Distillation as a meaningful water source for lithic lichens: the Negev case

Main conclusion: In the Negev, substantial vapor stems from the wet soil following rain events and therefore cannot be considered as dew but rather as distillation. Distillation provided ~ 35% and ~ 60% of the vapor-driven liquid for the cobbles and rock slabs, respectively, implying that lithobionts may benefit from vapor condensation also in non-dewy deserts. Abstract: Lithic chlorolichens (lichens with green algae as photobionts) and cyanobacteria cover almost all rock surfaces in the Negev Highlands, where chlorolichens are believed to mainly benefit from non-rainfall water (NRW), i.e., dew and vapor at high relative humidity. Since chlorolichens may also inhabit non-dewy deserts and vapor may also stem from the wet soil (which once condenses is termed distillation), we hypothesized that vapor that stems from the wet soil may also benefit lithic chlorolichens. To evaluate the potential amount accumulated on these rocky surfaces, whether by NRW or soil vapor plus distillation (jointly termed as indirect rain water, IRW), 3-year-long measurements were conducted in the Negev using cloths attached to a pair of rock slabs and a pair of cobbles. Taking 0.05 (reflecting vapor adsorption) and 0.1 mm (reflecting vapor condensation), which allows for net photosynthesis by chlorolichens and cyanobacteria, respectively, we found that: (1) the average number of days with NRW and IRW ≥ 0.05 mm was respectively 128.7 days and 28.0 days (for cobbles) and 37.3 days and 19.3 days (for rock slabs), with dew (which occurs along the year) and distillation (limited to days after rain events) occurring respectively for 36.7 days and 20.0 days (cobbles) and 28.0 days and 6.0 days (rock slabs), (2) average annual amounts of NRW and IRW ≥ 0.05 mm were respectively 11.5 mm and 3.9 mm (for cobbles) and 2.7 mm and 1.8 mm (for rock slabs), with dew and distillation being respectively 4.7 mm and 3.1 mm (for cobbles) and 0.5 mm and 0.9 mm (for rock slabs), (3) average annual daytime duration of > 0.05 mm for NRW and IRW were respectively 307.8 h and 83.9 h (for cobbles) and 81.0 h and 46.7 h (for rock slabs) with dew and distillation lasting respectively 103.8 h and 60.2 h (for cobbles) and 10.3 h and 17.6 h (for rock slabs). Given that daylight duration primarily dictates growth, we may conclude that: (1) cobbles receive substantially higher amounts of NRW and IRW than rock slabs, (2) the amount of distillation received on cobbles (3.1 mm) was not substantially lower than that of dew (4.7 mm). As far as the annual daylight wetness duration for cobble-dwelling lichens is concerned, distillation provided 36.7% of the total duration provided by vapor. Since IRW may occur also in dewless deserts, such as the Mojave Desert, it may partially explain lithic lichen inhabitation in the Mojave and other non-dewy deserts.