Saxicolous lichens are commonly viewed as agents of gradual weathering of their stone substrates. Their growth rates may be considered proxies of weathering potential, with lichen hyphae penetrating between mineral grains and contributing to physical and chemical rock disintegration. However, as long as lichens occupy their substrates, they also shelter the surface from erosion. Removing slow-growing organisms from lithic monuments may therefore accelerate erosion, opening up these surfaces for renewed invasion and thus renewed damage. In this context, assessing time scales at which weathering processes take place, investigating the dynamics of biodeterioration, biomineralization and erosion becomes fundamentally important. At two sites in Arizona, the Deer Valley Petroglyph Preserve north of Phoenix, and at V bar V near Verde Valley, we have recently started investigating lichens growth alongside and overgrowing petroglyphs, artifacts that native Americans carved into the rock. Currently petroglyph preservation strategies at both sites are generally "hands-off", documenting, but not eliminating lichens. But whereas sites at Deer Valley remain well preserved, petroglyphs at V bar V are increasingly overgrown, most likely a result of recent macrohabitat changes (increased shade and nutrients). Here we present preliminary results documenting lichen community dynamics at both sites. Recent developments in structure from motion (SfM) photogrammetry allows us to detail structural change of lichen growth in 3D. Combining these techniques with geological analysis of changes in substrate mineralogy will, over time, provide insights to better assess species-specific risks. Comparing lichen colonization rates and changes in mineralogy for different species in different microhabitats, on different rock types (basalt, sandstone), at different sites (low altitude desert, montane Juniper scrub) will allow us to better assess how lichen species contribute to substrate deterioration differently, assessing the time scales substrate alteration takes place, ultimately finding the best strategy to mitigate the effects of lichens deterioration on lithic monuments.
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