Is nonrainfall water a likely water source for arid and semiarid biocrusts? A critical perspective

It is commonly believed that nonrainfall water (NRW) serves as an important and even a central source of water for biocrusts in arid and semiarid regions. Known also as biological soil crusts, they mainly consist of cyanobacteria (that dwell within the upper 2 mm of the soil) and crustose chlorolichens that commonly protrude 1–2 mm above ground. It is commonly assumed that due to the prolonged dry season in deserts, a constant source of water is required for their survival. Overestimated, nonrealistic NRW values, whether due to the use of improper measurement devices or erroneous interpretation of the recorded values as well as instances during which distillation was regarded as dewfall, supported the notion that the amounts of NRW are sufficiently high to allow for biocrust activation. In the current commentary, a short account of the reasons that led scholars to believe that NRW may be utilized by biocrusts is presented, and the findings that led to this belief are critically analysed. It is argued that the amounts of NRW reaching the surface are below the critical thresholds of 0.05 and 0.1 mm required for net photosynthesis by the chlorolichens and cyanobacteria, respectively. Moreover, it is argued that even if these thresholds are seldom reached, their duration is too short to result in net carbon gain. As shown for the dewy Negev and may be applicable for other arid and semiarid regions, biocrusts rely on rainwater for their growth and survival.