Soil moisture correlates with shrub–grass association in the Chihuahuan Desert

The current ongoing expansion of shrubs at the expense of grasses in the Chihuahuan Desert is largely believed to result from a shift in competition between shrubs and grasses. Both groups, shrubs and grasses, occupy a gently sloping silty (loessial) soil within the Jornada Experimental Range (JER), northern Chihuahuan Desert, NM, USA. At the sites dominated by tarbush (Flourensia cernua), the shrubs seem to be randomly distributed in between barren surfaces, surfaces covered by biocrusts, and dense stands of burrograss (Scleropogon brevifolius), which are mainly confined to 2–4 m diameter and 0.02–0.1 m deep depressions. Often, the shrubs are accompanied by a dense stand of tobosa grass (Pleuraphis mutica) that occupies the under-canopy. In order to study shrub–grass interactions, four habitats were defined: playa habitats with bare silty material (PL), habitats with mature biocrust (CR), habitats with burrograss mainly confined to depressions (DP) and tarbush shrubs with a dense population of tobosa grass under the shrub canopy (UC). Periodic moisture measurements at the 0–30 cm soil and potential evaporation took place during the summers of 1999 and 2009. In addition, the grass biomass in these habitats was monitored. Moisture content and grass biomass followed the pattern PL < CR < DP < UC, while the evaporation rate followed the pattern PL ≈ CR > DP > UC. The findings point to a close link between grass biomass and the soil moisture content, with the high grass biomass at DP explained by runoff contribution, while that of UC by the higher moisture content mainly attributed to the reduced evaporation under shading conditions. The co-existence of shrubs and under-canopy grass does not support the notion of high competition between both groups of plants. It may rather indicate facilitation. The close link between soil moisture content and the grass habitats also suggests that lower water availability during summertime may explain current grass scarcity in JER.

► Shrub–grass interaction is a long ongoing enigma in the southwestern US.
► Higher moisture content during summer was found in grass habitats.
► Grass biomass either benefitted from runoff or from shading.
► In our site, shrub–grass relationship can be seen as facilitation rather than competition.
► Low moisture content in summer may explain the decrease in grass cover.