Discussion

Despite differences in degree of desertification, current vegetation and time since livestock removal, the results presented here indicate a pervasive effect of livestock removal on arid grassland soil properties across a set of sites in Arizona: water infiltration rate increases with time since livestock removal. There is roughly a 1.7% increase in relative water infiltration rate across a grazing fence each year following livestock removal over the span of several decades.

The data presented in this chapter both reinforce and extend the findings of Castellano and Valone [2007]. They reinforce those findings because the pattern reported here, increased relative water infiltration rate with time since livestock removal, is the same. More importantly, these findings extend Castellano and Valone [2007] in two ways. First, the sites here exhibited a wide range of current vegetation (degree of desertification) as evidenced by dramatic differences in the dominance of perennial grasses and shrubs (Table 1). Thus, the severe desertification of the San Simon valley per se is not an artifact that produced the patterns reported by Castellano and Valone [2007]. In addition, all data reported here were collected during the summer dry season, prior to the onset of monsoon rains and so soils were dry when infiltration rate data were collected. Castellano and Valone [2007] collected data when soils were wet. Soil moisture can strongly affect water infiltration rate [Gifford et al. 1977; Mapfumo et al. 1999]. To minimize this potential concern, all cross-fence data were collected on the same day within a narrow window of time. While measurements of water infiltration rate likely will differ at different times of the year as soil moisture varies, I do not expect the general pattern to change: infiltration rates should be higher inside compared to outside the grazing fences. Additional work, conducted at the same site but at different times of the year when soil is both wet and dry is necessary to test this assertion.

While I did not measure soil compaction at these five sites, I assume that differences in soil compaction is the mechanism that explains the differences in water infiltration rate observed across the five grazing fences studied. Castellano and Valone [2007] demonstrated that differences in soil compaction related directly to water infiltration rates (the higher the compaction, the lower the water infiltration rate). In addition, a large literature has demonstrated that the presence of livestock increases soil compaction and that soil compaction strongly and negatively affects water infiltration rate [e.g., McGinty et al. 1979; McCalla et al. 1984; Warren et al. 1986; Abdel-Magid et al. 1987].

Water infiltration rates can be measured in many ways [Brady and Weil 1999]. One advantage of the double ring infiltrometer method used here is that it eliminates the potential influence of canopy cover. Previous work has shown that canopy cover, via the interception of precipitation, can strongly affect water infiltration [e.g., Thurow et al. 1988]. Such results have had a strong influence on the development of desertification models [e.g., Rietkerk et al. 2002; van de Koppel et al. 2002; van de Koppel and Rietkerk 2004] that focus on the effects of vegetation canopy cover on water infiltration rate. The double ring infiltrometer method eliminates this effect of vegetation canopies and thus more directly examines how soil properties affect infiltration. In addition, while not a concern at these sites, the double ring infiltrometer method is particularly useful if vegetation differs strongly across a grazing fence.

Current models of desertification assume that the effect of livestock on soil compaction and water infiltration rate are minor relative to the importance of vegetation canopies in affecting water infiltration rate. While additional work is required to better understand the relative importance of these two factors, the data presented here suggest that the presence or absence of livestock can strongly affect water infiltration rate. It seems reasonable that to more fully understand the dynamics of vegetation in arid grassland systems, we will need to include effects of livestock on water infiltration rates via their effects on soil compaction.

Finally, the work here suggests that desertified systems are not necessarily stably desertified because water infiltration rate can increase substantially following the removal of livestock. If water infiltration rate plays a key role in perennial grass establishment, restoration of even highly desertified sites may be possible, given sufficient time for the recovery of soil properties such as release from compaction. I hope that at the very least, the work presented in this chapter will help to stimulate additional theoretical and empirical work to provide a better understanding of desertification, its reversibility, and the restoration of arid grasslands.

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