Contradictory evidence from biogeomorphological studies has increased the debate on the extent of lichen contribution to differential rock surface weathering in both natural and cultural settings. This study, undertaken in Côa Valley Archaeological Park, aimed at evaluating the effect of rock surface orientation on the weathering ability of dominant lichens. Hyphal penetration and oxalate formation at the lichen‐rock interface were evaluated as proxies of physical and chemical weathering, respectively. A new protocol of pixel‐based supervised image classification for the analysis of periodic acid‐Schiff stained cross‐sections of colonized schist revealed that hyphal spread of individual species was not influenced by surface orientation. However, hyphal spread was significantly higher in species dominant on northwest facing surfaces. An apparently opposite effect was noticed in terms of calcium oxalate accumulation at the lichen‐rock interface; it was detected by Raman spectroscopy and complementary X‐ray microdiffraction on southeast facing surfaces only. These results suggest that lichen‐induced physical weathering may be most severe on northwest facing surfaces by means of an indirect effect of surface orientation on species abundance, and thus dependent on the species, whereas lichen‐induced chemical weathering is apparently higher on southeast facing surfaces and dependent on micro‐environmental conditions, giving only weak support to the hypothesis that lichens are responsible for the currently observed pattern of rock‐art distribution in Côa Valley. Assumptions about the drivers of open‐air rock‐art distribution patterns elsewhere should also consider the micro‐environmental controls of lichen‐induced weathering, to avoid biased measures of lichen contribution to rock‐art deterioration.

Recent fieldwork carried out for the study of the lichen flora in the Upper Douro region (north-east Portugal) has resulted in a remarkable collection ofPeltulaspecimens, including one new species (Peltula lobataJ. Marques, M. Schultz & Paz-Berm.), three new species records for mainland Europe [Peltula bolanderi(Tuck.) Wetmore,Peltula farinosaBüdel andPeltula zahbruckneri(Hasse) Wetmore] and seven species new for mainland Portugal, that are reported here with a notes on their morphology and currently known distribution. The new speciesPeltula lobatais differentiated from any other knownPeltulaspecies by its thallus shape and size, presence of large, sessile apothecia with widely opened discs and by a complete absence of soredia. Identical specimens were also obtained from the nearby locality of Siega Verde in the province of Salamanca, and from the southern locality of Abla in the province of Almeria, both in Spain. New insights into the morphological diversity ofPeltula euplocaandPeltula bolanderiare also provided. The region is revealed to be a hotspot forPeltuladiversity and a very promising area for further research on the morphological and molecular variation among the former species. A revised key to European species of the genus is also presented.

Summary: This study addresses the primary and secondary bioreceptivity of schist used as a support for prehistoric rock art in the C ôa V alley A rchaeological P ark (north‐east P ortugal) and provides some parameters that can be related to the risk of biologically induced schist weathering. Samples of freshly quarried and naturally weathered schist were characterized in terms of their intrinsic properties and maintained in controlled environmental conditions after inoculation with biofilm‐forming cyanobacteria. The physical properties of the studied schist, as well as its abrasion pH , all varied according to the weathering degree of the samples and so did its susceptibility to colonization by biofilm‐forming cyanobacteria. Complete separation between weathered and unweathered schist samples in terms of laboratory‐induced photosynthetic biomass was obtained by measuring total colour change in the CIE (International Commission on Illumination) L *a*b* colour space. Weathered schist was more bioreceptive than unweathered schist, associated with increased open porosity, water saturation, capillary water and capillarity coefficient and decreased abrasion pH . In the future, it might be possible to determine the susceptibility of schist surfaces to biological colonization through evaluation of colour differences associated with the different weathering degrees presented by those surfaces prior to colonization.