Eastern hemlock typically grows in shady environments, but its world is now infested by hemlock woolly adelgids (HWA). The miniscule sap-sucking insects can kill mature trees in less than five years.
“Eastern hemlock is a shade-tolerant species,” says USDA Forest Service research entomologist Bud Mayfield. “But extra sunlight may help it survive HWA infestation.”
Extra sunlight equals fewer HWA, at least on potted hemlock seedlings grown under shade cloth.
Mayfield was part of the team that documented this relationship in 2017 and recently contributed to a follow-up study on the same potted seedlings. Marika Lapham, currently a graduate student at Indiana University, led the follow-up study, which was published as a brief communication in the journal Forest Science. The 2017 study was led by Steven Brantley, who also contributed to the follow-up, along with Chelcy Miniat and several other scientists from Coweeta Hydrologic Laboratory, and Robert Jetton of North Carolina State University – Camcore.
Some of the seedlings were grown under shade cloth that mimicked full shade conditions, while others received more sunlight. Before the shade cloth treatment, all seedlings had similar foliar nutrient concentrations. After nine months in the shade tents, a relationship emerged: more sunlight, lower nitrogen concentrations; less light, more nitrogen. For each 10 percent increase in shade, foliar nitrogen concentration increased significantly.
The team then artificially infested all 100 potted eastern hemlock seedlings with HWA. HWA settled on the shaded seedlings more than the seedlings in the sun. On seedlings with greater HWA infestation, foliar nutrient concentration continued to increase.
“These results suggest two things: that HWA infestation may be more severe in heavily shaded environments,” says Miniat. “And nutrients may be mobilized to the needles after infestation to create even greater susceptibility in these shaded environments.”
Like the 2017 study, the findings suggest that silviculture can play a role in saving hemlocks.
Silvicultural prescriptions would be a welcome addition to the current arsenal of chemical control, biocontrol, genetic resource conservation, and host resistance. The expansive approach is known as integrated pest management or IPM.
“We’re not ready to recommend that managers cut gaps around hemlocks in the forest – we’d like to continue our current study a few years to see if effects are consistent over time,” says Mayfield. “But our preliminary results do suggest that silviculture can help.”
Several field trials are underway. In a trial led by Miniat, the team is testing field gaps of a quarter acre around hemlock trees in infested and un-infested areas, as well as areas treated with biological controls. The project will test whether silvicultural treatments plus biocontrol efforts, which enlists predatory beetles to help control HWA populations, are more beneficial for eastern hemlock than silvicultural gaps or beetles alone.
In a second study, Mayfield and his colleagues are creating two sizes of gaps around target hemlock trees using two different methods (felling vs. girdling neighboring trees). The scientists are monitoring eastern hemlock response to the increased light. The first year of results suggests that the extra light is reducing numbers of HWA and improving hemlock growth.
“HWA is a heavy load for Eastern and Carolina hemlocks,” says Mayfield. “Silviculture is not a silver bullet, but it may be able to help hemlock bear that load.”