Article: Livestock, soil compaction, and water infiltration rate: Evaluating a potential desertification recovery mechanism
Authors: Castellano and Valone
Published: Journal of Arid Environments 2007
Much debate and controversy has surrounded the use of rest as a tool to restore degraded environments. In this context, rest usually means the exclusion of grazing animals from the land for extended periods. Traditionally, many people in the Holistic Management community have argued that rest will consistently and predictably exacerbate desertification and biodiversity loss, specifically and most acutely in brittle environments.
This claim has primarily been made based on direct observation and assumptions about the behavior of biological communities in response to management decisions. However, new research sheds some light on this debate in ways that may run counter to some of these assumptions. As the paper under consideration notes:
“Recently, there have been three independent reports of significant increases in perennial grass and vegetation cover (recovery from desertification) at desertified sites. All three cases have occurred following reductions in livestock grazing. Two of these reports are from southwestern North America. In southwest Texas, after approximately 25 years of livestock exclosure, Fuhlendorf et al. (2001) reported significant increases in perennial midgrasses. At a more arid site in southeastern Arizona, 39 years of livestock exclosure resulted in a 400% increase in perennial grasses (Valone et al., 2002). A third report, from the African Sahel, observed that after a 20 year reduction of human impacts, including livestock grazing, desertified sites experienced vegetation recovery (Rasmussen et al., 2001). Existing desertification models cannot account for these changes in vegetation or their timescale.”
Those who have made the case that livestock are often responsible for the continued deterioration and desertification of rangelands have focused on overgrazing and loss of soil cover as the primary mechanisms driving this process. On the other side of the debate, many HM practitioners have argued that over-rested perennial grasses and soil capping for lack of animal impact are the principal culprits. The authors of this paper suggest that soil compaction as a result of grazing may play a significant role, thus their conclusion that existing models cannot account for measured improvements in range quality.
The authors provide compelling empirical evidence to support their arguments. In southeast Arizona, three livestock exclosures were established in 1958, 1977, and 1993 “so that basic ecological studies could be conducted in the absence of livestock grazing.” The recent natural history of this site mirrors that of much of the West: desertification and shrub invasion began in the early 20th century after 50 to 100 years of severe overgrazing and drought.
Changes in vegetation were measured between the three exclosure sites and the surrounding landscape: “Perennial grasses, primarily Bouteloua eripoda (black gramma), are now significantly more abundant inside versus outside of the 1958 site but recovery required more than 25 years. Additionally, percent canopy shrub cover has significantly decreased in comparison to outside of the site.” Comparisons with the 1977 and 1993 sites showed no change in vegetation.
To test the hypothesis that “there is a significant relationship between livestock removal and water infiltration rate, mediated by changes in soil compaction”, the authors measure water infiltration and soil compaction “along cross-fence pairs of 60-100 m transects…established perpendicular to the exclosure fences on visually matched areas of topography, soil and vegetation.”
After statistical analysis of collected data, water infiltration rates within the exclosures increased with time and soil compaction decreased with time. “Infiltration rate was 98% higher inside compared to outside the 1958 exclosure, whereas it was only 51% and 11% higher inside the 1977 and 1993 exclosures, respectively.” The authors continue: “…the soil was approximately 84% more compact outside compared to inside the 1958 site but differed by only 39% and 27% across the fences at the 1977 and 1993 sites, respectively.”
Depth measurements indicate that soil compaction improves from the top down, and improvements in water infiltration occur after reductions in soil compaction. The authors postulate that wet-dry and freeze thaw cycles, along with biotic activity, are primarily responsible for reduction in soil compaction. In arid systems these processes are slow and barely perceptible on a human time-scale, hence the 25 year time period necessary to see any measurable changes.
The use of rest to reverse desertification and improve range quality is a scientifically interesting concept, but considering the long time periods require, the practical application of this principal in a management context is dubious. Most land managers need to improve biodiversity and range quality in periods that are measured in years, not decades.
The management implications of this research are worthwhile. Continued experimentation with grazing animals is required to further uncover strategies to accelerate the biological processes associated with land restoration. The soil compaction associated with animal impact can reduce water infiltration, therefore other steps should be considered to mitigate against these potentially negative consequences. Grazing planning must achieve ever greater levels of complexity, allowing for the consideration of factors like soil moisture, texture, and capping when considering the use of animal impact as a restorative tool. Decision-support models and GIS will have a role to play as we move towards the use of more sophisticated technological tools to facilitate the decision-making process.
In National Parks and other public lands, total rest may indeed be a useful tool for restoration in some circumstances, especially soils that are particularly vulnerable to soil compaction. These are areas where time-scale is less of a factor and experimentation with different tools can be attempted and monitored. Other exclosures, like the Drake exclosure in central Arizona, have shown results contrary to the results in this paper. Therefore, the effect of total rest on ecological processes may be very site specific.