Ecological traits of Saliceae and the species replacing them on the active floodplain

Abstract

AbstractThe dominant woody plants of active floodplains in the northern temperate zone are various species of Salix (willows) and Populus (poplars and cottonwoods) in the tribe Saliceae of the family Salicaceae. In this review, I consider the traits that enable these species to thrive in the dynamic floodplain environment and ask why they are now declining in the northern hemisphere, yet spreading rapidly in the southern hemisphere. I reach four main conclusions. First, floodplain Saliceae exhibit traits, notably huge numbers of minute, wind‐ and water‐dispersed seeds and an exceptional capacity for vegetative propagation, that uniquely fit them for life on the active floodplain. Second, there are no functional equivalents to floodplain Salix and Populus in the southern hemisphere, which accounts for their remarkable success in invading riparian ecosystems. Third, the replacement of Saliceae by other species in the northern hemisphere has been caused mainly by changes in flow through dam construction, regulation and abstraction. The species that replace Saliceae vary according to site conditions: fertile sites in humid climates become occupied by various broadleaved trees (e.g., Acer negundo, Fraxinus pennsylvanica, Ailanthus altissima); nitrogen‐fixing shrubs dominate in areas where nitrogen is limiting but phosphorus is adequate (Amorpha fruticosa, Robinia pseudoacacia, Elaeagnus angustifolia); sites in more arid regions are occupied by various Tamarix spp. The different ecological groups of woody species are associated with contrasting changes to the floodplain ecosystem.