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-m(2) depressions (termed IDDEP) in the interdune (ID), which are scattered within the undulating terrain (termed IDINC) of ID, and as previously found, serve as sink for short-distance runoff from IDINC to IDDEP. To test this, we randomly demarcated 50 5x5 m plots in the ID and semistable uncrusted dune crest (CR), and the cover of live HCs in the three habitats CR, IDINC, and IDDEP 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 IDDEP>CR>IDINC, HCs were almost absent from IDINC (above ground biomass of 0.3kg/ha) but relatively high in IDDEP (55.3kg/ha) and CR (113.0kg/ha). The high biomass at CR and IDDEP 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 IDINC due to increased evaporation, biocrusts assist the survival of relatively shallow-rooted perennials at IDDEP through runoff generation.