Introduction

The circumpolar boreal forest landscape reflects a combination of factors unique to high-latitude environments. A short growing season, strong seasonal fluctuations in air temperature and day length, low solar elevation angles, cold soil temperature, the presence of permafrost, poorly-drained soils, a thick forest floor, low nutrient availability, and recurring forest fires are thought to interact to produce the wide range in stand productivity characteristic of boreal forests (Bonan & Shugart 1989). Low soil temperatures and permafrost are perhaps the factors most unique to high-latitude environments. In this chapter, I examine the role of soil temperature and permafrost as ecological factors in the boreal forest.

Environmental controls of soil temperature and permafrost

Over 50% of Canada and the Soviet Union are underlain by permafrost, that is, the thermal condition of soil when its temperature remains below 0°C continuously for two years or more (Brown 1969, 1970; Brown & Pewe 1973). The existence of permafrost is the result of the historical and current state of the surface energy balance and geothermal heat flow (Lunardini 1981). Even if present energy conditions are not conducive to the formation of permafrost, it may still exist as relic permafrost if past conditions have been favorable. However, the current dynamics of permafrost depends on the current surface energy balance, which primarily reflects air temperature as modified to a secondary degree by solar radiation, vegetation, snow cover, and soil characteristics (Brown 1970; Lunardini 1981; Rieger 1983).

Air temperature and solar radiation are correlated with the heat load received on a surface. In Canada, the southern limit of permafrost corresponds with the −1.1°C mean annual air temperature isotherm (Brown 1969).

Introduction

The boreal forest biome is a broad circumpolar mixture of cool coniferous and deciduous tree species that covers approximately 17% of the world's land surface area. Throughout this region, the landscape is a mosaic of forest vegetation types. For example, in interior Alaska above-ground tree biomass ranges from 26 t ha−1 for black spruce (Picea mariana (Mill.) B.S.P.) forests growing on cold, wet, permafrost soils to 250 t ha−1 for white spruce (Picea glauca (Moench) Voss) forests on warm, permafrostfree sites (Van Cleve et al. 1983b). In northern Quebec, above-ground tree biomass averages 20 t ha−1 in nutrient-poor black spruce–lichen woodlands and 108 t ha−1 in more fertile black spruce–moss forests (Moore & Verspoor 1973). In the warmer jack pine (Pinus banksiana Lamb.) forests of New Brunswick, above-ground tree biomass ranges from 0.8 t ha−1 for recent burns to 91.1 t ha−1 for more mature stands (MacLean & Wein l976).

Numerous researchers have examined specific aspects of boreal forests, but no one has formulated a unifying model of the boreal forest biome, a paradigm to link the pattern of forest vegetation with causal environmental factors. Our most detailed understanding of the ecology of boreal forests comes from interior Alaska, where interactions among soil temperature, permafrost, soil moisture, the forest floor, litter quality, nutrient availability, and fire largely control forest productivity and vegetation patterns (Lutz 1956a; Heilman 1966; Viereck 1973, 1975; Van Cleve, Barney & Schlentner 1981; Van Cleve & Viereck 1981; Van Cleve et al. 1983a,b; Van Cleve & Yarie 1986).

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.