Seeing the woods and the trees

Terrestrial Ecosystems in Changing Environments
February 26, 1999

The landscape covered in this book is an interesting mosaic, dominated by woodlands, but with smatterings of all sorts of plant assemblages. Herman H. Shugart begins by emphasising the importance of understanding ecosystem change as a route to solving environmental problems. At present we are the problem and the cause of environmental change.

However, his fundamental tenet is that environmental change is omnipresent. This is certainly a reasonable characterisation of the past couple of million years, during which the earth has experienced repeated waxing and waning of continental ice sheets. At most spots on the land surface there have been great changes in the distribution of species concurrent with changes in the climate.

The unit of interest in this book is the ecosystem, but this turns out to be rather an elusive term that can be applied at a range of scales. The word ecosystem was coined in 1935 by A. G. Tansley, who noted that "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".

The problem is where to draw the boundary. To Shugart, "the ecosystem is an intellectual construct", its boundaries somewhat arbitrarily chosen to suit the requirements of a particular piece of research. However, we all know grassland from woodland. The problem arises because ecosystems are "open systems" that exchange both matter and energy with their surroundings. Their boundaries are diffuse and often blur into one another.

The coupling between organisms and their environment is not given the prominence that is suggested in the notion of an ecosystem. In introducing vegetation-environment relations, Shugart focuses almost entirely on how the environment determines the vegetation. Only later does he begin to consider how the vegetation alters the environment. Unfortunately, this reflects much of the history of vegetation ecology.

Natural landscapes are mosaics, and Shugart gives some lovely examples of spatial patterns in landscapes and how they arise. The most memorable is that of the tiger bush, in Niger, so called because from above the vegetation appears in bands that look like the stripes on a tiger. This spatial phenomenon has been effectively modelled with "cellular automata", a mathematical variant of chess, where the landscape is divided into a grid of squares (the cells) populated by model organisms (the automata), which interact according to simple spatial rules.

The core of the book provides a broad introduction to the different types of ecological model, although "this book is not intended as a treatise on ecological modelling". The reader is introduced to a hierarchy of types of model applicable at different spatial scales. These are often identified by carefully crafted acronyms such as Hybrid, Swamp, Forest and Biome. The wonderfully titled Triffid, which is just being unleashed in the UK Hadley Centre's climate model, is unfortunately too new to be included. It is difficult for a text to be up to date with the literature, but this is made up for by a thorough historical introduction to basic concepts.

Ecosystem models are broadly classified into individual-based and landscape models. Individual-based models "see the forests by looking at the (individual) trees", or indeed the lack of them - gaps in the forest canopy get a special emphasis for their effect on the light reaching below canopy level. Different tree species are classified as "functional types" according to whether they produce a canopy with gaps or not and whether they need a gap to regenerate or not.

These simple rules can produce rich patterns of woodland development. For example, mutualism arises when trees that produce gaps but do not require them to regenerate (eg maple and beech) alternate with those that require gaps to regenerate but do not produce them (eg birch).

Landscape models focus on the woods without worrying too much about the trees. Three disparate approaches to modelling a landscape are described, first assuming that its components do not interact, then assuming that they do, and finally assuming that they are so interactive that they respond as a uniform whole. The latter approach is most popular in attempts to model the whole earth system, and can involve scaling up from the responses of an individual leaf to that of a landscape. Some mathematical equations for the more important processes are given but by no means enough to build a model, and there are no clues as to whether the reader can get access to any of the models. This is unfortunate because, from experience, the only way really to get a handle on a model is to build it and play with it.

The final part of the book applies the different types of landscape models to evaluate the consequences of global change, especially at regional scales. This is a fertile area of research. Ground-based and satellite observations reveal a "missing sink" of carbon dioxide, inferred to be on the land surface, particularly in the temperate latitudes of the northern hemisphere. This removal of carbon dioxide can be partly accounted for by the direct stimulation of plant growth and partly as a response to warming. Current models are generally consistent with these observations, and interesting predictions are being made about what will happen to this carbon sink in the coming century.

It is unfortunate that Shugart only begins to address the global response of vegetation to contemporary change at the end of the book. The global response of the biota is clearly a composite of different ecosystem responses. For example, in a particularly warm year the growth of the boreal forests and tundra of the high northern latitudes is immediately stimulated, while growth in many tropical and arid ecosystems is being suppressed. Scaling up from ecosystem responses to the whole-earth system response is an important and non-trivial step. The lack of this synthesis left me disappointed. Despite these limits of scope, this book is a useful introductory text for undergraduates and an excellent pointer to the literature for interested researchers.

Timothy M. Lenton is an earth system modeller, Institute of Terrestrial Ecology, Edinburgh.

Terrestrial Ecosystems in Changing Environments

Author - Herman H. Shugart
ISBN - 0 521 56342 9 and 56523 5
Publisher - Cambridge University Press
Price - £75.00 and £.95
Pages - 537

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