Changing Forests - Nothing Succeeds Like Succession

July 7, 2012

The common notion that selecting the “no active management” option will perpetuate the status quo is wrong. When it comes to forested wildlife habitats, change is constant, it’s automatic. As long as the sun shines and trees grow, habitat changes accompany a centuries-long continuum of forest succession.

An example: hay meadows and livestock pastures quickly revert to rough weeds and small tree seedlings in the absence of active mowing or grazing. In a matter of months, coarse milkweed and thistle replace grasses. The next year, thorny shrubs and young tree saplings crowd out and shade sun-loving pioneers. Soon juniper, gray birch and poplar that colonized an open sunny pasture are choking in the shade of oak and fast-growing white pine that eventually yield to shade-tolerant beech, maples, hemlock and spruce.

Successional changes to former farm fields and young forests have decreased statewide abundance of avian habitat specialists. Whipoorwills and Brown Thrashers favor a mix of dry oak forests, open field edges and shrubs. Eastern Rufous-sided Towhees favor the dry blueberry and black huckleberry understory of once-bald mountains ringed by dry oak and pine forest. These dry heathlands develop beneath pitch pine and red pines on dry granite mountain tops prone to frequent lightning fires.

The Towhee’s melodic song – often described as “drink your teeee” – was once a fixture of the bald-topped, dry blueberry clad foothills of New Hampshire’s former hilltop pastures now nearly all grown back to trees. A few places protected by the Forest Society where you can still hear Towhees sing include Gap Mountain in Jaffrey and Pitcher Mountain Blueberry Farm in Stoddard and on Pine Mountain in Alton.

Old apple orchards interspersed with pasture and shrubs are favored by Blue-winged warblers and Eastern Bluebirds. More actively mowed hay fields and grazed pastures in regions of active dairy farming support Bobolinks. Eastern Meadowlarks need a threshold of no less than four hundred acres of contiguous open pasture or mowed hay meadows. These bird species have been in decline as their preferred habitats are less available. In order to sustain the full range of old field, orchard, shrubs and dry heathland birds, we must actively manage by mowing, grazing, burning or cutting encroaching forest to recreate their preferred habitats. Doing nothing does not maintain habitat or the views and blueberries which often compel people climb hills in summer.

The State’s forests are ever-changing – and they are aging. Like birds, some “specialist” trees might one day become less common or eventually counted among the missing. Foremost on the list is our State Tree: White birch, a tree cloaking the White Mountains as a legacy of turn of the century logging and fires. Century old paper birches are now declining throughout the White Mountains due to age-related effects.

The recent 10-year update to US Forest Service “Forest Inventory and Analysis” data reveals New Hampshire now has a higher percentage - 85% of the state now forested. Yet more than half the timberland in NH - 57% percent - is older than sixty-one years old. As forests age, they change composition. A higher percentage of our state’s forestland is now growing more shade-loving hemlock, beech, yellow birch and red or sugar maple. While many people think older forests and larger trees sound great, those species of wildlife and trees dependent on young forest conditions might disagree.

Only 13% of the State’s forest is now less than 40 years old. Seedlings, saplings and small-diameter, brushy forests of sun-loving trees: paper birch, pin cherry and poplar are less common. These “pioneer species” cannot establish in partial shade. They require larger cleared openings to regenerate and survive. Think “shopping mall parking lot” size without asphalt! Large clearings allowed to regrow new forest sustain hot, dry micro-climates these trees need to establish, compete and thrive.

Even-aged clearcut timber harvests have fallen out of public favor. Reticence by foresters and landowners to re-create open conditions favored by young forests of sun-loving pioneer trees and by bird species adapted to them will impact the abundance of both.

Healthy statewide forests communities include a range of different tree ages, sizes and species. In that sense, forests are analogous to healthy human communities with diverse age profiles.