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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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a b s t r a c tThe time-delayed extinction of plant species following habitat fragmentation is a well-known phenomenon in ecology. The length of the relaxation time until this 'extinction debt' is paid (i.e., until extinctions cease) depends on the ecosystem, species group and extent of fragmentation. Studies of grassland ecosystems have revealed that plant extirpations after fragmentation can occur rapidly when the degree of fragmentation is high. Studies of extinction debt in highly fragmented forests, however, are lacking. In this study, we evaluated the existence of an extinction debt in the Prignitz, Brandenburg, Germany, where 94% of the semi-natural forests have vanished since 1780. We surveyed the herb-layer species of 104 forest patches and fitted species richness as a function of the historical and present-day patch configurations. Models including the present-day habitat area and connectivity explained the present day species richness better than models including historical patch-configuration variables. There was no significant effect of the historical habitat area on the present day species richness. However, the effect of historical patch connectivity on the richness of forest specialists with short-distance dispersal potential was significant when excluding present-day habitat area from the models and habitat quality and heterogeneity were used as covariables. The extinction debt has largely been paid after approximately 160 years of relaxation time which contrasts with previous studies of temperate forests that have found extinction debts persisting 120-225 years after fragmentation. We demonstrate that extinction debts in temperate forests may be paid off more rapidly if the degree of fragmentation is high.

Questions: (i) Is plant species richness related to patch configuration, quality, heterogeneity and history in very small forest patches? (ii) Is there a similar effect of patch configuration, quality, heterogeneity and history on all plant species or does it vary for different plant groups? (iii) Does the importance of patch configuration, quality, heterogeneity and history vary for plant species richness between patch size classes?Location: Prignitz region, western part of Brandenburg, Germany Methods: Plant species lists for 183 forest patches (0.08-12.7 ha) were compiled. We conducted a hierarchical partitioning analysis to test which of the four patch variablesconfiguration, quality, heterogeneity and historyexplained most of the variance in plant species richness and richness of various plant groups.Results: Irrespective of patch size class, species richness is mainly predicted by patch quality and heterogeneity. In particular, generalists are primarily affected by the soil moisture and nutrient level, and specialists additionally by patch heterogeneity and history. A higher historical habitat quality, together with long habitat continuity, led to an increase in forest specialists and geophytes in all patches. In the >5-ha patches only historical habitat quality accounted for a significant increase in the number of species with short-distance dispersal capacity in all species. Regardless of patch size, the number of locally infrequent species increased significantly with historical habitat quality. The effect of patch configuration was negligible, except for space-filling trees and shrubs, when considering all patches. Conclusions:Our more comprehensive statistical approach shows, in general, that all complex variables have an effect on species richness and should be considered in future studies. The fact that patch heterogeneity and quality are the main predictors of species richness indicates that very small forest patches covered a range of highly structured patches and sites having various growth conditions. We assume that the century-old tradition of private ownership with smallholder social structure is the main reason for variability in stand structure and species assemblage, resulting in the uniqueness of each patch. Since the number of locally infrequent plant species increases significantly with historical habitat quality, we suggest that it is particularly important to consider this variable in future studies on the decline of infrequent plant species.