Aim Revisits of non‐permanent, relocatable plots first surveyed several decades ago offer a direct way to observe vegetation change and form a unique and increasingly used source of information for global change research. Despite the important insights that can be obtained from resurveying these quasi‐permanent vegetation plots, their use is prone to both observer and relocation errors. Studying the combined effects of both error types is important since they will play out together in practice and it is yet unknown to what extent observed vegetation changes are influenced by these errors. Methods We designed a study that mimicked all steps in a resurvey study and that allowed determination of the magnitude of observer errors only vs the joint observer and relocation errors. Communities of vascular plants growing in the understorey of temperate forests were selected as study system. Ten regions in Europe were covered to explore generality across contexts and 50 observers were involved, which deliberately differed in their experience in making vegetation records. Results The mean geographic distance between plots in the observer+relocation error data set was 24 m. The mean relative difference in species richness in the observer error and the observer+relocation data set was 15% and 21%, respectively. The mean “pseudo‐turnover” between the five records at a quasi‐permanent plot location was on average 0.21 and 0.35 for the observer error and observer+relocation error data sets, respectively. More detailed analyses of the compositional variation showed that the nestedness and turnover components were of equal importance in the observer data set, whereas turnover was much more important than nestedness in the observer+relocation data set. Interestingly, the differences between the observer and the observer+relocation data sets largely disappeared when looking at temporal change: both the changes in species richness and species composition over time were very similar in these data sets. Conclusions Our results demonstrate that observer and relocation errors are non‐negligible when resurveying quasi‐permanent plots. A careful interpretation of the results of resurvey studies is warranted, especially when changes are assessed based on a low number of plots. We conclude by listing measures that should be taken to maximally increase the precision and the strength of the inferences drawn from vegetation resurveys.

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Questions: How did plant species richness and spatial heterogeneity of the vegetation change across sub-montane forests over the past 50 years? Did the vegetation changes reflect eutrophication, acidification and management changes, which the area underwent during the second half of the 20th century?Location: Ordinary managed forests sampled across 2500 km 2 of a typical Central European sub-montane landscape in the southern part of the Czech Republic. Methods:We resampled 156 quasi-permanent plots sampled in the 1950s-1970s and covering the whole range of forest vegetation in the region. We compared understorey plant species richness and community composition between the surveys and tested for temporal changes in the spatial heterogeneity (β-diversity) of vegetation.Results: Species richness and dissimilarity in species composition among plots decreased significantly between the surveys. The vegetation of the plots also changed significantly, but there were no clear directional changes across all plots. The vegetation homogenization was driven by local extinctions of specialist species, especially competitively weak species adapted to nutrient-poor conditions. Few generalist species expanded between the surveys. Conclusions:We found evidence of taxonomic homogenization of forest understoreys across a large sub-montane region. This vegetation homogenization was probably driven by complex landscape changes that took place in the last century. Varied traditional management regimes were replaced with largely uniform management, and the once spatially diverse landscape mosaic has become simpler. The resulting transition to species-poor and less variable vegetation was probably further accelerated by environmental eutrophication. Our results thus complement previous forest herb layer resurveys, which focused mostly on lowland forests and small nature reserves, and suggest that landscape-scale taxonomic homogenization is occurring across Central European forests. K E Y W O R D Sbeta diversity, biotic homogenization, eutrophication, forest understorey, landscape change, semi-permanent plots, species richness, temperate forests, vegetation resurvey 374 |

Pronounced climatic warming associated with the Late Weichselian Pleniglacial‐to‐Lateglacial transition caused considerable environmental changes throughout the former periglacial zones (in Europe ~53°–46°N). During permafrost degradation and subsequent ground subsidence (i.e. thermokarst processes), the landscape changed rapidly. In this study we investigated a flat mid‐altitude area in south Bohemia, Czech Republic, lying close to the southern limit of the Weichselian permafrost. We discovered palaeo‐lake basins with sedimentary infillings up to 11 m in depth. According to radiocarbon and palynostratigraphical dating, these basins were formed at the onset of the Late Pleniglacial‐to‐Lateglacial transition, whereas the smaller depressions were formed later. We suggest that the basins resulted from thermal and fluvio‐thermal erosion of the former permafrost and represent remnants of discontinuous gullies and possibly collapsed frost mounds (pingo/lithalsa scars). The formation of this a fossil thermokarst landscape was climatically driven and multiple phased, with the major phase during the climatic warming and wetting at the onset of GI‐1e (Bølling) and the minor phase during GI‐1c (Allerød). This study enhances knowledge of the palaeogeography of the former European periglacial zone by showing that Late Pleistocene thermokarst activity could have had a significant impact on the evolution of the landscape of at least some regions of central Europe along the southern limit of the continuous permafrost zone. The research also points to a similar history for the physical transformation of the landscape of the former European periglacial zone and current thermokarst landscapes and could be a valuable source of information with respect to the future transformation of the Arctic under conditions of ongoing global warming.