Effect of intra‐seasonal variability on vegetation data

Abstract

AbstractQuestionsIt is generally acknowledged that the date of sampling partly determines the floristic composition and layer covers of vegetation plots. However, the effect of vegetation seasonality on the results of vegetation analyses is still insufficiently explored. Here, we investigated two data sets to examine how intra‐seasonal variability of vegetation influences the pattern of: (1) species composition; (2) species richness, number of unrecorded species, plant life‐form spectra; and (3) quantitative changes in cover of individual species and layers.LocationDeciduous forests and dry grasslands in SE Czech Republic.MethodsWe established 40 forest and 46 dry grassland permanent plots, each sampled in three periods (spring, summer and autumn). We compared species composition and β‐diversity patterns in multi‐dimensional space between different periods of the growing season using permutational multivariate analyses (PERMANOVA, PERMDISP). We used ANOVA for repeated measures and Tukey post‐hoc tests to assess differences in species richness, proportions of unrecorded species, changes in layer cover and plant life‐form spectra.ResultsIn the Forest data set, spring plots significantly differed in their species composition from summer and autumn plots, while we found significant differences between different periods of the growing season in the Dry grasslands data set. We confirmed, that the forest species richness was highest in summer plots, while the α‐diversity in spring and summer dry grassland plots were comparable and notably declined in autumn. Although most plant life forms were best recorded in summer, both forest geophytes and dry grassland therophytes were best recorded in spring.ConclusionsWe suggest that before analysing vegetation plot data, large databases should be carefully stratified according to date of sampling. Based on our results from temperate broad‐leaved forests and dry grasslands, we recommend excluding plots sampled before June, as they might be a potential source of misinterpretation of species composition analyses (with the exception of studies focused on vernal species). The use of autumn plots does not have any strong effect on the results, except in α‐diversity analyses in forest plots, while dry grassland plots are significantly different from the rest of the growing season.