Islands of fertility: Are they an artifact of grazing?

Background/Question/Methods

Desert shrublands and desertified grasslands exhibit a characteristic soil nutrient pattern, with nutrients concentrated under shrub canopies. Theory suggests that desertification is largely irreversible because positive feedback loops perpetuate islands of fertility under shrubs, preventing grass recovery. However, recent work failed to detect islands of fertility inside a long-term grazing exclosure that is experiencing reversal of desertification. To investigate further, we collected soil nitrogen data from two additional long-term livestock exclosures at sites with different vegetation history.  

Results/Conclusions

The fertile island pattern was absent inside both exclosures, while a strong fertile island pattern was present in the grazed areas outside each exclosure. These three examples of a lack of fertile islands prompted us to review the empirical literature on the fertile island pattern to examine its generality. We identified 49 studies covering 93 cases. In 88 cases, fertile islands were detected for at least one of three nutrients (N, C, P).  All cases where livestock grazing was present exhibited fertile island patterns for nitrogen and most did for carbon and phosphorus.  In contrast, fertile islands were less consistent where livestock grazing was absent.  All five cases where livestock have been absent for more than 50 years did not exhibit nitrogen islands under shrubs, suggesting a relationship between fertile islands and grazing history. Our review and empirical data suggest that the fertile island pattern may be an artifact of grazing rather than an inherent property of arid shrublands.  This has significant implications for our understanding of arid systems and grass restoration after desertification.