The contrasting effect of biocrusts on shallow-rooted perennial plants (hemicryptophytes): Increasing mortality (through evaporation) or survival (through runoff)

The effect of biocrusts on plants is ambiguous. At the Nizzana research site (Negev Desert, Israel), frequent droughts in the last 2 decades resulted in high shrub mortality, which has been explained by the negative effect of the low-albedo crusts on soil water evaporation. Nevertheless, shallow-rooted perennial plants, such as Stipagrostis plumosa and Cyperus sp., exhibit relatively little mortality. We hypothesized that these hemicryptophytes (HCs) grow at or adjacent to small 2- to 4-m² depressions (termed ID_DEP) in the interdune (ID), which are scattered within the undulating terrain (termed ID_INC) of ID, and as previously found, serve as sink for short-distance runoff from ID_INC to ID_DEP. To test this, we randomly demarcated 50 5 × 5 m plots in the ID and semistable uncrusted dune crest (CR), and the cover of live HCs in the three habitats CR, ID_INC, and ID_DEP were monitored. In addition, the available water content (AWC) for the 0- to 80-cm depth was measured during 2012–2014. Although AWC exhibited a clear pattern with ID_DEP > CR > ID_INC, HCs were almost absent from ID_INC (above ground biomass of 0.3 kg/ha) but relatively high in ID_DEP (55.3 kg/ha) and CR (113.0 kg/ha). The high biomass at CR and ID_DEP is explained by higher AWC due to lower evaporation or runoff addition, respectively. The findings highlight the contrasting role played by the biocrust: Although responsible for the mortality of most perennials at ID_INC due to increased evaporation, biocrusts assist the survival of relatively shallow-rooted perennials at ID_DEP through runoff generation.