American chestnut as an allelopath in the southern Appalachians
Prior to the chestnut blight (Crypkonectria parasitica), American chestnut (Castanea dentata (Marsh.) Borkh.) was the most common overstory tree in eastern deciduous forests. Chestnut's dominance has often been attributed to its resistance to fire and subsequent propensity to sprout vigorously and grow rapidly. Its role as an allelopath has rarely been studied.
Allelopathic qualities of chesnut leaves were tested with five native co-occurring tree species: red maple (Acer rubrum), sugar maple (A. saccharum), eastern white pine (Pinus strobus), eastern hemlock (Tsuga canudensis), yellow-poplar (Liriodendron tulipifera), a native shrub rosebay rhododendron (Rhododendron maximum), and a bioassay species lettuce (Lactuca sativa var. "black seeded Simpson"). For each species, six replicates of 100 seeds each were stratified for 90 days in distilled water or chesmut leaf extract, then germinated for 21 days. Six additional replicates of red maple, eastern hemlock, yellow-poplar, and rhododendron were germinated without stratification. Lettuce seed was not stratified. When germination percentage peaked, seeds were removed from the experiment and radicle length was measured. Chestnut leaf extract lowered germination rates of extract-treated lettuce, stratified and unstratified eastern hemlock, and unstratified rhododendron seeds. Radicles of extract-treated lettuce and unstratified rhododendron were significantly shorter than radicles of water-treated seeds. In general, radicles of extract-treated seeds were thinner, broke more easily, and were less likely to have developed secondary roots than radicles of water-treated seeds. This study suggests leachate from American chestnut leaf litter could have suppressed germination and growth of competing shrub and tree species and that allelopathy was a mechanism whereby American chestnut may have controlled vegetative composition and dominated eastern forests. Current vegetative composition in southern Appalachian forests may be attributable, in part, to the disappearance of American chestnut as an allelopathic influence.