Author: Enzai Du
Publisher: Elsevier
ISBN: 0323998488
Category : Nature
Languages : en
Pages : 306
Book Description
Atmospheric Nitrogen Deposition in Global Forests: Spatial Variation, Impacts, and Management Implications provides the most comprehensive knowledge on spatial variation and ecological impacts of reactive nitrogen deposition in global forests, as well as forest management options to mitigate the negative impacts. Written and edited by international experts in the field, this book synthesizes recent research developments and insights in monitoring and modeling nitrogen deposition in global forests. The book also assesses ecological impacts of enhanced nitrogen deposition on forest structure and function and responses of forest ecosystems to decreasing nitrogen deposition in regions such as the European Union and North America. Finally, the book reviews indicators and thresholds for nitrogen saturation in global forests and analyzes remediation options to reduce impacts of excess nitrogen deposition. This is an important resource for researchers in forestry and biodiversity conservation, as well as graduate students, policymakers and others who want to understand environmental issues of reactive nitrogen deposition in global forests. - Offers a systematic view of the ecological impacts of enhanced nitrogen deposition - Provides the most comprehensive knowledge on spatial variation and the ecological impacts of reactive nitrogen deposition in global forests - Presents expert research and findings on forest management options to remediate negative impacts
Atmospheric Nitrogen Deposition to Global Forests
Author: Enzai Du
Publisher: Elsevier
ISBN: 0323998488
Category : Nature
Languages : en
Pages : 306
Book Description
Atmospheric Nitrogen Deposition in Global Forests: Spatial Variation, Impacts, and Management Implications provides the most comprehensive knowledge on spatial variation and ecological impacts of reactive nitrogen deposition in global forests, as well as forest management options to mitigate the negative impacts. Written and edited by international experts in the field, this book synthesizes recent research developments and insights in monitoring and modeling nitrogen deposition in global forests. The book also assesses ecological impacts of enhanced nitrogen deposition on forest structure and function and responses of forest ecosystems to decreasing nitrogen deposition in regions such as the European Union and North America. Finally, the book reviews indicators and thresholds for nitrogen saturation in global forests and analyzes remediation options to reduce impacts of excess nitrogen deposition. This is an important resource for researchers in forestry and biodiversity conservation, as well as graduate students, policymakers and others who want to understand environmental issues of reactive nitrogen deposition in global forests. - Offers a systematic view of the ecological impacts of enhanced nitrogen deposition - Provides the most comprehensive knowledge on spatial variation and the ecological impacts of reactive nitrogen deposition in global forests - Presents expert research and findings on forest management options to remediate negative impacts
Publisher: Elsevier
ISBN: 0323998488
Category : Nature
Languages : en
Pages : 306
Book Description
Atmospheric Nitrogen Deposition in Global Forests: Spatial Variation, Impacts, and Management Implications provides the most comprehensive knowledge on spatial variation and ecological impacts of reactive nitrogen deposition in global forests, as well as forest management options to mitigate the negative impacts. Written and edited by international experts in the field, this book synthesizes recent research developments and insights in monitoring and modeling nitrogen deposition in global forests. The book also assesses ecological impacts of enhanced nitrogen deposition on forest structure and function and responses of forest ecosystems to decreasing nitrogen deposition in regions such as the European Union and North America. Finally, the book reviews indicators and thresholds for nitrogen saturation in global forests and analyzes remediation options to reduce impacts of excess nitrogen deposition. This is an important resource for researchers in forestry and biodiversity conservation, as well as graduate students, policymakers and others who want to understand environmental issues of reactive nitrogen deposition in global forests. - Offers a systematic view of the ecological impacts of enhanced nitrogen deposition - Provides the most comprehensive knowledge on spatial variation and the ecological impacts of reactive nitrogen deposition in global forests - Presents expert research and findings on forest management options to remediate negative impacts
Perspectives in Biophysical Plant Ecophysiology
Author: Erick De La Barrera
Publisher: UNAM
ISBN: 0578006766
Category : Science
Languages : en
Pages : 442
Book Description
Park S. Nobel pioneered the coupling of cellular physical chemistry with plant physiology, providing a sound physicochemical interpretation of the laws of diffusion to a rapidly expanding field of plant physiological ecology. His classical textbook is the only one of its kind to provide an extensive array of quantitative problems and solutions in the field of plant biophysics and ecophysiology, extending from the molecular to the ecological level. In this festschrift, former graduate students and postdocs, as well as colleagues of Prof. Nobel present a series of reviews that include scales from sub-cellular to global, and topics that range from desert succulent biology to the physiology of alpine plants, encompassing basic research and applications in agronomy and conservation biology. This state-of-the-field survey provides current and useful information for professionals and graduate students, while illustrating the broad span of the influence that Nobel's career has had on modern ecophysiology.
Publisher: UNAM
ISBN: 0578006766
Category : Science
Languages : en
Pages : 442
Book Description
Park S. Nobel pioneered the coupling of cellular physical chemistry with plant physiology, providing a sound physicochemical interpretation of the laws of diffusion to a rapidly expanding field of plant physiological ecology. His classical textbook is the only one of its kind to provide an extensive array of quantitative problems and solutions in the field of plant biophysics and ecophysiology, extending from the molecular to the ecological level. In this festschrift, former graduate students and postdocs, as well as colleagues of Prof. Nobel present a series of reviews that include scales from sub-cellular to global, and topics that range from desert succulent biology to the physiology of alpine plants, encompassing basic research and applications in agronomy and conservation biology. This state-of-the-field survey provides current and useful information for professionals and graduate students, while illustrating the broad span of the influence that Nobel's career has had on modern ecophysiology.
A History of the Ecosystem Concept in Ecology
Author: Frank B. Golley
Publisher: Yale University Press
ISBN: 9780300066425
Category : Science
Languages : en
Pages : 278
Book Description
The ecosystem concept--the idea that flora and fauna interact with the environment to form an ecological complex--has long been central to the public perception of ecology and to increasing awareness of environmental degradation. In this book an eminent ecologist explains the ecosystem concept, tracing its evolution, describing how numerous American and European researchers contributed to its evolution, and discussing the explosive growth of ecosystem studies. Golley surveys the development of the ecosystem concept in the late nineteenth and early twentieth centuries and discusses the coining of the term ecosystem by the English ecologist Sir Arthur George Tansley in 1935. He then reviews how the American ecologist Raymond Lindeman applied the concept to a small lake in Minnesota and showed how the biota and the environment of the lake interacted through the exchange of energy. Golley describes how a seminal textbook on ecology written by Eugene P. Odum helped to popularize the ecosystem concept and how numerous other scientists investigated its principles and published their results. He relates how ecosystem studies dominated ecology in the 1960s and became a key element of the International Biological Program biome studies in the United States--a program aimed at "the betterment of mankind" specifically through conservation, human genetics, and improvements in the use of natural resources; how a study of watershed ecosystems in Hubbard Brook, New Hampshire, blazed new paths in ecosystem research by defining the limits of the system in a natural way; and how current research uses the ecosystem concept. Throughout Golley shows how the ecosystem concept has been shaped internationally by both developments in other disciplines and by personalities and politics.
Publisher: Yale University Press
ISBN: 9780300066425
Category : Science
Languages : en
Pages : 278
Book Description
The ecosystem concept--the idea that flora and fauna interact with the environment to form an ecological complex--has long been central to the public perception of ecology and to increasing awareness of environmental degradation. In this book an eminent ecologist explains the ecosystem concept, tracing its evolution, describing how numerous American and European researchers contributed to its evolution, and discussing the explosive growth of ecosystem studies. Golley surveys the development of the ecosystem concept in the late nineteenth and early twentieth centuries and discusses the coining of the term ecosystem by the English ecologist Sir Arthur George Tansley in 1935. He then reviews how the American ecologist Raymond Lindeman applied the concept to a small lake in Minnesota and showed how the biota and the environment of the lake interacted through the exchange of energy. Golley describes how a seminal textbook on ecology written by Eugene P. Odum helped to popularize the ecosystem concept and how numerous other scientists investigated its principles and published their results. He relates how ecosystem studies dominated ecology in the 1960s and became a key element of the International Biological Program biome studies in the United States--a program aimed at "the betterment of mankind" specifically through conservation, human genetics, and improvements in the use of natural resources; how a study of watershed ecosystems in Hubbard Brook, New Hampshire, blazed new paths in ecosystem research by defining the limits of the system in a natural way; and how current research uses the ecosystem concept. Throughout Golley shows how the ecosystem concept has been shaped internationally by both developments in other disciplines and by personalities and politics.
Ecological Stoichiometry
Author: Robert W. Sterner
Publisher: Princeton University Press
ISBN: 1400885698
Category : Science
Languages : en
Pages : 465
Book Description
All life is chemical. That fact underpins the developing field of ecological stoichiometry, the study of the balance of chemical elements in ecological interactions. This long-awaited book brings this field into its own as a unifying force in ecology and evolution. Synthesizing a wide range of knowledge, Robert Sterner and Jim Elser show how an understanding of the biochemical deployment of elements in organisms from microbes to metazoa provides the key to making sense of both aquatic and terrestrial ecosystems. After summarizing the chemistry of elements and their relative abundance in Earth's environment, the authors proceed along a line of increasing complexity and scale from molecules to cells, individuals, populations, communities, and ecosystems. The book examines fundamental chemical constraints on ecological phenomena such as competition, herbivory, symbiosis, energy flow in food webs, and organic matter sequestration. In accessible prose and with clear mathematical models, the authors show how ecological stoichiometry can illuminate diverse fields of study, from metabolism to global change. Set to be a classic in the field, Ecological Stoichiometry is an indispensable resource for researchers, instructors, and students of ecology, evolution, physiology, and biogeochemistry. From the foreword by Peter Vitousek: ? "[T]his book represents a significant milestone in the history of ecology. . . . Love it or argue with it--and I do both--most ecologists will be influenced by the framework developed in this book. . . . There are points to question here, and many more to test . . . And if we are both lucky and good, this questioning and testing will advance our field beyond the level achieved in this book. I can't wait to get on with it."
Publisher: Princeton University Press
ISBN: 1400885698
Category : Science
Languages : en
Pages : 465
Book Description
All life is chemical. That fact underpins the developing field of ecological stoichiometry, the study of the balance of chemical elements in ecological interactions. This long-awaited book brings this field into its own as a unifying force in ecology and evolution. Synthesizing a wide range of knowledge, Robert Sterner and Jim Elser show how an understanding of the biochemical deployment of elements in organisms from microbes to metazoa provides the key to making sense of both aquatic and terrestrial ecosystems. After summarizing the chemistry of elements and their relative abundance in Earth's environment, the authors proceed along a line of increasing complexity and scale from molecules to cells, individuals, populations, communities, and ecosystems. The book examines fundamental chemical constraints on ecological phenomena such as competition, herbivory, symbiosis, energy flow in food webs, and organic matter sequestration. In accessible prose and with clear mathematical models, the authors show how ecological stoichiometry can illuminate diverse fields of study, from metabolism to global change. Set to be a classic in the field, Ecological Stoichiometry is an indispensable resource for researchers, instructors, and students of ecology, evolution, physiology, and biogeochemistry. From the foreword by Peter Vitousek: ? "[T]his book represents a significant milestone in the history of ecology. . . . Love it or argue with it--and I do both--most ecologists will be influenced by the framework developed in this book. . . . There are points to question here, and many more to test . . . And if we are both lucky and good, this questioning and testing will advance our field beyond the level achieved in this book. I can't wait to get on with it."
Atmospheric Reactive Nitrogen in China
Author: Xuejun Liu
Publisher: Springer Nature
ISBN: 9811385149
Category : Science
Languages : en
Pages : 334
Book Description
Atmospheric reactive nitrogen (N) emissions, as an important component of global N cycle, have been significantly altered by anthropogenic activities, and consequently have had a global impact on air pollution and ecosystem services. Due to rapid agricultural, industrial, and urban development, China has been experiencing an increase in reactive N emissions and deposition since the late 1970s. Based on a literature review, this book summarizes recent research on: 1) atmospheric reactive N in China from a global perspective (Chapter 1); 2) atmospheric reactive N emissions, deposition and budget in China (Chapters 2-5); 3) the contribution of atmospheric reactive N to air pollution (e.g., haze, surface O3, and acid deposition) (Chapters 6-8); 4) the impacts of N deposition on sensitive ecosystems (e.g., forests, grasslands, deserts and lakes) (Chapters 9-12); and 5) the regulatory strategies for mitigation of atmospheric reactive N pollution from agricultural and non-agricultural sectors in China (Chapters 13-14). As such it offers graduate students, researchers, educators in agricultural, ecological and environmental sciences, and policy makers a glimpse of the environmental issues related to reactive N in China .
Publisher: Springer Nature
ISBN: 9811385149
Category : Science
Languages : en
Pages : 334
Book Description
Atmospheric reactive nitrogen (N) emissions, as an important component of global N cycle, have been significantly altered by anthropogenic activities, and consequently have had a global impact on air pollution and ecosystem services. Due to rapid agricultural, industrial, and urban development, China has been experiencing an increase in reactive N emissions and deposition since the late 1970s. Based on a literature review, this book summarizes recent research on: 1) atmospheric reactive N in China from a global perspective (Chapter 1); 2) atmospheric reactive N emissions, deposition and budget in China (Chapters 2-5); 3) the contribution of atmospheric reactive N to air pollution (e.g., haze, surface O3, and acid deposition) (Chapters 6-8); 4) the impacts of N deposition on sensitive ecosystems (e.g., forests, grasslands, deserts and lakes) (Chapters 9-12); and 5) the regulatory strategies for mitigation of atmospheric reactive N pollution from agricultural and non-agricultural sectors in China (Chapters 13-14). As such it offers graduate students, researchers, educators in agricultural, ecological and environmental sciences, and policy makers a glimpse of the environmental issues related to reactive N in China .
Eddy Covariance
Author: Marc Aubinet
Publisher: Springer Science & Business Media
ISBN: 9400723504
Category : Science
Languages : en
Pages : 451
Book Description
This highly practical handbook is an exhaustive treatment of eddy covariance measurement that will be of keen interest to scientists who are not necessarily specialists in micrometeorology. The chapters cover measuring fluxes using eddy covariance technique, from the tower installation and system dimensioning to data collection, correction and analysis. With a state-of-the-art perspective, the authors examine the latest techniques and address the most up-to-date methods for data processing and quality control. The chapters provide answers to data treatment problems including data filtering, footprint analysis, data gap filling, uncertainty evaluation, and flux separation, among others. The authors cover the application of measurement techniques in different ecosystems such as forest, crops, grassland, wetland, lakes and rivers, and urban areas, highlighting peculiarities, specific practices and methods to be considered. The book also covers what to do when you have all your data, summarizing the objectives of a database as well as using case studies of the CarboEurope and FLUXNET databases to demonstrate the way they should be maintained and managed. Policies for data use, exchange and publication are also discussed and proposed. This one compendium is a valuable source of information on eddy covariance measurement that allows readers to make rational and relevant choices in positioning, dimensioning, installing and maintaining an eddy covariance site; collecting, treating, correcting and analyzing eddy covariance data; and scaling up eddy flux measurements to annual scale and evaluating their uncertainty.
Publisher: Springer Science & Business Media
ISBN: 9400723504
Category : Science
Languages : en
Pages : 451
Book Description
This highly practical handbook is an exhaustive treatment of eddy covariance measurement that will be of keen interest to scientists who are not necessarily specialists in micrometeorology. The chapters cover measuring fluxes using eddy covariance technique, from the tower installation and system dimensioning to data collection, correction and analysis. With a state-of-the-art perspective, the authors examine the latest techniques and address the most up-to-date methods for data processing and quality control. The chapters provide answers to data treatment problems including data filtering, footprint analysis, data gap filling, uncertainty evaluation, and flux separation, among others. The authors cover the application of measurement techniques in different ecosystems such as forest, crops, grassland, wetland, lakes and rivers, and urban areas, highlighting peculiarities, specific practices and methods to be considered. The book also covers what to do when you have all your data, summarizing the objectives of a database as well as using case studies of the CarboEurope and FLUXNET databases to demonstrate the way they should be maintained and managed. Policies for data use, exchange and publication are also discussed and proposed. This one compendium is a valuable source of information on eddy covariance measurement that allows readers to make rational and relevant choices in positioning, dimensioning, installing and maintaining an eddy covariance site; collecting, treating, correcting and analyzing eddy covariance data; and scaling up eddy flux measurements to annual scale and evaluating their uncertainty.
The Environmental Chemistry of Aluminum
Author: Garrison Sposito
Publisher: CRC Press
ISBN: 0429612486
Category : Science
Languages : en
Pages : 477
Book Description
The Environmental Chemistry of Aluminum provides a comprehensive, fundamental account of the aqueous chemistry of aluminum within an environmental context. An excellent reference for environmental chemists and scientific administrators of environmental programs, this book contains material reflecting the many recent changes in this rapidly developing discipline. The first three chapters discuss the most fundamental aspects of aluminum chemistry: its quantitation in soils and natural waters, including speciation measurements, and its stable chemical forms, both as a dissolved solute and in a solid phase. These chapters emphasize both critical assessments of and definitive recommendations for laboratory methodologies and measured thermodynamic properties relating to aluminum chemistry. The next four chapters in The Environmental Chemistry of Aluminum build on this foundation to provide details of the polymeric chemistry of aluminum: its polynuclear and colloidal hydrolytic species in aqueous solution, its complexes with natural organic ligands, including humic substances, and its role as an adsorptive and adsorbent in surface reactions. These chapters are grounded in experimental results rather than conceptual modeling. The final three chapters describe the chemistry of aluminum in soils, waters, and watersheds. These chapters illustrate the problems of spatial and temporal variability, metastability, and scale that continue to make aluminum geochemistry one of the great challenges in modern environmental science.
Publisher: CRC Press
ISBN: 0429612486
Category : Science
Languages : en
Pages : 477
Book Description
The Environmental Chemistry of Aluminum provides a comprehensive, fundamental account of the aqueous chemistry of aluminum within an environmental context. An excellent reference for environmental chemists and scientific administrators of environmental programs, this book contains material reflecting the many recent changes in this rapidly developing discipline. The first three chapters discuss the most fundamental aspects of aluminum chemistry: its quantitation in soils and natural waters, including speciation measurements, and its stable chemical forms, both as a dissolved solute and in a solid phase. These chapters emphasize both critical assessments of and definitive recommendations for laboratory methodologies and measured thermodynamic properties relating to aluminum chemistry. The next four chapters in The Environmental Chemistry of Aluminum build on this foundation to provide details of the polymeric chemistry of aluminum: its polynuclear and colloidal hydrolytic species in aqueous solution, its complexes with natural organic ligands, including humic substances, and its role as an adsorptive and adsorbent in surface reactions. These chapters are grounded in experimental results rather than conceptual modeling. The final three chapters describe the chemistry of aluminum in soils, waters, and watersheds. These chapters illustrate the problems of spatial and temporal variability, metastability, and scale that continue to make aluminum geochemistry one of the great challenges in modern environmental science.
Atmospheric Ammonia
Author: Mark Sutton
Publisher: Springer Science & Business Media
ISBN: 1402091214
Category : Science
Languages : en
Pages : 468
Book Description
Anthropogenic emissions of ammonia cause a host of environmental impacts, including loss of biodiversity, soil acidification and formation of particulate matter in the atmosphere. Under the auspices of the UNECE Convention on Long Range Transboundary Air Pollution, around 80 international experts met to review the state of scientific knowledge. This book reports their analysis. It concludes that threshold levels for ammonia effects have been underestimated and sets new values, it assesses the independent evidence to verify reported reductions in regional ammonia emissions, and it reviews the uncertainties in modelling ammonia, both in "hot spots" and at the regional scale.
Publisher: Springer Science & Business Media
ISBN: 1402091214
Category : Science
Languages : en
Pages : 468
Book Description
Anthropogenic emissions of ammonia cause a host of environmental impacts, including loss of biodiversity, soil acidification and formation of particulate matter in the atmosphere. Under the auspices of the UNECE Convention on Long Range Transboundary Air Pollution, around 80 international experts met to review the state of scientific knowledge. This book reports their analysis. It concludes that threshold levels for ammonia effects have been underestimated and sets new values, it assesses the independent evidence to verify reported reductions in regional ammonia emissions, and it reviews the uncertainties in modelling ammonia, both in "hot spots" and at the regional scale.
Critical Loads and Dynamic Risk Assessments
Author: Wim de Vries
Publisher: Springer
ISBN: 9401795088
Category : Science
Languages : en
Pages : 671
Book Description
This book provides a unique overview of research methods over the past 25 years assessing critical loads and temporal effects of the deposition of air pollutants. It includes critical load methods and applications addressing acidification, eutrophication and heavy metal pollution of terrestrial and aquatic ecosystems. Applications include examples for each air pollution threat, both at local and regional scale, including Europe, Asia, Canada and the US. The book starts with background information on the effects of the deposition of sulphur, nitrogen and heavy metals and geochemical and biological indicators for risk assessments. The use of those indicators is then illustrated in the assessment of critical loads and their exceedances and in the temporal assessment of air pollution risks. It also includes the most recent developments of assessing critical loads and current and future risks of soil and water chemistry and biodiversity under climate change, with a special focus on nitrogen. The book thus provides a complete overview of the knowledge that is currently used for the scientific support of policies in the field of air pollution control to protect ecosystem services.
Publisher: Springer
ISBN: 9401795088
Category : Science
Languages : en
Pages : 671
Book Description
This book provides a unique overview of research methods over the past 25 years assessing critical loads and temporal effects of the deposition of air pollutants. It includes critical load methods and applications addressing acidification, eutrophication and heavy metal pollution of terrestrial and aquatic ecosystems. Applications include examples for each air pollution threat, both at local and regional scale, including Europe, Asia, Canada and the US. The book starts with background information on the effects of the deposition of sulphur, nitrogen and heavy metals and geochemical and biological indicators for risk assessments. The use of those indicators is then illustrated in the assessment of critical loads and their exceedances and in the temporal assessment of air pollution risks. It also includes the most recent developments of assessing critical loads and current and future risks of soil and water chemistry and biodiversity under climate change, with a special focus on nitrogen. The book thus provides a complete overview of the knowledge that is currently used for the scientific support of policies in the field of air pollution control to protect ecosystem services.
Litter Decomposition: a Guide to Carbon and Nutrient Turnover
Author:
Publisher: Academic Press
ISBN: 9780120139385
Category : Science
Languages : en
Pages : 448
Book Description
Litter Decomposition describes one of the most important processes in the biosphere - the decay of organic matter. It focuses on the decomposition process of foliar litter in the terrestrial systems of boreal and temperate forests due to the greater amount of data from those biomes. The availability of several long-term studies from these forest types allows a more in-depth approach to the later stages of decomposition and humus formation. Differences between the decay of woody matter and foliar litter is discussed in detail and a different pattern for decomposition is introduced. While teachers and students in more general subjects will find the most basic information on decomposition processes in this book, scientists and graduate students working on decomposition processes will be entirely satisfied with the more detailed information and the overview of the latest publications on the topic as well as the methodological chapter where practical information on methods useful in decomposition studies can be found. Abundant data sets will serve as an excellent aid in teaching process and will be also of interest to researchers specializing in this field as no thorough database exists at the moment. Provides over 60 tables and 90 figures Offers a conceptual 3-step model describing the different steps of the decomposition process, demonstrating changes in the organic-chemical structure and nutrient contents Includes a synthesis of the current state of knowledge on foliar litter decomposition in natural systems Integrates more traditional knowledge on organic matter decomposition with current problems of environmental pollution, global change, etc. Details contemporary knowledge on organic matter decomposition
Publisher: Academic Press
ISBN: 9780120139385
Category : Science
Languages : en
Pages : 448
Book Description
Litter Decomposition describes one of the most important processes in the biosphere - the decay of organic matter. It focuses on the decomposition process of foliar litter in the terrestrial systems of boreal and temperate forests due to the greater amount of data from those biomes. The availability of several long-term studies from these forest types allows a more in-depth approach to the later stages of decomposition and humus formation. Differences between the decay of woody matter and foliar litter is discussed in detail and a different pattern for decomposition is introduced. While teachers and students in more general subjects will find the most basic information on decomposition processes in this book, scientists and graduate students working on decomposition processes will be entirely satisfied with the more detailed information and the overview of the latest publications on the topic as well as the methodological chapter where practical information on methods useful in decomposition studies can be found. Abundant data sets will serve as an excellent aid in teaching process and will be also of interest to researchers specializing in this field as no thorough database exists at the moment. Provides over 60 tables and 90 figures Offers a conceptual 3-step model describing the different steps of the decomposition process, demonstrating changes in the organic-chemical structure and nutrient contents Includes a synthesis of the current state of knowledge on foliar litter decomposition in natural systems Integrates more traditional knowledge on organic matter decomposition with current problems of environmental pollution, global change, etc. Details contemporary knowledge on organic matter decomposition