This post was originally published on BioDiverse Perspectives – a research blog aimed at fostering communication about biodiversity.
100 years ago, the Eastern United States was a lot different than it is today. Yeah, there was less urban development, and there were fewer paved roads, dams, and railroads, but by far the biggest difference (at least to an ecologist) was the makeup of the forests. 100 years ago, there stood a huge and dominant tree that is now a mere shrub. Prior to 1904, the mighty chestnut was one of the most dominant trees in the entire northeast, comprising as much as 40% of the canopy. Chestnuts grew up to five feet in diameter, and up to 100 feet tall, provided some of the best lumber, and produced some of the most valuable nuts to people and wildlife. But in 1904 in New York City, some chestnuts began to die. The blight, caused by the introduced fungus, Cryphonectria parasitica, quickly invaded North American forests as it encountered American trees with little natural resistance, girdling and eventually killing them. By the 1920’s, Cryphonectria had spread to the Appalachian forests and was quickly heading south, and by the 1930’s, the blight had entirely removed chestnuts from the southern Appalachian forests (McCormick and Platt 1980).
Twenty years after the demise of of the chestnut, researchers saw no evidence that it could ever recover, and until now they have been right. But what if the chestnut got a second chance? What if suddenly and unexpectedly, Cryphonectria disappeared in the temperate deciduous forests of North America? Because Cryphonectria appears to have been the only initial factor limiting the growth and abundance of chestnut, it is reasonable to believe that if it were to disappear, chestnut populations could recover back to their pre-blight status. And had humans contributed little else to change the forests since the early 20th century, disappearance of Cryphonectria could have as great an effect on Eastern US forests it had twenty years after its arrival to North America. However, due to current and past land-use, chestnut interacts with a different suite of co-occurring species and must contend with a shifting climate. Therefore, while deleting Cryphonectria may change deciduous forests, it would do little to restore the chestnut as a dominant species.
To illustrate my point, lets delve a little more into the biology of the pathogen and the host.
What is Cryphonectria?
Crhyphonectria, a member of the phylum Ascomycota, grows on the shoots of Castanea. The mycelium produces a canker inside the bark of the tree, and once the canker has encircled the entire stem, it girdles and kills it. This leads to the death of the majority of the tree. But Cryphonectria never enters the roots, leaving them intact to produce shoots known as stump-sprouts. As a result, chestnut is reduced to an understory shrub rather than being completely eked out.
What does this mean for the chestnut? It means that with the disappearance of Cryphonectria, Chestnuts would not have to rely solely on seed dispersal to begin returning back into the forests.
A Closer Look at the Chestnut Tree, Castanea
The chestnut is a highly efficient seed disperser. And this may be one of the primary factors that contributed to the past dominance of the Chestnut tree in North American forests. Chestnut seeds are nuts, which are protected by a thick husk that allows the seeds to survive past their most vulnerable stage to one that is more conducive to dispersal. Chestnuts also produce massive quantities of seeds, and they produce them mid-summer, which protects the nuts from the potential impacts of frost. The nuts are also highly palatable and rapidly harvested by animals, which aids in dispersal. This means that if chestnuts could grow large enough to produce seeds and become abundant enough to overcome seed predation, they could potentially proliferate quickly towards forest dominance. And because Castanea can produce shoots from already existing (and rather abundant) root stock, they should be able to produce seeds quickly, relative to reestablishing from seed.
What happened to North American deciduous forests after the Chestnuts died?
We don’t know a lot about chestnut ecology before the blight, but thanks to one plot of land on Beanfield Mountain, we know something about how forests responded to the chestnut’s demise. Prior to the blight, Castanea was a co-dominant species in all the sloped forests of Beanfield Mountain. In 1939, about twenty years after introduction of Chestnut Blight to the mountain, the only perceptible difference in composition was the absence of chestnut. After about 50 years, openings in the forest canopy were eventually filled by hickory, as Eastern US forests shifted from chestnut/oak to oak-hickory dominated. Then, largely due to fire-exclusion during the early 20th century, populations of red maple invaded forests, and now red maple is one of the most abundant trees in eastern forests (Lord 2004).
How did chestnuts get to North America last time?
Long before they were deciduous, North America was home to boreal forests. Then, around 16,000 years ago in the south, and 10,000 years ago further to the north, deciduous forest began to take over the landscape. But Castanea was the slowest species of tree to establish, expanding at roughly 100 meters per year, and reaching sites near Connecticut only as recently as 2,000 years ago, even though it was present near Memphis 13,000 years prior. Margaret Davis (1983) suggested that that despite high seed production and dispersal, the fact that Castanea is self-sterile could be a leading factor for such a slow rate of establishment, and still account for the huge proliferation once established.
What does this mean for the chestnut tree?
What does this tell us about reestablishment of chestnut following the disappearance of Chryphonectria? First, though chestnuts are not currently producing seed, they do so rapidly, yet they distribute very slowly as a species. Second, despite their slow dispersal, they still exist in many areas as small shrubs awaiting an opportunity to grow. And third red maple is their primary competitor in a world without chestnut blight.
Factors Limiting Reestablishment of Castanea
Ok, you’re saying, the chestnut’s chances don’t sound so dire. They are already in forests, and they produce massive quantities of seed when mature. What’s the deal?
Plant competition for light is asymmetrical. With a large enough canopy, trees can effectively block sunlight from reaching the branches of lower plants, slowing growth and seed production. The chestnut has been reduced to an understory shrub. And although plants can survive for decades in the shade, and despite being widely considered one of the fastest growing canopy trees in the Appalachian forest when under direct sunlight, the chestnut’s growth is heavily stunted by shade (Bass 2002). As a result, despite release from Chryphonectria, it would take chestnuts decades to grow large enough to produce seed and widely disperse throughout the forests.
Red maple, a current forest dominant, is one of the most sensitive forest trees to burning, and periodic fires resulting from Native American activities, lightning strikes and European settling practices were likely key factors in suppressing red maple populations in the past. Perhaps with the re-introduction of fire into Eastern US Forests, canopy openings could allow new chestnut stump-shoots to grow and become dominant. Sadly, current fire practices would need to change on a scale beyond any we’ve seen before, and so it appears that despite an absence of Cryphonectria as a shoot limiting factor, the red maple will still limit chestnut growth in its absence.
And human fire practices are not the only factors keeping chestnut trees in check. As a result of the excessive killing of Wolves in the past, deer populations have increased. And because deer tend to choose other shoots over red maple (Abrams 1998), there could be a problem of excessive deer browsing of chestnut shoots were they able to grow to substantial size and health. This could further deter them from recolonizing the forests and give the red maple another advantage.
Last, tree reestablishment would have to contend with shifting climate regimes. Suppose chestnut trees were able to overcome the shading from red maple and preferential selection by browsers. They might be able at maximum growth, to reach half the height of the canopy in twenty years, and reach the full height in 80 (Bass 2002). But in 80 years will current North American forests still be the ideal location for Chestnut trees? It was once predicted that range extensions due to climate would require a dispersal rate over 200km per century, that’s over 2km per year (Davis, 1989). With a dispersal rate of 100m per year, it seems unlikely that Castanea could keep up with increasing temperatures.
It’s sad to think that our introduction of Cryphonectria over 100 years ago, failed to serve as an effective warning, and that through historical resource use, we managed to prevent the forests from being successfully reinhabited by a once dominant and majestic tree.
Want more information about Chestnut blight and the chestnut tree? Check out these resources:
Bass Q. 2002. Talking Trees: The Appalachian Forest Ecoysystem and the American Chestnut. The Journal of The American Chestnut Foundation 16:42-55.
Davis M. B. 1983. Quaternary History of Deciduous Forests of Eastern North America and Europe. Annals of the Missouri Botanical Garden 70:550-563.
Delcourt H. R. 1979. Late Quaternary Vegetation History of the Eastern Highland Rim and Adjacent Cumberland Plateau of Tennessee. Ecological Monographs 49:255-280.
Keever C. 1953. Present Composition of Some Stands of the Former Oak-Chestnut Forest in the Southern Blue Ridge Mountains. Ecology 34:44-54.
Lord B. 2004. The Red Maple, An Important Rival of the Chestnut. The Journal of The American Chestnut Foundation 18:42-47.
McCormick J. F., R. B. Platt. 1980. Recovery of an Appalachian Forest Following the Chestnut Blight or Catherine Keever-You Were Right! American Midland Naturalist 104:264-273.
Paillet F. L. 2002. Chestnut: history and ecology of a transformed species. Journal of Biogeography 29:1517-1530.
Steele, M.A., McCarthy, B.C. & C. H. Keiffer. 2005. Seed Dispersal, Seed Predation, and the American Chestnut. The Journal of The American Chestnut Foundation 19:47-55.
VANDER WALL S. B. 2001. The Botanical Review; The Evolutionary Ecology of Nut Dispersal. 67:74.
Woods F. W., R. E. Shanks. 1959. Natural Replacement of Chestnut by Other Species in the Great Smoky Mountains National Park. Ecology 40:349-361.
*Disclaimer: This post is largely plagiarized from a paper that I wrote as an undergraduate, which is one reason why it’s so long, and also may explain why the citations are so old.