ARE STREET TREES BEING SUBSIDIZED BY HUMAN WASTE?

Urban landscapes are diverse spatial mosaics that represent a variety of growing conditions for trees. Street trees in particular are limited in growth by stressful urban environments and are often limited by the availability of nutrients, particularly nitrogen, and water due to limited rooting volumes and relatively high evapotranspiration rates. Despite these harsh environments, surveys conducted in urban landscapes find that a surprisingly high proportion of the total number of trees grow successfully in “tree pits” along streets or adjacent to parking lots. Tree pits typically have rooting volumes of 0.86 cubic m or less, which depending on the characteristics of the soil, severely limits the mature size of trees, if they indeed survive. For example, surveys of street trees in Baltimore City show that many street trees grow in maturity up to 75 cms in diameter.

In this study, we explore whether street trees in Baltimore City are subsidized by leaky infrastructure (sanitary and potable water) typically associated with urban streets. To test this hypothesis, we examined the stable isotopic composition of foliage from 24 trees of the species Pyrus calleryana in southeastern Baltimore City. Trees were chosen to represent two major categories: “street trees” and “park trees” located within a large park. Nitrogen isotopic composition of foliage was expected to provide insight on the relative contributions of waste-water derived nitrate to tree nutrition, with higher d15N values suggesting access to sewage. We observe a large range in d15N values (1.3-10.4), with statistical differences (p<0.02) between street (mean=4.3) and park (mean=2.3) trees. These preliminary analyses suggest that nitrogen from human waste can be a significant source of nitrogen (and water) for street trees in Baltimore City with important implications for other cities characterized by aging infrastructure.