Urban trees play a key role in managing stormwater volume, yet their litterfall leaches nitrogen and phosphorus into stormwater. Litter nutrient content can vary across tree species, and therefore the diversity and identity of species may influence nutrient inputs from litterfall. Although, research to date has focused on deciduous leaf sources from common species, we hypothesized that other seasonal inputs such as blossoms and seeds could contribute to nutrient loads in stormwater. We asked: how has species diversity and community composition changed across the urban forest within a mid-sized Western city? How do potential nutrient inputs from urban tree litterfall vary across species? And how do potential nutrient inputs vary across litter types?
We collaborated with city officials to survey 38,000 trees across the entirety of Pocatello, ID; a semi-arid, mid-sized city. This unique dataset includes city-managed street trees as well as residential trees located near the street; allowing us to characterize and compare community composition across the urban forest by individual neighborhoods and land use types. To assess potential nutrient inputs, we collected litterfall samples from ten common urban trees species. Leachates were analyzed for total nitrogen, total phosphorus, total dissolved nitrogen, nitrate, ammonium, and soluble reactive phosphorus.
Results/Conclusions
Species diversity has declined in Pocatello over the past 50 years, and street community composition has shifted from Ash (Fraxinus) and Elm (Ulmus) dominated to Linden and Maple (Tilia and Acer) with high species variation visible across neighborhood scales. These shifts may have important consequences for the spatial and temporal patterns of litter nutrient inputs. Nutrient leaching varied significantly among species, with litterfall from very common species such as Norway Maple and Siberian Elm litterfall leaching the greatest amount of nitrogen, and Green Ash leaching much smaller amounts. Leaching rates varied significantly by litter type and across species, with blossoms leaching up to three times as much nitrogen as whole leaves and twice as much phosphorus.
Understanding urban forest composition and its role in nitrogen and phosphorus loads to stormwater from litterfall can help cities better design street sweeping, tree planting, and overall urban design practices. The significance of blossom leachates is amplified by observed trends towards flowering ornamental tree use in residential and commercial locations near impervious surface. By incorporating this information, stakeholders can work towards building more sustainable urban landscapes with minimal impacts on water quality.