Eastern hemlock (Tsuga canadensis) is a coniferous tree species that plays a vital role in the forest ecosystems of eastern North America.
It provides habitat and food for many wildlife species, moderates stream temperatures and water quality, and enhances soil fertility and carbon sequestration.
However, this valuable tree is under threat from an invasive insect called the hemlock woolly adelgid (HWA), which feeds on the sap of hemlock branches and causes defoliation, reduced growth, and eventually death.
HWA was accidentally introduced to North America from Asia in the 1950s and has since spread to 20 states and four Canadian provinces, affecting millions of hemlock trees.
The insect has no natural enemies in its new range and can reproduce rapidly, making it difficult to control.
Chemical insecticides can be effective, but they are costly, labor-intensive, and potentially harmful to the environment.
Biological control agents, such as predatory beetles, have been released to reduce HWA populations, but their impact is still uncertain and variable.
In this context, a new study by researchers from North Carolina State University and the U.S. Department of Agriculture suggests that another strategy could help hemlock trees withstand HWA infestation: creating canopy gaps in the forest.
Canopy gaps are openings in the forest cover that allow more sunlight and other resources to reach the understory plants.
The study found that hemlock trees that had canopy gaps created around them showed improved crown health and shoot growth compared to control trees that did not have canopy gaps.
The Effects of Canopy Gaps on Hemlock Crown Health and HWA Infestation
Dan Kitwood/Getty Images
The study, published in the journal Forest Ecology and Management, involved 105 hemlock trees in national and state forests along the Appalachian Mountains from Maryland to Georgia.
The researchers created small or large canopy gaps around the trees by either felling or girdling the competing trees.
Felling is cutting down the tree outright, while girdling is killing the tree by removing its access to nutrients, but not cutting it down. The gaps around the hemlocks ranged in size from 0.05 to 0.15 acre.
The researchers monitored the health of the hemlock trees every six months from late 2017 to early 2021.
They measured the crown density, color, and dieback of the trees, as well as the number of new shoots produced annually.
They also assessed the HWA infestation level of each tree using a scale from 0 (no HWA) to 5 (severe HWA).
The results showed that all of the canopy gap treatments improved the crown health of the hemlock trees, especially in areas where large gaps were created by felling.
The canopy gap trees had higher crown density and color scores, lower crown dieback scores, and more new shoots than the control trees.
The canopy gap trees also had lower HWA infestation levels than the control trees, although this effect was not statistically significant.
The researchers hypothesized that the increased availability of sunlight (and possibly other limited resources) resulting from canopy gaps improved the physiological tolerance of hemlock trees to HWA infestation and could complement biological control by providing a more stable source of new shoots with adelgid prey for predatory insects.
The Pros and Cons of Canopy Gap Management for Hemlock Conservation
The study suggests that canopy gap management could be a useful tool for foresters and land managers who want to protect hemlock trees from HWA infestation.
By creating canopy gaps around hemlock trees, they could enhance their health and resilience, as well as their ecological functions and services.
Canopy gap management could also benefit other understory plants and animals that depend on hemlock forests.
However, the study also acknowledged some limitations and challenges of this approach. For instance, creating canopy gaps requires labor and equipment, which may not be available or affordable for some landowners or managers.
Moreover, creating canopy gaps may have unintended consequences for other aspects of forest structure and function, such as fire risk, wind damage, invasive plants, or carbon storage.
Therefore, canopy gap management should be carefully planned and monitored to ensure its effectiveness and sustainability.
The study also noted that canopy gap management alone may not be sufficient to save hemlock trees from HWA infestation in the long term.
Rather, it should be integrated with other pest management tactics, such as chemical insecticides or biological control agents, to achieve optimal results.
Furthermore, canopy gap management may not be applicable or desirable in all situations or locations where hemlock trees occur.
For example, it may not be feasible or appropriate in areas with high conservation value or cultural significance.
Therefore, the study concluded that canopy gap management should be considered as one of several options available for hemlock conservation and restoration in eastern North America.
By combining different strategies and adapting them to local conditions and objectives, foresters and land managers could increase the chances of preserving this important and threatened tree species.
