Urban trees drive stormwater nutrient pollution: The role of phenology and litter chemistry
Stormwater flowing over streets, similar to headwater streams, acquires substantial inputs of nutrients from tree litter and other materials. However, many urban streams and downstream water bodies suffer from excess inputs of nitrogen (N) and phosphorus (P), which lead to noxious algal blooms that cause lower water clarity and dissolved oxygen levels, bad odor and taste, and the loss of desirable species. In this study, we examined the role of urban boulevard trees driving N and P inputs to urban street gutters and stormwater. During the 2012 growing season (April-November), we hand-swept biweekly samples of material from the street gutters on city blocks dominated by one of four common tree species that differ in both litter chemistry and phenology (Acer platanoides, Fraxinus pennsylvanica, Quercus palustris, Tilia cordata), as well as blocks without tree canopy cover. We size-fractionated these samples (2mm), measured total carbon (C), N, and P, and also leached a subsample in lab as an index of soluble C, N, and P available for transport during rain events.
Differences among tree species in the total amount of nutrients in the street gutters were driven primarily by interspecific differences in the mass of litter dropped, which were much greater than differences in litter chemistry. Autumn litterfall transported 219.0-274.4 kg N km-2 and 14.2-20.6 kg P km-2 to the street gutters (range is across our study species). Total N and P concentrations decreased during the growing season, driven by changes in the chemistry of litter inputs, while soluble nutrients showed no clear seasonal patterns or differences among species. C and N were less soluble than P; generally less than 5% of total C or N leached out during our lab leaching measurements, in contrast with 10-20% or more of P. N leached out primarily in organic forms, whereas P leached out mainly in inorganic forms. We found that tree phenology is a much more important consideration than litter chemistry for choosing boulevard tree species and developing management recommendations. Cleaning up spring and autumn pulses of tree litter shortly after they fall has substantial potential to reduce nutrient inputs to stormwater. Because of the wide variation in species' litterfall timing, achieving this goal is likely to require adjusting both boulevard tree selection and litter cleanup strategies.