Shrubs have invaded extensive areas of grassland in the southwestern United States. The zones of nutri- ent-rich soil found beneath plant canopies, referred to as “islands of fertility,” are more intense and spaced farther apart in shrubland than in grassland. This dif- ference in the spatial pattern of soil nutrients may reinforce shrub invasion. Changes in water availabil- ity in the soil could also influence shrub invasion. Here we compare the spatial patterns of infiltration, defined as the total equivalent water depth entering the soil following individual rainfall events or summed over many events, at adjacent grass- and shrub-dominated sites in the Sevilleta National Wild- life Refuge. We use two infiltration data sets. First, following four rainfall events, we measured soil mois- ture and wetting front depth at 10-cm intervals along 24-m transects. We estimate infiltration from these data. Second, we use vertical arrays of soil moisture probes to compare infiltration between adjacent can- opies and interspaces following 31 storms. In both the grassland and shrubland, infiltration is typically greater beneath plant canopies than beneath in- terspaces. Canopies are oases where soil moisture is higher than in the surrounding areas. However, infiltration is not greater beneath canopies when surface runoff is limited. In the shrubland, the canopy interspace infiltration ratio increases as storm size, and therefore runoff, increases. This relationship also exists in the grassland, but it is not as strong or clear. The magnitude of spatial variability of infiltration is similar in shrubland and grassland. In addition, the distance over which infiltration is correlated is approximately 50 cm in both environments. Most of the spatial variability exists between the stem and canopy margin in the shrubland and straddling the canopy margin in the grassland. The most notable difference is that sub- canopy oases are spread farther apart in the shrubland because canopies are separated by larger interspaces in this environment.
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