A Green New Balance: Interactions among riparian vegetation plant traits and morphodynamics in alluvial rivers

The strength of interactions between plants and river processes is mediated by plant traits and fluvial conditions, including above-ground biomass, stem density and flexibility, channel and bed-material properties, and flow and sediment regimes. In many rivers, concurrent changes in (1) the composition of riparian vegetation communities as a result of exotic species invasion and (2) shifts in hydrology have altered physical and ecological conditions in a manner that has been mediated by feedbacks between vegetation and morphodynamic processes. We review how Tamarix, which has invaded many southwestern US waterways, and Populus species, woody pioneer trees that are native to the region, differentially affect hydraulics, sediment transport, and river morphology. We draw on flume, field, and modelling approaches spanning the individual seedling to river-corridor scales. In a flume study, we found that differences in the crown morphology, stem density, and flexibility of Tamarix compared to Populus influenced near-bed flow velocities in a manner that favoured aggradation associated with Tamarix. Similarly, at the patch and corridor scales, observations confirmed increased aggradation with increased vegetation density. Furthermore, long-term channel adjustments were different for Tamarix- versus Populus-dominated reaches, with faster and greater geomorphic adjustments for Tamarix. Collectively, our studies show how plant-trait differences between Tamarix and Populus, from individual seedlings to larger spatial and temporal scales, influence the co-adjustment of rivers and riparian plant communities. These findings provide a basis for predicting changes in alluvial riverine systems which we conceptualize as a Green New Balance model that considers how channels may adjust to changes in plant traits and community structure, in addition to alterations in flow and sediment supply. We offer suggestions regarding how the Green New Balance can be used in management and invasive species management.