By Joe Rankin
A forest, like all living things, is constantly changing. Sometimes the changes are big and abrupt and obvious and sometimes small and gradual and unnoticeable.
A few years ago I started an experiment in forest succession. Primary succession, to be precise, on about an acre and a half across the road. For several years we pastured sheep and goats there and they had groomed the field within an inch of its life. But when we got rid of the livestock, there was no easy way to keep it cleared, given the unruly piles of stones, old foundations, the ruins of a tumbledown barn and the fact that part of the field tended to be a boggy. We decided to let the forest come back, (mostly) naturally, and observe what happened.
It’s a process that has taken place on a much larger scale across much of Maine for hundreds of years, as trees march in to occupy abandoned farmland, starting in the middle of the 1800s as farmers headed to the black soil belt of the American midwest.
Forest succession is “a pattern, a sequence of tree species, over time,” says Kevin Smith, the supervisory plant physiologist at the U.S. Forest Service’s Northern Research Station in Durham, N.H. “The previous mix of tree species that came in, the depth of soil and its characteristics, the drainage, the temperature and the availability of seed sources all influence it.”
It’s a messy process, says Smith. And it’s never really over.
“Many people think that the end result will be a ‘climax forest’ — an unchanging pinnacle of forest evolution and that there is an ultimate climax forest, a right climax forest for Maine that is inexorable or inevitable,” he said. Not so. Climax “just represents a dynamic equilibrium that is stable over a land area for multiple centuries.” Individual trees may die, he said, but the overall composition is stable.
But 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. Smith defines that as “replacement of plant species due to differences in competitiveness due to different environmental conditions.”
Primary succession kicks it off. You might look at as the creation of a forest where there wasn’t one. By the forest itself. (A previous Forests for Maine’s Future feature looked at why the eastern U.S. “wants” to be a forest. Check it out in the archives on our website.)
In Maine the process starts with grasses, weeds and wildflowers, followed by raspberries and blackberries or staghorn sumac. This holds true for vacant lots, old ski slopes, and unmowed or ungrazed pastures. The earliest colonizers protest the soil from erosion, making it possible for shrubs and trees to join in.
Most of the primary succession tree species are light lovers and quick growers, vaulting above the blanket of weeds and grasses. They include white pine. “That’s why we have fairly uniform white pine stands in the state by the late 1800s. A lot of these are the artifact of the abandonment of agriculture. Before European settlement white pines were much more scattered throughout the landscape,” said Smith. “Still, individual white pine are occasionally ‘super-dominant’, overtopping the canopy of mature spruce-fir-hemlock and maple-beech-birch forest types.
Farther south in New England the role of white pine as a primary succession species might be played by red cedar; farther north it might be white spruce. Other important primary succession species here include the trembling and bigtooth aspens; red maple; paper and gray birches; and pin and black cherries.
If these trees have something in common, it’s that they adopt the “live quick, die young” approach, putting energy into growth rather than spending it making the chemicals that would help them enjoy a life measured in centuries rather than decades.
On our little primary succession parcel, the grasses came in strong after the goats and sheep left. Over the first years, tall wildflowers like asters and goldenrod multiplied. So did Virginia creeper. Wild grapes covered the rock piles. In one spot white pines cropped up, seeded in by an old “pasture pine” at the edge of the woods. Aspens marched into the field from one end. Interestingly, wild apple trees cropped up throughout the acre and a half.
Despite the fact that a mature birch was one of few mature trees in the field, little birches didn’t come up. Paper birch seeds don’t germinate well unless they land on mineral soil, said Smith. “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,” he said.
Other primary succession species have their own needs, and strengths. Pin cherry seeds can last for years in the soil, but only germinate if fire comes through. Thus their other name — fire cherry. Red maples are vigorous stump spouters, so if there were relict red maples mowed down by livestock or a brush hog, they might come back strong once the mowing stopped.
Invasive species such as Japanese barberry, tree of heaven and Norway maple can take over the role of native primary succession species in certain instances, said Smith. “Invasives are affecting natural succession,” and changing the arboreal landscape. But “they’re also just obeying their DNA,” he added. In fact, in an urban environment, “sometimes if it wasn’t for the non-native invasive trees there wouldn’t be any trees at all.”
The weeds and grasses; aspens, white pines and birches work together to create a new forest on bare ground and thus pave the way for a future forest of longer-lived species, what we think of as a climax forest. The crowns of primary succession tree species build a looser, richer soil the shade keeps soil moisture from evaporating. All that creates conditions where shade tolerant middle and late succession species, like sugar maples, American beech, or eastern hemlock, can grow.
Middle and late succession species tend to need more fertile soils, said Smith. Some of them are more than happy to grow slowly, year after year in the shade, until another disturbance grants them more light. Then they can shoot upward.
Eventually a climax forest could emerge. In Maine there are, broadly, two types — beech-birch-sugar maple. But few natural forests are pure when it comes to the trees growing in them. While beech and yellow birch and sugar maple may dominate in a so-called “hardwood” climax forest, paper birch may be sprinkled in, or silver maple, or eastern white cedar, white pine or even American elm. In a forest nothing is ever uniform, and nothing is ever unchanging.
And, given the fact that our planet’s climate is warming up, the very nature of northeastern forests is likely to change. Southern tree species are likely to move north and northern species move even farther north, or higher up mountainsides. That could mean different players in our forest succession parade.
I noted early in this piece that my primary succession observation plot was “mostly” natural. That’s because, in the inevitable way of people, I couldn’t help but stick my fingers into nature’s business. On year I set out about 60 red oak seedlings. But, because I didn’t keep the grass back, they didn’t make it. I did, however, set out a few red pines and white spruces that did grow. You could argue that I ruined the purity of the experiment by doing that. But I could make the argument that these are native species. Or, that people have been influencing forest succession for centuries on this continent and others.
We like to call it forestry. But to the forest we’re just another disturbance. Perhaps the biggest one of all.
Joe Rankin writes forestry and nature stories.