Sustain Our Nation's Forests and Grasslands

The promise of silvicultural treatments

A hemlock tree
Eastern hemlock released from shade in field experiment testing different silvicultural treatments to help control hemlock woolly adelgid (HWA). USDA Forest Service photo by Bud Mayfield.

NORTH CAROLINA – Eastern hemlock typically grows in shady environments, but its world is now infested by hemlock woolly adelgids. A miniscule sap-sucking insect that 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.”

Mayfield recently contributed to a study on the same potted seedlings, which was published as a follow-up study in the journal Forest Science to a 2017 study, co-authored by Mayfield along with Chelcy Miniat and a team of other scientists.

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 of the 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.”