Dr. Heather Throop is an ecosystem scientist and professor at Arizona State University who studies carbon cycling in dry rangelands. Recently, she presented the ninth webinar in an 11-part series called Grazing Land Management and Soil Carbon. The topic of her talk was “Long-term carbon responses to grazing and brush management in a semi-arid Arizona rangeland”, an ecosystem that Dr. Throop describes as drylands.
What are Drylands?
Drylands are regions that have an aridity index of less than 0.65. They are home to the majority of global livestock and cover more than 40% of Earth’s land surface. Drylands have relatively little rainfall, which affects plant life, but they’re still responsible for 35% of terrestrial photosynthesis. These semi-arid regions also account for 20% of soil carbon.
Overgrazing’s Effects
In southeastern Arizona, the Santa Rita Experimental Range (SRER) provides scientists like Dr. Throop with access to a dryland research environment. SRER was founded in 1902 during a time of growing concern about decreasing grass productivity and, in turn, livestock production. Native grasses are well-suited for the region’s climate, but overgrazing became problematic starting in the 1880s.
Drylands are Dynamic
Because the SRER is over 120 years old, it’s like an outdoor laboratory where scientists can study how grasslands change over time. Dr. Throop shared photographic evidence of what she called “woody encroachment”, mainly by mesquite trees but also by shrubs and cacti. There were once brush removal efforts to increase the grasslands’ size, but aerial herbicides are no longer applied.
Carbon Above and Below Ground
In her presentation, Dr. Throop explained how woody encroachment has affected carbon levels both above ground and below ground over time.
- Above ground: Woody encroachment has increased the amount of carbon in the visible parts of trees, shrubs, and cacti. There’s also a positive correlation between the amount of above ground carbon and the amount of precipitation.
- Below ground: The relationship between woody encroachment and below ground carbon (soil carbon) is less clear. With shrubs especially, there’s no clear relationship between mean annual precipitation (MAP) and percentage change in soil organic carbon (% SOC).
As Dr. Throop’s research has demonstrated, however, larger shrubs do store larger amounts of soil carbon. In addition, more soil carbon is stored toward the bole of the shrub than at the dripline.
Grazing and Carbon Levels
Finally, Dr. Throop considered whether soil carbon patterns are affected by dryland grazing. Her research indicates that 100+ years of grazing at low stocking rates have had little effect. Whether a dryland area was grazed or protected, soil carbon levels were very similar regardless of changes in canopy position or basal diameter. These are key parameters for woody encroachment.
