Emerald Ash Borer in National Capital Parks: 2021 Update

An ash swamp at Piscataway Park in 2007, before emerald ash borer (top), and in 2019 (bottom): the canopy is gone, light floods in, and green and pumpkin ash stumps are resprouting. NPS.

Key points

• The invasive emerald ash borer (EAB) has killed most of the 300,000-plus ash trees in the National Capital Region's national parks since 2014. Monitoring in 2017–2021 found fewer than about 80,000 still alive.
• Ash once grew at 17–18 trees per hectare across these parks; now it is no more than 5.
• In some ash swamps, root systems resprouted, turning forest into shrubland. In dry habitats, EAB killed trees faster.
• Standing dead trees (snags) have increased, but will decline as they fall.

The beetle

Emerald ash borer (Agrilus planipennis) is an invasive beetle that attacks ash trees. It has been active in the region's parks since at least 2014. Where it arrived in the early 2000s, it killed nearly 100% of ash trees. The National Capital Region is much the same — except in ash swamps, forested wetlands that flood regularly.

Ash keep declining

More than ten years into the invasion, ash density is still falling. Before EAB, the National Capital Region Inventory and Monitoring Network's forest plots put the parks' ash at more than 300,000 trees.

Ash density in National Capital Region parks. Green: living trees; black: standing dead trees; bars show estimated error. Each point covers four years, labeled by its last year (2021 covers 2017–2021). NPS.

The dead trees are swelling the number of snags, which may be near their peak. In the next few years they will decline — "both literally and numerically" — as they fall.

More seedlings

Field crews count ash of every size. While saplings and trees fell steeply, seedlings spiked. Possible reasons:

• a year or more of heavy seed production (a mast year);
• dying trees putting out a last burst of seed;
• park deer management letting more seedlings survive;
• more sunlight reaching the forest floor through dying canopies.

Ash trees and saplings (left axis) and seedlings (right axis), per hectare, at three survey periods. Seedlings are more than 15 cm tall but under 1 cm in diameter at breast height. Surveys paused in 2020 for COVID-19, so the last period spans five years. NPS.

Swamp ash become shrubs

In dry, upland forests the region matches the rest of the country: mostly white ash dying fast, seedlings proliferating. The ash swamps along the Potomac River are different.

These swamps help filter water flowing into the Chesapeake Bay. Their ash slow erosion, shelter birds, plants and semi-aquatic species, store carbon, and buffer storms — making them vital to the Bay's cleanup.

Swamps such as Piscataway Park's were dominated by pumpkin and green ash, "ecosystem engineers" that shape wetland habitat and thrive where almost no other native tree can, through twice-daily tides.

When EAB struck, many feared the swamps would lose all their trees. Instead, some canopy ash have so far outlived their trunks by resprouting from living roots as shrubs. Pumpkin ash is the only ash species whose saplings increased after infestation — from 15 to 25 per hectare in these swamps between 2013–2016 and 2017–2021.

Open questions:

• Could this help wetland ash survive?
• Will they cycle as shrubs indefinitely — and flower and seed — or is it a step toward death?
• How long until they are again attractive to EAB for egg-laying?
• Why only in green and pumpkin ash swamps, not drier areas — the swamp, or the species?

Wetlands, with fewer species, show change clearly; in drier, more diverse forests the effects are still unfolding.

More

• The National Capital Region Inventory and Monitoring Network
• Earlier updates: a 2017 article and print versions from 2017 and 2016
• NPS Species Spotlight: white ash
• Gabriel Popkin, "Can a Hidden World Be Saved From an Invasive Scourge?," New York Times, February 21, 2022

Sources

Based on Crystal Chen, "Ash Tree Update 2021," National Capital Region Inventory and Monitoring Network, National Park Service; a work of the United States government in the public domain. The article says fewer than 80,000 live ash remain, while its table totals 80,460. A photograph of the beetle credited to Bugwood.org is not reproduced.