6 minute read
NATURE’S VIEW
The Understory
Sun vs. shade is the variable for forest regeneration and wildlife habitat opportunities
By Dave Anderson
AAhhh summertime…I marvel at the billions of green pennants fluttering overhead in the forested canopy—two dozen shades of green. Tender young leaves of the hardwood canopy cast a seemingly sudden shade on the forest floor that will remain in shadow until autumn when the foliage falls. The summer’s shade-casting foliage is largely uncelebrated, but it is an equally dramatic transition in the woods. A season of filtered sun vs. dappled or deep shade reveals sunlight’s role in heating soils and regenerating seedlings. Sunlight dictates food availability and cover for nesting songbirds and wildlife inhabiting the forest understory.
Made in the shade? Meh, not so much in our forests. Dense shade beneath a 100 percent closed canopy of hemlock, pine, or dense hardwoods limits understory growth. At ground level, the forest appears open. Prime real estate for the limited plants that are best adapted to growing in full shade and acidic soil. Canada mayflower (aka false lily of the valley), starflower, and goldthread can often be spotted carpeting the forest floor beneath towering pines—a win by default.
Shade-tolerant hardwoods—beech, yellow birch, and sugar and red maple—reproduce from seed in the shade while sun-loving hardwoods—white birch, poplar, pin cherry, ash, and oak species—require full sunlight and warm soil to germinate. Among conifers, white pine and red pine require full sun to regenerate. Red spruce and hemlock can tolerate shade, but they require an opening in the canopy to grow. The more sun-loving trees are short-lived: pin cherry can live up to 40 years, followed by poplar (80 years), white birch (100–120 years), and white pine (perhaps 150 years max). Slower growing shade-tolerant trees can live centuries: beech and sugar maple (200-plus years), yellow birch (300-plus years), and hemlock (a whopping 400-plus years).
In response to the low-light levels in a hardwood understory, oaks and maples produce larger “shade leaves, ” outsized versions of much smaller canopy leaves, to capture more sunlight. Think of how much more energy an array of solar panels can create compared to just one. The same method applies for these larger leaves trying to grow big and strong in a dark forest. In a hemlock forest, the understory is barren, devoid of food for wildlife and cover for birds. Few plants are adapted to live in these harsh acidic soil conditions. Research has shown that people tend to appreciate an open understory for ease of walking and enhanced peripheral vision compared to a forest with more complexity that also obstructs views. But the latter provides more opportunities for wildlife feeding, nesting, and cover. It is a duality. But this is strictly a human construct— what we the people prefer. A wide field of view through an open understory yields better peripheral vision and provides a comforting sense of security, a throwback to the open Savannah where humans first evolved amid some formidable predators.
Conversely, sun-soaked openings ranging from small single-tree gaps to larger apertures quickly fill in with dense new growth. Fruit-bearing shrubs, ferns, wildflowers, and tree seedlings race to claim this valuable space. Foresters seem fond of admiring the strong regeneration of oak and pine seedlings; a new forest emerging in full sun.
With the addition of new sunlight, the understory transitions from an assortment of shade-tolerant ferns, witch hazel, striped maple, hemlock, beech, and red maple to a new growth of sweet fern, lowbush blueberry, staghorn sumac, pin cherry, white birch, poplar, and white pine seedlings. Pine, ash, and oak seedlings dominate former skid trails and timber landings surrounded by a matrix of shady closed canopy conditions. It’s no surprise that when people bushwhack, they tend to hike parallel to unmaintained skid trails to avoid the brush—and the ticks!
Once hardwood trees are fully leafed-out, nuances of sun versus shade are more apparent. Forest seedling regeneration and wildlife use of habitats for feeding or nesting reveal the influence of full sunlight or a lack thereof.
Wildlife Food and Cover
Warm light in forested openings attracts foraging insects drawn to sun-dependent grasses and flowers. In turn, insects attract aerial feeding by foraging songbirds, flycatchers, and bats. Tender leaves and twigs of seedlings and saplings are accessible to
browsing rabbits, deer, bears, and turkeys—a welcome salad after a winter diet of dwindling acorns, beechnuts, or bitter bark. The closed canopy of taller trees lies out of reach. Where deer population is high, including on some large lake islands and in suburban backyards, the survival of young hardwoods, particularly sugar maple, is always at odds with a deer’s hunger level.
Additionally, nocturnal wildlife habits change after leaf-out. Bears now concealed by dense ground cover travel readily in the daytime during their June breeding season. Deer often select locations close to human habitation for giving birth. The presence of house pets and the proximity to people deters bears and coyotes from venturing out of the forest to hunt. Newborn fawns can sometimes be spotted bedding down in our backyard gardens and hedges, which provide even more protection from predators. In June, deer with fawns stashed in thickets emerge to forage in sunny openings and meadows of tall grass.
Ground-nesting birds are protected from the high winds and heavy rains that buffet the upper canopy, but they face different challenges. Near-ground nests are vulnerable to four-legged predators: fishers, raccoons, skunks, and opossums prey on bird eggs and nestlings. Songbirds that nest in dense vegetation on the forest floor sing loud, ringing songs that carry long distances. Winter wrens, ovenbirds, and wood and hermit thrushes employ cryptic camouflage, drab plumage of speckled brown, rust, and olive, to blend in with the dappled sunlit forest and belt out their loud songs to attract mates and defend their forested nest territories.
It doesn’t matter if you’re hiking in the sun or the shade, swarms of biting insects by mid-summer are why one cynical forester once described the hazy, tropical forest to me as “the green hell. ” Heat and humidity sustain successive hatches of tenacious deer flies, particularly relentless insects which can take a tiny cube of flesh from an unguarded patch of skin and leave a painful welt. Deer flies are attracted to dark colors and movement and they target the top of the head, the back of the neck, and that space between your shoulder blades that is impossible to swat.
People might sympathize with deer and moose, who have limited refuge from insects save immersing themselves into a body of water. But that’s not a bad locale to seek regardless while the warm weather lasts.
Naturalist Dave Anderson is senior director of education for the Society for the Protection of New Hampshire Forests.
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