Resilient Forests Initiative - Forest Complexity

A log drive chokes the White River near Sharon, VT in the late 1800's.

VT Historical Society.

Enhancing tree regeneration and forest complexity protects the future forest.

Let there be Light!

Much of the forest in the eastern United States is around the same age, regrowing after widespread land clearing that peaked between the 1880's and 1920's. Throughout the twentieth century, forests began to regenerate, eventually spreading onto abandoned agricultural lands. Middle-aged forests, like those found in many eastern national parks, have mature tree canopies that soak in most light before it can reach the forest floor. Tree seedlings that manage to sprout in these closed-canopy forests struggle to get enough light to thrive and grow into young trees.

Tree species that are short-statured at maturity and will never reach full canopy height, or trees not typical of the surrounding forest type can be thinned from the lower canopy by park management. This helps preserve the native forest community and allows more light to reach the forest floor, while also benefiting wildlife and diversifying the forest making it more resilient to future disturbances. Trees can be thinned by girdling (which also creates ecologically beneficial standing dead tree-snags) or felled to create canopy gaps and increase the amount of coarse woody debris (CWD) on the forest floor. Snags are essential habitat for cavity-nesting birds and mammals, while CWD provides important habitat for insects, amphibians, reptiles, small mammals, and fungi. Downed trees also provide important habitat and nutrients for tree seedlings to germinate, especially as light is increased under the canopy.

Vital to the long-term success of a thinning program is that it should only occur in forests that are free from invasive plants and contain enough seedlings of desired canopy trees. Parks must make sure invasives are treated prior to (and continually managed thereafter) and desired tree regeneration is well-established before creating gaps. If these steps aren't taken, any invasive plants or undesirable woody plants that are present will thrive in a canopy gap. Forest health monitoring data from the Inventory & Monitoring program can help determine which forest stands have enough regeneration of desired trees to conduct thinning.

Once a forest contains advanced regeneration (taller seedlings and saplings of desired trees) small canopy openings can be made, one-tenth to one-quarter of an acre (0.05 to 0.1 ha) ideally, using chainsaws and hand tools so that forest floor disturbance is kept to a minimum. This closely simulates the natural forest regeneration process that takes place when single, large old trees die allowing seedlings and saplings around it to mature.

NPS image.

Canopy gaps allow light to reach the forest floor.