What is the future of hemlock? Many people think that hemlock is doomed. This article takes a closer look at some good news and some bad news.
Since 1952, eastern hemlock and its close relative Carolina hemlock have been under attack by the hemlock woolly adelgid, Adelges tsugae, or HWA. The insect, introduced from Japan, sucks the sap out of hemlocks and reduces their vigor. Many trees go into decline and die, or become susceptible to secondary agents of death, such as boring beetles. The insect has now spread to over half the range of eastern hemlock and most of the range of Carolina hemlock.
The spread of the insect has been slowed by a combination of cold winters that killed many adelgids in the north, the use of pesticides in high-priority forest areas, and the introduction of insects that prey on the adelgid. However, many observers predict the elimination of hemlock throughout most of its range.
The death of hemlock in the southern part of its range is compounded by the effects of climate change. Hemlock is very susceptible to heat and drought stress and both are increasing. Even in the absence of the adelgid, the loss of southern populations of hemlock is very likely.
The situation is dire – or is it? Two recent papers give contrasting views of what is currently happening to hemlock. It turns out that predictions about the future of hemlock depend on the scale of observations. Scale is a critically important component for understanding the fate of hemlock. A forester working in a single area of the southern Appalachians may think that the rate of loss of hemlock means that it will be wiped out. Zooming out to the entire range of eastern hemlock may give a different sense.
The Forest Inventory and Analysis (FIA) program of the US Forest Service provides us with the best range-wide view of hemlock, though it excludes Canadian forests. The FIA program is called the nation’s forest census, and that is an accurate description. Every year, state and federal foresters census permanent forest plots all over the country, on private as well as public land. This remarkable program provides forest managers, the timber industry and government with accurate information on the growth and health of our forests.
R. Talbott Trotter and other scientists with the USDA Forest Service analyzed 20 years of FIA data in eastern counties within the geographic range of hemlock, lumping eastern and Carolina hemlock, and compared infested and uninfested counties. FIA data show that forest growth of all species in these counties increased and that, surprisingly, the growth of hemlock increased on concert with the other species. In both infested and uninfested counties, the basal area (a measure of size and density) of live and dead hemlock increased. This was true for both infested and unifested counties.
This does not mean that hemlock woolly adelgid is not killing hemlocks, but that increased growth is offsetting death from HWA. Increased growth is due to natural reforestation of former farmlands, succession that favors shade-tolerant trees like hemlock, and possibly impacts of climate change (increased carbon dioxide and rainfall). Looking at the last 20 years throughout the range of hemlock, the impact of the death of hemlocks due to HWA is overwhelmed by the increased growth of the trees.
End of story? Not at all. Trotter et al found evidence that hemlock in long-infested counties may be at a tipping point, where death of trees due to HWA overwhelms increased growth. Nevertheless, throughout its range, hemlock is growing faster than it is dying. Does this mean there is reason for optimism? Well, yes and no. Let’s take a look at a different scale of analysis.
A group of US and Finland-based scientists led by Tuula Kantola analyzed the death of hemlocks at a much finer scale, looking at all the trees within the Linville Gorge, North Carolina. Some of their results confirmed the FIA analysis – over the entire landscape, the number of dead trees was fairly small. However, it is where the trees died that is important: most of the dead trees were concentrated near the Linville River and at higher elevations. Death of these trees could be catastrophic to the ecosystem of the river, even if the broader death rate is low.
What do we take away from this? It does not appear that hemlock as a species is at risk of extinction over short time scales of a hundred years or so. That may give scientists and forest managers time to develop strategies to avoid the extinction of hemlock. However, the loss of hemlock at specific spatial scales is already having a negative impact on forest ecosystems.