DOI: 10.5061/dryad.bg79cnp8v
title: Disentangling effects of disturbance severity and frequency: does bioindication really work?

description: Ecological disturbances are recognized as a crucial factor influencing the
 attributes of ecological communities. Depending on the specific adaptation
 or life cycle, plant species show different responses to disturbances of
 different magnitudes. Herben et al. (2016) proposed disturbance indicator
 values (DIVs) that describe the niche of each Central-European plant
 species along gradients of disturbance frequency and severity. Here we ask
 if the DIVs can be used in community ecology for bioindication of
 disturbance regime? We used a dataset of riparian forests sampled within
 mountain catchments (the Sudetes, SW Poland). As the regime of disturbance
 is driven by changes in floods from the spring towards the mouth, we
 calculated the position of every plot along longitudinal
 (upstream-downstream) gradient and used it as a proxy for the disturbance
 severity and frequency. We then calculated the community-weighted means
 (CWMs) for each of the six indices for each plot and analyzed whether
 these indices reflected the position of the plots along the rivers. We
 expected an increase in the severity indices and a decrease in the
 frequency indices downstream along the rivers. Moreover, we analyzed
 relationships between disturbance indices and species optima along
 longitudinal gradient. Surprisingly, means for all analyzed indices
 increased along the rivers. Severity indices showed the strongest
 association with the longitudinal gradient. The disturbance severity index
 for herbs was the only index that differed significantly among species
 with different responses along longitudinal gradient. On these results, we
 identified a strong correlation between the severity and frequency indices
 as the main problem. We conclude that the DIVs have considerable
 applicative potential; however, the determination of ecological niches
 separately for disturbance severity and frequency is difficult because
 different components interact to shape the realized niche of each species.
 All analyzed indices encompass different attributes of the disturbance
 regime including both severity and frequency.
description: The riparian forests were surveyed
 along mountain rivers in the Polish part of the Sudetes, SW Poland. The
 dataset used in this study was collected between 2006 and 2009 following
 the methods of the Central-European phytosociology (Dzwonko, 2007; Kent,
 2012). All plots were located close to the river bank; plots were
 rectangular (7.5 × 20 m) and oriented with the long dimension parallel to
 the upstream-downstream gradient of the river. The cover of vascular
 plants was estimated for each of three layers (trees, shrubs and herbs)
 using the Domin-Krajina scale with an ordinal transformation proposed by
 van der Maarel (1979). Plots were also located with a GPS receiver and
 recorded in a mobile GIS database. The precise localization and digital
 river network (Map of Hydrographic Division of Poland, MPHP) enabled the
 calculation of the distance from the river source for all the sampled
 plots. More details regarding study area and vegetation sampling are
 available in the previous study (Pielech et al., 2015). The full dataset
 is stored in the Forest Database of Southern Poland (Pielech et al.,
 2018). Values of DIVs are taken from Herben et al.
 (2016).
description: The extended dataset is stored in
 the Forest Database of Southern Poland (Pielech et al., 2018). It includes
 also spring forest and a broad array of environmental
 variables.