Colonization of Leaves Produced with Varying Concentrations of Juglone by Temperate Urban Stream Invertebrates

Interest is increasing to restore riparian buffer zones using native long-lived species, and black walnut trees (Juglans nigra) appear attractive candidates across a wide biogeographic range of the eastern United States of America. However, little research has investigated the potential unintended consequences of juglone (the allelopathic chemicals produced by these deciduous trees) on proximate stream communities. Our current investigation examined the colonization of stream macroinvertebrates into leaf packs containing chlorophyllous (spring canopy) leaves of one of four deciduous tree species differing in the levels of juglone and structural stiffness: black walnut, shagbark hickory, tulip poplar, and red oak. To evaluate the consistency of stream invertebrate community response across local streams, we deployed leaf packs for a week in a total of three different streams over three successive summers, then identified and enumerated the invertebrates within the packs. While there were significant differences across streams, the invertebrate communities colonizing the leaves of the different tree species was typically not different, and any differences present were unlikely caused by the level of juglones in the leaves. Thus, increased levels of juglones entering streams due to local plantings of black walnut are not likely to negatively affect the aquatic macroinvertebrates. However, the more rapid fragmentation of black walnut leaves compared with the other species examined suggests that the longevity of allochthonous coarse particulate material resources may be negatively impacted if other tree species are not interspersed with the black walnuts.