Is 9,000 tree seedlings per hectare enough? Comparing eastern national park forests

Is 9,000 tree seedlings per hectare enough for a park's forests to regenerate? Raw numbers like that mean little alone. The National Park Service's Eastern Rivers and Mountains Network sets them against other parks, and one yardstick it uses is a target of 14,000 seedlings per hectare, suggested by the U.S. Forest Service for forests with low deer browse pressure.

Image from the National Park Service's page.

Why compare parks

Understanding the condition of park natural resources is essential to management decisions, but raw data or statistics are hard to judge. A question such as how bad a park's invasive species problem is can only be answered in context. Comparing data from many parks lets managers see how a park's forest stands against others in the region.

Eight Inventory and Monitoring networks have collaborated on forest health monitoring since 2005. The collaboration includes 61 national parks in the eastern United States, 23% of all parks in the Inventory and Monitoring Program. Because they collect vegetation data in similar ways, the parks can be compared.

Inventory and Monitoring networks and parks in the regional collaboration. NPS / K. Miller.

Exotic invasive plants

Exotic invasive plants threaten many parks' forests because they can outcompete native plants, suppress forest regeneration, degrade wildlife habitat and affect soil fertility. Their diversity and spread vary among the network's parks and other parks in the region, as measured in forest monitoring plots between 2007 and 2010 in 40 parks that used comparable sampling methods.

• Delaware Water Gap National Recreation Area has more exotic invasive plant species in its forests than most other parks in the region.
• Johnstown Flood National Memorial has relatively few.
• Nearly all forest monitoring plots at Friendship Hill contain at least one exotic invasive plant species.
• Gauley River National Recreation Area has a lower percentage of plots with exotic invasive species than most other parks.

Number of exotic invasive plant species detected in forest monitoring plots, 2007–2010, in 40 national parks. NPS / J.P. Schmit.

Percentage of forest monitoring plots with at least one exotic invasive plant, 2007–2010, in 40 national parks. NPS / J.P. Schmit.

Forest regeneration

The number of tree seedlings on the forest floor shows whether a forest can renew itself as older trees die. Seedling density in monitoring plots varies greatly among parks. Measured from 2006 to 2009 in 39 parks with comparable sampling, the network's parks generally hold moderate densities compared with the rest of the region. Friendship Hill ranks in the lower 20th percentile of regional parks, and none of the network's parks reaches the 14,000-per-hectare target.

In the network's parks, regeneration is strongly shaped by elevation, fern cover, grass cover and the number of stems of indicator plants sensitive to deer browse:

• Forests at higher elevation generally have more regeneration.
• Where fern cover is above 20%, regeneration falls as fern cover rises.
• As grass cover increases, regeneration decreases.
• Sites with more stems of deer-sensitive herbaceous plants have more regeneration.

Average tree seedling density, 2006–2009, in 39 national parks; the line marks the suggested target for forests with low deer browse pressure. NPS / S. Perles.

Collecting forest monitoring data in New River Gorge National River. NPS / S. Murphy.

Sources

• National Park Service, Eastern Rivers and Mountains Network, “Forest Health in a Regional Context,” https://www.nps.gov/articles/forest-health-in-a-regional-context.htm; rewritten in hubnx's own words. The page's contact details and list of park acronyms are left out.