DEEP IN THE FORESTS of the southern coastal plains are places where trees rise up straight out of the ground, sometimes one hundred feet, their branches splayed all near the crown in a wide, high skirt, with clusters of glossy needles eighteen inches long exploding like fireworks from the hooked end of each branch’s tip. Together, these trees make a canopy that is wispy, airy, loose enough that sunlight touches and illuminates the needles, shining down the trunks and reflecting from the charred bark at the bases, and past that to the ground, to the seedlings that grow underneath like bunches of grass, the long needles clumped together to protect the delicate terminal buds from the fire on which the tree has evolved to depend.
This tree, the longleaf pine, is a “keystone species”—a term coined by the late ecologist Robert Paine to describe a species whose role is to maintain balance in an ecosystem. Although Paine originally saw keystone species as predators with “high trophic status”—that is, animals at the top of the food chain—the keystone species concept has since been applied more broadly: it not only describes the role of wolves in Yellowstone, for instance, but also elephants in African rainforests, and bison on the coastal prairie.
In the forests of the American Southeast, longleaf is a keystone species because it promotes fire in a fire-dependent ecosystem. Suffused with flammable oils, its needles catch fire and drop to the forest floor, where the fire spreads from dried pine straw to the grassy and herbaceous layer. Once the fire has cleared the scrub brush and hardwood saplings that would otherwise outcompete the longleaf, the seeds germinate, reaching their taproots deep into the ground, sometimes as far as fifteen feet. These taproots grow as wide as the trunk itself and stabilize the tree, protecting it from violent storms and damaging winds that blow in from the Gulf of Mexico and Atlantic Ocean. A strong taproot is the beginning of a healthy tree, and a healthy tree is an essential part of this ecosystem.
A seedling might stay in its “grass stage” for up to fifteen years, but when a fire burns the understory and opens the canopy, a seedling exposed to the sun can grow as much as three feet a year. This is the “rocket stage,” when the tree becomes an awkward, branchless juvenile, skinny as a broomstick but ten feet tall, covered in bark like dragon scales, topped by a plume of wild needles waving in the wind. It grows upward, while around its base grow prickly pear and trailing phlox, white firewheel and wire grass, and literally thousands of species of fungi, lichens, plants, trees, insects, birds, reptiles, mammals, and amphibians that rely on the tree for at least some portion of their survival. And the survival and health of the tree are linked to these same species, and to fire, in a complex web
