Letting in the Light: Creating Canopy Gaps for More Resilient Eastern Forests

A log drive fills the White River near Sharon, Vermont, in the late 1800s. Vermont Historical Society.

Part of the National Park Service's Managing Resilient Forests Initiative for eastern national parks.

Forests of one age

Many forests in the Appalachian and Mid-Atlantic regions are about the same age. They grew back after widespread land clearing that peaked between the 1880s and 1920s, regenerating through the twentieth century and spreading onto abandoned farmland. These middle-aged forests, like those in many national parks in the region, have mature canopies that soak up most of the light before it reaches the ground. Seedlings that sprout beneath them struggle to get enough light to grow into young trees.

Thinning the understory

Park managers can thin trees from the lower canopy: species that will never reach full canopy height when mature, or trees that are not typical of the surrounding forest type. This helps preserve the native forest community, lets more light reach the forest floor, benefits wildlife, and diversifies the forest so it is more resilient to future disturbances.

Trees can be girdled, which kills them standing and creates snags, or felled to open canopy gaps and add coarse woody debris to the forest floor. Snags are essential homes for birds and mammals that nest in cavities; downed wood shelters insects, amphibians, reptiles, small mammals and fungi, and provides habitat and nutrients where tree seedlings can take root, especially as more light reaches them.

Doing it right

Thinning works only in forests that are free of invasive plants and already have enough seedlings of the trees parks want in the future canopy. Invasive plants must be treated first, and managed continually afterward, and desired seedlings must be well established before gaps are opened; otherwise invasive or unwanted woody plants will take over the gaps. Forest health monitoring data from the Park Service's Inventory and Monitoring program can show which stands have enough regeneration to thin.

Once a stand has advanced regeneration, taller seedlings and saplings of the desired trees, small openings of one-tenth to one-quarter of an acre (0.05 to 0.1 hectare) can be cut with chainsaws and hand tools, keeping disturbance to the forest floor to a minimum. This closely imitates the natural process in which a single large old tree dies and the young trees around it grow up into the space.

Canopy gaps let light reach the forest floor. National Park Service.

Sources

• National Park Service, "Resilient Forests Initiative - Forest Complexity," part of the series Managing Resilient Forests Initiative for Eastern National Parks. https://www.nps.gov/articles/000/resilient-forests-initiative-complexity.htm
• NPS Inventory and Monitoring Program: https://www.nps.gov/im/index.htm