Observer and relocation errors matter in resurveys of historical vegetation plots

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 24m. 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

Rozmieszczenie i zmiany liczebności populacji Drosera anglica i Scheuchzeria palustris na torfowiskach Gór Izerskich =Distribution and changes of populations size of Drosera anglica and Scheuchzeria palustris in the peat bogs of the Izery Mts

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A contribution to the flora of the Włodarz Massif (Sowie Mountains, Central Sudetes) [in Polish with English summary]

<p>The paper presents the results of the studies on the forest flora of the Włodarz Massif, an isolated group of mountains in the western part of the Sowie Mts. The studied area covers 14 km2, which is covered in 77% by spruce monoculture and forest plantations. 195 species of vascular plants have been found, including 8 strictly protected and 8 partly-protected ones. The rarest species in the studied area are orchids: Listera ovata, Epipactis helleborine, Dactylorhiza majalis and Neottia nidus-avis, which were found in single localities only. The most common alien species is Impatiens parviflora. Pseudotsuga menziesii, Aesculus hippocastanum, Galinsoga parviflora, Myosotis arvensis and Melandrium album are much less common. The status of the two native species for Poland being alien for the Sudetes flora is unclear: both Telekia speciosa and Larix decidua are established in the local flora. Mountain species constitute less than 10% of all species which indicates a strong disturbance of the local flora due to the 19th century forest management. </p> <p> </p