Dating of mining-induced subsidence based on a combination of dendrogeomorphic methods and in situ monitoring

Mining-induced subsidence is a worldwide environmental problem that leads to damage to infrastructure and property; thus, knowledge of its past-recent development is critically needed during land-use planning. Moreover, the temporal occurrence of subsidence is greatly variable, with tendencies for gradual to abrupt movements. Subsidence rates can currently be achieved via radar interferometry, but these data usually cover short time periods and are less accurate in forested areas. In such cases, dendrogeomorphic dating can provide chronologies of past subsidence activity. In addition, comparisons of tree-ring-based data with in situ monitoring may answer the sensitivity and reliability of dating methods in these specific conditions of continuous ground movement. Geomorphic mapping and dendrogeomorphic dating were performed at two forested landslide sites affected by coal mining subsidence in the Upper Silesian Basin (Czech Republic), and the resulting chronology from one site was compared with levelling campaigns occurring from 1998 to 2015. Based on 284 increment cores from 71 trees, we were able to identify 7 subsidence events at the site dominated by a pioneering forest and 19 subsidence events at the site overgrown by a mature forest, with the most frequent responses occurring from 1956 to 1971 and 2005-2011. The occurrence of multiple compression wood growth intervals, some occurring in different directions of tree stems (more than 30% of sampled trees) indicated complex deformations presented by the flow-like mechanism of waterlogged surfaces, ongoing subsidence spreading, and fragmentation of larger blocks and grabens. A comparison of tree-ring-based chronology with in situ monitoring revealed good synchronicity with the periods of increased subsidence rates (between 1 and 3 m.year(-1)), thus indicating the high relevance of dendrogeomorphic methods for the determination of the most intense periods of ground subsidence. Our results show the first comparison of dendrogeomorphic methods with in situ monitoring of human-induced subsidence areas and provide an alternative option for reconstructing decadal to centennial development of the affected areas.