Ash Trees in the National Capital Region's Parks, 2022 Update

Ash trees keep declining

Emerald ash borer (EAB), an invasive beetle first found in the National Capital Region in 2014, has killed most of the 300,000 ash trees once growing in the region's parks. It attacks every ash species, laying eggs in cracks in the bark so the larvae can feed on the woody tissue that carries water and sugar through the tree. Nearly 100% of ash trees infested with EAB have died.

Monitoring by the National Capital Region Inventory and Monitoring Program (NCRN I&M) shows fewer than 56,000 ash trees still alive in the region as of 2022. Dead ashes keep adding to the number of standing dead trees, or snags, which may be close to peaking; over the next few years the snags will come down, in both senses, as they fall to the forest floor.

Ash tree density in the region's parks: living trees (green) and standing dead trees (black), in rolling 4-year monitoring periods labeled by their last year (5-year periods for 2021 and 2022). Vertical lines show estimated error. NPS.

But seedlings and saplings are increasing

NCRN I&M tracks ash of every size. While large trees keep collapsing, seedling density has spiked, and saplings are beginning to increase too.

The seedling boom likely has several causes:

• ash naturally produces many small seeds that sprout quickly;
• stressed canopy trees release an extra burst of seed before they die;
• park deer management is helping more seedlings survive;
• dying canopy trees let more sunlight reach the forest floor.

The recent rise in saplings is probably due to stump sprouting by green and pumpkin ash in wetlands, trees hit hard by EAB but not fully killed. The sprouts are big enough to count as saplings (1 to 10 cm diameter at breast height) but not yet big enough to attract the beetle. Whether these shrub-like ashes will stay in that state indefinitely, or eventually grow into trees that flower and set seed, is unclear.

Density of ash trees and saplings (left axis) and seedlings (right axis) over four monitoring intervals. Plots are sampled on a 4-year rotation; monitoring paused in 2020 for COVID-19, so the last interval spans 5 years. Seedlings are taller than 15 cm but under 1 cm in diameter at breast height. NPS.

What comes next

It is too early to be sure. I&M ecologists expect ash to keep declining for the next few years, and large ash trees are likely to become rare in the region's parks. Seedlings are plentiful, but EAB is unlikely to let any of them reach maturity. Wetland ash may follow the path of the American chestnut: seedlings grow into saplings or young trees big enough to attract the beetle, die, and start over, rarely reaching the canopy or producing viable seed.

Biological control could improve the outlook. The U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) has released several parasitoid wasps from EAB's native range in East Asia, where they are the beetle's natural enemies. By 2020 APHIS had released over 8 million of these stingless wasps in 30 states and the District of Columbia, and had recovered their offspring in 22 states, including Maryland, Virginia and DC — evidence that the wasps are establishing, reproducing and killing EAB in the region. The aim is a thriving wasp population in every county with EAB. Whether that lets ash recover its former place in the region's forests, only time will tell.

The four species of tiny, stingless parasitoid wasps APHIS released against emerald ash borer. USDA APHIS.

NCRN I&M, one of the National Park Service's Inventory & Monitoring networks, serves the national parks of the greater Washington, D.C., area; earlier ash updates appeared in 2017 and 2021.

Sources

Rewritten from the National Park Service article "Ash Tree Update 2022" by Nicholas Tait, NCRN I&M science communication intern (public domain), in hubnx's own words. Its cover photograph of an emerald ash borer carries no credit and is not reproduced.