Comment on ‘Kidron, G. J. (2018). Biocrust research: A critical view on eight common hydrological‐related paradigms and dubious theses. Ecohydrology, e2061’

Faculty of Organic Agricultural Sciences, Department of Soil Science, University of Kassel, Witzenhausen, Germany Agronomy Department, University of Almeria, Almeria, Spain Centro de Investigación de Colecciones Científicas de la Universidad de Almería (CECOUAL), Almeria, Spain Department of Food, Environmental and Nutritional Sciences (DeFENS), Milan, Italy Department of Environmental Sciences, Informatics and Statistics (DAIS), Cà Foscari University of Venice, Mestre, Italy Department of Ecology, Swedish University of Agricultural Sciences, Uppsala, Sweden Department of Agriculture, Food, Environment and Forestry, University of Florence, Florence, Italy U.S. Geological Survey, Southwest Biological Science Center, Moab, UT, USA School of Biological, Earth and Environmental Sciences, University of NSW, Sydney, NSW, Australia


| PARADIGM 2: PLANT ESTABLISHMENT IS NECESSARY FOR BIOCRUST ESTABLISHMENT ON DUNES
We know from recent extensive meta-analyses that the effects of biocrusts on plants are complex, strongly nuanced, and driven by crust type and biocrust traits (Havrilla et al., 2019). An overwhelming body of evidence points to the opposite view to that advanced in Paradigm 2 by Kidron (2018). Best available evidence indicates that biocrusts prime surface soils with water and nutrients and facilitate the establishment of vascular plants (Lan et al., 2014;Langhans, Storm, & Schwabe, 2009;X. J. Li et al., 2008;Rodríguez-Caballero, Chamizo, Roncero-Ramos, Román, & Cantón, 2018). We also know that some moss and lichen species benefit from their association with moisture-rich habitats, which could be provided by nurse plants (Jiang et al., 2018), but could equally result from shading by rocks, or their location at lower landscape positions that receive additional run-on water (Lan et al., 2014;Williams, Buck, Soukup, & Merkler, 2013;Yair, 1990). Thus, it is clear that plant establishment per se is not a precursor for biocrust establishment on dunes.

| PARADIGM 3: DEVELOPMENTAL STAGES OF BIOCRUSTS ARE OFTEN REGARDED AS SUCCESSIONAL STAGES
Paradigm 3 is based on the observation that filamentous cyanobacteria are the first pioneering photosynthetic organisms to colonize disturbed soils, hence, the putative link to biocrust successional stage, irrespective of desert type. As biocrusts develop, the trajectory of change and the resulting species composition will depend on abiotic factors such as climate, soils, and specific microclimatic conditions. In more mesic environments, high biomass and developmental stage are likely synonymous. In more arid environments, however, later stages of development are likely to be limited by the lack of moisture, so that biocrusts will remain dominated by cyanobacteria or cyanolichens. In many environments, a mixture of different patch types and resource levels will result in a mixture of successional stages within the same site, a phenomenon acknowledged by Kidron (2018). Thus, Paradigm 3 applies to some environments (Belnap & Eldridge, 2003) but is not globally consistent (e.g., Chilton, Neilan, & Eldridge, 2018). Successional theory advances that earlier successional species condition a site to favour late-successional species (Bowker, 2007). However, we are unaware of any empirical evidence to show that succession occurs in biocrusts past the cyanobacterial stage. Potential mechanisms by which biocrust species might affect different species directly by facilitation or competition (Li et al., 2013;Maestre, Callaway, Valladares, & Lortie, 2009;Soliveres & Eldridge, 2020) or indirectly by altering site conditions or facilitating different species are poorly understood (Soliveres & Eldridge, 2020). Nonetheless, if, as stated by Kidron (2018), "the role of cyanobacteria as precursor of lichendominated crusts is undermined," one cannot help but wonder where the photobionts in lichens originate from. In summary, the literature indicates that developmental and successional stages are synonymous in some contexts, but not in others, a view widely held within the biocrust research community.

| PARADIGMS 4-8: CLIMATE-DRIVEN CRUST MORPHOLOGY DETERMINES CRUST HYDROLOGY (PARADIGM 4); SOIL PORES AND EXOPOLYSACCHARIDES DETERMINE INFILTRATION (PARADIGMS & 7); SOIL PORES AND HYDROPHOBICITY DETERMINE RUNOFF (PARADIGMS & 6), AND MOSSES IMPEDE INFILTRATION AND PERENNIAL PLANT GROWTH (PARADIGM 8)
Paradigms 4-8 deal with what Kidron (2018) sees as the major drivers of hydrology (sens. lat.) or specific hydrological processes (infiltration and runoff). Given that the mechanisms behind these moderators exhibit several commonalities, we deal with these paradigms together.
That climate drives hydrology via crust morphology (Paradigm 4) is only partially correct and certainly not a universal paradigm. Climate can affect freeze-thaw relationships, alter porosity and sorptivity, and sometimes influence runoff indirectly through controls on vascular plant-biocrust composition (Belnap, 2006). Climate (aridity) can also influence moss cover (Ferrenberg, Reed, & Belnap, 2015), which may alter retention time (Eldridge & Rosentreter, 2004). Rainfall intensity can also affect hydrology, by influencing the runoff coefficient (the proportion of rainfall that does not infiltrate) and therefore the partitioning of rainfall between runoff and infiltration.
Since porosity is only one of many factors affecting hydrological function, reductions in total pore volume might be compensated for by changes in other hydrological drivers such as surface roughness or water repellency or pore shape and connectivity (a statement that we already made in Felde et al., 2014). Overall, therefore, there is strong evidence to suggest that soil pores determine runoff (Paradigm 5).
The effects of EPS on soil hydrology (Paradigm 7) are complex and not completely understood. EPS in biocrusts are complex macromolecules comprising different monosaccharide fractions with different molecular weight distributions and consequently different capability to interact with soil particles and with water molecules . It is not unreasonable, therefore, that any effects of EPS on hydrology should vary across different studies (e.g., Rossi, Potrafka, Garcia-Pichel, & De Philippis, 2012;cf. Colica et al., 2014), particularly where those studies are from different soil types, with diverse crust types of varying morphologies, and markedly different EPS chemical and macromolecular characteristics. Any claim that EPS determine infiltration (Kidron, 2018) is only part of the truth, which is that the hydrological effects of biocrusts are strongly nuanced and vary widely with abiotic and biotic factors.
Finally, Paradigm 8 contends that mosses impede infiltration and therefore perennial plant growth. There is almost no empirical evidence to support or invalidate this paradigm, so claims that this view is "common among hydrologists, ecologists, or microbiologists" are at best spurious. It is generally acknowledged, however, that mosses have variable effects on infiltration, either enhancing (Wu, Hasi, & Wugetemole,, & Wu, X., 2012) or suppressing (Xiao, Zhao, Wang, & Li, 2015) infiltration depending on ecological context and the nature of the moderators (soil texture, climate, level of disturbance, spatial scale, etc.) described above. Mosses can retain water due to the presence of specialized leaf architecture (leaf hair points, lamellae, and papillae) (Pan et al., 2016), which could reduce infiltration to deeper layers (Eldridge & Rosentreter, 2004), but this likely varies with moss species, seasonality, and soil type (Wu et al., 2012). Moss effects on the survival and growth of vascular plants are also variable and will depend on the balance of these contrasting effects. However, a global meta-analysis indicates that their overall effect on vascular plant performance is positive, but effects on germination are negative (Havrilla et al., 2019). There is little support for the contention therefore that mosses impede perennial plant growth.

| CONCLUDING REMARKS
It is clear from the preceding discussion that the eight putative paradigms advanced by Kidron (2018) cannot be upheld nor are they commonly held among hydrologists, ecologists, or microbiologists that investigate biocrusts. However, we are thankful that Kidron (2018) has formally published his viewpoint because it gives us the opportunity to critically examine the veracity of such arguments, an important stage in the scientific process. Any proposal for paradigms that report global phenomena for such idiosyncratic communities of organisms as biocrusts would require an examination of the literature across the whole spectrum of biocrust distribution and environmental settings. Unfortunately, Kidron's (2018) "critique" is unashamedly heavily reliant on his own knowledge of desert systems, largely from one dune field in southern Israel. In attempting to globalize the effects of biocrusts on hydrology, Kidron (2018) risks simplifying nuanced and complex conditions providing junior researchers a narrow view of biocrust effects on hydrology, while ignoring the full spectrum of effects in different environmental and experimental contexts and scenarios. This generalization risks trivializing the science of biocrust hydrology and ignores decades of established research undertaken globally on biocrusts.

CONFLICT OF INTEREST
The authors declare no conflict of interest.