Irregular Shelterwoods in a Rejuvenating Oak-Hickory Forest
Looking downslope from the perimeter of a recent irregular shelterwood cut in Connecticut’s Yale-Myers Forest on a mid-July day, forestry researcher Mark Ashton used his favorite phrase to describe the vigorous deciduous growth: “Everyone’s on the bus.”
In this 5-year-old cut, “the bus” is the acreage that had been harvested, allowing for frenetic tree regeneration. Dual raspberry canes reached out of the deciduous shadows, drooped with plump fruits. The stem growth was dense enough that little to no sunlight reached the soil: the point, Ashton noted, at which forbs and soft masts lose out. He pointed out saplings of tulip tree and sassafras, several black and red oaks, two black cherry trees, the odd pignut hickory, white oak, and sugar maple.
Ashton, senior associate dean of the Forest School, part of the Yale School of the Environment, also noted many gray birch and pin cherry trees – pioneering species that “get off the bus” about 30 years after a cut, when they’re sun-starved by competitors. Looking overhead at three trees near the cut’s edge, Ashton pointed toward the top of a white oak, with a sugar maple just below its crown and a pignut hickory below that. The trees were canopy and subcanopy remnants of a closed-canopy forest – and the distinguishing feature of an irregular shelterwood harvest.
Where clear-cuts remove everything, standard shelterwoods retain various seed-tree densities, expressed in basal area measurements, to encourage the regeneration of desired species.
Once those seedlings establish, loggers remove seed trees, sometimes doing successive work over the course of several years until most or all retained timber is gone. By contrast, irregular shelterwoods are a shelterwood/clear-cut hybrid, usually removing more trees initially while often doing less return work.
“They walk the line,” Connecticut Department of Energy and Environmental Protection (DEEP) forester Daniel Evans said of irregular shelterwoods. “They allow for higher structural retention while still allocating growing space for shade-intolerant species.”
This is optimal for Yale-Myers in the places where sun-loving oaks and hickories are regeneration targets. Since 1992, researchers here have diversified the 80- to 120-year-old, even-aged forests typifying southern New England, pacing such treatments at one to two a year across the 7,840-acre preserve, with each harvest covering 5 to 20 acres. This has both produced the desired regeneration and improved biodiversity, something researchers monitor fastidiously.
While irregular shelterwoods have been employed elsewhere for decades, management of the Yale-Myers Forest provides a 28-year timeline of similar treatments to better understand irregular shelterwood effects through the rapid succession that takes place.
“It’s called ‘chronosequencing,’” said Marlyse Duguid, director of research for the Yale School of the Environment. “It’s a substitution for looking at one plot for 25 years, which is hard to do. In one year, we can look at multiple cuts in various stages of succession.”
Many studies look at shelterwoods in the few years after a harvest, but research here covers from 0 to 28 years (and counting) and provides an expanded window to gauge how irregular shelterwoods affect both the growth of trees and the fauna that depend on them.
The Forest Then and Now
The Yale-Myers Forest is in Connecticut’s northeast quadrant, or “Quiet Corner,” a transition zone between oak-hickory and northern hardwood forests. Trounced by glaciers, ablation and basal till are the parent materials that have formed the predominant soil types across the ridge-valley landscape. One-third of the Yale-Myers Forest is in protected reserves, mostly wetland and riparian habitat.
The land follows the typical southern New England timeline. Native Americans – the Nipmuck people here – managed it thoroughly using a burn-grow-fallow sequence. European settlers through the early 19th century cleared the land pell-mell, mostly for pasture and fodder. After a prolonged period of farm abandonment from 1850 to 1920, young forest habitat prevailed through the mid-20th century. These forests matured, and by the mid-1980s canopy closure had squeezed out most early successional habitat.
Today, to reinvigorate young forests, Yale-Myers’ stand treatments mimic disturbances caused by large wind events, along with long-suppressed fire and beaver activity, all while seeking to maximize biodiversity. It’s intricate stuff, and Ashton and Duguid are studying irregular shelterwood effects on a range of faunal suites, with an overarching goal to create a forest age-class mosaic, or patchwork quilt, that supports the fullest spectrum of native wildlife.
The Trees
Southern New England oak-hickory forests are losing ground to the maple-birch type, which concerns researchers, foresters, and wildlife biologists. Red maple and black birch attract less wildlife, store less carbon, and currently have lower timber value. By contrast, the various species of oaks that converge here provide important acorn mast and do well in the region’s comparatively droughty climate and poor soil types. As saw timber, they’re much more in demand and grow more quickly than other species.
From 1985 to 1998, though, high grade harvesting pressure led Connecticut’s oak-hickory component to decline by 4 percent and Rhode Island’s by 11 percent. Additionally, from 2015 to 2017, gypsy moth outbreaks coincided with regional drought. From 2015 to 2016, 384,000 acres of Connecticut trees were defoliated, the majority oak, and while the Yale-Myers was mostly spared, nearby Pachaug State Forest suffered mightily.
“Defoliation occurred on all size-classes, especially the dominant and co-dominant oaks with good crown position,” said Daniel Evans.
The result is afflicted forests where oak corpses give a No-Man’s-Land sheen, making oak-hickory regeneration all the more urgent. Irregular shelterwoods are a good management tool for this in southern New England, where storms blowing in from the Atlantic periodically brutalize forests. For this reason, Yale-Myers treatments can seem in some ways like clearcuts, with the odd tree here and three-trunk cluster there looking more like the loggers missed a few spots. At higher elevations, where deep-rooting oaks have an advantage, higher retention is possible, but in low spots, including toe slopes with mesic soils that favor red maples, few trees remain.
Technique counts too. In some cuts a feller-buncher with a forwarder minimizes soil scarification. That’s generally in stands with a heterogenous understory, including advanced regeneration oak and hickory. Ashton said these seedlings can withstand large tire trauma, re-sprouting from roots if necessary. Sometimes, though, a skidder is required.
“That’s usually to knock back a recalcitrant understory, often clonal ones such as mountain laurel or hay-scented fern,” said Duguid. Skidding crushes these, providing a window that allows mast sources to take hold.
Retained stands always include oaks and may also include species less common to Yale-Myers, including tulip, basswood, tupelo, yellow birch, all wolf trees, and any American chestnut saplings. Return work is minimal. In some cuts every other seed tree is removed 7 to 14 years after the initial cut. But often crown thinning at 40 and 70 years, giving oaks and hickories further release, will be all that is needed. Ashton said they do very little “tinkering” to remove invasive flora such as autumn olive or multi-floral rose, relying on native hardwoods to shade them out.
Although forest products are only one component of the forest plan, oak veneer logs are the intended long-range yield. Healthy forests are the primary aim, which in turn succor wildlife.
Birds
Longtime birders and anyone who grew up hunting grouse and woodcock in southern New England before 1990 have likely noticed the loss of early successional habitat in the region. Along with upland gamebirds, species such as brown thrashers and chestnut-sided warblers have sharply fallen, although many species are responding to prescription forestry.
“Bird numbers have declined for decades,” Ashton said, standing in a months-old shelterwood surrounded by bare earth freckled with advanced regeneration seedlings. “But here our numbers have gone up in species diversity and population.”
Robert Craig, executive director of Bird Conservation Research who has surveyed Connecticut birds since the 1970s, said that although the reasons for Yale-Myers’ uptick in bird numbers are complex, mixing forest age classes has likely helped. “Bird densities have increased at Yale by an average of 24 percent since 1985, whereas continentally populations declined by 29 percent,” he said. “Habitat management appears to play some role.”
Mass Audubon ornithologist Jeffrey Ritterson looks approvingly on the Yale-Myers shelterwoods. “It’s a good strategy. Young forest is short-lived, so creating a shifting mosaic ensures its availability while accommodating species that prefer different ages [of forest],” he said. “For example, white-throated sparrows and indigo buntings prefer forests less than five years old, where eastern towhees and chestnut-sided warblers like them 7 to 10 years old. What’s more, having young forest near closed-canopy and pole stands supports species that use many ages during the breeding season, such as American woodcock, and species like wood thrush, that breed in older forests but use younger habitats afterward.”
Yale’s data support this. Ashton and others conduct point counts across the forested plots. Habitats are categorized to show how rapidly succession takes place and how quickly birds react. Early initiation, late initiation, early stem exclusion, late stem exclusion, and canopy closure describe the roughly 30-year march from bare ground to closed-canopy. Bird concentrations are greatest in the 2- to 5-year range, and lowest in the eight mature plots surveyed for control.
“Insect density has much to do with it,” Ashton said. “Early initiation stands have leaf densities equal to mature ones, but insects prefer saplings because they mostly lack the chemical pest protection found in older stands.”
Young stands also have abundant forbs and soft masts, and the stem density provides cover from predators such as Cooper’s hawks. In addition, birds normally thought of as closed-canopy nesters – such as scarlet tanagers and Baltimore orioles – readily nest in retained trees during early initiation. “We think it impedes nest raiders like squirrels, who prefer canopy travel and can’t do that in a shelterwood,” Ashton said.
Additionally, birds long thought of as mature forest obligates, such as black-throated blue warblers, use early successional habitat during fledging and migration. Ashton said that some also nest in recovering stands earlier than thought. “Black and white warblers and ovenbirds can be found in 5-year-old cuts alongside birds you’d expect, like prairie warblers and blue-winged warblers,” he said. “A lot of that is probably due to cut size, where mature forest isn’t far away.”
Irregular shelterwoods also benefit specific bird families where each species has different needs, such as flycatchers. In any early initiation, willow flycatchers would be expected along with phoebes. With just a few canopy and subcanopy vestiges, though, great crested flycatchers and eastern wood peewees nest, too, especially with the care Yale takes to keep cavity trees and snags, habitat characteristics preferred by great cresteds.
“Any tree with a cavity we leave,” Duguid said, adding that red maples with significant crown dieback are particularly valued, as the dead branches eventually create nesting sites for tufted titmice, black-capped chickadees, all woodpeckers, eastern screech owls, and others.
Amphibians
Amphibians are an unsung forest component, even though in some places they comprise two-thirds of faunal biomass. Some researchers, however, including Malcolm Hunter, professor of wildlife ecology at the University of Maine, have studied how timber harvests affect amphibians. Data show cutting to be detrimental, but with nuance.
“A typical New England forest has 10 to 20 amphibian species,” Hunter said. “Each species is effected differently. American toads tolerate dry, open conditions, but a clear-cut will wipe other species out.”
Hunter admires Yale’s shelterwood approach. “It’s rare to leave a third in reserve, so that’s great for amphibians,” he said. “And it’s good to see regeneration methods that avoid big openings and leave lots of coarse woody debris.”
Yale researchers prioritize coarse woody debris – such as downed trees and fallen limbs – that retains moisture and provides cover as it rots. As gray birch and pin cherry topple, they add material, and this moist, abundant matter can eclipse that found in mature forests. While timber harvests are generally anathema to amphibians, Yale’s chronosequence shows that small cuts littered with coarse woody debris will limit damage, particularly where canopy remnants retain moisture and micro-habitats.
This can eventually be beneficial to amphibians, as evidenced by Yale’s study of irregular shelterwood effects on two salamander species: red-backed salamanders and eastern newts. Red-backed salamanders prefer pole stands, while newts – like most amphibians – favor cooler, moister, mature forest. As expected, both species declined with initial canopy removal. While newts were densest in mature stands, however, researchers found surprisingly high numbers of red-backed salamanders in pole stands, cuts 25 to 30 years old.
The chronosequence studies in the Yale-Myers irregular shelterwoods are providing a wider data set than has been available. This will hone how researchers conduct future cuts, but the quilt they’ve been stitching since 1992 has already regenerated the flagging oak-hickory component, while buoying multiple bird suites and containing the damage harvests do to amphibians. It’s a rounded approach that should provide a bulwark against the many stressors faced by southern New England forests.