The dual role of soil crusts in desertification

Abstract

Vegetation cover in dry regions is a key variable in determining desertification. Soils exposed to rainfall by desertification can form physical crusts that reduce infiltration, exacerbating water stress on the remaining vegetation. Paradoxically, field studies show that crust removal is associated with plant mortality in desert systems, while artificial biological crusts can improve plant regeneration. Here it is shown how physical crusts can act as either drivers of or buffers against desertification depending on their environmental context. The behavior of crusts is first explored using a simplified theory for water movement on a uniform, partly vegetated slope subject to stationary hydrologic conditions. Numerical model runs supplemented with field data from a semiarid Long-Term Ecological Research site are then applied to represent more realistic environmental conditions. When vegetation cover is significant, crusts can drive desertification, but this process is potentially self-limiting. For low vegetation cover, crusts mitigate against desertification by providing water subsidy to plant communities through a runoff-runon mechanism.

Runoff from biocrust: A vital resource for vegetation performance on Mediterranean steppes

“…Physical soil crust formation can mitigate the negative impact of biocrust disturbance on downstream vegetation performance by providing similar or even larger amounts of water to downstream vegetation, as observed by Cortina et al () after two years of biocrust manipulation. However, this effect strongly depends on the interplay of runoff with local microtopography and slope gradient and the efficiency of downstream vegetation in intercepting and retaining runoff (Assouline et al, ; Chen et al, ). Thus, on longer time scales, and especially in highly erodible substrates, such as those in the Tabernas badlands (Cantón et al, ; Canton et al, 2014), high denudation rates in bare areas not covered by biocrust (Lázaro et al, ) and the large amount of sediment expected to be deposited under vegetation canopies under these conditions may promote the formation of microtopographical mounds below the plant canopy (Bedford & Small, ; Bochet, Poesen, & Rubio, ; Rossi & Ares, ), partially obstructing water and nutrient fluxes into vegetation.…”

“…Their improved soil properties combined with the direct effect of plant roots and stems make water infiltration and storage better than in open spaces (Thompson, Harman, Troch, Brooks, & Sivapalan, ). This hydrological dichotomy between open and vegetated patches controls the location of vegetation within the landscape and leads to a two‐phase system in which, during effective rainfall events, runoff is mainly generated on very often crusted open areas (sources) and redistributed towards vegetation patches (sinks) where it infiltrates (Assouline et al, ; Chen, Sela, Svoray, & Assouline, ; Ludwig et al, ; Mayor, Bautista, Small, Dixon, & Bellot, ; Puigdefabregas, Sole, Gutierrez, Del Barrio, & Boer, ; Rodríguez‐Caballero, Cantón, Lazaro, & Solé‐Benet, ; Thompson et al, ). Source‐sink interaction maximizes resource availability for plants and is critical for system functioning on a larger scale, reducing water, sediment and nutrient losses from runoff (Chen et al, ; Chen, Sela, Svoray, & Assouline, ; Magliano, Breshears, Fernandez, & Jobbagy, ) and increasing productivity (Ludwig et al, ).…”

Hydrologic refugia, plants, and climate change

“…Runoffrunon mechanisms can enhance infiltration volumes by a factor of eight times compared with rainfall (Galle et al, 1999;Niemeyer et al, 2014). These mechanisms are often essential for the maintenance of vegetation in otherwise arid regions (Thompson et al, 2010b;Assouline et al, 2015). They are often biologically mediated, primarily through the formation of biological soil crusts (Belnap, 2006).…”

Infiltration from the Pedon to Global Grid Scales: An Overview and Outlook for Land Surface Modeling

“…Moreover, re‐infiltration, called run‐on, is ignored in most LSMs; runoff production that occurs at sites where the infiltration capacity is exceeded may re‐infiltrate in the grid cell due to soil and land surface heterogeneity so that not all of the runoff that is generated at a grid cell needs to be routed out of the cell or to a receiving water body. A classic example is the run‐on in vegetation patches or bands (strips) in semiarid regions (Assouline et al, 2015). Roots can increase the local infiltration capacity so that runoff from sealed, unvegetated areas can infiltrate in vegetated areas (Nimmo et al, 2009).…”