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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<em>Arnica montana</em> L. is a critically endangered and highly valued medicinal plant species in Europe. We show the inter-relationships between arnica and accompanying plant species, as well as soil factors, that promote the persistence of the studied forest arnica populations in terms of active protection of this species in the northeast region of Europe. The population characteristics and plant species composition were assessed during a field study. Additionally, soil samples were taken and analyzed to assess variation in soil conditions in the habitats of arnica populations. Correlations between population characteristics and soil properties were highlighted. The forest habitats of arnica presented in this study differ from those described in other European mountain and submountain areas. The sandy and very poor soils are characterized by a very low content of macro- and microelements, and a strong acid reaction. The positive correlation between population characteristics and Ca and K indicates an important role of these macroelements in flower head production. Acidity, K, Ca, the sum of exchangeable bases, and base saturation play crucial roles in the persistence of arnica populations in pine forests. The level of acidity and its consequences result from soil-forming processes and climatic conditions rather than air pollution. When planning active protection scenarios, special attention should be paid to the frequency and cover of <em>Vaccinium myrtillus</em>, which can act as a competitor in forest habitats. Assessment of soil conditions that favor the persistence of the studied arnica populations and species relationships is important for improving knowledge of the ecology of the species and for the active protection of endangered plant species.

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Mountain arnica, <em>Arnica montana</em> L., an herbaceous plant species critically endangered in Europe, is the source of raw material, which is abundant in its secondary metabolites. During the field investigation conducted in Augustów and Knyszyn forests (Poland) and in Grodno Forest (Belarus), the plant species composition and population characteristics were measured. Additionally, to evaluate the edaphic conditions of arnica populations, soil samples were taken and analyzed. The sandy and very nutrient-poor soils are characterized by strong acidity and a very low concentration of macro- and microelements. The analyzed characteristics of the studied populations indicate a good status of populations located in Grodno Forest. However, the very small number of individuals and the very small proportion of flowering individuals in the populations in Augustów Forest and Knyszyn Forest indicate the need for active protective actions. <em>Calamagrostis arundinacea</em> can play the role of a competitor; therefore, during planning active protection, individuals of this species should be eliminated, and particular attention should be paid to the frequency and coverage of this plant species and the plant height of the herb layer. The dependence between population characteristics, especially the proportion of flowering stems and the concentration of available phosphorus, may indicate the effect of the concentration of this macroelement on flowering and, in consequence, provide a greater chance for the generative propagation of this plant species.