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Handbook of Functional Plant Ecology
edited by Francisco I. Pugnaire Arid Zones Experimental Station Spanish Council for Scientific Research Almeria. Spain
Fernando Valladares Center of Environmental Sciences Spanish Council for Scientific Research Madrid. Spain
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MARCEL
DEKKER
MARCEL DEKKER, INC .
NEW YORK· BASEL
23 Ecological Applications of Remote Sensing at Multiple Scales
ISBN: 0-8247-1950-6 This book is printed on acid-free paper.
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INTRODUCTION
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FUNDAMENTALS A. Sensors B. The Concept of Scale C. Vegetation Indices D . New Opportunities with Hyperspectral Sensors
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III. REMOTE SENSING AS A MULTIDIMENSIONAL MAPPING TOOL
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IV. LINKING REMOTE SENSING TO MODELS OF PHOTOSYNTHETIC PRODUCTION A. APAR and Radiation-Use Efficiency B . New Approaches To Resolving the Question of Efficiency
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REMOVING THE BARRIERS
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REFERENCES
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Gamon and Qiu
Ecological Applications of Remote Sensing
INTRODUCTION
We are restrained by our senses, which often limit our view to a particular favorite or accessible geographic location. Consequently, much ecological research has traditionally focused on specific organisms, populations, and communities defined by geographic region, and ecology is largely a collection of case histories in search of unifying principles. However, the growing human pressures on the planet's resources are altering ecosystem function at regional to global scales and placing a new urgency on studies that integrate information across spatial and temporal scales. Ecologists are now faced with the challenge of measuring and understanding ecological processes at these multiple scales. Answering this challenge will necessarily involve the use of remote sensing, which has the unique ability to provide an objective, synoptic view of the earth and its atmosphere. In this chapter, remote sensing will primarily be defined as the measurement of electromagnetic radiation (reflectance, fluorescence, or longwave emission) with noninvasive sampling (Figure 1). Remote implies measurements from a distance (e.g ., from an airborne or satellite platform), and many of the most spectacular examples have been from these great distances. However, the fundamental tools and plinciples of radiation measurement can be applied at almost any scale. Indeed, one of the strengths of optical sampling is that, unlike many other measurement methods, it is eminently' 'scaleable" -it can be applied at many spatial scales and used to analyze how information content changes with scale (Ustin et al . 1993, Foody and Curran 1994, Quattrochi and Goodchild 1997, Wessman 1992). This ability to bridge scales allows us to extend our otherwise limited perception to new domains and to reach a new understanding of complex ecological phenomena. Remote sensing has additional virtues, including the ability to sample nondestructively and without direct contact, thus avoiding the problem common to many measurement techniques of disturbing or destroying the object of measurement. Because digital remote sensing provides a consistent data format, it provides a degree of objectivity that is lacking in many other methods of data collection. Remote sensing per se is not new . Properly positioned, the eye is a powerful (if subjective) remote sensing device. Aerial photography has been used for many decades for mapping and reconnaissance. Continued advances in digital and optical technology are providing ever more powerful tools for collecting and analyzing remotely sensed data, and a vast array of sensors are now available for use by the ecological community. These technological advances are redefining the questions that can be addressed by ecologists and are contributing to paradigm shifts in both the concepts and methodology of ecology. Much of what is currently "new" about remote sensing is the way in which investigators are finding innovative ways of using these tools, often in combination with other methods, to address ecological questions at multiple scales.
. . e remote sensing, which is dependent on solar pass IV . t d (T) or reflected (R). Absorbed F igure 1 Schematic illustrating . b b bed (A) trans nut e , radiation. Radiation can either e a sor '(F) or more slowly as longwave, thermal as fluorescence f . dly radiation can be released rapl . f . formau' on about surface eatures .' ned nsors can In er In emission (E). Remotely posluo se . d fl resced or emitted radiation . In a real flected transnutte, u o , . d) , . lti Ie scene components (not Illustrate by detecting patterns 0 f re landscape, multiple scattering and Signals from mu p d significantly influence the pattern of radlaUon detecte .
. . fi ld combined with the uncertain future omprehensive review a difficult, The rapid pace of advance m thiS ; . ' of many current and planned sensors,m es a c being used to infer a dizzying if not impossible, task. Remote sensmg IS ~o:s and processes including surface array of earth surface and atmosphenc propde I . ral comp;sition, vegetation . . topography albe 0, nune temperature, mOisture,. 'c~ and land use change, atmospheric composlfluxes . Some of these (e.g., surfacecover and type, vegetatIOn dynarru h tion irramance, and surface-atmosp ere. f emotely sensed measurements , . th incorporatIOn 0 r atmosphere fluxes) reqUire e rface tern erature and vegetation cover) can with models, whereas ?thers (e.g., su a full s~rvey of these applications IS ",,:ell be deterrruned more directly . Clearly, thorough coverage of remote sensmg h' view For a roore , beyond the scope 0 f t IS re ' . . d d r might pursue current mformatechnology and applications, the mtereste rea e
Gamon and Qiu.
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tion on the World Wide Web or refer to any of a number of recent references and reviews on the topic (LiJlesand and Kiefer 1987, Hobbs and Mooney 1990, Richards 1993, Ustin et at. 1993, Foody and Curran 1994, Danson and Plununer 1995, Gholz et al. 1997, Kasischke et al. 1997, Quattrochi and Goodchild 1997, Sabins 1997). Instead of a comprehensive survey, we have chosen to present a few specific applications that illustrate the range, power, and potential of optical (visible and infrared) remote sensing for addressing ecological questions at severallevels of inquiry. We selected a particular focus on carbon stocks and fluxes associated with terrestrial vegetation. This topic is of critical concern today because human activities are perturbing vegetation cover, composition, and the carbon cycle in significant and measurable ways (Yitousek 1994, Amthor 1995, Houghton 1995). A central goal of global ecology is to clearly define vegetationatmosphere fluxes in the context of these perturbations. The chapter begins with a shOtt presentation of a few remote sensing fundamentals. It then illustrates recent examples of remote sensing as a mapping tool, perhaps the most obvious and traditional of remote sensing applications. Newer extensions of this mapping capability are now explicitly considering spectral and temporal aspects of landscapes, and linking this information to process models. We also examine some examples of the application of remote sensing to models of terrestrial carbon flux and cons(der how new developments offer to improve our understanding of carbon stocks and fluxes . An implicit message is that the relevant technology and appropriate data ' are increasingly becoming available, but are only slowly being adopted by the ecological community at large. The chapter ends by considering the removal of barriers to the use of remote sensing. The goal is to encourage ecologists to continue exploring innovative ways of applying this powerful and exciting technology .
809
Ecological Applications of Remole Sensing
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