Contrasting effects of native and exotic vegetation on soil infiltrability in the Sonoran Desert

Invasion by exotic grasses is transforming drylands across the planet, but the ecohydrological feedbacks of such invasions are not fully understood. For example, in the Sonoran Desert, previous studies have shown that buffelgrass (Cenchrus ciliaris) alters the spatial patterns of soil moisture, leading researchers to hypothesize that such alterations are related to the plants' effects on soil infiltrability. To evaluate this hypothesis, we compared field-saturated hydraulic conductivity (K_fs) in a native shrubland with that in a neighboring savanna extensively dominated by exotic buffelgrass. We measured K_fs during the dormant and growing seasons in both canopy and intercanopy zones. We found that K_fs was generally lower during the dormant season than during the growing season. There were no significant differences between sites during the dormant season, and at both sites, K_fs was 6-7 times higher under shrubs than in the intercanopies. During the growing season, K_fs for the exotic intercanopy was comparable to that for shrub cluster edges (140 mm h^-1) and was more than twice that for the native intercanopy. Both shrubs and buffelgrass improved K_fs by reducing soil bulk density (thus increasing porosity). Additionally, surface roughness in the exotic intercanopy was nearly 3 times higher than in the native intercanopy. The combination of greater surface roughness and higher infiltration rates during the growing season most likely alters hydrological connectivity in savannas invaded by exotic grasses such as buffelgrass. By capturing portions of the runoff generated in the intercanopy, these grasses reduce runon into shrub patches, with potentially substantial impacts on native vegetation dynamics and stability.