Allelopathic effects of black walnut on european black alder coplanted as a nurse species

Black alder trees were interplanted with black walnut on a 2.4 × 3.7-m spacing at four locations in Illinois and Missouri. In every plantation, the alders suddenly declined and died after 8–13 years. Alternative causes for the mortality were investigated–competition from walnut, adverse soil properties, frost, insects, disease, unsuitable seed source, and allelopathy—but only allelopathy could be substantiated. The decline in alder survival and diameter growth was strongly related to walnut above-ground dry biomass and walnut crown competition factor. We propose that the black walnut allelopathy most likely results from a combination of (1) sufficient walnut biomass to contribute substantial amounts of juglone to the environment, and (2) wet soil that greatly restricts aerobic metabolism by soil microorganisms, allowing juglone to build up to toxic levels.