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Allelopathic effects of black walnuts

Kelsey Nosek

Black walnut trees release juglone to inhibit germination or growth of surrounding plants.  My goal was to research the negative effects of black walnut allelopathy on surrounding vegetation.

Data were collected along Big Walnut Creek.  I paired each walnut tree with a box elder tree as a control, and measured tree and sapling density within a 3 meter radius from the densest part of the tree's canopy.  

The data showed the opposite pattern from what I expected.  The density of trees and saplings was higher around black walnut trees compared to box elder trees.

Possible explanations for these patterns

  • Other plants are tolerant of juglone toxin
  • Black walnut trees vary in production of juglone
  • Juglone needs to build up to a certain threshold for effective interference
  • Juglone in soil creates slightly basic environment
  • Microorganisms ingest and decompose toxins for energy, reducing the amount in the soil
  • Canopy of black walnut trees is relatively open, allowing more light to reach understory plants