VIRGIN . ERODED
Figure 31. Soil profile of a virgin area compared with that of an eroded area in the deciduous forest region. In the left picture, 1-2 represents the A I hOrizon, 3-4 the A2 horizon, and 5-6 the Bl layer (accumulation of leached material). Compare with Figure 32. (U. S. Soil Conservation Service Photo.)
130 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CH. 4 file. The upper horizon, or A horizon ("top soil"), is composed of the bodies of plants and animals which are being reduced to finely divided organic material by the process known as humification (see page 19). In a mature soil this horizon is usually subdivided into distinct layers representing progressive stages of humifica- tion. These layers (Fig. 31) are designated (from the surface downward) as A01, A02, Aos, Al, and A2, or, less technically, as litter, duff, leaf-mold, humus (Al), and leached (light-colored) zone (A2). The next major horizon, or B horizon, is composed of mineral soil in. which the organic compounds have been converted by decomposers into inorganic compounds by the process of mineralization, and thoroughly mixed with finely divided parent material. The soluble materials of B horizon are often formed in the A horizon and deposited, or leached by downward Bow of water, in B horizon. The dark band in Figure 31 represents the upper part of B horizon where materials have accumulated. The third horizon, or C horizon, represents the more or less unmodified parent materia1. This parent material may represent the original mineral formation which is disintegrating in place or it may have been transported to the site by gravity (colluvial deposit), water (alluvial deposit), glaciers (glacial deposit), or wind (eolian deposit, or loess). Soils which have developed on material trans- ported by glaciers, water, -and wind are often extremely fertile (witness the deep loess soils of Iowa and the rich soils of the deltas of large rivers).
The soil profile and the relative thickness of the horizons are generally characteristic for different climatic regions and for topo- graphic situations (Figs. 32 and 33). Thus, grassland soils diffel from forest soils in that humification is rapid, but mineralization is slow. Since the entire grass plant, including roots, is short-lived, with each year are added large amounts of organic material which decays rapidly, leaving little litter or duff, but much humus. In the forest litter and roots decay slowly and, since mineralization is rapid, the humus layer remains narrow (Fig. 32). The average humus content of grassland soil, for example, is 600 tons per acre, compared with 50 tons per acre for forest soils (Daubenmire, 1947). In the forest-grassland buffer zone (see Figure 33) in Illinois, one can easily tell by the color of the soil which cornfield was once prairie and which was forest: the prairie soil is muel]
blacker, due to its high humus content. Given adequate rainfall,
PRINCIPLES PERTAINING TO LIMITING FACTORS: §5 131 it is no accident that the "granaries of the world" are located in grassland regions.
Topographic conditions greatly influence the soil profile within a given climatic region. Hilly or well-drained land, especially if misused by man, will tend to have thin A and B horizons due to erosion (Fig. 31). In flat land water may leach materials rapidly into the deeper layers, sometimes forming a "hardpan" through which roots of plants, animals, and water cannot penetrate. Poorly drained situations such as bogs favor the accumulation of humus, since poor aeration slows down decay. The lack of oxygen and the accumulation of carbon dioxide and other toxic products be- come severe limiting factors. Sometimes soils developing on poorly drained sites are extremely productive if they are properly drained - witness the muck soils of the Everglades of Florida which rate as some of the most productive soils in the world, when properly handled. The catch is that "proper handling" involves difficult ecological, as well as engineering, problems.
In tropical climates, the rate of decay is so rapid that little humus accumulates. Thus, although other growing conditions are favorable, cleared land is often "worn out" in a couple of years and has to be abandoned. The problem of fertility of tropical soils is primarily an ecological one and needs further study.
Classification of soil types has become a highly empirical sub-
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132 BASIC ECOLOCICAL PRINCIPLES AND CONCEPTS: CH. 4
Figure 33. Soil-vegetation relalionships in a prairie-forest transition zone.
Distinctly difFercn t soils develop from the same parent material (calcareous loess, or wind-transported "C" horizon in this case) under influence of dif- ferent vegetation and climale. The decrease in organic matter, the develop- ment of a podzolic A horizon (with narrow humus layer, see Figure 32), and increased structural development of the B horizon are the main features differentiating the forest soils from the prairie soils. (AIter Crocker, 1952.)
jcct. The soil scientist may recognize dozens of soil types as occur- ring within a county or state. Type names for local soils, for example, Norfolk Sandy-Loam, usually include the geographical locaJjty where the series was described and an indicaUon of Lhe texture. The ecologist, of course, should do more than merely name the soil on his study area. At the very minimum, measure·
ments should be made of three important attributes in at least the A and B horizons as fo~ows: (1) texture, that is, the per cent of sand, silt and clay (or more detailed determination of particle size), (2) per cent organic matter and (3) exchange capacity, that is, an estimate of the amount of exchangeable nutrients; as emphasized in Chapter 2, the "available" minerals rather than the total amount determine potential fertility, other conditions being favorable. Variation in texture, organic content and exchange capacity in a series of soils, and the effect which thesc attributes have on the uptake of radiostrontium by plants is shown in Figure 156 (page 482).
Since soil is the product of climate and vegetation, a map of major (zonal) soil types of the world (Fig. 34) becomes a com- pOSite map of climate and vegetation (Fig. 35). Given a favorable parent material the action of organisms and climate will tend to build up a soil characteristic of the region (compare Figure 34 with map of biotic communities, Fig. 120, page 384). From a broad ecological viewpoint, the soils of a given region may be lumped into two groups, those which are largely controlled by climate and vegetation of the region and those whkh are largely
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controlled by local or edaphic conditions of extreme or unusual topography, water level or type of parent material. Thus, the local climate might favor a ,grassland community which in turn
"builds" a grassland type of soil, wherever conditions are not so extreme as to retard or prevent characteristic ecological action and reaction. VVhere some condition is extreme or limiting, as, for example, poor surface drainage, an entirely different soil may
PRINCIPLES PERTAINING TO LIMITING FACTORS: §5 135 develop, which in turn supports a different biotic community.
The extent to which local or edaphic factors influence soil devel·
opment varies with the region. In general, edaphic conditions are more important (1) in regions which are geologically young and in which climate and vegetation have not yet been able to "build"
a uniform soil cover, and (2) in regions where the climate is ex- treme, as in deserts, for example, where small differences in soil may make big differences in the resulting biotic community. Thus, ecologists working in the Middle West where soils are well de- veloped find that virtually the same sort of biotic community win, if given time, develop on a variety of soil types. Thus, these ecol- ogists tend to look upon the soil, unless very extreme in type, as not being a critical limiting factor in the long run. On the other hand, ecologists working on the coastal plain of southeastern United StatE..s, where the sandy soils are relatively undeveloped or
"geologically young," are very soil-conscious because they find that climate and vegetation have not yet been able to build a char- acteristic soil; more or less pelmanent differences in biotic com- munities are directly correlated with differences in soil types.o This difference in viewpoint in regard to soil as an over-all limit- ing factor is, of course, inherent in the principle of limiting factors in general. Both groups of workers are right as far as their re- spective localities are concerned. The particular aspects of the environment which function as primary limiting factors will vary with the region and the organisms being studied. The ecologist should not carry into a new region stereotyped ideas based on experience in another region.
For good general discussions of soil, reference may be made to Wolfanger (1930), Kellogg (1948), Russell and Russell (1950), Simonson (1951) and Russell (1957).
9. Fire as an Ecological Factor. Research during the last twenty or thirty years has made necessary a rather drastic re- orientation of our ideas about fire as an ecological factor. It is now evident that fire should be considered not a minor or abnormal factor, but a major factor which in many regions is, and has been for centuries, almost a part of the normal "climate." As with most ecological factors, man has greatly modified its effect, increasing its influence in many cases and decreasing it in others. Also, it has
o Translated into quantitative terms, 83 per cent of the soils of Marshall County, Iowa, are mature, compared with only 15 per cent in Bertie County, N. C., on the coastal plain, according to Wolfanger (1930).
136 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: Cli. 4 become clear that fire is not always detrimental to man's interest.
Properly used, fire can be an ecological tool of great value. Fire is thus an extremely important limiting factor if for no other rea- son than that mankind is able to control it to a far greater extent than he can many other limiting factors.
Fire is most important in forest and grassland regions of tem- perate zones and in tropical areas with dry seaSOnS. II) many parts of western or southeastern United States, it would be difficult to find a sizable area which does not give evidence of fire having occurred on it during the last fifty years at least. In many sections fires are started naturally by lightning. Primitive man, the North American Indian, for example, regularly burned woods and prai- ries for practical reasons. Fire thus was a limiting factor long be- fore the white man began to drastically modify his environment in an attempt to improve his status. Inadvertently, through care- less behavior white man has often so increased the effect of fire that the very thing he is seeking-a productive environment-is destroyed or injured. On the other hand, complete protection from fire has not always resulted in what was expected, namely, a more productive environment for man's purposes. Thus, it has become clear that flre should be considered an ecological factor along with such other factors as temperature, rainfall and soil, and should be studied with an open mind. Whether fire is a friend or a foe of civilization will depend on intelligent knowledge and control.
In speaking of fire as an ecological factor, the first thing to em- phasize is that there are several types of fire in nature which are tlifferent in effcct (Fig. 36). For example, crown fires often de- stroy all of the vegetation, whereas sU1jace fires have entirely dif- ferent elJects. The former is limiting to most organisms; the biotic community must start to develop all over again, more or less from scratch, and it may be many years before the area is productive from man's viewpoint. Swfacc fires, on the other hand, exert a selective effect; they are more limiting to some organisms than tc others and thus favor tlw development of organisms with high tolerance to the fire factor. Also, light surface fires aid bacteria in breaking down the bodies of plants and in making mineral nutri- ents more quickly available to new plant growth, Nitrogen-fixing legumes often thrive after a light bum. In regions especially sub- ject to fire, occasional light surface fires greatly reduce the danger of severe crown fires by keeping the combustible litter to a mini- mum. In examining an area in regions where fire is a factor, the
Figure 36. The two extremes of fire. Upper: Result of a severe crown fire in Idaho. Lower: A controlled burning operation on the Alapaha Experi- mental Range, Georgia, which removes hardwood competition, stimulates growth of legumes, and improves the reproduction of valuable pine timber.
Burning is done under damp conditions in afternoon (fire is stopped at night l-,y dewfall). Note that smoke is white and thin line of fire can be stepped over at almost any pOint. Insert: Long-leaf pine seedling; the terminal bud is well protected by long needles, which explains why this species is especially resistant to fire. (U. S. Forest Service Photos; insert by the author.)
Page 137
138 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CR. 4 ecologist usually finds some evidence of the past influence of fire.
Whether fire should be excluded in the future (assuming that it is practical) or should be used as a management tool will depend entirely on the type of community that is desired or seems best from the standpoint of regional land use.
A single example taken from a well studied situation will illus- trate how fire acts as a limiting factor, and how fire is not neces- sarily "bad" from the human viewpoint. On the coastal plain of southeastern United States the long-leaf pine is more resistant to fire than any other tree species. The terminal bud of seedling long-leaf pines is well protected by a bunch of long, fire-resistant needles (Fig. 36, insert). Thus, ground fires selectively favor this species. In the complete absence of fire, scrub hardwoods grow rapidly and choke out the long-leaf pines. Grasses and legumes are also eliminated, and the bob-white and other animals dependent on legumes do not thrive in the complete absence of fire in forested lands. Ecologists are generally agreed that the magnificent virgin, open stands of long-leaf pine of the coastal plain (Fig. 125, page 394) are fire-controlled, or a "fire climax," and that long-leaf pine and its associated animals cannot be maintained in the complete absence of fire. Since the long-leaf pine is one of the world's best timber trees and bob-white are generally considered more valu- able than blue jays (whic11 might thrive in the scrub oaks), it is obvious that "controlled burning" or the right use of fire is de- sirable from the economic standpoint. To put it another way, fire is a major limiting factor in the coastal plains pine region. 0
Among the best places to observe the long-term effects of in- telligent use of fire are the plantations of southwestern Georgia where for many years Herbert Stoddard and E. V. Komarek have been studying the relation of fire to the entire ecological complex.
As a result of these studies, Stoddard was one of the first to ad- vocate the use of controlled or "prescribed" burning for increasing both timber and game production. For years he has maintained high densities of both quail and wild turkeys on land devoted to highly profitable timber crops through the use of a system of
"spot" burning aided by a diversification in the land use (Stod- dard, 1936). All ecologists should visit one of these areas to see firsthand the "good" side of fire.
We emphasized that the example of the long-leaf pine men-
• For additional details, see Heyward (1939), Heyward and Barnette ( 1934), Stoddard (1936). and Garren (1943).
PRINCIPLES PERTAINING TO LIMITING FACTORS: §5 139 tioned above is well studied. In other regions fire-resistant species might be undesirable or lead to an unproductive environment.
Thus, conclusions regarding fire should be restricted to situations where study has been adequate. Sweeping statements should be avoided, at least until more information is available. Certainly there is no more fascinating and important field for ecological research.
10. Microenvironmen.t.o Not only are regional differences in temperature, moisture and other factors important, but also local horizontal and vertical differences. Organisms occupying the same general habitat may actually be living under very different conditions. The concept of microenvironment, the environment of small areas in contrast to large ones, has been developed only wilhin the last decade. Other terms, commonly applied to this concept but much more restrictive in scope, are microclimate and bioclimate. Since the term microenvironment is a relative one, it may signify the immediate environmental area occupied by a pine stand, or equally well of that occupied by a lichen within the stand. Critical studies at the microenvironmental level have sig-
nificantly sharpcncd our approach to the study of individual or- ganisms as well as to communities of organisms; the data are especially valuable in calculating energy flow of various popula- tions within the community.
Basic considerations and safeguards which have now come into fairly common use, but which were not well understood or gen- erally accepted a decade or more ago include: (1) the environ- ment of a particular organism at a particular time is not the same as that some distance removed, either by millimeters or by miles;
(2) data cannot be transposed in time any better than in space;
(3) environmental effects observed on an individual cannot be applied without question to the population or the species; (4) data obtained in greenhouses under unnatural conditions cannot be used for interpretation of the same organism when living in its natural environment and (5) vertical as well as horizontal gradi- ents are of far greater significance than point measurements of environmental conditions.
Figure 37 illustrates a microenvironmental laboratory located in southern Georgia, which consists basically of two fifty-foot wooden towers located at the ecological extremes of a dense wood- land and an open field. These towers have been equipped with
o This section has been prepared in collaboration with Dr. Robert B.
Platt, Emory University, Atlanta, Georgia.