Daniel A. Sims

Relationships between leaf pigment content and spectral reflectance across a wide range of species, leaf structures and developmental stages

Leaf pigment content can provide valuable insight into the physiological performance of leaves. Measurement of spectral reflectance provides a fast, nondestructive method for pigment estimation. A large number of spectral indices have been developed for estimation of leaf pigment content. However, in most cases these indices have been tested for only one or at most a few related species and thus it is not clear whether they can be applied across species with varying leaf structural characteristics. Our objective in this study was to develop spectral indices for prediction of leaf pigment content that are relatively insensitive to species and leaf structure variation and thus could be applied in larger scale remote-sensing studies without extensive calibration. We also quantified the degree of spectral interference between pigments when multiple pigments occur within the same leaf tissue. We found that previously published spectral indices provided relatively poor correlations with leaf chlorophyll content when applied across a wide range of species and plant functional types. Leaf surface reflectance appeared to be the most important factor in this variation. By developing a new spectral index that reduces the effect of differences in leaf surface reflectance, we were able to significantly improve the correlations with chlorophyll content. We also found that an index based on the first derivative of reflectance in the red edge region was insensitive to leaf structural variation. The presence of other pigments did not significantly affect estimation of chlorophyll from spectral reflectance. Previously published carotenoid and anthocyanin indices performed poorly across the whole data set. However, we found that the photochemical reflectance index (PRI, originally developed for estimation of xanthophyll cycle pigment changes) was related to carotenoid/chlorophyll ratios in green leaves. This result has important implications for the interpretation of PRI measured at both large and small scales. Our results demonstrate that spectral indices can be applied across species with widely varying leaf structure without the necessity for extensive calibration for each species. This opens up new possibilities for assessment of vegetation health in heterogeneous natural environments. D 2002 Elsevier Science Inc. All rights reserved.

Estimation of vegetation water content and photosynthetic tissue area from spectral reflectance: a comparison of indices based on liquid water and chlorophyll absorption features

Abstract Because of the high water content of vegetation, water absorption features dominate spectral reflectance of vegetation in the near-infrared region of the spectrum. In comparison to indices based on chlorophyll absorption features (such as the normalized difference vegetation index (NDVI)), indices based on the water absorption bands are expected to “see” more deeply into thick canopies and have a preferential sensitivity to thin as opposed to thick tissues. These predictions are based on the much lower absorption coefficients for water in the short wavelength water bands as compared to chlorophyll. Thus, the water bands may have advantages over NDVI for remote sensing of photosynthetic tissues. Previous studies have primarily related water band indices (WI) to leaf area index (LAI). Here we expand the definition of photosynthetic tissues to include thin green stems and fruits and measure a wide range of species to determine the influence of variable tissue morphologies and canopy structures on these relationships. As expected, indices based on reflectance in the water absorption bands in the near infrared were best correlated with the water content of thin tissues (less than 0.5-cm thickness). The choice of wavelength for a water index was much more important for thick than for thin canopies, and the best wavelengths were those where water absorptance was weak to moderate. We identified three wavelength regions (950–970, 1150–1260 and 1520–1540 nm) that produced the best overall correlations with water content. Comparison of these wavelength regions with the atmospheric “windows” where water vapor absorption is minimal suggests that the 1150–1260 and 1520–1540 nm regions would be the best wavelengths for satellite remote sensing of water content. We also developed and tested a new Canopy Structure Index (CSI) that combines the low absorptance water bands with the simple ratio vegetation index (SR) to produce an index with a wider range of sensitivity to photosynthetic tissue area at all canopy thicknesses. CSI was better than either WI or SR alone for prediction of total area of photosynthetic tissues. However, SR was best for prediction of leaf area when other green tissues were excluded. All of these relationships showed good generality across a wide range of species and functional types.

Optimum pixel size for hyperspectral studies of ecosystem function in southern California chaparral and grassland

Hyperspectral remotely sensed data are useful for studying ecosystem processes and patterns. However, spatial characterization of such remotely sensed images is needed to optimize sampling procedures and address scaling issues. We have investigated spatial scaling in ground-based and airborne hyperspectral data for canopy- to watershed-level ecosystem studies of southern California chaparral and grassland vegetation. Three optical reflectance indices, namely, Normalized Difference Vegetation Index (NDVI), Water Band Index (WBI) and Photochemical Reflectance Index (PRI) were used as indicators of biomass, plant water content and photosynthetic activity, respectively. Two geostatistical procedures, the semivariogram and local variance, were used for the spatial scaling analysis of these indices. The results indicate that a pixel size of 6 m or less would be optimal for studying functional properties of southern California grassland and chaparral ecosystems using hyperspectral remote sensing. These results provide a guide for selecting the spatial resolution of future airborne and satellite-based hyperspectral sensors.

Canopy quantum yield in a mesocosm study

Due to past limitations in experimental technology, canopy function has generally been inferred from leaf properties through scaling and/or indirect measurements. The development of a facility (EcoCELLs) at the Desert Research Institute has now made it possible to directly measure canopy gas exchange. In this experiment, sunflowers ( Helianthus annus) were planted in the EcoCELLs and grown under ambient (399mmol mol 1 ) and elevated (746mmol mol 1 )C O 2 concentrations. We continuously measured carbon flux during canopy development from which canopy quantum yield (C) was estimated. The results indicated that the total daily carbon flux was similar between elevated and ambient CO 2 treatments in the early stage of canopy development. After the canopy closed, carbon flux under elevated CO 2 averaged 53% higher than that under ambient CO2. Assimilation/incident irradiance (A/I) curves of leaves at different canopy positions were used to estimate leaf quantum yields (L), and A/I curves of canopies at late development stages were used to estimate C. Elevated CO2 enhanced L by 24%. There was little difference in L at different canopy positions, averaging 0.0542 at ambient CO2 and 0.0671 at elevated CO2. Canopy quantum yield (C) was higher by 32% at elevated than ambient CO2. It increased with canopy development and was strongly correlated with leaf area index (LAI) by C D 0.0094 LAI/(0.0829 C 0.1137 LAI) at ambient CO2 and C D 0.01382 LAI/(0.1129 C 0.1224 LAI) at elevated CO2. In addition, the curvilinear relationship between radiation and canopy carbon fluxes suggests that canopy radiation use efficiency (CRUE) varied with radiation availability. The variability in C and CRUE with canopy development and light levels warrants further research on the notion drawn from earlier work that CRUE in non-stressed conditions is relatively constant. ©2000 Elsevier Science B.V. All rights reserved.

Effects of gradual versus step increases in carbon dioxide on Plantago photosynthesis and growth in a microcosm study

This study investigated the effects of a gradual versus step increases in carbon dioxide (CO2) on plant photosynthesis and growth at two nitrogen (N) levels. Plantago lanceolata were grown for 80 days and then treated with the ambient CO2 (as the control), gradual CO2 increase and step CO2 increase as well as low and high N additions for 70 days. While [CO2] were kept at constant 350 and 700 mol mol − 1 for the ambient and step CO2 treatments, respectively, [CO2] in the gradual CO2 treatment was raised by 5 mol mol − 1 day − 1 , beginning at 350 mol mol −1 and reaching 700 mol mol − 1 by the end of experiment. The step CO2 treatment immediately resulted in an approximate 50% increase in leaf photosynthetic carbon fixation at both the low and high N additions, leading to a 20–24% decrease in leaf N concentration. The CO2-induced nitrogen stress, in return, resulted in partial photosynthetic downregulation since the third week at the low N level and the fourth week at the high N level after treatments. In comparison, the gradual CO2 treatment induced a gradual increase in photosynthetic carbon fixation, leading to less reduction in leaf N concentration. In comparison to the ambient CO2, both the gradual and step CO2 increases resulted in decreases in specific leaf area, leaf N concentration but an increase in plant biomass. Responses of plant shoot:root ratio to CO2 treatments varied with N supply. It decreased with low N supply and increased with high N supply under the gradual and step CO2 treatments relative to that under the ambient CO2. Degrees of those changes in physiological and growth parameters were usually larger under the step than the gradual CO2 treatments, largely due to different photosynthetic C influxes under the two CO2 treatments.