Novel flow regimes and novel plant communities: strategies of urban-adapted riparian plants
Urban riparian plant communities are expected to differ from rural counterparts given their differences in hydrogemorphology and landscape context. As a first step in understanding and explaining these differences, we asked “How do diversity and composition of riparian forests vary between urban and non-urban reaches, and between urban reaches with different flow regimes?” We also asked, “Do functional traits of the urban dominants differ from those in non-urban settings, and do traits of naturalized species diverge from those of regional species?” We focused on the Salt River in Phoenix, which is emblematic of an arid region urban river, and on non-urban (wild) rivers in the Gila watershed. We sampled tree abundance and size structure; quantified stream flow permanence and nutrient concentration; and assessed landscape context. We measured seed weight, dispersal phenology, and dispersal mechanism (as they relate to regeneration niche) and leaf size and specific leaf area (as they relate to survivorship in stressful environments).
The urban river supported a more diverse and more cosmopolitan riparian forest than the wild rivers. A total of 30 tree species were present along the urban river, about double that in the ‘reference systems'. The urban forests included a mix of remnant Sonoran riparian species, restoration plantings, globally-distributed riparian taxa, and landscape cultivars secondarily dispersed from the irrigated urban landscape after importation from dryland regions throughout the world. The dominant species along the urban river, irrespective of origin, had small, wind-dispersed seeds consistent with the continued presence of periodic flooding as an ecosystem disturbance. Tamarix, a species naturalized throughout the American West, was dominant in many urban reaches but co-existed with many other taxa. Riparian species in drier sections of the river had small leaves and large seeds, consistent with reduced water availability. The species in the wet sections of the river were sustained by novel riparian water sources including municipal effluent, storm drain runoff, and pumped groundwater.