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PART I
The Forest Today
While there are books about the great forests in national parks and national forests, there is little information on the fragments of forest that surround our homes and businesses. Similarly, there are many books and courses on identifying native plants and animals as well regional natural area guides, but the general public has very little understanding about the plight of regional forest systems and the adverse impacts, often unnecessary and inadvertent, that we have upon native plants and wildlife daily
The goal of Part I is to help the reader see the forest as a system that is not static but that changes both locally and pervasively, to help the reader read the landscape and recognize the patterns of both health and degradation. Despite our living in a culture that distances us from the natural world, we have actually seen more of the landscapes around us and across the face of the planet than we might be aware of, through the media and travel. With a slight change of focus, we can learn a lot about an environment simply by the way it looks.
The chapters in Part I provide essential background for understanding the condition of woodland habitats today — the fragmentation of the landscape caused by expanding growth and suburbanization, our habit of continuously altering natural patterns, and the failure of many native plants and animals to regenerate themselves sufficiently to survive, at the expense of vital natural functions and services that will be very costly to replace or restore. The chapters in Part I also examine how our management of the landscape, or lack thereof, affects the entire ecosystem as well as the particular site being used or maintained. Lastly, they discuss the impacts of our changing atmosphere and climate, which may have the most far reaching consequences of all for the forest.
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CHAPTER 1
A Landscape Overview
Earth is the “water planet,” and on the one-fifth of its surface area that is land, water is the most crucial factor in determining the character of the landscape. Where water is abundantly available for much of the year forests can grow, while grasslands occur where conditions become too droughty to support tree growth. Deserts are even drier landscapes. Sometimes cold restricts the availability of water by holding it as ice, unavailable to plants. Climate, and hence the availability of water, is determined in part by latitude, the position of a place between the equator and the poles, as well as by ocean currents and wind patterns and the ways in which they interact with the different sizes, forms, and positions of the landmasses.
These factors combined produce somewhat loosely banded patterns of similar landscapes around the globe (Figure 1.1). If we could look down from a satellite circling the Earth, we would be able to distinguish the forest landscape from the expanses of grassland. We could see where the northernmost forest ends and the low tundra begins or where grasslands give way to deserts. Scientists call these different bioclimatic zones “biomes,” a word that literally means “life-groups.”
These landscapes look dramatically different, and the way they look reveals their environment. As we observe plants more closely, their appearance tells us a lot about how water occurs in their habitat. and the patterns in which they grow. In the case of forests, differing kinds grow along a gradient of changing temperatures and moisture availability.
Figure 1.1. All landscapes occur in bands that correspond to climatic zones around the Earth reflecting the amount of energy and water available for development of vegetation.
Tropical Forest
The tropical forest is generally considered to be the richest terrestrial landscape. Like the oceans, tropical forests are cauldrons of life, out from which come most of the plants that have colonized, at least to some extent, almost every place on the planet. All landscapes are layered, but in the tropical forest the pattern of stratification reaches its greatest expression. Abundant rainfall and intense light year-round nurture forests that bind the environment’s nutrients into a lush tangle of plants from the forest floor to the tops of its 200-foot-tall trees. There are up to fifteen discrete layers, each supporting its specialized community of plants and animals, many of which live their entire lives in a single layer of the canopy, never reaching into layers above or below. Other aspects of life in the tropics are revealed by the look of these plants. We find thick and waxy leaves and thick, hard seed husks that provide some protection from rotting in the high humidity of the tropics.
Temperate Forest
In the middle of the global forest continuum are the temperate forests, the primary subject of this book. The temperate landscape is characterized by seasonal change in a year that includes a prolonged growing period that is warm and well watered, followed by a dormant period of cold and drought when water is frozen and hence unavailable to plants. Rather than having evergreen leaves that are adapted to extended drought, the trees of the temperate forests are predominantly deciduous, with leaves that are shed each fall and replaced each spring.
The complexity of the aboveground structure of the temperate landscape is limited by its shorter growing season, even where there is high rainfall, and as a result its forests have fewer layers than a tropical forest. A temperate forest generally has four layers. The canopy layer is composed of trees that are typically taller than 35 feet and often more than 100 feet. The understory is also largely woody and may include saplings of canopy trees and smaller trees, such as dogwood, that do not reach canopy height. The shrub layer also is woody and may include juvenile canopy and understory trees as well as shrubs, which are usually multistemmed and smaller than understory trees (less than 25 feet in height). The ground layer consists of herbaceous (nonwoody) plants, such as grasses, sedges, rushes, ferns, mosses, and seedlings of trees and shrubs.
Woody vines appear in every layer of the forest. Occasionally there is also a super-canopy, of higher, and usually older, trees. Another layer in the landscape is the litter layer, which is composed of the debris of leaves and tree limbs and other parts of vegetation as well as small animals, both living and dead, throughout this organic debris on the surface of the ground.
The predominance and distribution of species within the temperate forest has been influenced by land use as well as by the natural environment. Deciduous forest trees such as maple, basswood, and beech tend to predominate under the more mesic (moist) conditions in the temperate forest biome, while oaks and hickories are more typical at the drier end of the spectrum. The relative predominance of oaks today over presettlement conditions is due to past fire and logging history as well as the loss of the American chestnut in the early part of the twentieth century. Droughtier soils in temperate landscapes often support a mix of deciduous and evergreen trees, such as the mixed oak-and-pine forests of the sandy soils along the Atlantic coastal plain landscapes. The abundance of pine in these areas is maintained by fire, which in turn makes the landscape more fireprone. Fire suppression since settlement in areas with sandy soils is resulting in greater predominance of oaks and hickories. Beech and maple may become more numerous as fire is further suppressed by suburbanization.
The eastern deciduous forest, which is the temperate-forest biome type that covers most of the United States east of the Mississippi, reaches its most complex and diverse type in the southern Appalachians. Until European settlement, vegetation there persisted without interruption, over 300 million years, since the origin of flowering plants, when the land was still part of a supercontinent. While adjacent lands were either under water or scraped by glaciers for extended periods of time, the sheltered coves of what is called the “mixed mesophytic forest” served as both refuges for and sources of biodiversity in the region. The Ozark Mountains are nearly as rich biologically although somewhat drier.
Boreal Forest
At the other end of the global forest gradient is the boreal forest. Moisture is scarce in the frozen winter but is sufficient in the rest of the year to support a dense cover of pine, fir, and spruce trees. The long winters make their mark on the appearance of the landscape. The coniferous trees, whose name, incidentally, means “cone-bearing,” reflect the relatively dry conditions. Their compact, spiky needles serve to limit the surface area from which moisture can be lost. The persistence of these needles for several seasons is also an adaptation to the harsh environment. It requires a great deal of energy to produce a new leafy canopy every spring, and when the fuel for that energy is in short supply it is better directed to growth and reproduction. Even the shape of the coniferous trees reflects the conditions of their environment. We can hardly look at the low downward-sloping boughs without seeing them bending even farther under the weight of winter snowfall.
The trees in the boreal forest are also shorter, and the ground layer is far less dense than is usually seen in milder climates. The relatively bare forest floor is partly the result of the low levels of light and limited physical space that remain year-round beneath the dark evergreen boughs, but the paucity of herbaceous (nonwoody) plants, shrubs, and saplings also reflects the effect of the long droughty winter and the difficulty the smaller plants have in competing with the large established trees for the limited resources.
Grassland
Trees will grow where there is adequate moisture and a long enough growing season. The character of the grassland biome is shaped by a critical period of drought during the growing season (in addition to the winter drought) that precludes tree growth except along water courses.
Like trees, grasses seem to be little affected by extremes of temperature, growing abundantly from northern Canada to the tropics. It is interesting to note that grasses do not require droughty conditions; it is just that they survive them better than trees can generally. We have more than enough evidence that grasses will grow perfectly well in regions where resources will support a forest, but, as most homeowners know, grass does not grow well in competition with trees. The extensive root and shoot systems of trees are more effective than grass at marshaling available light, water, and nutrients, so grasses persist in the more humid areas only where forests are held back by human interventions. Often grasslands border woodland or precede forest establishment.
In the great prairies of western North America, flat, open grassland mixed with a rich variety of forbs (broadleaved herbaceous plants) once extended for miles on end. The land here is sometimes so flat and so devoid of trees that it often seems we can see the Earth curving away. The open land also allows the wind to sweep across the vast spaces, blowing and howling relentlessly, further desiccating the landscape. Only along the corridors of streams or in the occasional low, wet areas do trees break the view of the oceanlike grass.
Grasses can prosper in such habitats. Their buds are safely underground, rather than on aboveground branches, during the winter freeze as well as during the frequent summer fires that have had a strong hand in shaping this landscape. Their canoe-shaped blades capture the rain when it comes, sending it directly to the roots.
The root systems of the great prairies formed a mat that became legendary among the early pioneers known as the “sodbusters,” who left the eastern deciduous forest to farm the midwestern lands. This mat held the soil in place despite droughts and high winds and was so thick it inhibited the germination of trees. It even resisted breaking up when a great herd of buffalo passed, a trampling that was reported to lower the level of the ground in places by as much as 4 feet.
Just as there are major differences between boreal, temperate, and tropical forests, similar distinctions can be seen in the grassland biome. The American grassland is usually divided into three types: a tallgrass prairie (with grasses between 6 and 10 feet tall) in its eastern range, a shortgrass prairie (in which the grasses are about 1 to 2 feet tall) to the west, and a mixed section in the middle.
Landscapes in Transition
Some areas in the landscape are in fact called transitional between two different biomes. The boundary between two biomes is generally quite broad, with elements of both biomes blended together. For example, a savanna, which is a mix between a grassland and woodland, often occurs between the forest and grassland biomes, where there are woody thickets and small woodlands woven into a prairie fabric. Trees thin out and vanish in the drier areas of a savanna and become more dense at a water source and along streams. The boundary of the transition area shifts over time and is also affected by human activities such as burning and animal activities such as grazing. We are presently seeing a critical process of desertification taking place in sub-Saharan Africa, where grasslands and forests at the edge of their climatic range are being exploited, destroying a fragile balance and causing the desert to expand.
The transitional area will also shift locations with long-term climatic changes. During periods of abundant rainfall, for instance, forest may become established in areas that had been grasslands. Then, when droughts return, some of the trees may survive because they draw moisture from deep in the ground, but new trees cannot grow and the region will gradually return to grassland. Evidence indicates that several thousand years ago (but since the last glaciers) the United States was drier than it is today. At that time the great prairie extended...
