HYPERSPECTRAL REMOTE SENSING OF TROPICAL AND SUBTROPICAL FORESTS
CONTENTS CHAPTER TROPICAL DRY FOREST PHENOLOGY AND DISCRIMINATION OF TROPICAL TREE SPECIES USING HYPERSPECTRAL DATA CHAPTER REMOTE SENSING AND PLANT FUNCTIONAL GROUPS: PHYSIOLOGY, ECOLOGY, AND SPECTROSCOPY IN TROPICAL SYSTEMS HYPERSPECTRAL DATA FOR ASSESSING CARBON DYNAMICS AND BIODIVERSITY OF FORESTS EFFECT OF SOIL TYPE ON PLANT GROWTH, LEAF NUTRIENT/CHLOROPHYLL CONCENTRATION, AND LEAF REFLECTANCE OF TROPICAL TREE AND GRASS SPECIES SPECTRAL EXPRESSION OF GENDER: A PILOT STUDY WITH TWO DIOECIOUS NEOTROPICAL TREE SPECIES SPECIES CLASSIFICATION OF TROPICAL TREE LEAF REFLECTANCE AND DEPENDENCE ON SELECTION OF SPECTRAL BANDS DISCRIMINATING SIREX NOCTILIO ATTACK IN PINE FOREST PLANTATIONS IN SOUTH AFRICA USING HIGH SPECTRAL RESOLUTION DATA HYPERSPECTRAL REMOTE SENSING OF EXPOSED WOOD AND DECIDUOUS TREES IN SEASONAL TROPICAL FORESTS ASSESSING RECOVERY FOLLOWING SELECTIVE LOGGING OF LOWLAND TROPICAL FORESTS BASED ON HYPERSPECTRAL IMAGERY CHAPTER 10 A TECHNIQUE FOR REFLECTANCE CALIBRATION OF AIRBORNE HYPERSPECTRAL SPECTROMETER DATA USING A BROAD, MULTIBAND RADIOMETER CHAPTER 11 ASSESSMENT OF PHENOLOGIC VARIABILITY IN AMAZON TROPICAL RAINFORESTS USING HYPERSPECTRAL HYPERION AND MODIS SATELLITE DATA CHAPTER 12 HYPERSPECTRAL REMOTE SENSING OF CANOPY CHEMISTRY, PHYSIOLOGY, AND BIODIVERSITY IN TROPICAL RAINFORESTS CHAPTER 13 TROPICAL REMOTE SENSING—OPPORTUNITIES AND CHALLENGES 1 TROPICAL DRY FOREST PHENOLOGY AND DISCRIMINATION OF TROPICAL TREE SPECIES USING HYPERSPECTRAL DATA 1 1 INTRODUCTION TROPICAL TREE SPECIES DISCRIMINATION USING HYPERSPECTRAL DATA IS AN EMERGING FIELD IN WHICH PROGRESS IS CLOSELY LINKED TO THE AVAILABILITY AND ADVANCEMENT OF REMOTE SENSING TECHNOLOGY INITIAL INTEREST IN THE OPTICAL PROPERTIES OF TROPICAL PLANTS WAS EXPRESSED IN THE LATTER HALF OF THE 1980S [1], AND THE POTENTIAL FOR USING THESE PROPER- TIES FOR TREE CLASSIFICATION WAS RECOGNIZED AT LEAST A DECADE LATER [2,3] WHILE STUDIES TO THAT POINT FOCUSED ON LEAF-TO-BRANCH SCALE REFLECTANCE, THE FIRST ATTEMPT TO CLASSIFY TREE CROWNS USING HYPERSPECTRAL IMAGERY FROM AIRBORNE SENSORS WAS MADE ONLY RECENTLY [4] THIS SMALL BODY OF RESEARCH TO DATE HAS APPROACHED TREE IDENTIFICATION OVER MULTIPLE SCALES AND HAS BEEN LARGELY EXPLORATORY IN NATURE UNDERSTANDING THE FACETS OF TREE CROWN REFLECTANCE AND THE CHALLENGES INVOLVED IN TREE CLASSIFICATION BASED ON SPECTRAL REFLECTANCE BEGINS AT THE LEAF LEVEL INCIDENT LIGHT ON A LEAF IS DESTINED FOR ONE OF THREE MAIN PATHWAYS: REFLECTANCE, TRANSMITTANCE, OR ABSORP- TANCE A PORTION OF THE INCIDENT BEAM WILL UNDERGO SPECULAR REFLECTANCE AT THE LEAF’S SURFACE, OR CUTICLE ANOTHER PORTION WILL ENTER THE LEAF AND BE INFLUENCED BY BEER’S LAW OF RADIATIVE TRANSFER—ITS PROGRESS SLOWED ACCORDING TO THE ABSORPTION COEFFICIENTS OF THE CELLULAR MATTER IT ENCOUNTERS AT CELLULAR INTERFACES, SUCH AS THE BOUNDARY BETWEEN CELL WALLS AND AIR SPACES, RAYS MAY BE REDIRECTED NUMEROUS TIMES, GOVERNED BY SNELL’S LAW OF REFRACTION RED LIGHT INTERCEPTED BY CHLOROPHYLL ALONG THE WAY IS USED TO INITIATE THE ELECTRON EXCITATION PROCESS THAT DRIVES PHOTOSYNTHESIS AT SLIGHTLY LONGER WAVELENGTHS (700–1300 NM), LIGHT IS OF LITTLE USE TO A LEAF AND IS EITHER REFLECTED BACK OUT OR TRANS- MITTED ON THROUGH THE LEAF, WHERE IT MAY ENGAGE WITH LEAVES LOWER IN THE CANOPY AND CONTRIBUTE TO LEAF ADDITIVE REFLECTANCE [5] STILL LONGER WAVELENGTHS (1300–2500 NM) ARE LARGELY ABSORBED BY WATER, LEADING TO DOMINANT ABSORPTION BANDS AT 1400 AND 1900 NM OVERALL, SPECTRAL REFLECTANCE OF LEAVES HAS BEEN WELL STUDIED IN REVIEWS [6–9] LIGHT INTERACTION WITH A LEAF IS A COMPLEX PHENOMENON, AND SCALING OUT TO BRANCHES AND EVENTUALLY TREE CROWNS MULTIPLIES THIS COMPLEXITY THE SIMPLER ISSUE OF LIGHT TRAVEL THROUGH A SINGLE LEAF BECOMES COMPOUNDED BY POTENTIAL ENCOUNTERS WITH A MULTITUDE OF LEAVES AND ADDITIONAL VEGETATIVE (FLOWERS, FRUITS, BRANCHES, STEMS, TRUNK, EPIPHYTES, LIANAS, UNDERSTORY PLANTS, AND LITTER) AND NONVEGETATIVE (SOIL) FOREST COM- PONENTS THE EXPRESSION OF FOLIAR CHEMISTRY IN CANOPIES IS LARGELY INFLUENCED BY THE AMOUNT OF LEAFY MATERIAL, MEASURED AS LEAF AREA INDEX (LAI), AND THE ORIENTATION OF THE LEAFY MATERIAL, MEASURED AS LEAF ANGLE DISTRIBUTION (LAD) [10], WHICH HAVE BEAR- ING ON THE SHAPE AND AMPLITUDE, RESPECTIVELY, OF THE CUMULATIVE RETURN WHEREAS LAI AND LAD ARE THE DOMINANT CONTROLS OF CANOPY REFLECTANCE IN FULL CANOPIES, SOIL AND UNDERSTORY VEGETATION HAVE A MAJOR IMPACT ON REFLECTANCE OF SPARSE CANOPIES [10] IN TROPICAL DRY FORESTS, FOLIAR CHEMISTRY, LAI, AND LAD CAN VARY SIGNIFICANTLY— THE FORMER TWO SEASONALLY AND THE LATTER DIURNALLY THESE VARIATIONS ARE MORE DISTINCT IN PARTIALLY TO FULLY DECIDUOUS TROPICAL DRY FORESTS THAN IN EVERGREEN WET FORESTS THAT ARE BY THEIR VERY NATURE EVERGREEN SEASONAL CYCLES IN TROPICAL DRY FORESTS THAT LEAD TO PHENOMENA SUCH AS LEAF DROP, LEAF FLUSH, AND FLOWERING EVENTS SHOULD PROVIDE UNIQUE OPPORTUNITIES FOR REMOTE SENSING OF TREE SPECIES OR FUNCTIONAL TYPES THE OBJECTIVES OF THIS CHAPTER, THEREFORE, ARE TO (1) REVIEW THE CURRENT STATE OF REMOTE SENSING FOR SPECIES IDENTIFICATION IN TROPICAL FORESTS IN GENERAL, (2) DISCUSS POTENTIAL UNIQUE OPPORTUNITIES FOR TREE SPECIES DISCRIMINATION IN TROPICAL DRY FORESTS, AND (3) CONSIDER THE CHALLENGES AND FUTURE DIRECTIONS OF SPECIES IDENTIFICATION USING REMOTELY SENSED DATA IN TROPICAL FORESTS HYPERSPECTRAL REMOTE SENSING OF TROPICAL AND SUBTROPICAL FORESTS LEAF OPTICAL PROPERTIES OF TROPICAL FOREST PLANTS: LOOKING FOR DIFFERENCES ALTHOUGH LEAF OPTICAL PROPERTIES HAVE TRADITIONALLY BEEN MEASURED AS DIFFUSE REFLEC- TANCE AND DIFFUSE TRANSMITTANCE USING A SPECTROMETER WITH AN ATTACHED INTEGRATING SPHERE, IT IS BECOMING INCREASINGLY COMMONPLACE TO SOLELY MEASURE LEAF BIDIREC- TIONAL REFLECTANCE USING A SPECTROMETER WHILE THE FORMER PERMITS THE COMPUTATION OF ABSORPTANCE (1-REFLECTANCE-TRANSMITTANCE), A MEASURE OF THE RADIATION USED BY A LEAF IN PHOTOSYNTHESIS, THE LATTER IS SIMILAR TO THE BIDIRECTIONAL REFLECTANCE RECORDED BY AN ELECTRO-OPTICAL AIRBORNE OR SATELLITE-BORNE SENSOR LEAF SPECTRAL REFLECTANCE MEASUREMENTS ARE EASILY OBTAINED IN THE FIELD USING A PORTABLE SPECTROMETER, OR IN THE LAB USING DETACHED LEAVES, PROVIDED THAT THE LEAVES ARE KEPT FRESH [11] BY THE VERY NATURE OF THE SCIENTIFIC METHOD, HYPOTHESIS TESTING FOCUSES ON FIND- ING DIFFERENCES WITHIN OR BETWEEN GROUPS RESEARCH ON THE LEAF OPTICAL PROPERTIES OF TROPICAL PLANTS IS NO DIFFERENT IN THIS RESPECT AND HAS FOCUSED ON FINDING DIFFERENCES WITHIN SPECIES, BETWEEN SPECIES, OR BETWEEN GROUPS OF SPECIES, SUCH AS BETWEEN LIFE FORMS OR BETWEEN SUN- AND SHADE-ADAPTED PLANTS INTRASPECIFIC DIFFERENCES IN LEAF OPTICAL PROPERTIES ARE RECOGNIZED TO COMPLICATE THE MATTER OF FINDING INTERSPECIFIC OR INTERGROUP DIFFERENCES [12] AN EXAMPLE OF VARIABILITY IN LEAF OPTICAL PROPERTIES WITHIN SPECIES OCCURS AS LEAVES AGE ROBERTS ET AL OBSERVED TYPICAL CHANGES IN SPECTRAL REFLECTANCE OF NEW LEAVES OF AN AMAZON CAATINGA SPECIES, INCLUDING DECREASED VISIBLE REFLECTANCE AND INCREASED NIR OVER THE FIRST 6 MONTHS, WHICH RELATE TO INCREASING CHLOROPHYLL CONTENT AND THE DEVELOPMENT OF THE MESOPHYLL, RESPECTIVELY [13] AS LEAVES AGED FURTHER, COLONIZATION BY EPIPHYLLS (FUNGI, LEAFY LIVERWORTS, LICHENS, ALGAE, BACTERIA) CAUSED FURTHER SPECTRAL CHANGES IN THE LEAVES OLDER EPIPHYLL-LADEN LEAVES REFLECTED AND TRANSMITTED LESS LIGHT AND ABSORBED MORE, ESPECIALLY IN THE NEAR-INFRARED REGION INTRASPECIFIC DIFFERENCES IN LEAF SPECTRA OF TROPICAL FOREST SPECIES—IN THIS CASE, ACROSS MULTIPLE SITES AND MULTIPLE SEASONS—WERE ALSO FOUND BY CASTRO-ESAU ET AL [12] CLASSIFICATION ACROSS EITHER BOUNDARY WAS CONSIDERED FUTILE DUE TO THE HIGH DEGREE OF VARIABILITY WITHIN SPECIES WITHIN THESE BOUNDARIES (I E , SPECTRA GATHERED FROM A SINGLE SITE WITHIN A SINGLE SEASON), HOWEVER, CLASSIFICATION OF TROPICAL PLANT SPECTRA WAS MUCH MORE PROMISING INDEED, EFFORTS AT CLASSIFYING TROPICAL TREES BASED ON THEIR LEAF SPECTRA HAVE BEEN LARGELY SUCCESSFUL, WITH OVERALL ACCURACIES CONSIS- TENTLY EXCEEDING 85% FOR UP TO 27 SPECIES [3,4,12,14] WHILE INTERSPECIFIC DIFFERENCES IN LEAF OPTICS CAN BE USED TO DISCRIMINATE BETWEEN SPECIES, DIFFERENCES IN LEAF OPTICS BETWEEN GROUPS OF SPECIES CAN PROVIDE INSIGHT INTO THE ECOLOGY AND PHYSIOLOGY OF THOSE GROUPS AT PARQUE NATURAL METROPOLITANO, PANAMA, DIFFERENCES IN LEAF SPECTRAL REFLECTANCE BETWEEN TWO LIFE FORMS, LIANAS AND TREES, WERE FOUND TO BE RELATED TO DIFFERENCES IN THEIR CHLOROPHYLL CONTENTS AND POTEN- TIALLY LINKED TO DIFFERENT STRATEGIES OF PHENOLOGY AND COPING WITH DROUGHT STRESS [15,16] THESE TWO GROUPS COULD BE READILY CLASSIFIED USING HYPERSPECTRAL LEAF REFLEC- TANCE DATA DURING THE DRY SEASON THE SAME COULD NOT BE SAID FOR LIANAS AND TREES AT A TROPICAL WET FOREST SITE, WHERE THE TWO LIFE FORMS COULD NOT BE RELIABLY DISTINGUISHED AND DID NOT MANIFEST DIFFERENCES IN CHLOROPHYLL CONTENT IN AN EARLIER STUDY [17], ANOTHER DIFFERENCE BETWEEN THE LEAF OPTICAL PROPERTIES OF LIANAS AND TREES AT PARQUE HYPERSPECTRAL REMOTE SENSING OF TROPICAL AND SUBTROPICAL FORESTS