(Perma)Culture and Sanity
Around the world, lands which have been laid bare by fires or humans (often the two go hand in hand) follow a similar pattern of recovery. This pattern is referred to as "natural succession."
Natural succession describes the ecological stages which the land, plants and animals move through in returning to a state of relative stability after they have been disturbed (natural succession is not the same as "natural selection," which describes plants' and animals' evolutionary adaptations to particular ecosystems over a long period of time).
The cycle of natural succession begins after some major disturbance, such as fire, has removed an area's vegetation and left the soil exposed.
The first plants to move into the new bare ground are a relatively-small number of species of fast-germinating, leafy herbaceous plants known as forbs (wildflowers and "weeds"). These fast-growing plants quickly germinate, grow and produce a new crop of seeds (in the desert, these same plants would be called 'ephemerals,' due to their habit of germinating, growing, making seeds and then disappearing from sight, all after a single hard rain). Grasses follow, often within the same season.
Although slower to start initially, grasses soon replace most of the forbs since grasses outcompete forbs during summer dry spells. Grasses have phenomenal root systems, and merely go dormant during extended drought, whereas forbs survive drought as seeds and must resprout when rains return. Each time the drought cycle repeats, grasses have covered more ground and soon begin to prevent the forbs from re-establishing themselves. Grasslands, however, do not themselves form a stable, climax state (without the periodic intervention of fire) and reign only a few years. Soon, "pioneer" shrubs and trees begin to move into the grassland.
Pioneer trees are hardy species—relatively small, slow-growing, unpalatable to livestock and able to mature at greatly reduced sizes when lack of moisture or nutrients (i.e., lack of soil) is a problem. These characteristics of unpalatability, slow growth and small size (though larger than the grasses) are important traits for dealing with harsh climates, poor soils and the over-grazing that seems invariably to plague arid lands. While grasses must compete with cattle, the trees are able to grow relatively unmolested (which is why burning was important to Plains-dwelling Native Americans and early settlers—both were pastoral, and burning was needed to keep the land from progressing past the grassland phase).
Once pioneer trees begin to germinate and establish themselves among the fields, the existing grassland is on its inexorable way toward becoming a scrub forest. Individual trees manage to sprout, growing and spreading out above the grasses to shade them out. Grazing pressures, shading and poor soil conditions (which grasses can tolerate but will not thrive in) continue to favor the trees and soon they are rampant.
Scrub trees eventually become the dominant vegetation, completely filling in the area with their crowns (often a single aggressive, extremely hardy species of tree—for instance mesquite or juniper—will predominate for years). The thick canopy of scrub shades the grasses underneath mostly out of existence, and lays down a thick layer of needles or leaves to produce mulch and, eventually, soil. By the time they have matured (commonly 40 or more years), mesquite, juniper or other scrub trees will have created and be protecting a new layer of relatively rich, friable topsoil under a continuous mulch of forest litter.
With their crowns offering continuous cover, the forest soil will now be shaded and cool, and humidity in the trees will be much higher than in surrounging open areas. Compare this moist, rich topsoil with the dusty-surfaced, water-repellent, compacted exposed soil which exists in the overgrazed grasslands which now predominate in the Southwest (this set of degraded conditions—invariably accompanied by 'pedestalling' of grass plants which indicates rapid surface erosion—will be seen in almost any grazed arid land).
As a scrub forest reaches maturity and evolves with it a rich forest soil, taller trees and vines begin to move in, taking advantage of the now more-hospitable conditions. Though these taller trees or vines could not have survived under the conditions the scrub trees started in, they are able to prosper in the enhanced soil and air created by the scrub forest and soon grow above and start to shade out the older, shorter, slower-growing trees.
Eventually climax forests are formed with tall trees and some large vines almost completely shading out understory growth (except at edges such as where forest meets grassland or large rivers—here understory prospers in the light and makes edges the richest part of the forest both in terms of number of species and total productivity). It is to prevent trees and shrubs from replacing pasturage with a forest that grasslands were periodically burned in the past.
Eventually, a mature forest will begin to decline in health (nothing in Nature lasts forever). For one thing, too great a proportion of the trees are old—old trees spend a lot of energy just maintaining their huge bodies, leaving little energy left over for growth. Furthermore, in a climax forest most organic matter and nutrients become unavailable since they are tied up in the trees. This is why rainforests do not support farming for long after the trees are burned—burning destroys organic matter, and converts the nutrients that were held by the trees into ash which is quickly leached out of the soil. New trees—which would sprout and grow to rejuvenate the older trees—cannot obtain light to become established under the high, dense forest canopy.
The old forests thus slowly decline until they begin to leave open areas in their canopies due to tree death, fire, wind, humans, or other event. Then the cycle of natural succession begins again, whether in a whole forest newly-burned, or in a clearing created by the death of a single large tree.
Almost anywhere, each of these successional states (forbs, grassland, scrub or thicket, pioneer forest, mature forest) can be seen in different parts of the same area. Even the initial, short-lived post-fire state dominated by forbs, now uncommon since burning is no longer practiced, can still be seen in any newly cleared garden bed.
Each successional state, if left undisturbed, will move toward the same "climax" state depending on the local ecosystem—and each climax state in turn leads eventually and inexorably to a fresh start. Even deserts follow this pattern of recovery, though at a very slow pace and with a climax vegetation that we might not recognize as 'trees.'
Natural succession is the reason that gardeners and ranchers must both work so hard to maintain the conditions they prefer. Gardeners must constantly pull forbs, grasses, and tree seedlings from their gardens, and ranchers must burn (or—more commonly in these times—resort to bulldozing and the use of toxins) every few years to kill tree and shrub seedlings and maintain their land as grassland.
Burning of grasslands (practiced until the 1950's in Texas, and regaining popularity) is a complex skill, and a controversial and complex subject. Since arid grassland soils are severely depleted in general, with low organic matter and such major nutrients as phosphorous and calcium often-or-usually completely missing, burning valuable organic matter to produce easily-lost ash has to be seen as a questionable practice. Far better to evolve agricultural systems that mix, and simultaneously or successively hold, the better qualities of both forest and grassland.
Trees are mixed with crop by most traditional cultures in complex agroforestry systems. Long-term rotation systems can use the natural tendency of crop and grassland areas to move toward forest, to obtain varied, sustainable yields safe from the sorts of blights that plague monoculture. Mature climax forests (excepting wilderness areas) could be started over and returned to cropland periodically by almost-complete harvesting (leaving some climax forest intact for windbreak, wildlife corridors and islands, etc.).
Modern systems of alternating bands of trees and crop or pasture can even be created which allow machine cultivation and harvesting while still conferring all the combined benefits of cropland, rangeland and forest.
Reprinted from (Perma) Culture and Sanity Website