A false paradigm? Do biocrust types necessarily reflect ‘successional stages’?

Abstract

The term ‘succession’ was first proposed to describe the gradual development of plants from an initial stage such as a bare ground to a well-developed plant community, which at its peak, may reach a climax (primary succession). Accordingly, the earlier and fast growing stage (such as an annual plant community) may grant stability, organic matter and nutrients to the latter, high-biomass and slow-growing stages, such as trees. Commonly, reference to the different successional stages is also made once intact and disturbed communities (such as due to mechanical disturbance, tillage, fire, etc.) are compared (secondary succession). The concept was borrowed by many ecologists to describe variable biocrust types. Cyanobacterial or algal biocrust is regarded as an initial stage before turning to a later, more mature biocrust, whether composed of lichens or mosses. The underlain assumptions are that (a) the cyanobacteria provide essential stability and (b) nutrients that are required for the development of the later stages; (c) the initial biocrusts improve the water regime for the later successional stages; (d) cyanobacteria promote the lichen symbiosis; (e) due to substantial differences in the recovery time, a linear succession is inevitable, commonly from cyanobacterial/alga to lichen and lastly to moss; and (f) the cyanobacterial/algal biocrust is a temporary stage, just before being outcompeted by a later stage. It is argued hereafter that the above-mentioned assumptions are not necessarily correct. As with higher plants, unless a direct comparison between disturbed and intact crusts justifies a reference to successional stages, different types of biocrusts commonly reflect the abiotic conditions at their site, and as such, unless the abiotic conditions change, they reflect stable communities of variable crust types. This paradigm shift may have important implications regarding inoculation efforts and directions and may explain the low success thus far obtained following inoculation experiments once performed with the more developed biocrusts, lichens and mosses.