The contradictory role of lichens in the weathering of limestone

Experimental evidence suggests that coverage by certain lichens may actively reduce the rate of calcium loss from limestone surfaces subject to natural dissolution, through shielding by the thallus and the binding and waterproofing of the rock surface and subsurface by fungal hyphae. However, the subsequent decay of hyphae, induced by changes in microenvironmental conditions, necrosis, parasitism or the natural physiological traits of particular lichen species, may expose a chemically and physically weakened substrate to dissolution, triggering relatively rapid weathering-related calcium loss and surface lowering . Consequently, certain crustose and endolithic lichens may induce a period of surface stability throughout the course of their lifespan, followed by a phase of instability and rapid episodic microtopographical evolution after their death and decay. A series of conceptual models is proposed to illustrate this idea over short (single lichen lifespan) and long (multiple lichen lifespans) timescales. The models suggest that the microscale biogeomorphological system of lichen-rock interaction is underpinned by non-linear dynamical system theory as it exhibits dynamical instability and is consequently difficult to predict over a long timescale. Dominance by the conditions of net biodeterioration or net bioprotection may be altered by changes in lichen species or in environmental conditions over time.EThOS - Electronic Theses Online ServiceGBUnited Kingdo