The Influence of Spatial Patterns of Soil Moisture on the Grass and Shrub Responses to a Summer Rainstorm in a Chihuahuan Desert Ecotone

Abstract

The cycling of surface water, energy, nutrients, and carbon is different between semiarid grassland and shrubland ecosystems. Although differences are evident when grasslands are compared to shrublands, the processes that contribute to this transition are more challenging to document. We evaluate how surface redistribution of precipitation and plant responses to the resulting infiltration patterns could contribute to the changes that occur during the transition from grassland to shrubland. We measured soil water potential under grasses (Bouteloua eriopoda), shrubs (Larrea tridentata) and bare soil and changes in plant water relations and gas exchange following a 15 mm summer storm in the grasslandshrubland ecotone at the Sevilleta National Wildlife Refuge in central New Mexico USA. Following the storm, soil water potential (W s ) increased to 30 cm depth beneath both grass and shrub canopies, with the greatest change observed in the top 15 cm of the soil. The increase in W s was greater beneath grass canopies than beneath shrub canopies. W s under bare soil increased only to 5 cm depth. The substantial redistribution of rainfall and different rooting depths of the vegetation resulted in high W s throughout most of the rooting volume of the grasses whereas soil moisture was unchanged throughout a large portion of the shrub rooting volume. Consistent with this pattern, predawn water potential (W PD ) of grasses increased more than 5 MPa to greater than -1 MPa whereas W PD of shrubs increased to -2.5 MPa, a change of less than 2 MPa. Transpiration increased roughly linearly with W PD in both grasses and shrubs. In grasses, assimilation was strongly correlated with W PD whereas there was no relationship in shrubs where assimilation showed no significant response to the pulse of soil moisture following the storm. These data show that preferential redistribution of water to grass canopies enhances transpiration and assimilation by grasses following large summer storms. This process may inhibit shrubland expansion at the ecotone during periods without extreme drought.

Long-Term Dynamics and Hotspots of Change in a Desert Grassland Plant Community

“…The remaining precipitation comes as a mixture of snow and rain during winter and spring. Thunderstorms during the monsoon season provide intense but highly localized rainfall (Gosz et al 1995;Pockman and Small 2010;Petrie et al 2014), and monthly and daily rainfall extremes typically occur during this period. Although highly variable, there was no general trend in annual or monsoon precipitation during the 20 years of this study ( fig.…”

Soil enzyme responses to varying rainfall regimes in Chihuahuan Desert soils

“…These systems are characterized by patches of vegetation interspersed with unvegetated soils often occupied by biological soil crusts. Soil nutrients, organic matter, and soil moisture are generally greater (Schlesinger et al 1990, Kieft et al 1998, McCrackin et al 2008) under plant canopies than beneath adjacent unvegetated soil patches (Bhark andSmall 2003, Pockman andSmall 2010). Assuming that soil resources, along with root exudates, influence the rate of microbial processes (e.g., Vargas et al 2012, enzymatic activities are likely to be higher in soils beneath plant canopies than beneath unvegetated patches.…”

Positive feedback between microclimate and shrub encroachment in the northern Chihuahuan desert

“…1) by increasing bare soil area Musick 1990, Huenneke et al 2002). The mechanism behind the change in microclimate is revealed by differences in vegetative cover with 50-60% cover in grass-dominated vegetation, roughly 25% cover in shrub-dominated areas, and large patches of bare soil around shrubs in the ecotone between grassland and shrubland (Bhark andSmall 2003, Pockman andSmall 2010). During the day, the large areas of bare soil in shrubland absorb greater amounts of solar radiation, which is transformed into thermal energy and stored in the soil.…”