By Ken Bevis, Stewardship Wildlife Biologist, Washington State Department of Natural Resources, ken.bevis@dnr.wa.gov
The forest canopy is a reassuring thing of beauty. Lovely green conifers form an arching roof of green, with blue sky behind it, or rain gently dripping from the needles. A full blanket of conifer feels right, but it is not one of our highest quality forest habitats. In fact, robust, young, healthy conifers, especially in dense stands, can be one of the least valuable habitat features in our forests. Why?
Who eats conifer needles? Only some special insects, a few birds and mammals, (but not many, and often only when they are new, with tender surfaces).
Who eats conifer cones? Mature cones, not much, but lots of wildlife (Douglas and red squirrels, crossbills, finches) eat the seeds hidden in the cones. Older conifers, especially those with deeper crowns, a product of room to grow, produce the most cones. Live conifers are very good at resisting herbivory and insect or fungal attack. It has even been suggested that conifers evolved with lots of resin and non-digestible tissue to resist herbivory from dinosaurs! And structurally, a dense even-aged conifer forest contains limited understory vegetation, and often little canopy diversity, or large dead trees.
Some of the best quality forest wildlife habitat niches are provided by trees dying. They create snags, logs and even dead branches. These dead wood elements provide suitable places for fungi and insects to reside, (usually unavailable in living trees). In special, soft-enough trees and snags, there are places for woodpeckers to drill their amazing nesting cavities. When this death occurs in an intact forest – in small groups or individuals, as opposed to a large catastrophic event such as fire – the forest structure is changed in more subtle ways.
When trees die in mature forests, or are significantly damaged, they lose their foliage and eventually topple over. A “gap” is created in the canopy. These breaks between trees in the forest canopy allowing light to get through, and the resulting vegetative changes can be striking. A flush of understory shrubs occurs, and sometimes fully occupy the gap. Young trees become established. These locations can be wildlife and vegetative hot spots with higher plant species and structural diversity than the surrounding forest.
A couple of years ago, the U.S. Forest Service research group put out an interesting publication about how these gaps function. The article discusses the variability of types of gaps, and the trees and shrubs that come in afterwards. They report on a research project whereby created gaps of different sized are being assessed long term.
Wildlife is a function of habitat, and these gaps often create rich, small-scale niches for forest wildlife to feed, nest or rest in rich shrubs or small trees in the gaps. For example: red elderberry, vine maple, thimbleberry, or bitter cherry (among many others) may make a gap appearance and provide a seasonal food bonanza in berries and edible foliage for different wildlife species. The gap creates microenvironments that allow for this touch of habitat diversity to exist.
Over time, the forest canopy fills back in and the temporary gap is gone. But in normal forest successional dynamics, these small gaps will periodically occur by way of windthrow, root rot, insects, or simple mechanical failure of trees.
As the forest matures, this “gappy-ness” is one of the characteristics that naturally develops. The heterogeneity in natural forests is one of the ways the best, diverse habitats are provided. Go to an old growth forest and look up and across. The number of gaps will surprise you.
Bird species using canopy gaps in our forests include Swainson’s or hermit thrushes, black capped chickadees, or ruffed grouse feeding on the berries. Mammals such as black-tailed deer or black bear might focus on feeding on the lush shrubs in the gap, or small mammals such as red backed voles or shrews may find improved forage in a gap. Birds and bats forage overhead in the opening and foliage edges around the gap. Many animals will take advantage of these small-scale habitat niches.
Some managers and foresters with objectives of moving younger forests (particularly planted conifer) more quickly towards old forest habitat conditions are utilizing deliberate gap creation. They use variable density thinning and small scale, even-aged patch harvests to add this diversity to younger forests. When using these tools for habitat, managers may consider utilizing an approximate dominant tree height (150 feet in diameter = half acre), or a proportion of it, as the target gap size. Smaller can be good, say one-tenth of an acre (about 65 feet across), which could provide enough filtered sunlight to allow for shrub and new tree establishment. When introducing gaps to a forest, keep noxious weeds in mind. Creating large gaps will result in lots of sunlight and potentially good habitat for some less desirable plant species. Get ahead of them by planting in a host of native, wildlife-friendly shrubs and trees.
Washington State Parks performed two experimental timber harvests aimed at creating structural diversity using some of these techniques in the early 2020s. WSU Extension Forester, Patrick Shults, hosted two workshops at Seaquest and Nisqually State Parks in the spring of 2022 to highlight these projects and how variable density thinning can be applied on private forestland. Forest Service scientists helped lay out these projects working closely with WA State Parks, with some interesting results.
Shults said of these projects, “Both stands were originally single species conifer plantations. State Parks wanted to increase stand health and promote habitat diversity with variable density thinning, including gap creation. It was a win-win for State Parks supporting both education and recreational goals.”
Sometimes, ironically, root rot, drought stress, or even bear damage (all at a minor scale) creates these gap habitat opportunities for small landowners. A small pocket of overstory mortality might not be such a bad thing after all!
Most small forest landowners have high interest in maintaining healthy forests with quality wildlife habitats. Many, if not most, of the forests owned by our clients fall into the younger age classes. Helping develop mature forest characteristics faster through gap creation (or acceptance) is one of the active management tools we can use to help promote forest health and biodiversity in our forests over time.
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