Salix Regeneration in Fluvial Landscapes: Empirical Findings Based on a Systematic Review

Riparian vegetation is highly dependent on dynamic river systems at all life stages. However, anthropogenic pressures have modified natural river landscapes, reducing their extent and causing severe limitations to the natural recruitment of riparian species. In particular, the recruitment of Salix species (willows) is highly dependent on fluvial processes. A comprehensive understanding of the drivers of Salix recruitment is crucial to effectively restore and conserve riparian biodiversity. To date however, there has been no systematic review of the drivers of Salix recruitment in riparian ecosystems. We provide a synthesis of Salix recruitment requirements from the scientific literature throughout the world. Our results show that the proportion of species studied compared to existing species is low worldwide, revealing a need for more research on Salix recruitment. We identified 19 abiotic factors concerning Salix recruitment, which we classified into four groups. The most studied abiotic factors are those related to soil (35%) and hydrology (25%). Within those categories, soil moisture and flow regime are the most frequently studied, while interstitial soil temperature, precipitation, geomorphologic dynamism, and atmospheric CO2 content are among the least studied factors. We identified 6 types of environmental pressures. Flow regulation followed by invasive vegetation competition are the most studied pressures, while the effects of climate change and extreme climatic events on willow recruitment have received little attention. Additionally, there has been little consideration of the interactions of multiple anthropogenic pressures. Therefore, we recommend comparative studies and experiments including several species to fill this knowledge gap. Our synthesis of the factors that influence Salix recruitment, such as optimal temperature ranges and elevations, can be incorporated into management practices to promote successful native riparian tree regeneration. Further research on how both abiotic factors and anthropogenic pressures affect Salix regeneration would greatly contribute to conservation of fluvial landscapes, and increase the success of restoration initiatives.