1. data for field-checking remotely sensed image data of various surfaces so that regional models can be developed for monitoring the land surface changes over time.

USGS Geomet sites are natural earth-science laboratories for understanding ecosystem interactions of the land surface, climate, vegetation, and animal conditions; for quantifying temporal (year-to-year and month-to-month) variability as well as spatial (site-to-site) variability; and to provide historical data that enhances short-term, intensive studies at the sites. Changes in the land surface of the desert southwest due to climate changes and/or human impacts can reduce the productivity of the rangeland and contribute to increased dust production, which negatively impact the health, the economy, and the ecosystems of this fast-growing region.

The areas around the Geomet sites show climate-related and/or human-related impacts. Data from the sites collected during the 1980's and 1990's, a period of climatic extremes including wet El Niño years and very dry La Niña years, show that biologic components come and go, and return again to various desert habitats, and that this ebb and flow can be natural part of life in some arid and semiarid environments of the United States. However, if a major disruption of the surface occurs during this ebb and flow, such as can be produced by severe eolian events, the ecosystem can be altered forever.

Description of Selected GEOMET Sites

Yuma

annual precipitation at the Yuma site

Annual Precipitation at the Yuma GEOMET site. Click on a red year to see a photo of the site taken during that year.

The Yuma Geomet site is in the most arid region of the United States. The Yuma desert responds dramatically to the fluctuations of the ENSO (El Niño) weather pattern. The El Niño rains, which generally fall in the early winter, produce a luscious spring growth of vegetation that in turn provides food and shelter for burrowing animals, birds, insects, and reptiles.

The El Niño of 1982-83 produced dense vegetation (Figure 1) with grasses measuring a meter high where previously there had only been a barren sand sheet (Figure 2). The creosotebush and white bursage also flourished during this wet period.

Figure 3.

By 1987, during the La Niña (dry) phase of the ENSO weather pattern, the barren sand sheet had returned, the vegetation and inhabitants of the new ecosystem had disappeared, and the sand was moving (Figure 3), destroying all remnants of the vanished grassland.

During the dry period from 1985 through 1991, the drought was so extreme that twenty-five percent of the white bursage (H.B. Musick, pers. comm.) died (Figure 4, Figure 5), while the creosotebush went into dormancy and the big galletta was reduced to stumps of dry, withering bunch grass.

Figure 4. Figure 5.

With the El Niño of 1991-1992, the rains returned, producing increased vegetation cover including the exotic (non-native) grass species, schismus barbatus (Figure 6). This grass, measuring a uniform 15 cm high, covered the sand sheet completely, protecting the surface from sand movement and providing new habitat for the burrowing mammals, owls and golden eagles, snakes and other reptiles. Holes and tunnels for the burrowing animals permeated the area, and the remains of the root system of the dead white bursage were very popular with the new inhabitants.

Figure 6. Figure 7.

Once again, by 1996, the vegetation and animals were gone, the tunnels and holes were collapsing, and the barren sand sheet returned, producing extensive moving sand (Figure 7). By monitoring these ecosystem responses to changes in the weather, models are produced that provide baseline understanding of natural land surface processes as well as models of potential, dramatic changes that can occur in this area with a global climate change.

Jornada

annual precipitation at the Jornada site

Annual Precipitation at the Jornada GEOMET site. Click on a red year to see a photo of the site taken during that year.

Figure 8.
Figure 8
Figure 9.
Figure 9

The Jornada Geomet site is a semiarid rangeland used mainly for cattle grazing. The area was originally covered with grasses, mainly black grama (Figure 8). Cattle grazing began in the 1880's and has been one of the main industries in the area ever since. Over time, new vegetation species such as mesquite, snakeweed, and yucca began to invade the grassland (H.B. Musick, pers.comm.). Mesquite, which is a woody shrub in this area, is a sand-catching plant that builds huge, meters-high coppice dunes by capturing all the moving sand under its branches, creating a large, nutrient- and water-rich dune for its own use (Figure 9). The mesquite changes the dynamics of the moving sand, which discourages grass cover during dry periods. With less grass cover, the moving sand kicks the fine surface material into the air, producing dust clouds. These fine particles often are removed from the area by the wind. With the sand captured by the mesquite and the fine particles of soil removed, the grasses no longer have the necessary soil composition, grain size, or chemistry to regrow after the drought period. Then the mesquite coppice dunes continue to build, creating a totally new and irreversible ecosystem, one that no longer supports the main industry of the area, cattle grazing.


Figure 10.
Figure 10
The Jornada Geomet site is situated in a transition zone between the remaining, endangered grassland and a large, coppice dune field (Figure 10). Long-term monitoring of the site has produced data on the climatic conditions during mesquite invasion and changes in the ecosystem. The 1991-1992 El Niño years produced above normal rainfall in the area, supporting grass growth over a meter high (Figure 11). Nevertheless, the mesquite also used the increased moisture to grow and proliferate. During the dry period of 1992-1996, the grasses struggled to survive while the invading species continued to grow (Figure 12).

Figure 11.
Figure 11
Figure 12.
Figure 12

The mesquite in this transition area have built bigger dunes and have also increased in number. The sand now moves easily around the mesquite dunes because of the decrease in the surface protection provided by the native grasses. Monitoring of this site continues to show the climatological conditions and the resultant vegetation response and sand movement that produce changes in the ecosystem, causing a negative impact to the economy of the entire area.


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