Pioneer annual vegetation of gravel-bed rivers: first insights on environmental drivers from three Apennine streams

Abstract

A huge knowledge gap exists on riverine pioneer herbaceous vegetation. Despite its relevance in regulating the C metabolism at the catchment scale, and the triggering role in shrubs and trees establishment along riverbanks, little data is available on its environmental determinants. Indeed, most existing knowledge in this field refers to woody species or aquatic macrophytes neglecting the ecosystem relevance of ephemeral herbaceous vegetation. Focusing on three gravel bed rivers located in northern Italy (Baganza, Nure and Parma streams), the present study is aimed to evaluate the riverine ephemeral plant richness, considering both native and alien taxa, and the role of hydrogeomorphological disturbance and sediment quality in the observed richness patterns. At higher disturbance rates (e.g., larger river sizes), our data indicates a progressive decrease in overall plant richness, but also an increase in the coverage-abundance rates mainly due to alien species. This evidence confirms that variations in hydrology imply changes in pioneer plant species richness at in-stream periodically exposed sediments. More attention must be given to the vulnerability of pioneer vegetation to climate change and direct human impacts to fully understand the functioning of lotic ecosystems, especially the non-perennial ones.

A longitudinal snapshot of pioneer plant patterns along lowland temperate rivers

“…Comparable results have been recently recorded by Bolpagni and Dalla Vecchia (2021) for a set of intermittent streams in the same hydrological basin of the current rivers studied (Po River plain, Northern Italy), although under completely different geomorphological and hydrological conditions (e.g., intermittent vs. permanent regimes). The authors indicated the progressive replacement of the typical ‘pioneer riverine plants’ by ruderal taxa not strictly riparian, following the categories defined by Tabacchi et al (1996): a dynamic vegetation process that may be ascribed to the progressive increase in hydrological disconnection of seasonally exposed fluvial forms.…”

“…This wide hydrological variability alters the ‘natural’ dynamism of rivers and, in turn, the ‘natural’ structure of riverbeds lowering the complexity of riverine mesohabitats, including the spatial representativeness and the structure of islands, sandbars and scree deposits. All this can strongly affect riverbed vegetation dynamics and functioning, especially of pioneer plant communities able to establish on seasonally exposed sediments (Bolpagni & Dalla Vecchia, 2021).…”

“…establish on seasonally exposed sediments (Bolpagni & Dalla Vecchia, 2021). Furthermore, due to the growing meteorological uncertainties the structure and the services provided by instream vegetation could change significantly as the harshness of drought events rises (Bolpagni, Folegot, Laini, & Bartoli, 2017;Bolpagni, Laini, Mutti, Viaroli, & Bartoli, 2019;G omez-Gener et al, 2021).…”