Succession: How a forest creates and re-creates itself

Illustration by Adelaide Tyrol

A few years ago, I started an observational experiment in forest succession on a couple of acres where we once pastured sheep and goats. Rocky and wet, without livestock it was hard to keep cleared. So, I let the forest recreate itself and just watched the process unfold.

It’s a process that has taken place across much of the Northeast since the mid-1800s.

Forest succession is, simply, a “sequence of tree species, over time,” the “replacement of plant species due to differences in competitiveness due to different environmental conditions,” explained Kevin Smith, the supervisory plant physiologist at the U.S. Forest Service’s Northern Research Station in Durham, N.H.

In northern New England, it starts with grasses, weeds and tough wildflowers, followed by raspberries and blackberries and staghorn sumac. The earliest colonizers protect the soil from erosion. Most of the first tree species to follow are light lovers and quick growers. They include white pine, virtually pure stands of which sprouted in abandoned fields when farmers moved on to the black earth country of the Midwest.

Farther south in New England, red cedar plays a larger role than white pine; farther north, it might be white spruce. Other important early species are the trembling and bigtooth aspens; red maple; paper and gray birches; and pin and black cherries.

On our experimental parcel, the grasses came in strong. Asters and goldenrod multiplied. Virginia creeper and wild grapes blanketed the rock piles. A thicket of baby white pines cropped up, aspens marched in, and wild apple trees sprouted throughout.

A mature birch was one of few older trees in the field, but little birches didn’t appear. Paper birch seeds don’t germinate well unless they land on mineral soil. “If you had had a fire go through to burn off the organic matter above the mineral soil, then you would be setting up a situation for birches,” Smith said.

Other early succession species have their own needs, and strengths. Pin cherry seeds can last for years in the soil, and germinate if fire comes through. Thus their other name: fire cherry. Red maples are vigorous stump sprouters, so if they are cut down by livestock or a mower, they might come back strong afterward.

The weeds and grasses, the wildflowers, shrubs and fast-growing trees work together to create a new forest on bare ground and thus pave the way for a future forest of longer-lived species, the so-called climax forest. The crowns of early successionists create annual layers of leaf litter, building up humus and looser, richer soil. Their shade keeps soil moisture from evaporating.

All that creates conditions for shade-tolerant middle- and late-succession species, like sugar maples, American beech, or eastern hemlock, can grow, said Smith. Some of these trees grow slowly year after year in the shade until a disturbance or death grants them more light so they can shoot upward.

The succession process in a forest is unending. Eventually a new forest emerges, usually two types in northern New England: beech-birch-sugar maple and red spruce-balsam fir-eastern hemlock. As long as there is disturbance – fire, windstorms, insect infestations, disease outbreaks, logging or development – there will always be change. And where there is change there will be succession. Grasses and wildflowers, anyone?

Joe Rankin writes forestry and nature stories. The Outside Story is assigned and edited by Northern Woodlands magazine.

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