A data-driven evaluation of lichen climate change indicators in Central Europe

Lichens are widely utilized as indicators of air quality, forest health and climate change. In Central Europe, specific lichens have been designated as climate change indicators; however, the lichen biota of central Europe has been substantially altered by air pollution and only re-established during the past decades—complicating the interpretation of recent changes in lichen composition. To assess their validity as climate change indicators, we aggregated georeferenced records of these taxa and compared their historic and modern distributions. Modern distributions substantially differed for fewer than half of the indicator taxa with sufficient data to enable evaluation—reinforcing their utility as climate change indicators. However, modern distributions for approximately half of the taxa evaluated were largely confined to historically suitable climates—raising questions about their utility as climate change indicators. We were unable to model historic distributions for nearly two-thirds of all indicator taxa due to insufficient data. About one-third of these had multiple modern records but one or fewer historic records, suggesting they may indeed be expanding their range; however, about half had comparable or greater numbers of historic records relative to modern records, complicating their interpretation as climate change indicators. Together, our work illustrates that distributions for fewer than half of the lichen climate change indicators have substantially shifted in the recent past, and calls into question whether the remaining designated taxa are indeed strong positive indicators of climate change. We argue that more quantitative, evidence-based derivations of climate change indicators are required to accurately detect climate change.