SCIENCE OBSERVER
Into the Woods
Frank Diller
If a tree once felled other trees in the woods, but was no longer
around to be studied, would it make for good research?
The American chestnut was once the most common canopy tree in the
deciduous forests of the eastern United States. It shaded areas from
New England to Georgia until the fungus Cryphonectria
parasitica wiped out the species in the infamous blight of the
early 20th century. The fungus continues to kill chestnuts before
they can mature.
The void left by the chestnut's demise is now filled with competing
species. Considerable research has gone into understanding what
happened to the American chestnut. But some scientists remain
interested in getting to the root not of its demise, but rather of
the centuries of dominance enjoyed by the massive, fragrant and
economically important tree. David B. Vandermast, a biology graduate
student at the University of North Carolina at Chapel Hill, and his
colleagues recently posited that Castanea dentata may have
been engaged in the chemically charged competition known as allelopathy.
An allelopathic plant releases potentially toxic substances into the
environment through its roots, its leaves or processes such as
evaporation. Black walnut, sycamore and sassafras trees are just a
few known allelopaths that limit the germination of competitors.
It's likely that the list will soon branch out to include the chestnut.
Vandermast's study, described in the July 15 issue of Forest
Ecology and Management, found that extracts from the
American chestnut leaf significantly limited the germination of
lettuce, eastern hemlock and unstratified rhododendron seeds. The
tree's rapid growth, dense foliage and slowly decaying leaf litter
may have allowed regular rainfalls to douse competitors with
leachate that defended against encroachment. The blight not only
ended the chestnut's dominance but also, as Vandermast notes,
contributed to changes in vegetative composition—and may have
even put chestnuts on the allelopathic receiving end.
Chestnut sprouts can reach five inches in diameter before the fungus
cuts them down, according to Vandermast. The sprouts are still
prolific, but anecdotal evidence suggests that something is limiting
their growth.
"It's possible to find sprouts and dead trees in Southern
Appalachia," Vandermast says. "In areas where there are
dense rhododendron, there are old chestnut stumps, but no
sprouts." The same holds true in areas where hemlock grows
around chestnut stumps. "The hemlock didn't grow alongside the
chestnut [in its pre-blight days], even though it's
shade-tolerant," he says. But now, where there's hemlock,
there's a noticeable absence of chestnut sprouts.
The publication of the allelopathy study hints at the chemical
warfare waged throughout the forests. Coincidentally, it also
commemorates the 25th anniversary of David E. Flora's 1977 master's
thesis, "The American chestnut as an allelopath,"
completed at the University of Tennessee, Knoxville. Flora, under
the supervision of Frank W. Woods (two men whose names suggest they
were destined for careers in forestry), showed that the American
chestnut hindered the germination of radish seeds.
Flora also demonstrated in his discussion section how long it can
take for ideas to germinate into completed research when he noted:
"Tests of germination of native seeds ... might reveal whether
toxins are released from decaying leaves."
Vandermast says he learned of Flora's work after submitting his own
manuscript for publication. After reviewing it, he was relieved to
find that he hadn't duplicated someone else's work. "I was glad
to see that his results compared favorably with mine, and it is nice
that he suggested that someone should do what I
did!"—Frank Diller
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