Visit almost any location in southern mountains, however, and you will likely witness not just a forest covered in hemlocks but a forest in which those hemlocks are dying. The cause is a foreign insect pest native to Asia and introduced to the U.S. in the 1950s called the hemlock woolly adelgid (HWA for short). HWA adults are so small that they can barely be seen with the naked eye, but the effects an infestation can have on a single tree or forest of hemlocks can be devastating. Infested trees can be identified by HWA’s characteristic white “puffballs” that form on the trees' branches and serve as the insects’ egg sacs. Adults attach to hemlock branches and feed on nutrients from the trees’ sap. Once a tree becomes heavily infested, it begins to brown and lose needles. If an infestation continues unchecked, the tree will almost certainly die.
In just a decade or so, HWA has moved rapidly down the spine of the Appalachians from Virginia to Georgia. What once were healthy, shaded forests of old growth hemlocks in the Great Smoky Mountains, for example, are now virtual graveyards of bleached, standing dead trees. In the wake of this infestation, foresters and biologists are trying several approaches to managing the pest, but many are beginning to realize that part of their focus should be on protecting some existing trees as a source for the repopulation of forests once the adelgid has done its damage.
HWA egg sacs on a hemlock frond (Photo: Connecticut Agricultural Experiment Station Archive, Connecticut Agricultural Experiment Station / © Bugwood.org / CC-BY-3.0-US)
Since foresters cannot protect every existing hemlock for this effort (due to the constraints of time and money), several groups are specifically borrowing an evolutionary approach to determine which trees or stands of trees represent the best candidates for seed-gathering once many of our hemlock forests have declined. This approach is based on examining the genetic diversity of hemlock forests across the Appalachians, selecting stands with the highest amounts of genetic variants, called “alleles,” as priorities.
Genetic diversity relates to evolutionary biology for several reasons, primarily because the pillar of evolutionary theory is that all species change over time and across generations. Genetic diversity relates to this change because diversity is the “stock” through which change can occur; our genes code for the assembly of proteins, which ultimately make up our cells and help direct processes within our cells and our bodies, at large. In a very basic sense, more genetic diversity means that a species can adapt more easily to change – in other words, more “code” exists that is capable of assembling new proteins, and therefore helping organisms survive new environments or threats.
If you’re a hunter or fisherman, you’re likely already familiar with this evolutionary concept, even if you’ve never heard it placed in technical terms. One reason that “bag limits” are placed on wildlife species, for example, is to keep wildlife populations large enough to prevent genetic diversity from crashing and allowing rare genetic defects to predominate. One purpose of wildlife translocations – in which organisms like deer are moved from one forest to another, or when fish are transferred into a stream from a hatchery – is to introduce new alleles (forms of the same gene) into a population and ultimately make those organisms more adaptable to change.
This idea relates to current efforts with the hemlock because biologists want to select seeds from trees with high genetic diversity, or trees which have the capability to better adapt to new conditions when planted in a new environment. The hope is that if HWA does succeed in wiping out many of the Appalachians’ hemlock forests, these seeds could be collected ahead of time, protected until the HWA infestation has lessened, and ultimately replanted in areas where hemlock forests used to exist. In this sense, genetic diversity serves as a form of insurance for the hemlocks’ survival, ensuring that any trees that must be replanted have at least the genetic background to survive their new life.
Biologists have specifically identified several areas within the southern Appalachians where such “hotspots” of genetic diversity exist. Several of these are located near the fringes of the hemlock’s habitat, such as the Blue Ridge Escarpment of South Carolina. Outside of their applied use a conservation tool, these areas may also help illustrate the recent evolutionary history of the species, since the pattern shown by the hemlock suggests a relatively recent expansion of the hemlock’s range across Appalachia since the last ice age. Genetic diversity can therefore act as both a window into the future for conservation efforts, as well as a glimpse into the species’ past. ______________________________________________________________________
See it for yourself
Sipsey Wilderness (AL): Hemlock forests untouched (for now) by HWA
Cataloochee Valley (Great Smoky Mountains NP, NC): Hemlock forest heavily impacted by HWA
Roaring Branch Cove (VA): Old-growth hemlocks; some over 300 years old - smartphone guide available here
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Relevant articles from the scientific literature:
Nuckolls, A. et al. 2009. Hemlock Declines Rapidly with Hemlock Woolly Adelgid Infestation: Impacts on the Carbon Cycle of Southern Appalachian Forests. Ecosystems. 12:179-190.
Potter, K.M. et al. 2008. Allozyme variation and recent evolutionary history of eastern hemlock (Tsuga canadensis) in the southeastern United States. New Forests. 35:131-145.
Zabinski, C. 1992. Isozyme variation in eastern hemlock. 22:1838-1842.
