Disentangling effects of disturbance severity and frequency: Does bioindication really work?

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. (Journal of Vegetation Science, 27, 628-636) proposed six disturbance indicator values (DIVs) that describe the niches of 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 toward 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.