Allelopathy in the southern woodlands appears to be more prevalent than previously thought. Black walnut was once considered the principal antagonist, exuding juglone from its roots, leaching the natural herbicide from its foliage, and washing it from the green shucks of walnuts in late summer and winter rains. Even black walnut seedlings growing beneath parent trees succumb to the effect of the chemical. Sugar maple also releases a chemical that inhibits growth of yellow birch when the two species are growing together, providing an advantage for the former species (even though the two often occur together in northern hardwood forests). Unknown is whether southern sugar maple behaves similarly when found with river birch along southern streams. Black cherry also may be allelopathic, its foliage upon wilting releasing cyanide from the bound form, cyanide glycoside. This exudate is toxic to cattle; deer eat only wilted foliage. Several other chemicals, including citrol and a form of camphor, released by sassafras, seem to enable that species to maintain itself in pure stands in old fields. (An extract from roots provides an ingredient for perfume.)

Allelopathy occurs in the Louisiana Delta country when broomsedge grass grows among red oak, cherrybark, and sweetgum trees. There, foliar and root extracts of the low-quality grass detrimentally affect some species. Sweetgum appears to be excluded from these sites because of salicylic acid leached from cherrybark oak crowns by rain. This chemical is tied so loosely in foliage that it can be leached with cold water. Toxic products are also released by the action of microflora in the decay of their own roots.

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