Effects of subsurface flow on dunefields: Doubling the amount of soil moisture and substantially increasing the biomass of annual plants

Abstract

Plants in deserts exhibit heterogeneous distribution, which may stem from water redistribution following runoff. However, heterogeneous plant distribution is also common in dunefields regardless of runoff. In the Hallamish dune field in the hyper-arid Negev Desert, dense vegetation belts are noted in loci where an abrupt change in slope angles of uncrusted slopes (interface, IF) occurs, attributed to subsurface flow (SSF). While SSF was previously noted in dunefields, quantification of SSF is problematic, especially in arid regions where precipitation is low, evaporation is high, and the amount of SSF is not sufficiently large to facilitate direct measurement (in water volume). Here indirect measurements of SSF are reported based on near weekly measurements of available water content (AWC) during 2012/13, 2013/14 and 2015/16 at five slope positions along two dune catenas and the adjacent interdunes: flat semi-stable top (TOPs), flat crusted interdune (IDc), semi-stable (NFs) and crusted (NFc) north-facing slope, and IF. In addition, the hydrological rain and monthly surface stability were also measured, and additionally plant characteristics (cover, number of species, density, and biomass) were used as indicators for AWC. Higher AWC characterized the uncrusted slope positions (especially at IF) in comparison with crusted slope positions of similar slope angles. Although receiving the lowest amount of rain, AWC at IF was ∼ 2-fold higher than at most other slope positions, resulting in ∼ 3- to 5-fold higher cover and biomass, serving therefore as 'islands of fertility'. As verified by the Generalized Linear Regression Model, plant performance was mainly dictated by AWC and to a lesser degree by surface stability. Our findings may have broader implications for water redistribution in other dunefields, where a unique distribution of water has at times been attributed to factors such as the effect of biocrusts on infiltration rather than to SSF.