Using LiDAR to assess transitions in riparian vegetation structure along a rural-to-urban land use gradient in western North America

Riparian forests are essential for stream ecological processes in arid and semiarid regions, however, they are often highly altered by the rapid expansion of urban areas. To maintain riparian ecosystems services, it is important to better understand the effects of urbanization on riparian forests. We quantified the three-dimensional (3D) structure and woody species composition of a riparian corridor in Utah, USA, to evaluate patterns of vegetation along stream reaches that flow through distinct hydrologic domains (with gaining and losing reaches) and through a rapid rural-to-urban gradient. By using LiDAR imaging and field observations, we explore the extent to which the riparian vegetation structure follows patterns of topography linked to energy and water subsidies and patterns of human influence along the stream. Whereas natural reaches of Red Butte Creek were characterized by native vegetation and typical riparian species (e.g., Betula occidentalis), urbanized reaches had higher numbers of introduced plants (e.g., Acer platanoides) and more upland species (e.g., Quercus gambelii). Urban reaches were also characterized by exceptionally high trees (>18 m) in older residential neighbourhoods. In the natural area, canopy height was negatively correlated with height above the river (HAR). Additionally, we found higher cover and taller canopies on north-facing aspects. These results show that LiDAR data, in combination with ground observations, can reveal strong influences of hydrology as well as land use in different canopy layers of riparian forests. We suggest that the decision making of individual landowners shapes vegetation beyond natural hydrological patterns, with implications for riparian forest management and restoration.