Sunday, 15 September 2019

Global Change

Global Change

Both scientists and nonscientists are aware that the earth and it's ecological systems are dynamic when one considers relatively long time-scales. That there was a major continental ice cap a few tens of thousands of years ago is sufficiently part of the common wisdom that children's cartoons can use a "caveman" motif in the context of an "ice age" without any need for explanation. The occurrence of major extinctions of biotic groups (such as the "dinosaurs") in the past is not difficult for most people to imagine. In general, we are quite willing to believe that, viewed over the eons, the earth has been an extremely dynamic planet.

What is novel-newsworthy to the public and challenging to the scientist are observations of shorter time-scale, relatively rapid changes in atmospheric and surface features of the earth, and the strong evidence that some of these changes are being induced by human activities. It appears that we are producing measurable changes in major earth systems, but we have relatively little knowledge as to how the earth's systems actually operate. The scientific challenges that attend these topical concerns have inspired a major international research program (the International Geosphere-Biosphere Program-IGBP), which was chartered by the International Council of Scientific Unions in 1986, "to describe and understand the interactive physical, chemical, and biological processes that regulate the total earth system, the unique environment that it provides for life, the changes that are occurring in this system, and the manner in which they are influenced by human activities."

This article treats one of several problems related to global change, namely, the prediction of the response of the global pattern of vegetation to a change in the climate. Changes in the composition of the earth's atmosphere (such as the change in the amount of carbon dioxide-C0 2 ) are, sensu strict, components of global climate change, as are possible changes in global temperature and precipitation
patterns. There are several observations of global-scale change that shape the scientific issues of vegetation dynamics and global change. Three examples are our increasing appreciation of the nature and magnitude of past changes in vegetation in response to climatic change, the measured variation of the atmospheric CO 2 concentration over the past three decades, and the ability to survey the global terrestrial surface by using sensors carried on orbiting platforms. The past vegetation dynamics were discussed recently for boreal regions by Solomon (1992). The other two examples are discussed briefly in the two sections that follow.

co2 levels in the atmosphere

In 1957, Keeling (e.g., Keeling 1983) began measuring the concentration of CO 2 in the atmosphere at Mauna Loa, Hawaii. He has continued these observations to this time. The initial observations indicated an annual fluctuation in the amount of CO 2 in the atmosphere over Mauna Loa. Further observations confirmed that the amount of CO 2 in the atmosphere had been increasing exponentially since the initial 1957 measurements. This exponential increase was found to be correlated with the release of CO 2 into the atmosphere from the burning of fossil fuels.

Climate Change

 Fig. Selected monthly mean CO 2 concentrations from continuous measurements at National Oceanic and Atmospheric Administration/Geophysical Monitoring for Climate Change  OANGMCC) stations at four locations (Barrow, Alaska [BRW]; American Samoa [SMO]; Mauna Loa Hawaii [MLO]; South Pole [SPO]) from 1973 to 1983 (from Harris and Bodhaine 1983).

Figure 1.1 shows the pattern of variation in CO 2 measured at four different stations (Barrow, Alaska [BRW]; American Samoa [SMO]; Mauna Loa Hawaii [MLO]; South Pole [SPO]) from 1973 to 1983 (Harris and Bodhaine 1983). While there is a considerable difference in the amount of oscillation in these data, there is a clear tendency for the measured levels of CO 2 to increase regularly throughout this record. This figure can be thought of as illustrating the "breathing of the planet." It shows regular oscillations of the CO 2 concentrations in the Northern Hemisphere over three annual cycles and a much more constant pattern in the Southern Hemisphere (Harris and Bodhaine 1983).

Carbon dioxide is an essential component of plant photosynthesis, and when one sees a seasonal or systematic multiple-year change in CO 2 levels, questions immediately arise as to whether these changes might in some way be altering the way plants function. If the functioning of plants is changed in some way, what other changes might ensue if the CO 2 in the atmosphere continues to increase? There is evidence that the number of stomata per unit area on plant leaves has decreased in response to the increase in CO 2 concentration since the industrial revolution (Woodward 1987; see also). At the different levels of CO 2 in the atmosphere that were thought to prevail in the past, were conditions so different from those today as to confound our ability to interpret historical accounts of the vegetation?

The important role of biological processes in affecting the atmosphere also arises when considering the level of CO 2 in the atmosphere. When one computes the amount of CO 2 that has been released into the atmosphere through the burning of fossil fuels, subtracts the amount of CO 2 thought to be taken up by the oceans, and further subtracts the amount that appears to remain in the atmosphere (producing the regular increase shown in Fig. 1.1), about half of the CO 2 produced by fossil fuel burning remains unaccounted for. Is this "missing carbon" being taken up by land plants or is our understanding of the processes that transfer CO 2 from the atmosphere to the ocean incomplete?
At the annual time-scale, we would like to know to what degree the terrestrial vegetation is involved in causing the annual oscillations in CO 2 in the Northern Hemisphere (Fig. 1.1,). An important consequence of an increased ambient level of CO 2 in the atmosphere is related directly to the role of this relatively rare atmospheric constituent in the heat balance of the earth. CO 2 is a "greenhouse gas" that is transparent to visible light but is opaque to long-wave radiation. Since much of the radiation that penetrates the earth's atmosphere is reradiated as long-wave radiation, CO 2 in the atmosphere functions much like the glass in a greenhouse, allowing light to enter and trapping heat (Fig. 1.2a). 

energy balance
Fig. 1.2 .a. Schematic diagram of the global average components of the earth's energy balance (from MacCracken and Luther 1985). b. Schematic illustration of the climatic cause-and-effect (feedback) linkages that are typically included in numerical models of the earth's climate system (from MacCracken 1985; based on Robotech 1985).

Clouds absorb long-wave radiation (a "greenhouse" function) but also reflect incoming radiation back to space (a cooling function). Other atmospheric components are also active in the earth's energy balance, notably ozone (0 3 ), man-made chlorofluorocarbon compounds, and water vapor which acts as "greenhouse gases." Some of these other atmospheric components are increasing at rates that exceed the rate for CO 2, An estimate of the possible effects of these atmospheric components based on projected increases from 1980 to 2030 is shown in figure 1.3a (Bolin et at. 1986). 

Global Change
Fig 1.3 .a. Cumulative surface temperature warming due to an increase in CO 2 and other trace gases from 1980 to 2030 as computed by a one-dimensional model (from Ramanathan et al. 1985). In these calculations, the atmospheric concentration of CO 2 is expected to double in this time period. b. Estimates of GCM models that include dynamic feedbacks for the global warming expected from a doubling of CO 2 (from Bolin et al. 1986).

These computations are based on relatively simple models that do not attempt to take into account the interactive nature of and the feedback effects among the components of the atmosphere (Dickinson 1986).
These feedback effects have been incorporated into massive computer models (general circulation models, or GCMs) in an attempt to understand the global climate system and to assess the effects of changes in the atmosphere such as might occur from the observed change in atmospheric CO 2 (Dickinson 1986). 
The resultant climate models are complex (Fig. 1.2b) and, at present, even the largest and fastest computers are unable to solve the equations at a fine spatial level (solutions are typically for an earth divided into large blocks ca. 2° latitude by 2° longitude) or with representations of the effects of the oceans which change dynamically as the climate changes.
The GCMs vary with respect to the way that processes important to the earth's climate are represented in a given formulation and in terms of assumptions used to approximate conditions that cannot be simulated in the models directly (e.g., the formation of clouds, the energy exchange between the atmosphere and the ocean, or the formation of ice at sea). Despite these model differences, the models converge in that they predict an increase in the earth's temperature as a consequence of increased atmospheric CO 2, The models show substantial differences in the patterns of temperature change in space and particularly with regard to precipitation (Dickinson 1986).
There is a considerable level of uncertainty in the predictions of the rates of increase of CO 2 over the coming years, in the reliability of the GCMs and in the effects of the behavior of the oceans under a changing climate. Nevertheless, there is a need for a clearer understanding of the response of vegetation to climate change. This is true both with respect to the effect of climate on the vegetation and with respect to the role of the terrestrial biota in the carbon cycle.
Because of the short time-scale of predicted environmental change (Schneider 1989a & b), the most important issues related to ecosystem response in response to climate-change scenarios should concern transient responses and factors affecting these responses. These include direct consideration of the magnitude and time-scale of climate change, shifts in the seasonality of temperature and precipitation, and changes in climate-mediated disturbance regimes (Solomon et al. 1984; Neilson et al. 1989; Smith and Tirpak 1989). 
Important ecosystem factors that must be considered include the ecosystem state in terms of age structure or successional status of the vegetation (Davis and Botkin 1985; Urban and Shugart 1989; Urban et al. 1989) and soil characteristics (Pastor and Post 1988; Bonan et al. 1990). These considerations underscore the fundamental concern that the transient responses of ecosystems maybe
dramatically different from the responses extrapolated from studies of plant species or communities in isolation.

Observations of Vegetation Dynamics from Satellites

The manned space programs and the development of orbiting platforms with sensors capable of monitoring features on the surface of the earth have provided images that have allowed a generation to "see" what the earth looks like when viewed from space. Making the earth visible as a whole has indubitably amplified the interest in global ecological studies much as the invention of the microscope electrified the scientists of another era. Along with the important role of making global studies more tangible, the use of satellites and remote sensing of the earth's surface has provided fundamental observations that fuel a need to better understand global vegetation patterns and dynamics.
For example, Tucker et al. (1986a) related the variations in global CO 2 shown in Figure 1.1 to the variation in the "greenness" of the total earth surface. Tucker et al. (1986b) used the advanced very high-resolution radiometer (AVHRR) that is carried on the National Oceanic and Atmospheric Administration's "weather" satellites (NOAA-6, NOAA-7, and NOAA-8 sun-synchronous, polar-orbiting, operational satellites) to map the greenness of vegetation at the continental and global scale. These same satellites are used to develop the satellite maps that are frequently used as part of the weather-report section of many television evening news programs.
Two of the spectral bands detected by the satellites (one in the visible-red part of the spectrum and one in the near-infrared) have been used to map green leaf area of intercepted photosynthetically active radiation (Tucker 1979; Curran 1980; Kumar and Monteith 1982). By analyzing data collected over the past several years by these satellites, estimates have been obtained of the rates of clearing of
tropical rainforest (Tucker et al. 1986b), of the types of vegetation cover in Africa (Tucker et al. 1985), and of the world distribution of green leaf biomass (Tucker et al. 1986a).
In this latest application, the periodic variation in the amount of green leaf biomass (as indicated by the NOAA-7 satellite AVHRR sensor) was determined for the same period as is shown in the data of Harris and Bodhaine (1983; see Fig. 1.1). The resultant pattern of "greenness" varying over 3 years and with latitude is shown in Figure 1.4. One notes a striking similarity in the pattern of Figure 1.4 and that seen in Figure 1.1a for global CO 2 dynamics. Tucker et al.
(1986a) state in their conclusions that "Our analysis demonstrates the measurable link between atmospheric CO 2 drawdowns and terrestrial NDVI [greenness index, ed.] dynamics and suggests that there may be quantitative relationships between multi-temporal satellite data and atmospheric CO 2 drawdowns." This is indeed the case and serves as but one example of the scientific challenges being produced by the observational capacity provided by satellites.

Global Ecological

Many of the basic concepts used in global ecology originate in the works of Clements (1916; emphasis on dynamic interactions), Gleason (1926; the importance of species attributes in dynamic systems), Tansley (1935; the ecosystem concept), and Watt (1947, the relationship between internal dynamics and spatial patterns). The theories that were developed by these early ecologists proved difficult to apply in a formal mathematical fashion to the complex natural systems for which they were intended. The eventual development of mathematical models based on these concepts has clearly been catalyzed by the increased availability of computers. Some of the that follow will discuss the application of several different models to problems in global ecology with the overall intent of providing an impression of the issues, capabilities, and challenges involved in understanding vegetation dynamics and climatic change.

process in ecological studies

A fundamental theme in the biological sciences that is echoed in cellular biology, genetics, morphology, population biology, and ecology involves the relationship between geometrical structures of living things and the processes that attend these structures. The organization of tissues is intimately related to their physiological function. It is important to the function of the DNA molecule that it is a spiral helix. The adaptive implications of the morphology of plants and animals are central to both taxonomy and evolutionary biology.

process in ecological
Fig. 1.4. A weighted index based on the visible-red and the near-infrared channels of the NOAA-7 satellite and indicative of green leaf biomass plotted against time and latitude zone. The seasonal effects seen in the northern latitudes in the dynamics of CO 2 are also seen in the northern latitudes of this graph of "greenness" dynamics. Note also the relatively constant values for the equatorial latitudes and the influence of deserts in depressing greenness in the 20° to oN latitude zone (from Tucker et al. 1986a).

Pattern and Process

In biology, these themes are variously described as "structure and function" or "pattern and process." Depending upon the examples chosen, there may be an emphasis on the manner in which processes influence pattern. For example, what changes in the morphology of the vertebrate limb development in the evolution of flight? Or, how do the seeds of plants vary between arid and moist environments? In other examples, the emphasis is on the pattern or structure modifying processes.
Pattern and process are linked in a biological yin and yang in which each causes and is caused by the other.
The relationship between form and function, or pattern and process, is a classic ecological theme (Lindeman 1942; Watt 1947; Whittaker and Levin 1977).
Bormann and Likens (1979a) pointed out the effects of changes in forest structure on processes such as productivity and nutrient cycling. Many ecologists recognize that pattern and process are mutually causal, with changes in ecosystem processes causing changes in pattern and modifications in ecosystem pattern changing processes. Nonetheless, it is difficult to investigate directly the feedback between pattern and process.
In global ecological studies, what is of particular importance is a knowledge of the degree of dominance of particular causal factors at particular scales. The knowledge of which factors are important at a given scale is also involved in the determination of the "rules" for deciding what should be included in the formulation of a given model. That different phenomenon may be invoked when developing models of analogous phenomena at different scales is to a degree responsible for what is categorized as the "art" (as opposed to the science) of ecological modeling. While the determination of the importance of processes at a given time- or space-scale is central in model formulation and evaluation, it is a consideration that is neither trivial nor unique to the developers of computer models. For example, in the case of "hierarchy theory" (Allen and Starr 1982;
Allen and Hoekstra 1984; O'Neill et al. 1986; Urban et al. 1987), one sees a focus on expressing relevant mathematical developments in a manner that can provide insight into the ways ecosystems are structured at different scales.
The categorization of controlling factors important at different space- and timescales in particular ecosystems has been the topic of several reviews (Delcourt et al. 1983; Pickett and White 1985). Historically, this interest is evident in A.S. Watt's (1925) early work on beech forests and elaborated in his now-classic paper on pattern and process in plant communities (Watt 1947). These themes have
been reiterated by several subsequent ecologists (Tansley 1935; Whittaker 1953; Bormann and Likens 1979a,b).
The factors governing vegetation structure and ecosystem processes vary considerably within and among biomes. In mesic forests, a frequent constraint is the availability of light. As a forest environment tends from mesic to xeric or nutrient-poor conditions, the effective constraint shifts from above- to below-ground factors (Webb et al. 1978; Tilman 1988; Smith and Hudson 1989). Under still drier conditions, forest changes to grassland in which the principal constraint is below-ground, suggesting patterns in the influence of environmental constraints in structuring ecosystems across broad environmental gradients.
Pastor and Post (1986) used the LINKAGES model to evaluate the principal constraints in forests through time and among trees of different stature. The principal constraint changed from below-to above-ground as trees grew and the canopy closed. The principal constraint also varied with tree size and time in the simulated succession. The dominant individuals were limited by below-ground constraints, while understory individuals shifted from below- to above-ground constraints as they were overtopped. This result cautions against oversimplifying ecosystems in terms of "the primary constraint" and also suggests parallels in patterns of constraint through time and over spatial gradients.

Spatial Scale

Consideration at the basis of the relationship between pattern and process is the intrinsic scale of phenomena. This theme is fundamental to modem science.
Phenomenological scale can be considered in spatial or temporal dimensions. In either case, scale refers to the range with respect to a fundamental dimension (usually in time or space) associated with a pattern or process of interest. For example, at the time- and space-scales considered in quantum mechanics, the effects of gravity are sufficiently small to be ignored. At the scales typically considered in astrophysical studies, the gravitational effects are a paramount consideration and cannot be ignored.
As is the case with pattern and process, the concept of scale is well developed in the biological sciences. Infield studies in ecology, a frequent manifestation of scale involves the determination of the appropriate size of a sampling quadrat to survey a given sort of vegetation. A typical question in this regard might be, "Is it more accurate (or more convenient) to survey the variety of trees in a forest by determining the number of trees of different species in a random set of sample areas of 0.1 ha or 1.0 haT' The appropriate scale of sample quadrats is influenced by the spatial distribution of the tree species so that what might appear to be a straightforward sampling question is actually involved with understanding a fundamental property of a forest.
One of the most intellectually stimulating aspects of global-scale ecological studies is a consequence of the interaction of environmental scientists (oceanographers, meteorologists, climatologists, etc.) with a strong orientation toward physics. This has simultaneously served to create an interest in phenomenological scale in the ecological sciences and to present problems in understanding phenomena at space- and time-scales that are relatively nontraditional to the ecologists. For example, for an atmospheric process to exert a significant successional effect, it must impact the ecosystem in a manner that directly allows for the alteration of local community composition. Some meteorological processes (windfalls, lightning strikes) appear as prominent and persistent. Others, such as differential mortality of trees of different species resulting from unusual temperatures, are much more subtle. Fujita (1981) provides strong evidence for five relatively discrete scales of atmospheric motion (Fig. 1.5), some of which can influence ecological succession. Fujita's five scales are each separated by approximately two orders of magnitude in spatial extent, with the largest, or "A" scale, corresponding to the planetary and synoptic circulations. It is generally the "E" ("mesoscale") circulations which come to mind when one thinks of the direct effects of the atmospheric circulation on forests. The determination of how changes in the frequencies of tropical cyclones (Neumann et al. 1981) and differences in the spatial patterns of the paths of such cyclones (Hayden 1981; Shapiro 1982) might be manifested as a change in a forest region is related to considering the forest at an appropriate scale (ca. regions on the order of 10 2 to 10 3 km on a side).
The problem of understanding spatial scale arises in interfacing ecology with physical dynamics of the Earth's surface, and it is also an important feature of global ecological studies as such. For example, in the problem of understanding the gas, water, and heat exchange of the surface of a leaf (a problem typically considered at space-scales of a few square centimeters or less) at the scale of a surface several kilometers on a side is discussed in some detail. This is but one of a large number of scale-related problems that have been engendered by the interest in global ecological studies.

Atmospheric scales
Fig. 1.5. Atmospheric scales of motion according to Fujita (1981),

Temporal Scale

There are also scale-related responses in time. Temporal scale can be discussed n an example case by considering the response of dynamic systems to periodic variations in important external variables. There is a tendency for many dynamic systems to display what is termed by engineers as "band-limited" behavior. In such systems, the dynamics response of the system to very-high-frequency and
very-low-frequency periodic variation is almost zero, and only in an intermediate range of frequencies of variation does the system actually respond. One biological example is the human ear, which does not respond to extremely high- or low-frequency sounds (which we consequently do not hear), but does respond to an intermediate frequency of sound waves (in the audible range). The ear of a dog has a different response, and a dog hears a different range of sound waves from a human. The response of ecological systems to different periodicities of variation in time may also in many cases be band-limited, with a range of frequencies of external factors to which the system is responsively bordered by frequencies either too high or too low to produce a response.
The important consequence of band-limited behavior in ecological systems involves the identification of the typical periodicities in the external factors that are important to the ecological system. As will be discussed in the that follow, the range of important frequencies in external variables as well as the particular external variables that are important depends to a great degree on the nature of the ecological processes under consideration.

Temporal Scale
Fig. 1.6.a. Biotic disturbance regimes viewed in the context of space/time domain. The scale of a particular process shown in the figure reflects the sample intervals typically required to observe the process. 
b. Space/time domain of representative biological processes. 
c. Space/time domain of ecological patterns (from Delcourt et al. 1983).

Scale, Pattern, and Process

Delcourt et al. 1983 illustrate the relationship between the scale of several important external environmental conditions (disturbances, Fig. 1.6a) with the scale of important biological processes (Fig. 1.6b). The interaction of these disturbances and processes produces ecological patterns (Fig. 1.6c). The fundamental concept of ecological scale is that the processes that are most important in producing ecological patterns change as a direct function of the temporal and spatial domain. The importance of a particular process when predicting the response of an ecological system to change is related to the time- and space scales of interest.

The Ecosystem Concept

In a classic paper written by Tansley in 1935, the term "ecosystem" was first defined as an arbitrary system with respect to both its spatial extent and the phenomena considered. This can be seen in the first use of the term by Tansley:
The more fundamental conception is, as it seems to me, the whole system (in the sense of physics), including not only the organism-complex but also the whole complex of physical factors forming what we call the environment-the habitat factors in the widest sense. Though the organisms may claim our primary interest, when we are trying to think fundamentally we cannot separate them from their special environment, with which they form one physical system.
It is the systems so formed which, from the point of view of the ecologist, are the basic units of nature on the earth... These ecosystems, as we may call them, are of the most various kinds and sizes. They form one category of the multitudinous physical systems of the universe, which range from the universe as a whole down to the atom.
In this, the first use of the word "ecosystem" in the English language, Tansley stressed that ecosystems are of "various kinds and sizes." This relative arbitrariness and abstraction were viewed by Tansley as a necessary step to the formulation of environmental science that was on a par with physics and other more established sciences. The value of the ecosystem concept has been proven in the 50 years that have ensued since Tansley coined the term.
If the three panels shown in Figure 1.6 are separated and stacked one on top of the next, then an ecosystem, as defined by Tansley, can be depicted as the set of interacting external variables, ecological processes, and patterns all with equivalent space-time domains. An ecosystem may be large or small with respect to either temporal or spatial scales but there should be an equivalency in the time and space domains in the patterns and processes considered.

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