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R. F. Mueller
November, 2005What is a Forest?
A forest is more than a collection of trees. While trees are generally so big that they fill our eyes, a forest also includes all the wild flowers, ferns, mosses and mushrooms present, as well as all the animals, birds and microscopic life-such as bacteria- in it. It includes the soil out of which it grows, and even the atmosphere that circulates through it. All these parts are linked to each other. Which brings us to ecology.
What is Ecology?
According to one dictionary (American Heritage) ecology "is the science of the relationships between organisms (living things) and their environment". Of course, this also includes mutual relationships among the organisms themselves. Also, while some of these relationships are of the family type, as between parents and children, others involve food, shelter and other interactions with the environment. For example, some small trees are specially suited to grow in the shade of large trees, and some wild flowers on the forest floor can bloom only in the early spring, before tree leaves appear to rob them of light.
What is a Species?
All living things fall into related groups and each organism has two scientific names. The first of these names always begins with a capital letter and indicates the "genus", or smallest grouping of related organisms, while the second name, which begins with a small letter, refers to the "species", or a particular type of organism. Thus all maples fall in the genus Acer, and the common Red Maple is Acer rubrum. Similarly, oaks fall in the genus Quercus, and the White Oak is Quercus alba.. In some cases species are further divided into "varieties" or "sub-species" on the bases of small variations in appearance or other characteristics. There are also larger groupings in this scientific classification. Thus a grouping of closely related genra (the plural of genus) fall into the same "family". Still larger groupings are also part of the system, but are left to the reader to look up. At this point we also arrive at the theory of evolution, which states that those species survive and do best that have the capacity (the right genes) to adapt to their environment and can pass this capacity on to their offspring. A habitat. or living place for a community of species, is said to have high "biological diversity" when it contains a large number of different species.
How Do Forests Differ?
There are many different kinds of forests, and they fall into large groups. A convenient grouping is to divide them into needle-leaf and broad-leaf types. Familiar examples of the needle-leaf type are the evergreen coniferous (seeds produced in cones) spruce (genus Picea ) and pine (genus Pinus ) forests, which are so common in cool climates. The "deciduous" (drop leaves in a season such as autumn) hardwoods, like those of eastern North America, East Asia and Europe, are examples of the broad-leaf type. However, some needle-leaf types, such as the larches (genus Larix, also known as tamaracks), drop their needles in the fall as well. Additionally, many broad-leaf types, such as many species of the tropics, are also evergreen. Here however, we discuss as examples mainly the forests of eastern North America, since they are the most familiar to this writer.
Forests may also be divided into moist and dry climate types. Thus the "mesic" (medium moist) as well as dry (also known as "xeric") deciduous forests are common types in the eastern US. Tropical dry forests also drop their leaves, in this case, during the dry season. Rain forests are extreme examples of the moist climate type. Most familiar are those of the tropical regions, such as the Amazon Basin, but some, composed of spruce, fir (genus Abies ), cedar (genus Thuja ) as well as other conifers, also occur along the northwest Pacific coast of the US (including Alaska) and Canada (Fowells, 1965).
Mesic and dry forests have different species, or some of the same species in different proportions. While the leaf cover of mesic forests tends to be dense, that of dry forests is frequently thin, allowing more light to reach the underlying vegetation and forest floor. Also, the tree, shrub and non-woody plants { herbs }of mesic forests tend to have thin, juicy leaves that decompose rapidly after death, while those of dry forests are generally relatively thick and leathery and are resistant to decay, so in dry forests the forest floor is usually covered by a thick mat of dead leaves. This tends to create an acid soil condition that discourages the lush non-woody plant growth that is found in mesic forests. Examples of temperate climate mesic forests are those with tree layers composed of maples, beech (genus Fagus), birches (genus Betula) and basswoods (genus Tilia) Examples of dry forests are some composed dominantly of oaks, hickories (genus Carya) and pines.
The uppermost branches and leaves of a tree are known as the "crown." Together the crowns of the tallest trees form the "canopy", which shades all lower trees and other plants. If the crowns of individual trees are in contact with each other, they form a "closed-canopy" forest, and when the crowns are separated, the canopy is said to be "open." When the separation is great, the forest is called a "woodland."
Trees and other plants that grow in forests do not all tolerate shade equally well. It is no surprise that many that comprise the densely-foliaged mesic forests, such as the maples, beech and basswoods, are among the most tolerant, while the oaks, hickories and pines of the thinly-foliaged dry forests are among the least tolerant However, this division is not absolute, since the most complex North American mesic forests, the "Mixed Mesophyte Type" also contains some quite shade intolerant trees such as Tuliptree ( Liriodendron tulipifera) and White Ash (Fraxinus americana ). These trees manage to grow in natural openings formed by tree-falls, as discussed below. Additionally, some trees, including White Ash, are quite tolerant of shade when they are still seedlings and small saplings. This makes it possible for them to survive until an opening appears.
Forest Stability and Equilibrium
If a thing, such as a forest, remains unchanged for a long time, we may suspect that it is in equilibrium with its surroundings, that is, all forces which might change it are balanced. However, there are different types of equilibrium, namely stable and unstable (we ignore a third type that adds little to our explanation). We may illustrate with a small ball and a bowl. If the ball is placed in the bowl and the latter is given a slight push, the ball will roll part way up the bowl's side, but will again come to rest in the bowl's center. This is an example of stable equilibrium. If the bowl is now turned upside down and the ball carefully balanced on the bottom, it will remain so until disturbed. However, if the bowl is now pushed, the ball will roll off, never to return. This illustrates unstable equilibrium. In the case of a forest, or indeed any natural vegetation, a disturbance such as a high wind or logging may wipe out part of the vegetation, but in time the original type of vegetation will re-grow, although. this may be preceded by a succession of unlike vegetation. This is an example of stable equilibrium in nature. However, we can't think of any example of unstable equilibrium in forest vegetation, since no unstable system can remain unchanged for any length of time under natural conditions.
The best known example of equilibrium in forests is the concept of the "climax forest." A climax forest is one in which the vegetation has reached its fullest development and death is balanced by reproduction. The mature trees have an average age that is the maximum for the forest type, and all plants reproduce themselves, although their proportions may vary with time as a consequence of disturbances. There is a balance between living and dead wood, and soils are mature and characteristic of the forest type. Furthermore, there is a close relation between the type of vegetation and the type of soil. Usually also, the soil type is closely related to the underlying "sub-soil" or rock. Exceptions are soils that have been altered by rainwater leaching. When such forests have suffered little or no alteration by humans they are called "virgin" and when they contain old vegetation, such as their trees, they are classified as "old growth."
We noted above that the re-growth of mature vegetation is preceded by unlike vegetation. The first of this vegetation to form after a disturbance is known as "early successional vegetation" and usually consists of non-woody plants, shrubs and tree seedlings. Many of its plants can only grow in the abundant light of forest openings. This earliest vegetation is soon (on the scale of 5-10 years) replaced by vegetation somewhat like that of the original forest, and this in turn by a forest with a species composition almost identical to the original.
Although stable equilibrium is not difficult to consider in the case of the climax forest, the concept may also be applied to the rapidly changing successional vegetation of a disturbed forest (Mueller, 2000). Thus, if a forest that was adapted to and grew on acidic (poor in calcium and magnesium) soils is cut, the first and all later successional plants that replace it will also consist of species adapted to such soils. Similarly, if a forest adapted to alkaline ( rich in calcium and magnesium) soils is cut, all successional plants will also consist of species adapted to these soils. Of course, it is possible that in some cases the disturbance itself might bring about a change in soil acidity or other factors, and thus change the floral composition, as from alkaline to acidic, for example. Note that in all cases these are examples of stable equilibrium between plants and the soil in which they grow. The only difference from the equilibrium of the climax forest is that it is established rapidly and lasts only a short while for each successional stage.
Forest Disturbances
Forests are subject to many disturbances, which include strong winds, fire, ice, diseases and insects that feed on them. A common type of disturbance is the "blow-down", in which the wind up-roots or breaks individual or a large number of trees. In the deciduous forests of the eastern US, individual tree blow-down is the dominant disturbance, but in the northern coniferous forests large blow-downs and fire are much more important (Runkle, 1996). Insects and diseases may cause great disturbances and kill many trees, and many of the most destructive have been introduced by humans from overseas. An example is a blight or disease of American Chestnut (Castanea dentata) that eliminated the species as a canopy-sized tree in our eastern forests, although it continues to send up sprouts, Another is an insect that is killing our beautiful evergreen hemlocks (genus Tsuga) practically everywhere in these same forests. You may also have heard of Dutch Elm Disease that has destroyed so many American Elms (Ulmus americana), the fungus that afflicts Flowering Dogwood (Cornus florida) and the Gypsy Moth, that, in its caterpillar stage, feeds on the leaves of many trees. It should be mentioned that all these tree diseases and insect parasites also have genus and species names, which we have omitted only in the name of simplicity, but which may readily be found in references such as the internet.
We have seen that, while disturbances may be highly destructive and even threaten the existence of certain species, others have adapted, with beneficial results, as when openings created by disturbances let in the light required by some species. Of particular importance is the role of fire. Most destructive are "crown fires", in which flames leap from crown to crown in a "firestorm" of destruction. Crown fires usually result from the accumulation on the forest floor of debris from logging operations or from the absence of ground fires for a long period in unlogged forests. Unfortunately, humans have been late in recognizing the beneficial role of fire in controlling natural (not logging) debris accumulation. Studies have shown that when fire burns along the forest floor periodically, it tends to be of low intensity and benefits the vegetation, particularly in dry forests. Thus the seeding and growth of plants such as blueberries (genus Vaccinium) and huckleberries ( genus Gaylussacia ) is aided in dry oak forests, to the benefit of wildlife.
Many plants and other organisms are adapted to fire. Some trees of eastern North American forests, such as Chestnut Oak (Quercus prinus), have adapted to periodic fires by developing, not only thick bark, but a greater thickening of the underside bark of leaning trees, where exposure to flames is greatest. Similarly, fire adapted pines such as the Pitch Pine (Pinus rigida ) have developed bark consisting of numerous thin layers to increase heat insulation. These pines also have cones that open to release their seeds when exposed to the heat of fires, thus assuring reseeding of burned areas.
Not all forests are subject to fire when in their natural condition. Thus the lush coniferous rainforests of the northwestern US and Canada are far too wet to burn under most conditions. Even in the eastern US, the original high elevation virgin Red Spruce (Picea rubens) forests of West Virginia showed little evidence of fire as a consequence of the cool, moist climate. However, when logging opened up such forests and produced great quantities of dead wood debris, they burned catastrophically.
Forest Animals
We have already mentioned a number of forest animals, namely some of the insects. However, the usually most conspicuous are mammals, particularly tree squirrels. Without discussing all of these, which number five common species in eastern North America, we may consider one of the most widespread, namely the Red Squirrel (Tamiasciurus hudsonicus ), which inhabits a wide variety of forests in the eastern US as well as the coniferous forests of Canada. Another kind of tree squirrel of special interest are the flying squirrels (genus Glaucomys ). These squirrels don't actually fly, but glide from tree to tree or to the ground with the aide of a membrane they spread between hind and front legs. An important ecological role of squirrels is their habit of burying and otherwise spreading seeds such as acorns and pine cones, and many trees and other plants have seeds that evolved especially to appeal to squirrels. Squirrels are also important spreaders of various mushrooms, many of which form intergrowths with tree roots to the tree's advantage.
Another forest animal of great ecological importance is the Beaver, in America, Castor canadensis. As is well known, the Beaver builds dams, which have several purposes. The dams create ponds and raise the water in all creeks that feed into them upstream. Since Beavers need to cut trees for the bark they feed on, as well as for building material, the raised water level allows them to reach many more trees. The dam is constructed of interwoven sticks plastered with mud. Since Beavers have a number of predators, major uses of the pond are to provide a safe place for the Beaver house or "lodge." to be built, and as a storage site for the winter food supply, which., like the lodge entrance, is in water deep enough so it will not freeze in the coldest winter days. Additionally, the pond provides habitat for a wide variety of animals, including fish, turtles, frogs and birds. Beaver ponds are abandoned after a number of years, usually because the local food supply has been used up. After the Beaver are gone, the dam usually fails, since it requires constant repair. Following this, the pond is replaced by a meadow, and new set of ecological functions is initiated. Because the former pond. was a catch basin for so much life and its wastes, as well as forest debris, these meadows have richer soils than those of the surrounding forest. Also, their vegetation and the animals that live there are quite different from those of the forest, with the result that the biological diversity of the entire area is increased..
Many forests not long ago had more species of animals, in particular large predators, such as the Gray Wolf (Canis lupus) and large cats such as the American Cougar (Felis concolor), than they now have. The elimination of these large predators by humans has had a great ecological impact, in that it allows small predators, formerly hunted by them, such as the Red Fox (Vulpes vulpes), skunks (several genra) and Raccoons (Procyon lotor) to flourish. Since all of these feed on birds, their eggs and a variety of other small animals, the loss of the big predators is a major cause for decreases in the populations of certain species, and most particularly those of many of the rarest song birds.
Another consequence of the loss of the large predators is the increase in numbers of deer, particularly the White Tail (Odocoileus virginiana) of eastern North America. The overpopulation of this animal is also a consequence of overcutting of forests, which makes available for food an abundance of early successional vegetation that springs up when the trees are cut.. The additional deer also feed on rare plants of the nearby uncut forest, and many, such as the beautiful evergreen shrub Canada Yew (Taxus canadensis), have practically been eliminated in places in which they were once common.
The overpopulation of deer also poses a direct threat to human health, since one result is the spread of an illness similar to mad-cow disease, that for a long time has been present over large parts of Western and Midwestern North America, and recently has appeared in the East as well.
Although mammals and birds are the best known forest animals, there are many smaller and generally concealed species, such as salamanders, that play equally important ecological roles. Examples are the lungless woodland salamanders, particularly of the genus Plethodoni, in eastern North American forests. These salamanders seldom are seen in the open, but comprise a large fraction of the vertebrate biomass (the mass or weight of living things) in these forests. They also are large consumers of the abundant small invertebrates that live on the forest floor.
Some Examples from the Appalachian Forests
The Appalachian Mountains vary enough in latitude, bedrock, elevation, and topography to allow a variety of forest types to occur in them. The major types were originally classified and compared by the brilliant forest ecologist Lucy Braun in her classic Deciduous Forests of Eastern North America (1950),to which this reporter is greatly indebted. Here we confine ourselves largely to the Southern and Central Appalachians since they show the greatest variation in forest types.
The Southern and Central Appalachians fall in four physiographic provinces. the Appalachian Plateaus, the Ridge and Valley, the Blue Ridge. and the Piedmont. Of the four, our emphasis is largely on the first three, since these include the mountains themselves and show the greatest variation in the physical environment. The rocks of the Appalachian Plateaus and the Ridge and Valley Regions are of Paleozoic age, while those of the Blue Ridge and the Piedmont are mostly much older. Those of The Appalachian Plateau have nearly flat-lying or slightly folded beds, while those of the Ridge and Valley are mostly closely folded and much faulted. All of the Paleozoic rocks are only slightly metamorphosed (changed by heat and pressure), while those of the Blue Ridge and the Piedmont are generally complexly folded and faulted and are more highly metamorphosed. As might be expected, the mountain peaks and ranges consist of hard, erosion-resistant rocks such as sandstone, quartzite and granite, while the valleys have formed in weaker rocks such as shales and limestones.
A salient characteristic of the Appalachians of Virginia, West Virginia and Western Pennsylvania is the NE to SW orientation of the regional isotherms, which is essentially parallel to the mountain ranges. In Central Pennsylvania their orientation changes to roughly E-W, which again approximates the orientation of the ranges there. Consequently, regional temperatures in Virginia's Ridge and Valley generally decrease from east to west and reach their lowest values on the Allegheny Plateau of West Virginia. By contrast temperatures in Central Pennsylvania decrease from south to north.
Of the Appalachian forest types the most complex and richest in species is the Mixed Mesophyte Type, which we referred to earlier, and which is best developed in the Cumberland Mountains of Kentucky,. but also occurs in many other locations. It has as indicator trees White Basswood (Tilia heterophylla ) and Yellow Buckeye (Aesculus octandra), which are usually accompanied by ten or more other canopy species, as well as a number of small "understory" trees, shrubs and hundreds of species of herbs, including especially ferns and colorful wildflowers. Some other canopy species are American Beech (Fagus grandifolia), Sugar and Red Maples, Tuliptree, Northern Red Oak, Black Oak (Quercus velutina), Shagbark Hickory (Carya ovata), White Ash and Canada Hemlock (Tsuga canadensis). Common understory trees and shrubs are Hophornbeam (Ostrya virginiana), Muscletree ( Carpinus caroliniana), Striped Maple (Acer pensylvanicum),. Spicebush (Lindera benzoin) and Black Haw Viburnum (Viburnum prunifolium), among others.
While herb species are too numerous to list in detail, some immediately catch our eye. Among these are May Apple (Podophyllum peltatum), with its umbrella-like leaves, and the trilliums (genus Trillium), with their large, colorful blooms in early spring. Impressive also, are the many medicinal herbs, among them Black Cohosh (Cimicifuga racemosa), American Ginseng (Panax quinquefolius) and Goldenseal (Hydrastis canadensis ).
Mixed Mesophyte forests almost invariably occur on rugged, stream dissected topography, in the case of the Cumberland Mountains, on terrain with several thousands of feet of relief. Their soils are frequently colluvial (formed by down-slope movement) in origin. Consequently these soils have in most cases been in contact with, and include rock fragments, and other soils from up-slope layers richer in nutrients. In the bottoms of stream valleys and ravines they may also benefit from deposits of alluvium (stream deposited sediments), which frequently bear nutrient-rich material from up-stream. When Mixed Mesophyte forests occur on flatter terrain, abundant moisture and beds of nutrient-rich material such as limestone, occur near the surface. They also grade into less rich forests which we earlier called simply "mesic."
Mesic forests in turn grade into the Oak-Chestnut type, which characterizes the driest ridges and slopes, The dominant tree in them currently is the Chestnut Oak, but Northern Red and Black Oaks and the hickories are also important. Almost omnipresent is Red Maple, except in the driest stands, where Pitch Pine is common and is sometimes accompanied by Table Mountain Pine (Pinus pungens) Before the Chestnut Blight the American Chestnut was frequently as important as the oaks in this forest type.
Because these dry forests have thin and sometimes open canopies and inhospitable soils that discourage herbs, shrubs frequently form dense layers beneath the trees. Chief among these may be Witch Hazel (Hamamelis virginiana), or, in the most acidic soils, a variety of ericaceous species such as Mountain Laurel (Kalmia latifolia), Minnie-bush (Menziesia pilosa), various deciduous rhododendrons or azaleas, blueberries and huckleberries. Also small sub-shrubs with thick, evergreen leaves, such as Teaberry (Gaultheria procumbens) and Spotted Wintergreen (Chimaphila maculata) are common, as is Bracken Fern (Pteridium aquilinum). While herbs are of scattered occurrence, a few, such as Whorled Loosestrife (Lysimachia quadrifolia), Rattlesnake Weed (Hieracium venosum)) and Partridge Berry (Mitchella repens) regularly occur.
Just as Mixed Mesophyte forests are characteristic of rich colluvial slopes or limestone valleys, the Oak-Chestnut type is typical of linear or winding ridges of hard sandstone or siltstone, with minor interbedded shales. Where these ridges are cut by ravines, and somewhat more nutrient-rich rocks are exposed, it grades into a more mesic type, in which such species as Tuliptree and the maples, as well as herbs are common.
The range of Chinquapin Oak. After Fowells (1965).
The range of Chestnut Oak. After Fowells (1965).
The range of Black Oak. After Fowells (1965).
The distribution of the different species of oak illustrate well their relation to soils and the underlying bedrock. As we have seen, Chestnut Oak generally occurs on nutrient poor soils of sandstone ridges. However, a better indicator of an environment favorable to this oak is soil acidity, with the species being excluded by low acidity and high concentrations of calcium and magnesium. This oak in fact does well in quite rich, moist or mesic environments as long as soil acidity is above a certain level. The opposite is however true of another member of the genus, namely Chinquapin Oak (Quercus muhlenbergii), which demands soils with low acidity and high concentrations of calcium and magnesium and is frequently confined to the vicinity of limestone outcrops. A third species, the Black Oak, thrives in very acidic as well as low acid soils and all acidities between these. All three oaks have similar distributions with respect to latitude. However, their distributions in longitude are markedly different (see distribution maps). The basis for this difference is related to the prevalence of limestone bedrock and lime-rich soils west of the Appalachians as well a decrease in rainfall westward, which leads to greater concentrations of calcium and magnesium in upper soil layers.
Chestnut Oak is confined to eastern North America and virtually to the generally highly acidic soils of the Appalachians and adjacent regions. A minor but instructive feature of this oak's distribution map is the conspicuous exclusion of the Kentucky Bluegrass, a region notable for its limestone bedrock.. By contrast, although Chinquapin Oak occurs in the East, in limestone areas, it also ranges as far west as Kansas, Oklahoma and Texas. Black Oak is common throughout the East and occurs virtually as far west as does Chinquapin Oak. It thus seems clear that the geographic distribution of these three oaks is a direct reflection of climatic conditions as well as their soil preferences and tolerances.
The distribution of the Hemlock- White Pine- Northern Hardwoods forest type in the Appalachians is governed largely by latitude and elevation. In the Southern and Central Appalachians it is almost entirely confined to the higher elevations, which in the south it may exceed 4000 feet (1200 meters) above sea level. However, in New York, New England and Eastern Canada it is stable near sea level.
Although they have great prominence in the name of this forest type, Hemlock and White Pine are generally subordinate in the Appalachians to northern hardwoods such as Sugar and Red Maples, Beech, Yellow and Black Birches and Northern Red Oak.
Like the other forest types discussed above, the species of trees as well as the flora in general, reflect not only the climate but soil character. In glaciated regions the composition of the underlying till (glacially eroded, ground up rock), with respect to nutrients and acidity, is important, while in un-glaciated areas bedrock character may dominate. In many areas of eastern North America soils immediately above limestone bedrock or lime-rich sub-soils, are nonetheless acidic in character. This feature is the result of the downward leaching of calcium and magnesium by rainwater, which is naturally acidic as a consequence of its carbon dioxide content. Of course this acidity may be further increased by industrial pollutants captured by the rain from the air. These soil characteristics influence not only the canopy species but all understory trees, shrubs and herbs.
At higher elevations and lower northward, spruce of several species as well as Balsam Fir (Abies balsamea) may be present as minor components. While deciduous understory trees are mostly of the same species throughout the region, northerners such as Mountain Maple (Acer spicatum) have greater prominence in cooler environments. Shrubs also have many species in common with other forest types. However some, such as Hobblebush (Viburnum alnifolium), distinguish this forest type. Many herbs are also of wide distribution, but many others, such as Canada Mayflower (Maianthemum canadense), Goldthread (Coptis groenlandica }and Mountain Woodfern (Dryopteris campyloptera) occur only in cool northern or montane {mountain } habitats.
At even higher elevations and northward, the Hemlock- White Pine- Northern Hardwoods forest type grades into sub-alpine or boreal spruce-fir types. In the Southern Appalachians of North Carolina, Tennessee and Virginia, Fraser Fir (Abies fraseri) is endemic and occurs at the highest elevations In the Central Appalachians of West Virginia and Virginia there is a variety of Balsam Fir ( Abies balsamea var, phanerolepus ) that is distinguished from the typical variety Abies balsamea var balsamea, which is found from northern Pennsylvania northward. Red Spruce is associated with all of these firs along the Appalachian chain, but the proportion of fir generally increases with elevation. At the highest elevations, where most spruce-fir forest occurs, these conifers dominate almost to the exclusion of deciduous species. However, Yellow Birch is a frequent minor component in the Southern and Central Appalachians.
Although Fraser Fir occurs at the highest elevations in the Southern Appalachians and under a severe climate, it does not occur farther north in similar climates, even where Balsam Fir is absent. This characteristic is held in common with a number of montane species. For example Southern Mountain Cranberry (Vaccinium erythrocarpum ) is a common high elevation and cool habitat shrub of the Southern and Central Appalachians, but is found no farther north than West Virginia. It may be that some factor such as light intensity may be involved here.
Other common vegetation types that occur within the Hemlock-White Pine Northern Hardwoods forests are the acidic bogs. In them the dominant tree is usually a species of spruce, either Red Spruce, or more typically Black Spruce (Picea mariana). As distinguished from the former, which is essentially montane in ditribution, Black Spruce is boreal and ranges beyond the Arctic Circle. Commonly associated conifers are Eastern Larch (Larix laricina) and Balsam Fir. While Red Spruce is the dominant bog tree species south along the Appalachians in Virginia and West Virginia, it is joined by Black Spruce in Central Pennsylvania, and this species becomes more frequent northward.
An important feature of acidic bogs is the number of northern species, of which Black Spruce is only the most conspicuous. Especially abundant are northern ericaceous shrubs such as Bog Rosemary (Andromeda glaucophylla) Labrador Tea (Ledum groenlandicum), Leather-leaf (Chamaedaphne calyculata ) and Large and Small Cranberries (Vaccinium macrocarpon and V. ozycoccos ). Also abundant are the lichens and mosses, such as the numerous species of the genus Sphagnum in particular. Although grasses are present, sedges are more common, and one type, "Cotton Grass" or more properly "Cotton Sedge" (genus Eriophorum ) is particularly conspicuous with its cottony seed heads in late summer.
Northern bog species can grow farther south than northern upland plants because bog soils are naturally cool as a consequence of water saturation, the insulating properties of the vegetation and the tendency of bogs to act as traps for the cold air that flows from adjacent uplands. It is interesting to compare the distribution of the upland species White Spruce (Picea glauca) with that of Black Spruce., which is well adapted to bogs. White Spruce ranges almost as far north as Black Spruce. However, while the latter is common in bogs in northern Pennsylvania, White Spruce occurs no farther south than northeastern New York.
Although wetlands are not common in unglaciated parts of the Appalachians, various types occur on the edges of flood plains, in association with springs and in areas of flat-lying rock strata. Of special interest are those which result from large artesian springs that well-up in underlying fractured limestone. This type of wetland is rich in calcium and magnesium and other nutrients and has low acidity. These conditions, combined with a constant source of cool spring water, favor the stable occurrence of a variety of rare species, including some far south of their normal ranges. Thus the Folly Mills Calcareous Wetland of Virginia's Shenandoah Valley, that was first identified by this reporter, is home to one of the southernmost colonies of the Arctic-ranging herb Buckbean (Menyanthes trifoliata), the State's only occurrence of the boreal Pussy Willow (Salix discolor) and a number of other rare plants. Buckbean also occurs in acidic bogs, but does not thrive as well in them as it does in calcareous wetlands. This illustrates an ecological principle, namely, that the normal climatic range of a species, such as that the cold-loving Buckbean, may be expanded by the increased availability of nutrients or other favorable environmental factors.
Although the specific examples of ecological relationships presented here are virtually all from the Appalachians, conclusions drawn from them should be applicable to forests in general. However, different forest types, such as those of other mountain regions and the tropics in particular, likely embody relationships not encountered in this limited survey.
Acknowledgements
The writer greatly appreciates the many happy days in the field with Dr. Robert Hunsucker, as well as his instruction on the Appalachian Flora that contributed so much of the background for this publication. He also wishes to express his deep appreciation for the work of Dr. P. D. Strausbaugh and Dr. Earl L. Core, both now deceased. Their book Flora of West Virginia (1977) has been a constant companion in the field and at his desk.
Selected References
Braun, E. Lucy (1050) Deciduous Forests of Eastern North America. Macmillan Publishing Co. New York, N. Y.
Fowells, H. A. (1965) Silvics of Forest Trees of the United States, Agricultural Handbook 271, US Dept. Of Agriculture, Forest Service, Washington, D. C., 20250.
Mueller, R. F. (2000) Stability Relations in Forests. Forests of the Central Appalachians Project, Virginians for Wilderness Web Site.
Runkle, James R.(1996) Central Mesophytic Forests, in Mary Byrd Davis, editor, Eastern Old Growth Forests. Island Press, Washington, D. C.
Strausbaugh, P. D. and Earl L. Core (1977) Flora of West Virginia, Second Edition, Seneca Books Inc., Grantsville, West Virginia.
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