Ecological Succession and Its Application to Forestry
Succession is the process by which the communities we see in nature were established. It consists of a series of stages which result in the establishment of a climax community. These stages involve changes in both the biological and physical components of the habitat. The climax community represents the combination of plants and animals which make the most efficient use of available resources and conditions. In other words, the community established by nature is the one which works best given the climate, topography and other characteristics of the area. Climax communities are designated according to the dominant plant type, but many other plants as well as animals make up the community. Examples of types of climax communities are: spruce-fir forest, ponderosa pine forest, beech-maple forest, creosote bush desert, etc. However, even these can vary from one place to another with regard to the specific organisms found and their relative abundance.
Succession on barren terrain, i.e. rock or other inorganic material, is called primary succession. Primary succession initially formed all climax communities, but we can only see it at work on recently deposited volcanic land, rock exposed by retreating glaciers, and inorganic sand, silt., or cinder which has little or no organic material present. The chart below shows primary succession that results in a forest community. In grassland, desert, or tundra the process would lead to a grass or shrub stage, stopping short of the establishment of trees. The types of organisms are shown in bold with some examples shown in plain type.
The process must begin with organisms that form organic soil, the pioneers or soil builders. This soil will be necessary to provide for the next group of plants to succeed. Lichens are found in most every habitat, wet & dry, hot & cold, and come in a wide variety of shapes and colors. They are a symbiont composed of fungus, algae and , sometimes, bacteria. If cyanobacteria are involved the lichens are nitrogen fixers. In wet climes mosses and liverworts are important pioneers. In dry sand or clay soils cryptogamic organisms are critical to the buildup of organic materials and the ability of the soil to hold water. Cryptogamic soil consists of a combination (not a symbiont) of lichens, fungi, and mosses which form a crust on the surface which is very susceptible to physical damage. Cryptogamic soil is critical to the sustainability of desert soils in the Southwest and the damage resulting from off road travel and overgrazing is a major factor in soil loss.
Once the soil is produced the quick-growing grasses and weeds come in. Some of these are significant, like fireweed which supports a wide variety of wildlife with its nectar, foliage and seeds, and lupine which harbors nitrogen-fixing bacteria. The next group to arrive is the shrubs which often includes food-rich blueberry, salmonberry, raspberry, blackberry etc. Eventually the early trees gain a foothold. They are called early trees because they are the first to arrive and are species which generally grow quickly and well in new and disturbed soils. They are NOT simply young or small members of the climax trees. Early trees include types such as willow and alder which thrive in wet habitats, and aspen and birch which are important species in the habitats recovering from fire and other disturbance. Both are capable of growing back quickly from their roots after the standing trees are cut or burned. The early tree stage will be firmly established by about 20 years after the start of succession.
The late trees produce
the climax forest. (See
Figure 2) They require the
most soil support and are
the slowest to grow, but
once established they will
dominate the forest,
usually in about 65 to 75
years. Old growth is a
climax forest with a
preponderance of trees
200 years old or greater.
Most of the earlier stages
are still represented in the
climax and old growth
forests to some degree.
This results in increased
diversity which is often enhanced by the natural disturbances which return a portion of the forest
to an earlier stage (secondary succession). A natural fire for instance will burn a small section of forest
returning it to a stage containing grasses, weeds, and shrubs. This will increase habitat value for
the herbivores which depend on these foods, while the unburned portions of forest will continue
to support the species which live in and on the old trees, and the species which eat them. A
natural forest is heterogeneous with earlier and later stages of succession forming a mosaic of
diversity. One problem with fire suppression by man is that it allows lots of fuel to build up so that
when a fire eventually does occur (and in the dryer forests it always does) it will be very
destructive and burn a much greater portion of the forest than the small cyclical fires which would
occur naturally. Even so, the forest will grow back if left to itself.
In a typical forest the diversity increases as shown in Figure 2 as succession proceeds to climax. In a moist coniferous forest like the forests of the Pacific northwest, diversity is maintained and even increases as the forest proceeds to old growth and the organisms of rot and decay foster a new community of species. This biological turnover of old trees helps to keep the entire system rejuvenated. In drier forests such as pine, however, the pines tend to suppress other species on the forest floor. The pine needles and cones are very acidic and inhibit the organisms which produce organic soil (humus) and therefore the weeds and shrubs are also inhibited. An old growth pine forest tends to be a less diverse uniform stand of old trees. Fire and other physical disturbances are the important agents in these forests in maintaining an overall heterogeneous forest.
The production of wood from a forest (the productivity line on the graph) depends upon the climax trees reaching what is called the young, mature status. This means that they are mature enough to have harvestable wood, but they are not yet beginning to succumb to disease and rot. This peak of productivity occurs well before the forest has reached its maximum diversity. These days hybrid varieties of douglas fir and other trees are being developed which reach maturity at ever younger ages. In the near future we will have plantation forests which mature at less than 60 years.
And therein lies the conflict of "modern forestry" with forest ecology. Younger maturing forests are more productive and more profitable for timber companies and help to satisfy our society's increasing demand for wood. But they reduce the diversity and other ecological values of the natural forest by never allowing it to return to the original diverse climax, let alone old growth. Add to this the fact that "type conversion" changes the natural forest over to a plantation monoculture as uniform rows of even-aged hybrid trees replace the random diversity of the original forest. The push in recent years by the timber industry to cut the last remaining old growth forests is as much due to its desire to replace them with these quick maturing monocultures as for the wood they contain. It is these techniques, together with the use of herbicides and other chemicals, removal of competing vegetation and decaying wood, erosion and other edge effects, that threaten the integrity of our forest resources.
Edge effects occur when forests are chopped up and criss-crossed with roads, exposing much of the forest to edges along roads and cutover lands. Some edges are good. Natural forests benefit from the edges created between meadow and forested land. But edges also allow the freer movement of insects and plant diseases and manmade edges along roads are prime areas of erosion and exposure to pollution such as solid waste, air pollution, oil and gas residues etc. The straight edges created along clearcuts and roads allow insects and fires, otherwise natural phenomena when in limited amounts, to spread great distances and become more destructive than they otherwise would.
In addition chopping the forest up causes isolation or fragmentation of the remaining pockets of uncut forest from one another. A branch of ecology called island biogeography investigates the relationship of isolation to diversity and other ecological factors. Isolation or occurs not just on islands surrounded by water but also when a particular habitat type is isolated from other areas of the same type. This can occur when forestry results in isolated pockets of old growth surrounded by cut-over forest. The results of theses studies show that the resulting diversity is related to three factors:
1) remoteness - the farther the "island" is from other similar areas, the lower the
resulting plant and animal diversity will be.
2) size - the smaller the "island" is, the lower its diversity will be.
3) habitat - the more varied the potential habitat, the greater its diversity will be.
There are techniques which can be utilized to reduce the impact of isolation, edge effects and other threats to the integrity of diverse natural forests. Roughly encompassed by the terms alternative forestry or sustainable forestry [See Nez Perce article] these techniques aim at maintaining the natural complexity [biological legacy] which gives rise to both forests and wood. They function by trying to mimic nature, letting timbering have the same effect as natural disturbance and by maintaining the original pre-logging conditions as much as possible. Specifics will be discussed in class but these techniques involve what is cut and what is not, where cutting occurs, how cutting occurs, etc.
There are two basic approaches to forestry: even aged management, and uneven aged management. Uneven aged management is the sounder approach because it results in a more diverse forest, one with a variety of ages and types of trees.
1) Selective cutting is the ideal. In this technique one cuts only select trees of a certain type and age, leaving the rest to mature or to round out the forest's habitat and diversity. But much of the damage to forests comes from building the roads to get to the trees, and from the large scale machinery needed to get the trees out. These operations destroy much of the remaining forest and lead to erosion and damage as described above. And since a selectively cut forest will require much more forest land than clearcutting to yield the same amount of timber, it is only a sound environmental option when it can be done without large scale machinery and road building. An operator in the Manzano mountains, for example, is able to manage his woodlot selectively by hand cutting trees and dragging them out with horses. This is not an option for large timber companies.
2) Shelterwood management involves a series of cuts over several decades which leave a greater variety of ages and types of trees than clearcutting. In the first cut all trees of a harvestable age and type are cut, leaving everything else, including older trees past their maturity but good for habitat, younger trees that will be maturing in the next several decades, and non-commercial species. Then on a ten year cycle over the next two decades trees are cut which have subsequently matured. Eventually everything will be cut, but the ages of trees will vary from 10 to 30 years or so and some non-commercial and old trees can persist.
3) Seed tree management is a simpler form of #2 with only one cut that removes all harvestable timber and leaves some old trees for habitat value. It results in less diversity than the other two methods but still is not as damaging as clearcutting.
The above methods are very useful in pine forests such as those in the southwest where diversity is not that great to begin with, and the forest therefore yields sufficient product with these methods. Unfortunately they do not work well in the more diverse forest of the northwestern U.S. where less-commercial varieties dominate and timber companies feel the pressure to turn forests into tree farms for douglas firs.
4) Clearcutting is the type used mostly in these diverse forest of the Pacific Northwest. Everything is cut, often the stumps and debris are burned, herbicides and fertilizers are used, and a uniform stand of tree, usually douglas firs bred to maximize their growth rate, is planted.
The other approach to the problem is found in alternatives to reliance on timbering to supply all our demands for building materials and fiber. The total solution involves recycling wood and paper, alternatives to wood in construction and other uses, more efficient use of wood byproducts, and production of more finished or value-added products which provide more jobs than logging.