UA News

The intricate veil of molecules that is Earth's atmosphere has been stained over the years with a variety of man-made substances. Elevated levels of carbon dioxide and other greenhouse gases associated with fossil fuel use and industrialization are thought to be to blame.

Many scientists believe these and factors such as the deforestation of tropical regions, may contribute to global warming and an increase in the frequency and magnitude of extreme climatic events. Range managers, meanwhile, have seen grasslands become overgrown with trees and shrubs that constrict water flow, alter soils and ultimately change how the land can be used.

Investigating how vegetation changes may have affected the accumulation and processing of carbon and nitrogen on southwestern rangelands has been UA plant ecologist Steve Archer's passion for more than two decades.

Mesquite (Prosopis velutin) now dominates many of today's rangelands. It has expanded from its historical ranges ("enclave" refers to human occupation) in bosque, riparian and arroyo ecosystems into higher elevations over the past century.

The thorny mesquite often grows in dense thickets and, depending on local soils, can reach heights of 10-20 feet with trunks 12-24 inches in diameter. Mesquite and woody shrubs such as creosote alter wildlife habitat and compete with native vegetation for water and nutrients, changing grasslands forever. One reason for the spread of mesquite is that many animals savor its sugary seed pods. Pods consumed by cattle, along with range grasses and forbs, are deposited (complete with a dollop of fertilizer) wherever the animals roam. Scientists also wonder if the spread of mesquite and other woody plants might actually increase the absorption of atmospheric carbon dioxide.

Compared to grasses, woody plants typically have much greater leaf areas, and lose more water to transpiration. They also have deeper and more extensive root systems able to tap sources of water that might otherwise go to groundwater recharge and stream flow.

"There has been a long-standing interest in how mesquite affects livestock production, wildlife habitat and water availability," Archer says. "We know much less about how it affects the manner in which ecosystems process carbon and nitrogen, two important elements linked to climate change. We're interested in understanding how carbon and nitrogen stocks in ecosystems change as you shift the composition from grassland to shrubland."

The Santa Rita Experimental Range (SRER) southeast of Tucson provides an extraordinary outdoor laboratory for studying this topic. Land use patterns have been well documented at the UA research facility for more than 100 years. Research there has measured plant introduction and reseeding, invasive weed control, livestock/wildlife interactions, insect studies and the effect of livestock grazing.

For example, there are large plots of land that have been protected from livestock grazing; other plots where grazing took place for years and then was curtailed; and still others with continuous or rotational grazing. This rich array of contrasts allows Archer and his colleagues to evaluate how real-world land uses might affect the encroachment of woody plant and their impact on the carbon and nitrogen cycles. The elevation on SRER's 51,000 acres (nearly 80 square miles) ranges from 2,900 to 5,200 feet, with varying soil types and rainfall amounts. That allows the results from the research to be broadly extrapolated to other regions.

During the first year of the research project Archer and his research team are measuring the carbon and nitrogen content of mesquite and other rangeland plants. They are taking plant and soil samples from pastures that vary in both elevation and management history. They expect these measurements provide a more complete record of how carbon and nitrogen pools vary as vegetation changes in response to climate or land management.

Archer previously researched two areas of Texas - the temperate northern part of the state and the subtropical zone near the Gulf of Mexico. Combining those data with the SRER project will allow Archer and other researchers to make regional assessments and determine if brush encroachment has similar effects in different regions of the Southwest.

Carbon and nitrogen are contained in plant biomass and in soils. Biomass is the total weight of plants above and below the ground: leaves, stems, trunks and roots. Soils contain carbon and nitrogen that is derived from the death and decomposition of plant leaves, stems and roots. Working early in the day during the warm growing season, field crews collect "biomass" by clipping and weighing low-growing vegetation such as shrubs and groundcover. Since the mesquite production of an entire ecosystem is too large to cut and weigh, scientists estimate using equations that relate plant height, basal stem diameter and canopy dimensions to biomass.

At the laboratory, plant and soil samples are ground into fine particles by a series of mechanical grinders. An elemental analyzer then measures the concentration of carbon and nitrogen in the particles. The levels of carbon and nitrogen in the plants and soils of the ecosystem are calculated by multiplying the biomass values by concentration values. By measuring different pastures, Archer can assess the impact of land use and vegetation change on carbon and nitrogen abundance.

New funding from the USDA's Managed Ecosystems Program makes possible more detailed and extensive sampling, including the use of dynamic simulation models. These models refine estimates of how the carbon and nitrogen mass in plants and soils have changed over time as a result of woody plant encroachment and brush management practices aimed at reducing it. Archer also is collaborating with scientists at the University of Colorado and the Carnegie Institute at Stanford University on a NASA project at the SRER.

"We are currently working on approaches that will allow us to use aerial photography and satellite imagery to make large-scale assessments of the carbon and nitrogen content of soils and vegetation," Archer says.

Atmospheric nitrogen occurs naturally when organic matter decays. It also adds to greenhouse gases and to trophospheric ozone. Mesquites are legumes, which use bacteria to "fix," or convert, atmospheric nitrogen to forms of nitrogen necessary for plant metabolism and growth.

"Changes in vegetation from non-nitrogen fixing species such as grasses to nitrogen-fixing species such as mesquite might change fundamental ecosystem properties with regard to nitrogen availability," Archer says. "Both ecosystem productivity and greenhouse gas concentrations are affected."

Archer's research ultimately will lead to some answers about how much carbon gets sequestered, or tied up, and for how long, in various vegetative types and under various land management practices. This is important for carbon credit programs being considered in some parts of the United States. Such programs compensate landowners and land managers for promoting vegetation growth or restoring degraded range and pasture land, to increase carbon sequestration in the ground.

"This might be a win-win situation because it could be a source of capital for range improvement practices that have many spin-off benefits, such as increasing soil organic matter and improving water-holding capacity and fertility," Archer says.

The biodiversity of grassland ecosystems, the capacity for the soil to retain water and the ability of the land to sustain wildlife and livestock are some of the issues at stake. "With woody plant encroachment, we're losing the plants and animals indigenous to our grasslands," Archer says. "That's one downside of woody plant encroachment that offsets the possible benefits of carbon sequestration."