Friction threshold velocities (FTVs) were determined for biological soil crusts in dierent stages of recovery. Particles on the surface of crusts that had been relatively undisturbed for at least 20 years were found to have signi®cantly higher FTVs than those that had been disturbed 5, 10 or 1 years previously (376, 87, and 46 cm secÿ1, respectively). FTV's for crust breakage was also much higher for undisturbed crusts when compared to the previously disturbed crusts (573, 148, and 88 cm secÿ1, respectively). All crusted surfaces were more stable than bare sand, which had an FTV of 16 cm secÿ1. Disturbance treatments were then applied to the three crustal classes. Disturbance signi®cantly reduced the FTVs of all classes by 73±92 per cent. Comparing crustal FTVs with mean and high monthly wind speeds found in this region, it was observed that only crusts that had been undisturbed for approximately 20 years or more were able to protect soil surfaces from wind gusts expected on the average of once a month. Other crustal classes, as well as all disturbance treatments, had FTVs lower or equal to that of commonly occurring winds in this region. Because most of the crustal biomass occurs in the top 0.3 mm of soils, even slight soil loss can negatively in¯uence stability and nutrient inputs to this ecosystem.
