Can urban trees help protect our lakes and streams? Species effects on nitrogen and phosphorus leaching

Background/Question/Methods

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 examined how fourteen common urban tree species affect nitrogen and phosphorus leaching to groundwater, a major pathway supplying water and nutrients to local waterways.  We sampled thirty-three trees (23 hardwoods and 10 conifers) 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, 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.

Results/Conclusions

Although a prolonged drought in 2011 prevented us from collecting sufficient lysimeter water to date, initial data from soil and leaf chemistry indicate strong species effects on biogeochemical cycling.  These urban soils show the typical pattern of decreasing %N and %C and increasing soil C:N with depth.  C:N ratios provide the clearest link between tree tissue chemistry and surface soil chemistry.  Leaf C:N ranged from 25.2-34.5 (mean=29.5) for conifers, and from 11.2-22.1 (mean=16.4) for hardwoods.  Soil C:N in 0-10cm ranged from 12.6-16.5 (mean=13.9) under conifers, from 10.9-14.6 (mean=12.4) under hardwoods, and from 10.9-12.9 (mean=11.7) under open grassy areas. Even though leaf litter in these parks comes from a mix of nearby species, we found a positive relationship between tree leaf C:N and 0-10cm soil C:N (p=0.0003, R²=0.34).  We are currently analyzing fine roots from our sample trees, as well as tissue phosphorus chemistry, to further understand the effects of tree tissue chemistry on soil nutrient cycling.