Gregory A. Carter

Leaf optical properties in higher plants: linking spectral characteristics to stress and chlorophyll concentration

A number of studies have linked responses in leaf spectral reflectance, transmittance, or absorptance to physiological stress. A variety of stressors including dehydration, flooding, freezing, ozone, herbicides, competition, disease, insects, and deficiencies in ectomycorrhizal development and N fertilization have been imposed on species ranging from grasses to conifers and deciduous trees. In all cases, the maximum difference in reflectance within the 400-850 nm wavelength range between control and stressed states occurred as a reflectance increase at wavelengths near 700 nm. In studies that included transmittance and absorptance as well as reflectance, maximum differences occurred as increases and decreases, respectively, near 700 nm. This common optical response to stress could be simulated closely by varying the chlorophyll concentration of model leaves (fiberglass filter pads) and by the natural variability in leaf chlorophyll concentrations in senescent leaves of five species. The optical response to stress near 700 nm, as well as corresponding changes in reflectance that occur in the green-yellow spectrum, can be explained by the general tendency of stress to reduce leaf chlorophyll concentration.

Ratios of leaf reflectances in narrow wavebands as indicators of plant stress

Abstract Ratios of leaf reflectances that were measured within narrow wavebands (2nm) were evaluated as indicators of plant stress. Wavebands used in ratio computation were based on earlier studies that determined the wavelength regions in which reflectance was most affected by 8 stress agents among 6 plant species. Several ratios, such as reflectance at 695 nm divided by reflectance at 670 nm (R695/R670), were affected by some but not all stress agents. However, R695/R420, R605/R760, R695/R760 and R710/R760 were significantly greater (p≤0·05) in stressed compared with non-stressed leaves for all stress agents. The ratios that most strongly indicated plant stress were reflectance at 695 nm divided by reflectance at 420 nm or 760 nm.

Early detection of plant stress by digital imaging within narrow stress-sensitive wavebands

Digital images of soybean canopies [Glycine max (L.) Merrill] were obtained within selected narrow wavebands (6–10 nm bandwidths) to determine their capability for early detection of plant stress. Images and physiological measurements of stress were acquired 2 days, 4 days, and 7 days following application of control, drought, and herbicide [(3,4-dichlorophenyl)-1, 1-dimethylurea, or DCMU] treatments. As a result of frequent rainfall, drought stress never occurred. However, exposure to herbicide rapidly induced plant stress. By day 4, the ratio of variable to maximum leaf fluorescence (Fv/Fm) decreased and leaf water potentials (ψw) increased in the herbicide treated soybean, indicating damage to the photosynthetic apparatus and stomatal closure. Also, Munsell leaf color had increased from approximately 5GY 4.6/5.7 to a lighter green-yellow value. Canopy reflectances at 670 nm, 694 nm, and in the 410–740 nm band (Rvis), as well as reflectance at 694 nm divided by reflectance at 760 nm (R694/R760), detected stress simultaneously with the physiological measurements and increased consistently with stress through day 7. Reflectances at 420 nm and 600 nm, together with R600/R760 and Rvis/R760, did not increase until leaves were yellow or brown and wilted and canopies had begun to collapse on day 7. None of the reflectance or reflectance ratio images detected stress prior to visible color changes. This was attributed primarily to the rapid inducement of chlorosis by the herbicide. Reflectance in narrow wavebands within the 690–700 nm region and its ratio with near-infrared reflectance should provide earlier detection of stress-induced chlorosis compared with broad band systems or narrow bands located at lesser wavelengths.

Remotely Sensed Spectral Heterogeneity As a Proxy of Species Diversity: Recent Advances and Open Challenges

Abstract Environmental heterogeneity is considered to be one of the main factors associated with biodiversity given that areas with highly heterogeneous environments can host more species due to their higher number of available niches. In this view, spatial variability extracted from remotely sensed images has been used as a proxy of species diversity, as these data provide an inexpensive means of deriving environmental information for large areas in a consistent and regular manner. The aim of this review is to provide an overview of the state of the art in the use of spectral heterogeneity for estimating species diversity. We will examine a number of issues related to this theme, dealing with: i) the main sensors used for biodiversity monitoring, ii) scale matching problems between remotely sensed and field diversity data, iii) spectral heterogeneity measurement techniques, iv) types of species taxonomic diversity measures and how they influence the relationship between spectral and species diversity, v) spectral versus genetic diversity, and vi) modeling procedures for relating spectral and species diversity. Our review suggests that remotely sensed spectral heterogeneity information provides a crucial baseline for rapid estimation or prediction of biodiversity attributes and hotspots in space and time.

Variability in leaf optical properties among 26 species from a broad range of habitats.

Leaves from 26 species with growth forms from annual herbs to trees were collected from open, intermediate, and shaded understory habitats in Mississippi and Kansas, USA. Leaf optical properties including reflectance, transmittance, and absorptance in visible and near infrared (NIR) wavelengths were measured along with leaf thickness and specific leaf mass (SLM). These leaf properties and internal light scattering have been reported to vary with light availability in studies that have focused on a limited number of species. Our objective was to determine whether these patterns in leaf optics and light availability were consistent when a greater number of species were evaluated. Leaf thickness and SLM varied by tenfold among species sampled, but within-habitat variance was high. Although there was a strong trend toward thicker leaves in open habitats, only SLM was significantly greater in open vs. understory habitats. In contrast, leaf optical properties were strikingly similar among habitats. Reflectance and reflectance/transmittance in the NIR were used to estimate internal light scattering and there were strong relationships (r1 > 0.65) between these optical properties and leaf thickness. We concluded that leaf thickness, which did not vary consistently among habitats, was the best predictor of NIR reflectance and internal light scattering. However, because carbon allocation to leaves was lower in understory species (low SLM) yet gross optical properties were similar among all habitats, the energy investment by shade leaves required to achieve optical equivalence with sun leaves was lower. Differences in leaf longevity and growth form within a habitat may help explain the lack of consistent patterns in leaf optics as the number of species sampled increases.

Reflectance wavebands and indices for remote estimation of photosynthesis and stomatal conductance in pine canopies

Abstract A field experiment determined the reflectance wavebands and indices which corresponded most strongly with photosynthetic capacity in a mixed stand of loblolly pine (Pinus taeda L.) and slash pine (P. elliottii Engelm. var. elliottii). The 5-year-old pines ranged in height from 3 m to 6 m and formed an optically dense canopy. Variation in photosynthetic capacity was amplified by soil application of the photosystem II herbicides diuron and bromacil to three of six experimental plots. Field measurements began on 23 August 1994 and continued through 20 December 1994. Canopy reflectance and leaf physiological data were acquired during mid to late morning for the sides of trees that generally received full sunlight. Net CO 2 assimilation rate first regressed significantly with reflectance on 5 October at wavelengths near 700 nm. In the 5 October to 2 December period, assimilation rates approximated photosynthetic capacity. When data were combined over this period, the ratio of reflectance at 701±2 nm with reflectance at 820±2 nm, or a normalized difference vegetation index (NDVI) computed from these values, regressed more strongly with photosynthetic capacity than first derivatives of spectral reflectance or wavelength at the red edge inflection point. The narrow band NDVI accounted for 17–29% more variability in the data than NDVI that were based on simulated TM, MSS, AVHRR, or SPOT bands. As a result of its linear relationship with assimilation rate, stomatal conductance to water vapor also regressed strongly with the narrow band ratio. These results are explained by the high sensitivity of reflectance near 700 nm to leaf chlorophyll content. The use of a narrow band centered near 700 nm along with a narrow or broad near-infrared band in vegetation indices should provide increased accuracy in estimates of photosynthetic capacity and corresponding conductance for optically dense canopies. Evaluation of the influence of leaf area index on these relationships will require further study.Published by Elsevier Science Inc., 1998

Hurricane Degradation—Barrier Development Cycles, Northeastern Gulf of Mexico: Landform Evolution and Island Chain History

Abstract Before its western sector was stranded and/or buried ca. 4.0–3.8 ka BP (3.9–3.7 ka 14C), the Mississippi–Alabama chain of regressive barrier islands extended well into present southeastern Louisiana. Westward-directed net littoral drift, ebb-deltas, and microtidal inlet bypassing were instrumental in the formation of elongated, narrow, sandy barrier platform sectors on which these islands, mostly of strandplain topography, have originally emerged. The development of sizable subtidal–intertidal berm basins, ringed by swash and foreshore berm ridges that emerged after storms, then filled by storm-mobilized sand, has aided posthurricane recovery. These processes are linked to discrete stages in aggradational barrier genesis. Increasingly frequent and destructive cyclones reduced island areas to laterally extensive subtidal barrier platform intervals. Enhanced overwash across lengthened platform sectors reduced drift volumes and consequently island progradation. Deepened ship channels facilitated sand loss from littoral drift to offshore seafloor areas. This and the apparent reduction in the longshore sand flux point to natural and human interference with the drift supply. Comparisons of charts, aerial photos, and satellite images provide a quantitative record for the dynamic changes that occurred. Abrupt widening of Petit Bois Pass in 1916 and periodic island diminution and attrition episodes during at least nine hurricanes since had a decisive impact on all the islands. Breaches across low-lying central and eastern (updrift) sectors contributed to long-term island reduction. Starting with Hurricane Betsy (1965), more frequent and destructive tropical cyclones resulted in accelerated island diminution. Damage from wind, salt toxicity, and overwash, combined with shore retreat, seriously impaired the vegetation of several islands in 2005. Because of extensive low and narrow island sectors, Ship and Petit Bois were the most vulnerable. Between 1848–49 and 2005, they suffered 66% and 52% area loss, respectively. Despite recurring but limited post-storm recovery, East Ship now may be approaching extinction. Despite its higher relict beach ridges, secondary dunes, and historically substantial downdrift progradation, even Horn Island has undergone considerable attrition (23%).

Detection of solar-excited chlorophyll a fluorescence and leaf photosynthetic capacity using a Fraunhofer line radiometer

Abstract A Fraunhofer Line Radiometer (FLR) measured solar-excited chlorophyll a fluorescence ( F ) as an indicator of photosynthetic capacity in leaves of Washingtonia robusta (palm) and Vitis vinifera (grape). Under clear skies, the FLR utilized the Fraunhofer line-depth principle (FLDP) to detect F within the O 2 absorption band centered at 687 nm wavelength. On 20 April, the morning after soils were treated with the photosystem II herbicide DCMU F increased significantly ( p = 0.05) in palm, while leaf reflectance at 687 nm ( R ) did not change. By 21 April, leaf herbicide concentrations were lethal. Increased F and R in palm corresponded with net photosynthetic rates near zero. Photosynthesis in grape also decreased greatly, but F and R remained similar to the controls. Results for palm indicate that F could be measured while leaves remained exposed to full sunlight, and support earlier work to indicate a potential role for the FLDP in remotely sensing F and photosynthetic capacity.

Remote Sensing and Mapping of Tamarisk along the Colorado River, USA: A Comparative Use of Summer-Acquired Hyperion, Thematic Mapper and QuickBird Data

Tamarisk (Tamarix spp., saltcedar) is a well-known invasive phreatophyte introduced from Asia to North America in the 1800s. This report compares the efficacy of Landsat 5 Thematic Mapper (TM5), QuickBird (QB) and EO-1 Hyperion data in discriminating tamarisk populations near De Beque, Colorado, USA. As a result of highly correlated reflectance among the spectral bands provided by each sensor, relatively standard image analysis methods were employed. Multispectral data at high spatial resolution (QB, 2.5 m Ground Spatial Distance or GSD) proved more effective in tamarisk delineation than either multispectral (TM5) or hyperspectral (Hyperion) data at moderate spatial resolution (30 m GSD).

Going with the flow or against the grain? The promise of vegetation for protecting beaches, dunes, and barrier islands from erosion

Coastlines have traditionally been engineered to maintain structural stability and to protect property from storm-related damage, but their ability to endure will be challenged over the next century. The use of vegetation to reduce erosion on ocean-facing mainland and barrier island shorelines – including the sand dunes and beaches on these islands – could be part of a more flexible strategy. Although there is growing enthusiasm for using vegetation for this purpose, empirical data supporting this approach are lacking. Here, we identify the potential roles of vegetation in coastal protection, including the capture of sediment, ecological succession, and the building of islands, dunes, and beaches; the development of wave-resistant soils by increasing effective grain size and sedimentary cohesion; the ability of aboveground architecture to attenuate waves and impede through-flow; the capability of roots to bind sediments subjected to wave action; and the alteration of coastline resiliency by plant structur...