The Louisiana Environment
The Louisiana Environment
by B. E. Fleury
Many northern species of plants and animals retreated before the ice sheet. When the glaciers retreated, those species that were readily dispersed, like Maple trees, quickly reoccupied their former range. Other species, which were not good dispersers, took much longer to reach their old range. A few, like Beech trees, are still migrating north, still recovering from the effects of the last glacier.
As the glaciers formed and reformed, they locked up huge quantities of water. As a result, sea levels during the Pleistocene varied as much as 400 feet. Vast sections of the low-lying southern terrain were under the sea at various times. These areas had originally been raised above sea level during the Mesozoic by the great Cretaceous uplift. This uplift raised the Atlantic slope above the waves, forming the Gulf Coastal Plain. Today we can still find marine features in the soils of the Carolinas at elevations up to 100 feet above sea level, and we find these same features in Louisiana up to 160 feet above present sea level.
This changing coastline left a thick layer of beach sand over much of the southeastern states, in a series of glacial terraces. These sandy soils form a belt of sand hills in Georgia and the Carolinas, and extend across western Florida, southern Alabama, Mississippi, and Louisiana. Louisiana soils are a mixture of very fine and heavy deltaic soils (lots of clay), and areas of immense deposits of beach sand. The city of New Orleans sits right on top of an old barrier island, with beach sand about 20-30 feet down.
The dominant vegetation throughout the coastal plain today depends on soil type and elevation. Even a relatively small change in elevation can lead to a very big change in the dominant species. In flooded lowland areas, cypress-tupelo forests are dominant. As you go to a somewhat higher elevation, these swamp forests grade into forests dominated by red maple, black gum (Nyssa sylvatica), spruce and pine.
The upland areas are usually dominated by one of three forest types, depending on the amount of moisture:
1) Bottomland Hardwood - some flooding
2) Mixed Mesophytic forest - damp
3) Pine Forest, or Oak and pine/hickory/chestnut - dry
Hardwood bottomland forests occur where the surface stays dry for most of the year, but there is some seasonal flooding. In our area, these forests are dominated by live oak, water oak, sweet gum, green ash, red maple, hackberry and tupelo (black gum) The understory of these forests is dominated by dense stands of Palmetto (Sabal minor). Bottomland forests are usually dense, with many large trees (up to 3 feet in diameter or larger), and lots of vines (lianes).
In areas of intermediate elevation and moisture, we find a type of deciduous forest called the mixed mesophytic forest. The dominant species includes beech, magnolia, white oak, and red maple (as co-dominant species). In Louisiana, the beech-magnolia forest seems to have been the original climax community, with only the tiny remnant of the Zemurray tract spared by the timber industry. This small forest, is located northeast of Hammond, up near Folsom. It is part of what was supposed to be a United Fruit Company plantation, although the plantation never got off the ground. Zemurray's wife got bored out in the boondocks, and started planting azaleas, dogwoods, and other ornamentals, and the Zemurray Gardens are a popular tourist attraction today.
These mixed mesophytic forests represent a mingling of northern deciduous forest species and typical southern broad-leaved evergreens, like magnolia. In dryer upland areas, the forest is usually dominated by an Oak-Hickory or Oak-Chestnut association, with a few pines and spruces mixed in. But most of this dry upland southern habitat is covered by an immense belt of pine forest.
Coniferous evergreen forest stretches 1,200 miles across the coastal plain, and covers a greater area than all other forest types combined. South of Lake Pontchartrain, a thick layer of delta mud overlies the sandy soils, so south of the lake hardwood trees are the dominant forest species. North of the lake, where soils are very sandy, pines are the dominant forest species.
Angiosperms are generally better competitors than gymnosperms. When angiosperms first evolved, they quickly replaced gymnosperms as the dominant vegetation over most of the planet. But in areas that are very dry or very cold, gymnosperms are better competitors. Gymnosperms evolved in the Permian, during a relatively cool and dry period, and are very well adapted to cold and arid conditions. Plants draw water up through their roots. The water passes through a system of small tubes (xylem cells). These cells are very different in flowering plants (Angiosperms) and pines (Gymnosperms). Gymnosperms have hollow tubes called tracheids, while Angiosperms have larger hollow tubes called vessels. The larger diameter angiosperm vessels are not especially well adapted to dry conditions. Water movement up from the roots to the leaves relies on an unbroken column of water, pulled up in part by capillary action. If that tube of water is broken, a process called cavitation, then it is very difficult for angiosperms to form a new unbroken column of water. Gymnosperms, on the other hand, do not cavitate as easily as angiosperms, and reform the rising water column more quickly when it does break.
Pines can therefore outcompete angiosperms in sandy soils. Sandy soils pass water through very quickly. Soils with a very high clay content, with fewer and smaller pore spaces, can hold more water, and retain it for longer periods of time. We call this property of soils their field capacity. Field capacity is the amount of water the soil can hold after being completely saturated. Clay soils have a very high field capacity, but sandy soils have a very low field capacity. So pines and other gymnosperms do much better in areas where there is often little or no water in the soil, in habitats that are very dry.
From the standpoint of plant physiology, cold is the equivalent of dry, because getting enough water is a problem in both environments. Forest in the far North, or at higher elevations, are also dominated by gymnosperms because water in the soil may be mostly frozen most of the year. Sandy soils are also poor in nutrients. Sand passes water quickly, and the water leaches out the nutrients from the sand as it filters through. Pines do better than angiosperms in these nutrient-poor soils. Pine needles are also an a adaptation to reduce water loss. And they’re the perfect shape for wind-pollinated plants, like conifers.
The major natural disturbances in the pine forest are fire, wind, and man. The combination of fires and sandy soils are the most important factors explaining the dominance of pine forests throughout the south. Fire is essential in maintaining pine forests. In the absence of disturbance by fire, hardwoods may replace the pines by gradually getting a foothold in the forest understory.
Not many species can tolerate the physical conditions on the floor of the pine forest. The decaying pine needles produce a very acid leaf litter. The few deciduous species that can tolerate these conditions, and grow in the understory of the pine forest, grow much faster than the pines. Their shade-tolerant seedlings will shade out the sun-loving pine seedlings, preventing the pines from regenerating, and ultimately clearing the way for angiosperms to take over. Whereas angiosperm seedlings can establish and grow in the pine forest, pine seedlings need so much light that they cannot grow well in the shady understory of the typical hardwood forest. Fire burns up the less-resistant seedlings and shrubs of the deciduous species that normally sprout in the understory. Palmettos are also very fire resistant, so fire leaves pines and palmettos standing, and wipes out almost everything else. Fires also kill many insect and fungal pests (e.g. fusiform rust), but not the pine tree.
Historically, the most characteristic species in this vast southern pine forest belt is Pinus palustris, the longleaf pine. Longleaf pine thrives under both wet or dry conditions, although it cannot tolerate actual flooding. In wet areas it is joined by other pine species, the slash pine (Pinus elliotti) and the loblolly pine (Pinus taeda). The same loggers who clearcut the great forests of the northeast, and are now working on the Pacific Northwest, also clearcut the southeastern pine forests, turning them into vast pine plantations. Longleaf didn’t seem to grow as quickly as other species, so when they replanted for the next harvest in the southeast, they replaced longleaf pine with slash pine and loblolly pine. Loblolly pine is sometimes called "oldfield pine", because in nature it does very well in secondary succession. It is one of the major sources of domestic lumber and pulp wood.
But longleaf pine is starting to make a comeback in the south, thanks to research by the Forest Service and others. It is much better adapted than other species of pines. Although longleaf pine takes longer to mature, for example, it is more resistant to fire and disease than other species. And the apparently slow growth of longleaf pine seedlings turns out to be illusory. In fact, it is a superb adaptation that helps it to survive fires.
Longleaf pine seedlings spend 3-6 years developing a huge tap root. During this “grass stage”, only a small tuft of needles is visible above the ground. The most intense heat during a fire will be closest to the ground where the leaf litter and young angiosperm plants are burning. So when a longleaf pine seedling finally starts to grow above ground, it grows very rapidly, drawing on the stored reserves in its big tap root. This quickly carries the terminal bud and needles above the most dangerous zone.
The young needles of these saplings have an outer coating that gives them a silvery color. This silvery color helps reflect some of the heat away from the vulnerable terminal bud, protecting the young plant from dying during a fire. Longleaf pines are also protected from fire by their very thick corky bark. They also have very few lateral branches until they have grown relatively high into the air, because lateral branches are very vulnerable to fire damage. During this broom stage of their growth, they look like upside down broomsticks stuck in the sand.
So the dominance of the pine forest over most of the southern coastal plain is primarily due to three factors:
1) Pines do better in sandy soils (low field capacity) because
of their primitive xylem (tracheids)
2) Pines do better in nutrient poor soils (sandy soils are rapidly
leached of nutrients by water)
3) Pines do better when fire is the main source of natural disturbance
The pine forest can be considered as a "sub-climax" of sorts. If there
are regular fires, pines remain the final community in succession. But
in the absence of this disturbance, the ultimate climax community will
be a hardwood forest. One of the few surviving tracts of hardwood climax
forest in the south is the Zemurray tract. The forest is surrounded by
a small stream, which has historically kept this area from regular burning.
Selected References
Where to Observe Hardwood and Pine Forests
Links
Return to B. Fleury's Home Page