麻豆国产

Before the Europeans arrived on the Atlantic seaboard of North America, the longleaf pine (pinus palustrus) was the principal tree growing on about 90 million acres of the upland soils of the coastal and Gulf of Mexico plains of the Southeast. These trees were valuable for poles, lumber, ship masts and spars and naval stores. They grow straight and with little taper except as found on the growing top of the tree. The heartwood is very resinous and because of low nutrient sandy soils they grew slowly thus creating a hard wood with tight growth rings. Why did the longleaf pine dominate the landscape? The reason is that longleaf, more than many other southern trees, learned to live with fire and it not only survived fire, it depended upon fire caused by lightening or by indigenous people for hunting and agriculture. It has characteristics that start and sustain fires which regularly burn every 2-3 years.

The longleaf pine evolved marvelous physical adaptations to tolerate fire when young. Instead of growing upward right away as most saplings do, longleaf seedlings sit low to the ground in what is termed the grass stage for periods of 3-15 years. During this time the young trees use their energy to grow a long, heavy taproot that reaches down in the sandy soil into more constant moisture. When the young plant has good moisture and has stored food in its thickening taproot it then shoots rapidly upward. While it is growing to get above the frequent normal fires it delays putting out side branches giving it a distinctive bottle-brush appearance. By growing rapidly upward in a single spurt, the young tree minimizes the amount of time its growing tip is vulnerable to ground fires.

A variety of adaptations help longleaf pines resist death by fire, including that the flakes of bark on the tree will sacrifice themselves to fire and while burning, peel away, rising in the hot air away from the trunk. These adaptation utilities are eclipsed by a fantastic secondary function of the needle. This fire-resistant tree produces needles that have more volatile resins and oils than other southern pines, rendering the dry needles flammable.

The seeds of the longleaf require open sunlight and bare mineral soil on which to germinate. Beneath the trees in the forest the ground is dense with wiregrass and many other native grasses and herbs. The only open ground patches available to longleaf seeds are small bare patches of soil created by burrowing animals and the tip-up mounds of wind-thrown trees. However, there are not enough of these bare patches for a longleaf pine community to keep regenerating. What, then, opens up the groundcover so the seeds have open ground to sprout?

It is fire that brings renewed life to the longleaf pines. Fire burns back the young hardwoods that otherwise would take over and fire prepares the ground so the seeds can grow into it. The longleaf pine times its seeding to the fire cycle and the seedlings key their growth to fire.    

In the longleaf pinelands, summertime was the season of fires. Thunderstorms swept across the landscape accompanied by lightning strikes. Summer fires prepared the soil bed for longleaf by burning off the ground cover. Longleafs drop their cones with seeds in autumn, and the seeds germinate in October and November, synchronized with the summertime lightning cycle of fires. The impact of natural fires historically depended mainly on two major factors: the number of local lightning ignitions and the expanse of broad sweeping fires. The amount of lightning varies from summer to summer. About once every decade, summer lightning reaches a peak during which there are enough lightning storms to set local fires to burn off most of the longleaf sites in the coastal plains.

The original longleaf forest did not need a lightning strike in each unburned section to burn off the whole forest. In pre-settlement times the only barriers to fires were wetlands and the steeper stream-valley bottomlands although under drought conditions these areas were burned. In the past longleaf forests stretched for tens upon tens of miles. Fires burned for weeks, slowly but inexorably incinerating most of the pinelands in their path. Acreages burned were often several counties in size.

The longleaf community thrives and even survives by fire. In the absence of fire on the drier, sandier sites, longleaf occurs naturally in the company of several 鈥渟crub oaks鈥� such as Turkey, Blackjack and Bluejack oaks. The above ground parts of these trees are vulnerable to fire, which readily kills the branches and stems. They later sprout new stems. This readiness of scrub oaks to respond to fire is their way of coping with fire. Without fire, they keep growing to form a scrub oak thicket, shading out the ground and adding smothering leaf litter. Longleaf seeds and seedlings which need sunlight, can鈥檛 prosper. The big pines die of age and without new pines to succeed them, the prolific oaks inherit the forest.

Natural fires keep the scrub oaks under control, pruning them back, keeping most of them as small scrubs, no bigger than herbs. Photographs of longleaf sites at the turn of the 20th century show this, as do recent experiments in controlled burning. All over the coastal plains and sandhills, you can see dense, 30鈥� plus stands of scrub oak. This condition is what took over after people cut pines and disrupted natural fires. In essence, such scrub oak forests are human creations.

Longleaf communities can survive timbering if enough, mature seed-bearing trees remain and the ground cover can still carry fire. Longleaf communities can even survive altered fire cycles for a while. The natural ground cover competes with young hardwoods and can retard their encroachment. If hardwoods do get established among the pines and begin to suppress the pine seedlings, a summer fire or two can burn the hardwoods and rejuvenate the longleaf forest provided that the groundcover community has survived. Nevertheless, in spite of its toughness, resilience and marvelous adaptations, longleaf communities are defenseless before the plow and bulldozer.

The ground cover beneath the tall pines may be more important than the trees from many points of view. The dominant wiregrass (Arista strict may have more control over which plants are found in the community than longleaf does. In fact, the nearby presence of longleaf itself may be controlled by the presence or absence of wiregrass. Any seed falling near the wiregrass faces three problems. First, the shallow but dense root system is a formidable barrier to newly germinating roots. Second, wiregrass produces a dense cluster of leaves which sprawl over the ground and over the tops of adjacent plants to form a low canopy, shading out new seedlings of pines and herbs. Finally, if a colonizing plant does become established, it is most likely to be burned by the next blaze fueled by a highly flammable wiregrass. Indeed, summer fires trigger wiregrass and many other ground cover plants to engage their flowering.

In the past 200 years the settlement of the sandhills and flatwoods coastal plain has drastically altered the normal summertime fire cycle. Suppression of fire has resulted in a gradual drift from majestic, open longleaf pine forests with dense wiregrass and numerous herbs to thick hardwood forests and scrub oak thickets of negligible groundcover and with little wildlife. However, even, after decades of dormant states, longleafs, wiregrass and forbs can rejuvenate and grow vigorously after fire. This creates the ideal habitat for many animals and plant types as well as longleaf specialists such as fox squirrel, red cockaded woodpecker, gopher frog, gopher tortoise and the diminutive pixie moss.

Turpentining and clear-cutting for lumber destroyed most of the once-great longleaf pine forest. Today, less than five percent of its former acreage remains. But conservation groups across the Southeast are working to protect and restore this critical tree and the communities it lives in. Without these efforts the beautiful longleaf pine ecosystem will vanish. The longleaf pine isn鈥檛 just a legacy of the past: it鈥檚 a tree of the future.

NOTE: This copy was edited and adapted by Vince Zucchino from an article originally written by Julie Moore in 1989. At that time, she was involved in the NC Natural Heritage Program at the NC Department of Environment, Health and Natural Resources.

Longleaf: The Heart of Pine

For a broader look at the longleaf pine ecosystem, the documentary below explores both its cultural and natural history across the American South. It highlights the once vast longleaf forests, their role in early industry and settlement, and the dramatic decline that followed. The film also examines ongoing efforts to restore this unique and vital landscape.