What grows back when a field is left alone

Stop mowing a field in the eastern United States and it becomes a forest without any help from you. The process is reliable enough that ecologists have watched it for a century and named it old-field succession, and it follows the same script from New Jersey to Georgia. It is worth knowing whether you want to speed it up, slow it down or steer it.

The first ten years

The first summer belongs to annual weeds: horseweed, ragweed, foxtail. Within two or three years the perennials take over: goldenrod, asters and, on poorer ground, the tawny broomsedge that makes an old field glow orange in winter. Blackberry and sumac creep in from the edges.

The first trees arrive at the same time, most of them from the air. Eastern redcedar comes in along fence lines, where birds that ate its blue, berry-like cones have perched. Sweetgum, tuliptree and red maple blow in on winged seed from any tree within a few hundred feet. Black cherry, sassafras and persimmon arrive in bird and raccoon droppings, and black locust by seed and root suckers from the nearest colony. Along a ditch, sycamore seedlings line up in the wet ground. In the South, and on sand or worn-out Piedmont clay anywhere, pines take the field instead: loblolly southward, Virginia and shortleaf pine farther north. Ten years on, a field nobody planted is a thicket of saplings taller than a person, and by twenty it is a young forest.

Why the pioneers win

None of these is the best tree in the region. They win the field for other reasons:

• Their seed travels. Winged seeds and bird-carried fruit arrive by the thousand, years before an acorn or hickory nut, which mostly falls under its parent.
• They grow fast in full sun. Pioneers put their energy into height, and in the open they outrun everything.
• They tolerate bad soil. Old fields are compacted, eroded and short on nitrogen. Redcedar and persimmon shrug that off, and black locust fixes its own nitrogen through bacteria on its roots, grows where nothing else will, and leaves the soil richer than it found it.
• They spread on their own. Sassafras, persimmon and black locust send up root suckers, so one seedling becomes a patch.

The same traits make them poor at holding what they win: nearly all need full sun to regenerate, so their own seedlings die in the shade they cast.

How fast they really grow

The pioneers’ reputation for speed is deserved, but the numbers get inflated. On a decent site in full sun, once established, tuliptree, sweetgum and sycamore typically add two to three feet of height a year through their first couple of decades, more in the best years on rich bottomland, a foot or less on a dry shale bank. Black locust is quick in its first years, sometimes as quick as tuliptree, but it slows sooner, and on the thin ground where it usually appears a foot or two a year is typical. Persimmon and redcedar are slower, often a foot a year, sometimes two.

A planted tree does not hit those rates at once; its first two or three years go into roots. The growth estimator turns species, site quality and years into a range rather than one number, and the spread is the point: the same species can be fifteen feet tall or forty at ten years old.

The second act

By thirty or forty years the pioneers are a closed canopy, and what happens next happens beneath it. Oaks, hickories, beech, black gum and, in the north, sugar maple come up in the shade from nuts carried in by squirrels and jays, and grow slowly, a few inches a year, waiting. Black locust dies out first, riddled by borers and shaded out, often within fifty years. Redcedar and pine follow as the hardwoods overtop them. Sweetgum and tuliptree last much longer, tuliptree for two centuries or more, but they cannot replace themselves in shade, and the forest slowly turns over to the trees that can. A century after the last hay was cut, the field is an oak and hickory woods with a few enormous tuliptrees in it, and only a stone wall or a plow terrace shows what it was.

That is the script when it goes well. Two things stall it. Deer eat oak and hickory seedlings by preference, and in much of the East there are enough of them to remove nearly every one, leaving a forest with no next generation. And invasive shrubs and vines (autumn olive, multiflora rose, bush honeysuckle, privet, oriental bittersweet) can seize an old field in its first decade and hold it as a thicket that trees cannot get through.

If you own an old field

The pioneers will come whether you plant them or not, so put your effort into what does not come on its own.

• Keep some of the pioneers. A light canopy of redcedar, locust or young tuliptree breaks the wind and gives young oaks the partial shade they do well in.
• Plant the later species into the young stand: oaks, hickories, black gum, beech, whatever the site and region call for. Small seedlings planted well outgrow big ones planted badly, and the planting itself is where most are lost.
• Protect each one from deer with a tube or cage until it is above browse height, usually five to eight years.
• Pull invasive shrubs while they are small, and keep at it. A thicket of ten-year-old autumn olive is a project; a hundred seedlings are an afternoon.
• Stop mowing the parts you want to become forest. Every pass resets the clock to year one.
• Be patient. The pioneers will show you something in five years; the oaks will take thirty to look like anything. That is the pace forests have always run at.