Introduction

The boreal forest and the Arctic tundra are two major biomes which cover a total of 30 million square kilometers in the Northern hemisphere. They are characterized by contrasting dominant life forms and productivity levels associated with the presence of trees in the boreal forest and their general absence in the Arctic tundra. The same general differences exist between forest and grassland but the ecological factors responsible for the position of the forest–grassland transition zone are likely to differ from those operating for the boreal forest–tundra transition zone. A causal explanation of the climate–vegetation pattern of the northern forest border is as yet obscured by the poorly known ecophysiology of the tree species (Oechel & Lawrence 1985). Despite the lack of knowledge of the exact mechanism involved, there is general agreement that thermal characteristics of the climate are of major importance in determining the northern edge of the boreal forest. Somewhere at the interface between boreal forest and tundra, tree species face severe climatic conditions, which reduce their growth and development. This induces changes at the plant population and community levels, which can be observed in the transition zone between these two biomes. Forests of this boundary area are likely to be sensitive to environmental changes and often display low regenerative capacity. Various types of ecological alteration, for example climatic changes and wildfires, take place over a wide range of areal dimensions and in different periods. These result in a temporal and spatial vegetation patterning typical of the northern forest border.

Introduction

In this chapter we will examine the nature of the southern boreal forest, explicitly its transition with the cool temperate deciduous forest that occurs to its south. Although this mixed deciduous and conifer forest occurs on portions of several continents in the northern hemisphere, it is most intact and most studied in North America as the northern hardwood and conifer biome (Braun 1950; Curtis 1959). Therefore, although we will describe this zone in other regions, we will concentrate on the North American expression of it. For convenience, we will also adopt the shortened common name ‘northern hardwoods’ for the cool temperate and largely deciduous forest to the south while recognizing that certain conifers are also important components of this forest.

The southern boreal forest contains extreme contrasts in ecosystem properties corresponding to the locations of different species within a mosaic landscape. These properties are accentuated at its boundary with the northern hardwoods and conifers, which are transitional to the deciduous forests to the south. Within these two biomes, each genus, and in some cases different species within each genus, affects ecosystem properties in different ways. Such properties include nutrient cycling and productivity, disturbance type and frequency, and suitability as an animal habitat. Consequently, there are strong feedbacks between succession, species dominance, resource limitation, disturbance regimes, and trophic structure (Fig. 8.1). These feedbacks may be stabilizing or destabilizing, the latter often causing cycles at various times and in various places.

In the Introduction, we expressed a hope that this book would represent a point of departure for subsequent studies of the world's boreal forest ecosystems. We have presented the physical (Chapter 15) and biological (Chapter 16) elements of a computer model designed to simulate the local changes in any forest in the boreal zone. For purposes of future identification, we will refer to the merged version of this model as the BOFORS model and we expect these versions of the model to be updated as further work and information are incorporated into the model structure. Detailed information on the model is available in Bonan (1989a) for the biophysical features of the model and in Leemans & Prentice (1989) for the biological and silvicultural features of the model. Listings of the computer program for the BOFORS model are available on request from the University of Virginia's Science and Technology Library (Charlottesville, Virginia, USA) on interlibrary loan. The model is available in two implementations: Bonan (1990c) is a version of the model with all of the physical subroutines included (see discussions in Chapter 15); Leemans (1990) is a version of the model that has been used to simulate several locations in the USSR and Fennoscandinavia (see Chapter 16). The FORSKA model (Leemans & Prentice 1989) is available (in English) upon request from: Meddelanden från Växtbiologiska Institutionen, Uppsala University, Uppsala, Sweden, and on interlibrary loan from the University of Virginia library mentioned above.

Introduction

In this chapter we will present our achievements in developing a spatial model of long-term forest fire dynamics. By ‘long-term’ we mean dynamics over hundreds or thousands of years rather than changes in forest patterns over one fire season. By the word ‘spatial’ we denote a model that describes the dynamics of a large, non-homogenous (from the ecological viewpoint) forested territory, taking into account interactions between adjacent landscape units.

Three types of spatial interaction are known to take place during longterm dynamics of boreal forests. These are the spread of fire, seed propagation and the spread of insects. We will consider only the first two types, for the following reasons.

1. To us the methodologically correct way of developing a new model is to include new mechanisms and to verify the model at each stage of development. Because of the absence of spatial models describing forest dynamics on such time and space scales it is desirable to decrease the number of new spatial interactions to be included in the model.

2. According to the majority of forest researchers, wildfires are a dominant factor controlling formation and maintenance of boreal forest communities. In fact, present boreal forests represent a mosaic of different areas, each of post-fire origin. Thus the spread of fires seems to be the first type of spatial interaction to be included in the model. On the other hand, taking into account the spread of insects without considering wildfires is not quite correct because the spread of insects inevitably leads to burning of damaged forested territory.

3. […]

Introduction

The problem of predicting ecological responses to global environmental change has generated a rich array of scientific challenges for ecosystem ecologists. The International Geosphere–Biosphere Program (IGBP), chartered by the International Council of Scientific Unions in 1986, has as its objective:

IGBP is one of the most exciting scientific opportunities of our time. The intellectual challenge is ultimately to unify ecological and geophysical sciences at the global scale: a large undertaking in which ecosystem ecology must play a central part.

One of the major tools for this multidisciplinary undertaking will be ecosystem simulation models implemented on high-speed computers. Global dynamics models exist for the atmosphere and oceans as physical systems, but not yet for ecosystems and their interactions with the atmosphere and oceans. This book represents the first attempt to develop a unified model of a major biome (the boreal forest) at the global scale. The model described in the chapters by Bonan (Chapter 15) and Leemans (Chapter 16) belongs to a well-established class of models that simulate the dynamics of forests by projecting the change in size of individual trees over time, through explicit representations of the interactions between arrays of individuals and their local resource environment.

Introduction

“I am large. I contain multitudes”, said Walt Whitman to explain why he sometimes contradicted himself. Every human being contains multitudes. I have never been more aware of the multitudes within me than at this conference of field ecologists describing their work.I am not a field ecologist. Most of the research reported here is new to me. It strikes me at many levels, revealing both the multitudinousness of my own responses, and the inherent complexity of the planetary ecosystems, which can touch a person in so many ways.

I was trained in biophysics, which, although it is a very different science, allows me to understand and to admire field work, not only for its difficulty (to a lab scientist like me, studies in the Arctic, or under the ocean, or in a peat bog look difficult), but also for its inherent elegance.

I am a systems analyst, and I am fascinated with the feedback systems that have been described here. They are full of synergisms, exponentials, non-linearities; they are beautiful systems. They make me want to run to my computer and start modeling. My systems experience also makes me sensitive to the vital role of information in changing system behavior, a bias that you will find permeating my comments here.

I am a journalist, a syndicated columnist, and I have been wondering how best to communicate in 800 words or less the important stories that were told which the public needs to know about.

Editor's Note: The oceans cover two-thirds of the surface of the earth, support primary production variously estimated as one-third to more than one-half of the global total, have by far the largest fraction of the volume of an obviously finite biosphere, and support a diversity of living systems that is only now being revealed as new techniques of exploration are developed. The oceans are simultaneously accumulating disturbance from human activities, even before the extent and role of life in the oceans have been defined. Disturbance includes the removal of fish, such as the tuna from the pelagic food-webs of the Pacific, and the accumulation of wastes, now pervasive. An ocean free of human influences is of the past.

The wastes are a nagging problem whose effects remain obscure, at least in the open ocean. Some of the wastes are dumped deliberately as sewage and other offal from cities such as New York and Boston and Rio and Athens. Some are industrial wastes such as the radioactivity dumped into the Irish Sea from the British reprocessing plant at Sellafield and into the Bay of Biscay by the French. Some are carried by the great rivers of the world, collected from cities and industries along their shores, and mixed into the sediments and waters of coastal regions globally, gradually to enter the complex circulation of the abyssal seas. Others are carried much less conspicuously to the oceans in air to be scrubbed from the atmosphere in rain.

Editor's Note: The bottom of the deep sea is dark and cold and a very old habitat by most terrestrial standards. Dr. Grassle and his colleagues have shown that the benthos contains an extraordinary diversity of life with different life histories and adaptations to habitat. A combined sample from 1,500 m to 2,500 m off New Jersey that covered a mere 21m2 yielded 798 species, a diversity that approaches the upper limits of what can be found on land anywhere. Another sampling off the East Coast of North America yielded nearly 600 species. Most deep-sea species are rare, and species once recognized as “cosmopolitan” are now seen as groups of species with restricted distributions. Experience in study of this extraordinary diversity remains minuscule in proportion to the area, and estimates of the total number of species in the oceans now exceed by many orders of magnitude the earlier estimate of 160,000. The experience is similar to that in tropical forests where access to the crowns of trees has recently revealed not only new populations of birds, but thousands of new insect populations. The knowledge has caused estimates of the total number of species on earth to soar to 10 million or more. Experience with the benthos may push this number higher still.

Chronic disturbance of either the benthos or the water column increases the abundance of a few species.

Editor's Note: Occasionally a circumstance arises in nature that, treated imaginatively by a talented scholar, allows unusual insights into cause and effect. Yossi Loya, an Israeli ecologist, recognized such an opportunity in his studies of coral communities in the Gulf of Eilat and has used the chance to gather further insights into the patterns of diversity and dominance in natural communities under various types of stress. His observations not only reveal further details of the structure and function of these communities, but reconfirm the importance of long-continued studies of specific sites to determine changes under way in response to intensified human influence, details that would otherwise be lost as the biota moves inexorably through various stages of impoverishment in response to uncontrolled chronic disturbance.

Loya offers a case history study of a coral reef exposed in different places to oil pollution and climatic anomalies. While the circumstances seem specialized, they are increasingly common, and the observations Loya makes are emergent generalities, broadly applicable to natural communities under stress.

Introduction: The Reef at Eilat

One of the central questions of ecology deals with the mechanisms that generate and maintain the diversity of organisms. There have been many varied opinions and large numbers of publications on this subject. I review here briefly our studies of community structure of corals at Eilat, Red Sea, describe changes that have occurred in coral diversity due to human versus natural disturbances, and discuss mechanisms that promote and maintain high diversity of corals on the reef-flats of Eilat.

Life occurs, as far as we know, only as part of the earthly biosphere. There is no life elsewhere in the solar system, and if there is life beyond it on one of the tens of billions of planets of the universe, it is not likely to be of more than momentary academic interest in a world preoccupied with human crises.

Life exists at once as diverse entities, individuals, populations, and species, apparently independent, yet dependent for survival itself on a web of interconnections with other species immediately available – a community. And, in a context only slightly larger, dependent for place and time and habitat on the totality of other life that sustains the biosphere. Could we imagine a forest without decay? Or a ruminant without its cellulose-digesting flora?

The annual fluctuation in the carbon dioxide concentration in the atmosphere offers one of the most impressive bits of evidence of the importance of the biota in maintaining a habitat suitable for humans. Each year during the spring and summer the carbon dioxide concentration of the air of the northern hemisphere is reduced by several parts per million as forests of the temperate zone store carbon compounds through photosynthesis. Each year the carbon dioxide content rises through the fall and winter as respiration of forests and soils exceeds photosynthesis.

Editor's Note: Evolution has been for nearly a century and a half the intellectual core of biology. More recently evolution has been recognized as having shaped and stabilized the surface of the earth as the only habitat for life. The concept is attractive: evolution is building an open-ended, developmental system, self-guided, self-repairing, capable of accommodating all travails in due course, even to the point of building into the biosphere more room for life, more diversity, and greater stability of habitat. Surely, according to this concept, biotic resilience is sufficient to accommodate the activities of Homo sapiens, one species of the several million now on earth, all the product of this magnificently versatile and effective process.

But the timing is off. In the long term of thousands of millennia the glorious vision of a benign and effective evolutionary resilience may be appropriate. In the time of our lives it is the principles of succession and impoverishment that apply.

Succession enjoys all the optimism of any developmental progression, no matter how mean the origins: a bank account, growing with the accrual of interest. Growth is salutary, almost by definition.

Editor's Note: Lakes and streams receive a disproportionate share of the disruption that follows the spread of human influences over the globe. The disruption reaches from the direct effects of dams and modifications of water courses to the disposal of sewage and the introduction of toxins including agricultural poisons. There is in addition the series of problems associated both with the depletion of the fish populations through harvest and with the introduction of exotics. And, more recently, we have recognized the seriousness of the problems associated with air pollution, including the acidification of rain and the cumulative effects on lakes. Now we see the further possibility that genetic engineers will be adding to the burdens already well known from the introduction of exotics by producing novel combinations of genes that may escape into nature to produce totally unforeseen effects.

In many ways, among all of science, the limnologists have led in defining the transitions under way in nature. The process of eutrophication was recognized first in aquatic systems and has since been recognized as a general phenomenon, the enrichment with nutrients mobilized by human activities, a first step in the continuum of change that is gradualy recognized as pollution, and, in a twist peculiar to nature, impoverishment.

The full array of effects and patterns laid forth so explicitly by Schindler is almost overwhelming. But the other message, only slightly less powerfully articulated, is that the changes are a continuum away from the status quo toward progressive impoverishment.

Editor's Note: Glacial advances and retreats over several hundreds of thousands of years have sorted and resorted the plant populations of the middle and high latitudes of eastern North America and have winnowed from those populations a flora of hardy survivors. The survivors among trees are almost uniquely singular species, unencumbered by dependence on other species, by the existence of a community, by a narrow dependence on edaphic perfection, or on special mechanisms for pollination or dispersal of seeds. The species of fir, spruce, birch, oak, poplar, hemlock, and pine can occur in almost any mixture in closed forests or in open woodlands, even on occasion in savannas, and on a range of habitats. To be sure, the species are not interchangeable, have different distributions and requirements and roles in succession, but, as wind-pollinated species (in contrast to tree species of the tropics that may depend on insects, birds, or mammals for pollination), they have few direct dependencies on other species that restrict them to any special community. We might go so far as to suggest that they are preadapted to climatic change as a result of selection by some of the most rapid climatic changes that we know of beyond the catastrophe of the Cretaceous/Tertiary boundary.

Margaret B. Davis, a distinguished scholar who has specialized in retrospective studies of vegetation, has addressed the challenge of estimating the rate of migration of one of these glacial survivors, Tsuga canadensis, under the climatic changes anticipated for the next decades.

Editor's Note: Lichens, many of them, are surface dwellers. They have evolved to occupy, among other places, the surface of rocks, the surface of the soil, the bark of trees. These surfaces offer some of the most difficult habitats for life, habitats marked by extremes. The surface of a rock, for example, may bake in the sun during the day and rise in temperature to 70° Celsius or more, dropping at night to freezing or below. Similar extremes in water availability accompany such changes. Yet such habitats may support communities of lichens that garner water and nutrients directly through the surfaces of their thalli. Despite the apparent hardiness of lichens, their populations have long been considered sensitive indicators of air pollution: certain lichens appear to be particularly vulnerable to air pollution and their vigor appears to offer potential as an early index of impoverishment.

The difficulty is that under disturbance the populations are ephemeral: they often disappear quickly and the index becomes neither morbidity nor other active change, but mere absence. Professor Hawksworth warns, to add to the difficulty, that it is not always appropriate to transfer experience with lichen communities from one place to another. Nonetheless, he provides compelling documentation of the sensitivity of lichen populations to airborne pollutants. The effects are unquestionably impoverishment, a reduction in diversity that is virtually universal and that applies in particular to fruticose and foliose, less to crustose, lichens.

I recall speculating years ago with the distinguished Russian scholar, Victor Kovda, at dinner in his Moscow apartment, about the productivity of the currently impoverished vegetation surrounding the modern city of Tblisi in Soviet Georgia. I had seen in a museum in Tblisi an ancient oaken cart with wheels fashioned of single disks of oak cut from the end of a log fully 4 feet in diameter. Large oak trees or forests of any description do not exist in the region now. Kovda guessed that the current productivity of the region, measured as ecologists would measure it as “primary production”, is 1% of what it once was. The cause was deforestation followed by the grazing of goats, sheep, cattle, and, later, over much of the land, by tillage, all of which contributed to the erosion of a thin soil.

Tblisi, a landscape that has sustained human occupation from time immemorial, is but one example of the cumulative effects of long-term human activities on the capacity of the landscape for sustaining life. The conference that was the basis for this book emerged from many years of puzzlement over how ecologists address the human-caused transitions that now dominate plant and animal communities globally. For many ecologists, perhaps most, the answer seems to be to focus on some of the more fascinating aspects of the evolution and extinction of species. But human effects and disturbance reach much farther than to extinction alone.

Editor's Note: Islands have provided some of the most fundamental of insights into evolution and ecology while bearing some of the greatest burdens of human depredation. What biologist can remain long innocent of the saga of Darwin's finches as spun out by Darwin himself and David Lack and others who have been puzzled by the diversity of life of the Galapagos? Whose curiosity is not piqued by the biotic anomalies of New Zealand, Australia, Madagascar, Easter Island, Surtsey? And yet, while scholars have found extraordinary insights in the biota of islands, the biota was often devastated early in the period of human expansion through the introduction of goats or other ungulates thought to benefit seamen on future visits.

Both the puzzles and the depredations grow more complicated as human influences spread. Dieter Mueller-Dombois addresses the current transitions in the forests of Pacific Islands. He describes the rain forests of the Hawaiian Islands as “originally impoverished and secondarily enriched.” They were impoverished by comparison with other large islands of the Pacific that are closer to continental areas of higher diversity; they have been enriched by high endemism, a product of their insularity and period of isolation. Mueller-Dombois introduces a concept new in this treatment, progressive development through a peak period of a thousand years or more followed by regression as soils deteriorate.