Can urban trees help protect our lakes and streams?

From New York City to Los Angeles, cities throughout the United States are aiming for the “Million Trees” mark and expanding their urban forests.  Urban trees are known to enhance human well-being in many ways, from improving air quality to reducing crime rates, but less is understood about how urban trees can affect the water quality of local lakes and streams.  Many urban waterways suffer from excess nitrogen and phosphorus feeding algal blooms, which cause lower water clarity and oxygen levels, bad odor and taste, and the loss of desirable species.  The expansion and turnover of urban forests present a large-scale opportunity for homeowners, city foresters, and other land managers to select species that reduce nutrient pollution and improve the water quality and ecosystem service provisioning of local lakes and streams.

In this study, we examine how common urban tree species affect nitrogen and phosphorus inputs to stormwater and leaching to groundwater.  To study nutrient leaching to groundwater, we are sampling thirty-three trees of fourteen species, and seven open grassy areas, across three city parks in Saint Paul, Minnesota.  We installed lysimeters at 60cm depth to collect soil water and measure nitrogen and phosphorus concentrations approximately biweekly, and installed tensiometers at 45cm and 75cm to measure matric potential gradients and calculate water flux.  We collected soil samples from 0-10cm, 10-20cm, 20-40cm, and 40-60cm as well as leaf, root, and leaf-litter samples, for carbon, nitrogen, and phosphorus analyses.

To study how street trees affect nutrient inputs to stormwater, we are monitoring blocks lined with four different species of street trees, as well as blocks without canopy cover, in a residential neighborhood of Saint Paul, Minnesota.  We are collecting material from the street gutter biweekly throughout the growing season, size-fractionating and quantifying it, and measuring total and 30min-leachable carbon, nitrogen, and phosphorus.  We are also measuring throughfall volume and chemistry under the different species, as well as snowmelt and runoff chemistry entering the storm sewers on our study blocks. 

Preliminary results:

Leaching to groundwater: A prolonged drought in 2011-2012 prevented us from collecting lysimeter water during leaf-drop and snowmelt to date, the seasons we expect to have the highest nutrient leaching to groundwater.  Data from July-August 2011 and April-June 2012, however, show significant differences in total N and P among grass, conifer, and hardwood sites (TN mg/L: grass=8.7, conifer=5.9, hardwood=3.5; p=0.004; TP ug/L: grass=116.4, conifer=73.8, hardwood=41.0; p=0.0004).  Total P concentrations are significantly higher than expected for most soils, with a grand mean of 60ug/L that is higher than the lake-eutrophication standard of 50ug/L.  P concentrations are best predicted by leaf C:N ratio, not leaf P, whereas N concentrations are best predicted by a combination of soil C:N ratio (0-10cm) and 10d net mineralization rate.  N has a significant pattern with date (p=0.01) and appears to decrease during April-June 2012, whereas P shows no seasonal pattern so far.

Street trees and stormwater: Our curbside-sweeping samples are still being processed and analyzed, but runoff (September 2011) and snowmelt (2012) sampling show significant differences among species.  In particular, city blocks lined with pin oaks have the highest N and P during September runoff, and the highest P but lowest N during snowmelt events.

Related Materials and Graphics: 

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