The role of trees in mediating land-water nutrient flows in urban landscapes

Background/Question/Methods

Eutrophication is a major environmental problem in aquatic ecosystems influenced by urbanization.  Understanding the sources of nutrients to urban waters is critical to improving urban water quality and enhancing the ecosystem services provided by urban lakes and streams. We present a conceptual model of the myriad mechanisms by which city trees influence land-water nutrient fluxes – through leaching to groundwater, and interception, throughfall, and litterfall that alter both water and nutrient inputs to impervious surfaces.  Then we present evidence that litterfall inputs to streets can be a significant source of nutrients to stormwater.

We estimated litterfall into streets from a study of nutrient recovery by street sweeping. Litterfall was estimated from the coarse fraction (>2mm) recovered when streets with varying canopy cover were swept four times per month for the entire snow free season. After sweeping, we analyzed swept material for nitrogen (N) and phosphorus (P), and developed regressions between annual litterfall N and P and canopy cover using remote sensing-derived canopy mapping over streets. Then, we used these relationships to estimate canopy cover and litterfall to streets for six watersheds in St. Paul, Minnesota, USA. For each of these watersheds, we have estimates of warm-season (April – October) N and P export in stormwater from a six-year monitoring program.

Results/Conclusions

When compared to growing season export of N and P in stormwater, litterfall N and P fluxes were substantial, equal to 30 to >100% of total stormwater nutrient export from the watershed. Consistent with watershed flux results, tree cover over streets was significantly related to flow-weighted average stormwater nutrient concentrations across watersheds. Seasonally, average stormwater N and P concentrations peaked in spring and fall, consistent with the timing of litterfall inputs to streets. These results suggest that tree litterfall into streets is an important source of nutrients to urban surface waters and that street sweeping could be modified to be a more effective strategy for reducing nutrient loading of stormwater and improving urban water quality.