The Advanced Series in Agricultural Sciences is designed to ftll a long-felt need for advanced educational and technological books in the agricultural sciences. These texts, intended primarily for students of agriculture, should also provide up-to-date technical background reading for the many agricultural workers in extension services, educational systems, or international bodies. The editors of Advanced Series in Agricultural Sciences will select key subjects relating to the agricultural environment, agricultural physics and chemistry, soil science, plant sciences, animal sciences, food technology, and agricultural engineering for a critical and synthetic appraisal. An initial theoretical presentation will be used by authors of individual volumes in the series to develop a technical approach-including examples and practical solutions- to each subject. In addressing the advanced undergraduate and early graduate student of agriculture, selected authors will present the latest information, leavened with the lessons learned from their own experience, on precise and well-defined topics. Such books that widen the horizons of the student of agriculture can serve, too, as useful reference sources for the young specialist in the early years of his career. Many specialists who are involved in teaching agricultural science are isolated from universities and research institutions. This series will bring them up-to-date scientific information, thus keeping them in touch with progress. The basic objective of Advanced Series in Agricultural Sciences is to effect a structural integration of the theoretic and technical approaches to agriculture.

As a result of the green revolution, the use of yield-increasing inputs such as fer tilizer and pesticides became a matter of course in irrigated rice farming in Southeast Asia. Pesticides were applied liberally, both as a guarantee against crop failure and as a means of fully utilizing the existing yield potential of the crops. However, since outbreaks of pests, such as the brown planthopper (BPH) or the tungro virus, continued to occur despite the application of chemicals, a change of approach began to take place. It is now being realized more and more in Southeast Asia that crop protection problems cannot be resolved solely by the application of chemicals. In the past several years, increasing efforts have there fore been made to introduce, as a first step, supervised crop protection, leading gradually to integrated pest management (Kranz, 1982). Although the crop protection problems naturally differ in the different devel oping countries in Southeast Asia, the economic situation prevailing in these countries can nevertheless be regarded as an important common determinant: pesticide imports use up scarce foreign currency and thus compete with other imports essential to development. For the individual rice farmer, the problem is basically the same: his cash funds are limited and he must carefully weigh whether to use them for purchas ing pesticides, fertilizer or certified seed. In view of this constraint, it is becom ing necessary to abandon the purely prophylactic, routine calendar spraying and instead, employ critically timed and need-based pesticide applications."

Mechanisms of resistance to plant viruses are diverse, and probably involve different types of recognition events. Often, a cascade of changes affecting broader aspects of defence and metabolism is switched on progressively after the initial recognition event. Virulence, i.e. resistence-breaking behaviour of the virus, involves a failure or alteration of recognition or subsequent signalling. Consequences of these recognition events are the ways in which the pathogenic effects on the host are exerted: formation of visible symptoms and control of plant growth. This volume offers a comprehensive coverage of the recognition and signalling events between plants and viruses whereby the particular attraction of viruses (and viroids) is that they can now be completely defined in molecular terms: they offer excellent opportunities for studying the molecular biology of signalling, and may even provide useful guidelines on how plants and cellular pathogens interact.

The concept of controlled release has attracted increasing attention over the last two decades, with the applications of this technology proliferating in diverse fields in cluding medicine, agriculture and biotechnology. Research and developmental efforts related to controlled release are multiplying in both industry and academia. The reason for this phenomenal growth is obvious. The use of a variety of biologically active agents, such as drugs, fertilizers and pesticides, has become an integral part of modern society. Along with the use of these reagents has evolved an awareness that their uncontrolled application almost inevitably induces harmful effects on the health of humans and their surrounding environments. To eliminate or minimize these harmful effects necessitates the controlled release of these chemicals. Moreover, the controlled release of substances, not usually considered toxic or hazardous, e.g., some catalysts and nutrients, can enhance their effectiveness. The number and variety of controlled release systems, differing in their physical and chemical makeup, are increasing rapidly. Proliferation almost always demands correlation, generalization and unification; it requires both the development of underlying theories of their behavior and the mechanistic interpretation of their performance. This, in turn, requires a statistical and mathematical (quantitative) treatment of the scientific information and technical data pertaining to them. A quantitative treatment can also facilitate the formulation of procedures for computer-aided design of these systems through a priori prediction of their per formance for a variety of design parameters.

The cambium has been variously defined as follows: "The actively dividing layer of cells that lies between, and gives rise to, secondary xylem and phloem (vascular cambium)" (IAWA 1964); "A meristem with products of periclinal divisions commonly contributed in two directions and arranged in radial files. Term pre ferably applied only to the two lateral meristems, the vascular cambium and cork cambium, or phellogen" (Esau 1977); and, "Lateral meristem in vascular plants which produces secondary xylem, secondary phloem, and parenchyma, usually in radial rows; it consists of one layer of initials and their undifferentiated deriva tives" (Little and Jones 1980). Clearly, the cambium is a diverse and extensive meristem, and no one defini tion will encompass all manifestations of what anatomists consider cambium. Its diversity and extent are further exemplified by a single plant, such as a temperate zone tree, in which procambium is initiated in the embryo and perpetuated throughout every lateral, primary meristem before giving rise to cambium in the secondary body. The cambium thereafter performs its meristematic task of producing daughter cells that differentiate to specialized tissue systems. The cam bium, however, does not remain static. Its derivatives vary either in form, or TImc tion, or rate of production at different positions on the tree, with age of the tree, and with season of the year. Moreover, the cambium responds both to internal sig nals and to external stimuli such as environment or wounding."

The vascular tissue of higher plants has been an object of continuous detailed study since the invention of the light microscope. As relevant new physical, chemical and biochemical techniques and concepts have appeared over the years, they have regularly and immediately been applied to this field of enquiry. The reasons are not far to seek. Vascular tissue provides the long-distance transport system for water and nutrients within the plant, and an understanding of the structure and development of the pathways within it is essential for an under- standing of growth in all higher plants, including crop plants. Moreover, parts of the vascular tissue - the whole of the xylem and the fibers of the proble- have been, and still are, in high demand commercially for their unique properties and as the only renewable structural material on earth. There are, in addition, more subtle ways in which xylogenesis, particularly as it allows the development of tree species, contributes to the environment. A few years ago, while walking in the foothills of the Pyrenees, I found all this expressed more elegantly than I can express it, in a plea carved in wood at the entrance to a forest: Homme!! Je suis la chaleur de ton foyer par les froids nuits d'hiver, l'ombrage ami lorsque brule Ie soleil d'ete, l'eau des rivieres et des sources. Je suis la charpente de ta maison, la planche de ta table.

The world needs for food and fiber continue to increase. Population growth in the developing countries peaked at 2. 4% a year in 1965 and has fallen to about 2. 1%. However, in many developing countries almost half the people are under 15 years of age, poised to enter their productive and reproductive years. The challenges to produce enough food for this growing population will remain great Even more challenging is growing the food in the areas of greatest need. Presently the world has great surpluses of food and fiber in some areas while there are devastating deficiencies in other areas. Economic conditions and the lack of suitable infrastructure for distribution all too often limit the alleviation of hunger even when there are adequate supplies, sometimes even within the country itself. World hunger can be solved in the long run only by increasing crop production in the areas where the population is growing most rapidly. This will require increased efforts of both the developed and developing countries. Much of the technology that is so successful for crop production in the developed countries cannot be utilized directly in the developing countries. Many of the principles, however, can and must be adapted to the conditions, both physical and economic, of the developing countries.

change is simply described by the rate of income and rate of loss. Our home's energy budget, our firm's inventory, our nation's debt, and humanity's numbers all have accounts that change at rates that are equal to the inputs minus the outputs. Jenny's "system view" of the soil was carried into the fertile fields of Midwestern American prairies from the laboratories of Switzerland in the late 1920s. Jenny's rate equations provided the other paradigm or world view that, I recall, brought us to the threshold of systems ecology as it later evolved in the second half of the twentieth century. As if world renown in the specialties of pedology and soil chemistry were not enough for one lifetime, excerpts below remind us that Hans Jenny has also been a perceptive outdoor field ecologist since his early Alpine expeditions with Braun Blanquet in the mid 1920s. Jenny's ecosystem studies in the pygmy forest, a further classic example of a soil-plant system "run down" over hundreds of thousands of years since its origin, continue to occupy some of the vigorous retirement time near his farm in Mendocino County. But each specific, quantitative case study, and each research area conserved (with additional hard work) for further study by future generations, fits into Jenny's coherent world view. It is that view, and its legacies of discovery and of tangible landscape preserves, which we are privileged to share with their originator in this volume."

Roger C. Dahlman Environmental Sciences Division U.S. Department of Energy Washington, D.C. The potential for humans to alter Earth's atmosphere has been recognized since the end of the 19th century when Arrhenius estimated that a doubling of atmospheric carbon dioxide could alter the atmospheric radiation balance and raise average global temperature. Today, atmospheric CO concentrations play an important part in the 2 climate-change debate. Sources and sinks of CO associated with land use can be 2 significant determinants of the rate and magnitude of atmospheric CO change. 2 Combustion of fossil fuels and the deforestation associated with land-use change both contribute CO to the atmosphere; in contrast, biological processes on land create 2 potential sinks for the excess CO . Thus, land-use change and associated biological 2 processes become important elements in assessments of future atmospheric CO 2 increase; land-cover properties also affect the Earth's albedo, which is a climate feedback.

"Reviews of Environmental Contamination and Toxicology" contains timely review articles concerned with all aspects of chemical contaminants (including pesticides) in the total environment, including toxicological considerations and consequences. It attempts to provide concise, critical reviews of advances, philosophy, and significant areas of accomplished or needed endeavor in the total field of residues of these and other foreign chemicals in any segment of the environment, as well as toxicological implications.

Developments in statistics and computing as well as their application to genetic improvement of livestock gained momentum over the last 20 years. This text reviews and consolidates the statistical foundations of animal breeding. This text will prove useful as a reference source to animal breeders, quantitative geneticists and statisticians working in these areas. It will also serve as a text in graduate courses in animal breeding methodology with prerequisite courses in linear models, statistical inference and quantitative genetics.

Living organisms are separated from the outside world by their membranes, which perform important roles for maintaining their lives. Sterols are indispensable com- ponents in eukaryotic organisms for stabilizing membranes composed of phospho- lipid bilayers. Most prokaryotic organisms, on the other hand, do not contain sterols in their membranes, although recent studies indicated that the bacterium Methylo- coccus capsulatus produces sterols 1) and many strains of Mycoplasma, bacteria without cell walls, require sterols as an essential growth factor 2). Some bacterial species synthesize hopanoids which are hypothesized to function like st~rols in eu- karyotic organisms 3). Significance of ergosterol in fungi has been recognized from the fact that the yeast Saccharomyces cerevisiae requires ergosterol for growth, when cultured in a strictly anaerobic condition "'. l:.lectron microscopic studies showed that membrane structures disappeared in the yeast grown anaerobically without a supplement of ergosterol 5.6), indicating the necessity of ergosterol for membrane biogenesis. Further confirmation of the significant role of ergosterol in the membranes was derived from the studies on polyene antibiotics. In the presence of polyenes such as nystatin, filipin and amphotericin B, fungal cells lose selective permeability of the membranes, since polyenes interact with ergosterol, such that the structure of the fungal membranes is disrupted 7).

The increasing use of biotechnology for the improvement of agricultural crop species has gained momentum, and recent developments (see Crops J) have shown beyond doubt the far-reaching implications of biotechnological approaches for future agricultural research and plant- breeding programs. The production of novel plants and somaclones showing resistance to pests, diseases, herbicides, and salt and the early release of disease-free as well as improved cultivars have become reality. The present volume comprises 31 chapters and deals with the impor- tance, distribution, conventional propagation, micropropagation, and methods for the in-vitro induction of genetic variability in various fruits, vegetables, grasses, and pasture crops such as grapes, strawberry, brambles, red raspberry, currants, gooseberry, kiwifruit, blueberry, cran- berry, cauliflower, cabbage, brussels sprouts, broccoli, cucumber, chico- ry, taro, rhubarb, lettuce, spinach, quinoa, kale, fescue, bromegrass, Ber- mudagrass, napier grass, foxtail millet, turtle grass and others. (The cere- als and other vegetable crops are discussed in Crops J, Vol. 2 of the series). Micropropagation of some fruit crops such as strawberry, grape, and raspberry is already being practiced on a large scale in various countries. Likewise, test-tube-derived plants of certain crops such as brassicas, let- tuce, and taro and improved pastures are being utilized, while the technology for mass propagation of certain other crops is being worked out. These recent developments emphasize the urgent need to arouse awareness among horticultural scientists and plant breeders to enable them to incorporate these modern innovative approaches into routine crop improvement programs.

Over the past years, a great deal has been learned about variation in wood prop erties. Genetic control is a major source of variation in most wood properties. Wood is controlled genetically both directly in the developmental or internal pro cesses of wood formation and indirectly by the control of tree form and growth patterns. Emphasis in this book will be on the internal control of wood production by genetics although there will be two chapters dealing with the indirect genetic control of wood, which was covered in detail in the previous book by Zobel and van Buijtenen (1989). The literature on the genetics of wood is very variable, SO'lle quite superficial, on which little reliance can be placed, and some from well-designed and correctly executed research. When suitable, near the end of each chapter, there will be a summary with the authors' interpretation of the most important information in the chapter. The literature on the genetics of wood can be quite controversial. This is to be expected, since both the environment and its interaction with the genotype of the tree can have a major effect on wood properties, especially when trees of similar genotypes are grown under widely divergent conditions. Adding to the confusion, studies frequently have been designed and analyzed quite differently, resulting in conflicting assessments of results."

From the beginning of agriculture until about 1950, increased food production came almost entirely from expanding the cropland base. Since 1950, however, the yield per unit of land area for major crops has increased dramatically. Much of the increase in yields was because of increased inputs of energy. Between 1950 and 1985, the farm tractor fleet quadrupled, world irrigated area tripled, and use of fertilizer increased ninefold. Between 1950 and 1985, the total energy used in world agriculture increased 6. 9 times. Irrigation played a particularly important role in the rapid increase in food production between 1950 and 1985. The world's irrigated land in 1950 totaled 94 million hectares but increased to 140 million by 1960, to 198 million by 1970, and to 271 million hectares in 1985. However, the current rate of expansion has slowed to less than 1 % per year. The world population continues to increase and agricultural production by the year 2000 will have to be 50 to 60% greater than in 1980 to meet demands. This continued demand for food and fiber, coupled with the sharp decline in the growth rate of irrigation development, means that much of the additional agricultural production in future years must come from cultivated land that is not irrigated. Agricultural production will be expanded in the arid and semiarid regions because these regions make up vast areas in developing countries where populations are rapidly rising.

While working in the laboratory of Professor Dr. Jacob Reinert at the Freie Universitat Berlin (1974-1976), I had the opportunity to become deeply involved in studying the intricacies of the fascinating phenomenon of somatic embryogenesis in plant cells and protoplasts. In numerous stimu lating discussions with Professor Reinert on this subject, I was fully convinced that somatic embryogenesis would become one of the most important areas of study, not only regarding basic and fundamental aspects, but also for its application in crop improvement. During the last decade, we have witnessed tremendous interest and achievements in the use of somatic embryos for the production of synthetic seeds, for micro prop a gation, genetic transformation, cryopreservation, and conservation of germplasm. The en masse production of somatic embryos in the bioreactors has facilitated some of these studies. Somatic embryos have now been induced in more than 300 plant species belonging to a wide range offamilies. It was therefore felt that a compilation ofliterature/state of the art on this subject was necessary. Thus, two volumes on Somatic Embryo genesis and Synthetic Seed have been compiled, which contain 65 chapters contributed by International experts. Somatic Embryogenesis and Synthetic Seed I comprises 31 chapters, arranged in 3 sections: Section I Commitment of the cell to somatic embryogenesis; early events; anatomy; molecular basis; gene expression; role of polyamines; machine vision analysis of somatic embryos. Section II Applications of somatic embryos; technology of synthetic seed; fluid drilling; micropropagation; genetic transfor mation through somatic embryos; cryopreservation."

Modem Methods of Plant Analysis When the handbook Modern Methods of Plant Analysis was fIrst introduced in 1954 the considerations were: 1. the dependence of scientifIc progress in biology on the improvement of existing and the introduction of new methods; 2. the difficulty in fInding many new analytical methods in specialized journals which are normally not accessible to experimental plant biologists; 3. the fact that in the methods sections of papers the description of methods is frequently so compact, or even sometimes so incomplete that it is difficult to reproduce experiments. These considerations still stand today. The series was hIghly successful, seven volumes appearing between 1956 and 1964. Since there is still today a demand for the old series, the publisher has decided to resume publication of Modern Methods of Plant Analysis. It is hoped that the New Series will be just as acceptable to those working in plant sciences and related fIelds as the early volumes undoubtedly were. It is difficult to single out the major reasons for success of any publication, but we believe that the methods published in the fIrst series were up-to-date at the time and presented in a way that made description, as applied to plant material, complete in itself with little need to consult other publications. Contributing authors have attempted to follow these guidelines in this New Series of volumes.

The trend in forestry is toward shorter rotations and more complete utiliza tion of trees. The reasons are: (1) financial pressures to obtain rapid returns on the forestry investment made possible by an earlier harvest; (2) enforced harvest of young plantations to maintain a continuing supply of cellulose for mills where wood shortages are experienced; (3) thinning young plantations, both because they were planted too densely initially and because thinning is done where long rotation quality trees are the forestry goal; (4) more intensive utilization is being done using tops and small diameter trees; and (5) there is interest in using young (juvenile) wood for special products because of its unique characteristics and the development of new technologies. The largest present-day source of conifer juvenile wood is from thinnings of plantations where millions of hectares of pine were planted too densely. Because of the better growth rate resulting from improved silviculture and good genetic stock, plantations will need to be thinned heavily. As a result of this trend, young wood makes up an increasingly larger proportion of the total conifer wood supply each year. Large amounts of juvenile wood from hard woods are also currently available, especially in the tropics and subtropics, because of the fast growth rate of the species used, which results in shorter rotations and ess ntially all juvenile wood."

It is apparent that wilt diseases continue to be a major problem in crop production because of the number of crops affected, the number and genetic variability of pathogens involved, and their widespread occurrence throughout tropical and temperate regions under a variety of cropping systems. It is also apparent, however, that new understandings and approaches, often in combinations not previously discerned, offer exciting new prospects for research, understanding and practical control methods. The current state-of-the-art and fields for further studies were discussed by researchers actively engaged in a wide range of areas from ecological studies of physical and biological factors in the host-parasite-environmental interactions in the soil, through physiological and biochemical studies of host-parasite recognition and interaction that determine relative colonization of the host, through genetic-molecular studies of these interactions, to the most practical field studies of disease control.

7. 2 The Pilot Zone of the FerIo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 7. 2. 1 Geographical Zoning and Administrative Setup. . . . . . . . . . . . . 157 7. 2. 2 Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 7. 2. 3 The Substratum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 7. 2. 4 Surface Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 7. 2. 5 Vegetation and Rangelands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 7. 2. 6 Wildlife. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 7. 2. 7 Livestock. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 7. 2. 8 Evolution of Land Use and History of Development . . . . . . . . 170 7. 3 Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 7. 3. 1 Principles, Problems and Methods . . . . . . . . . . . . . . . . . . . . . . . . 181 7. 3. 2 Evaluation of Green Herbaceous Biomass by Orbital Remote Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 7. 3. 3 Evaluation of Range Production from Ground Sampling . . . . 207 7. 3. 4 Low Altitude Systematic Reconnaissance Flights (SRF) . . . . . . 211 7. 3. 5 Practical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 7. 4 Conclusions on Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 8 General Conclusions: Towards an Ecological Management of the Sahelian Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 of Scientific Names. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 . . . . . . . . . . . . Index Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 VII Abbreviations and Acronyms AVHRR Advanced Very High Resolution Radiometer CILSS Comite Inter-Etats de Lutte contre la Secheresse au Sahel CIPEA Centre International pour l'Elevage en Afrique CNRS Centre National de la Recherche Scientifique CP Crude Protein (N x 6.

Based on papers presented at the International Symposium on Sexual Reproduction in Higher Plants, this volume covers the topics: micro- and macrosporogenesis, the activation and recognition of mature pollen, pollen germination and tube emission in vivo and in vitro, pollen and pollen tube cytoskeleton, stigma and style morphology, pollen/stigma interactions, incompatibility mechanisms and gene expression. The reinvestigation of classical topics using modern methods such as immunofluorescence, micromanipulation, freeze-substitution, electron microscopy, etc., is the common basis of all results presented. Especially applied aspects of sexual reproduction important e.g. for crop improvement, are discussed in detail.

Crassulacean acid metabolism (CAM) represents one of the best-studied metabolic examples of an ecological adaptation to environmental stress. Well over 5 % of all vascular plant species engage in this water-conserving photosynthetic pathway. Intensified research activities over the last 10 years have led to major advances in understanding the biology of CAM plants.
New areas of research reviewed in detail in this book include regulation of gene expression and the molecular basis of CAM, the ecophysiology of CAM plants from tropical environments, the productivity of agronomically important cacti and agaves, the ecophysiology of CAM in submerged aquatic plants, and the taxonomic diversity and evolutionary origins of CAM.

This book is directed at foresters who work, or have an interest, in the developing world, and at development analysts and theorists who are concerned with the forestry sector. Most readers will be aware that in recent years, some fundamental changes in thinking about the development process in very poor countries have occurred. At one level, the underdevelopment problem has been explained as a lack of absorptive capacity, or implementation ability in very poor countries. However, it now seems that these are only symptoms of a more profound ailment in the whole economic structural and philosophical approach to development. The idea that poor countries could transform their economies through an accelerated process of industrialisation has proved largely incorrect, or at least highly premature. Within the rural sector, emphasis on productivity and aggregate income growth have been shown to have had little effect or, worse still, negative effects, on the burgeoning group of poor and landless rural dwellers.

Soil is formed from physical and chemical weathering of rocks - processes described historically because they involve eons of time-by glaciation and by wind and water transport of soil materials, later deposited in deltas and loessial planes. Soil undergoes further transformations over time and provides a habitat for biological life and a base for the development of civilizations. Soil is dynamic -always changing as a result of the forces of nature and particularly by human influences. The soil has been studied as long as history has been documented. Numerous references to soil are found in historical writings such as Aristotle (384-322 B. c. ), Theophrastus (372-286 B. c. ), Cato the Elder (234-149 B. C. ) and Varro (116-27 B. c. ). Some of the earliest historical references have to do with erosional forces of wind and water. The study of soils today has taken on increased importance because a rapidly expanding population is placing demands on the soil never before experienced. This has led to an increase in land degradation and desertification. Desertifica tion is largely synonymous with land degradation but in an arid land context. Deterioration of soil resources is largely human induced. Poverty, ignorance, and greed are the indirect causes of desertification. The direct cause is mismanage ment of the land by practices such as overgrazing, tree removal, improper tillage, poorly designed and managed water distribution systems, and overexploitation."

This book brings together research into the process of stream acidification and its impact on Welsh surface waters, carried out over the past decade or so. It is perhaps surprising that not until the 1980's was clear evidence of stream acidification assembled. In Wales, concerns over pollution had focused water quality sampling principally on the areas of traditional heavy industry and large urban popula tions served by inadequate sewerage systems and sewage disposal arrange ments. Mistakenly, it had been assumed that, with its prevailing westerly winds, Wales would receive precipitation substantially unpolluted by the industrial and urban emissions from Britain and mainland Europe. Assurance of the high quality of Welsh upland streams, the traditional nursery ground of salmonids, was eroded particularly by studies in the vicinity of Llyn Brianne reservoir in the catchment of the River Tywi of Central Wales. These demonstrated a clear correspondence between the biological quality and fisheries of streams in the catchment and aspects of stream chemistry, par ticularly pH, aluminium and calcium on the one hand, and catchment land use on the other. It is salutary to record that the first signals were of an inexplicable failure of the runs of migratory salmonids into the upper catchment, occupied by the Llyn Brianne reservoir and its influent streams, and the failure to restore the fishery by re-stocking with eggs and fry. Only then did the significance of the recent decline in some other upland lake and reservoir fisheries in Wales become apparent.