The more lichens, the merrier for rock art conservation? Relationships between lichen diversity, biodeteriorative potential and conservation condition

Lichens are frequently regarded as agents of biodeterioration on exposed rock surfaces and are commonly incorporated into conservation assessments of rock-hewn cultural heritage. However, whether lichen diversity and biodeteriorative potential are actually associated with conservation condition remains poorly evaluated. This study investigated relationships between lichen communities and the conservation status of 28 engraved schist outcrops in the Côa Valley, north-eastern Portugal, a UNESCO World Heritage Site since 1998. Conservation condition was assessed using the Intervention Urgency Scale (IUS), while lichen assemblages were characterised through species richness, diversity indices, estimated cover, community composition and the Lichen Potential Biodeteriorative Activity index (LPBA). The results revealed consistent negative relationships between lichen diversity and intervention urgency, indicating that richer and more diverse lichen communities were associated with better conservation condition, while there was little evidence linking estimated biodeteriorative potential to intervention urgency. Relationships with topographic and physical weathering variables supported the interpretation that parallel patterns in lichen diversity and conservation condition reflect a common underlying gradient of outcrop stability. These findings challenge the assumption that greater lichen colonisation necessarily reflects poor conservation condition and instead suggest that lichen diversity provides complementary ecological information on the long-term geomorphological context of rock-art-bearing outcrops. More broadly, the study demonstrates that indices operating at different spatial scales capture distinct ecological processes, highlighting the need for hierarchical conservation assessment frameworks that integrate complementary ecological and geomorphological indicators.