Development and testing of a mechanistic potential niche model of riparian tree seedling recruitment

Seedling recruitment is an important reproductive process for sustaining riparian tree populations in arid and semi-arid environments. Riparian tree species such as cottonwoods and willows are highly adapted to the dynamic riparian environment; their seed dispersal and germination patterns are tied with climatic signals that also drive hydrology. The magnitude and timing of seasonal hydrologic components determine environmental conditions that either promote or limit seedling establishment processes. This article presents the development and testing of a potential niche model for riparian seedling recruitment. The presented Riparian Seedling Recruitment Model (RSRM) identifies spatially explicit locations of suitable habitat for seedling recruitment driven by relevant hydrophysical processes. This model extends previous seedling recruitment algorithms by accounting for seedling mortality due to scour by sediment mobilization, the reduction of potential germination sites due to existing forest canopy shade, and the incorporation of engineered channel and substrate modifications. The model is integrated into the open-source river analysis software River Architect. Potential future applications for the model include assessing seedling recruitment patterns for existing conditions, under alternative flow regimes, or for designs with topographic modifications. A canonical test channel and five scenarios with relevant hydrographic features are used to perform an implementation verification. Predictable recruitment patterns for the canonical test channel demonstrate model functionality and establish a dataset for future benchmarking. Finally, results from a site on the lower Yuba River, California are presented to illustrate the usefulness of a simplified test site given the complexity of results from real-world data.