Showing papers on "Leaf area index published in 2002"

Modeling vegetation as a dynamic component in soil‐vegetation‐atmosphere transfer schemes and hydrological models

Abstract: [1] Vegetation affects the climate by modifying the energy, momentum, and hydrologic balance of the land surface. Soil-vegetation-atmosphere transfer (SVAT) schemes explicitly consider the role of vegetation in affecting water and energy balance by taking into account its physiological properties, in particular, leaf area index (LAI) and stomatal conductance. These two physiological properties are also the basis of evapotranspiration parameterizations in physically based hydrological models. However, most current SVAT schemes and hydrological models do not parameterize vegetation as a dynamic component. The seasonal evolution of LAI is prescribed, and monthly LAI values are kept constant year after year. The effect of CO2 on the structure and physiological properties of vegetation is also neglected, which is likely to be important in transient climate simulations with increasing CO2 concentration and for hydrological models that are used to study climate change impact. The net carbon uptake by vegetation, which is the difference between photosynthesis and respiration, is allocated to leaves, stems, and roots. Carbon allocation to leaves determines their biomass and LAI. The timing of bud burst, leaf senescence, and leaf abscission (i.e., the phenology) determines the length of the growing season. Together, photosynthesis, respiration, allocation, and phenology, which are all strongly dependent on environmental conditions, make vegetation a dynamic component. This paper (1) familiarizes the reader with the basic physical processes associated with the functioning of the terrestrial biosphere using simple nonbiogeochemical terminology, (2) summarizes the range of parameterizations used to model these processes in the current generation of process-based vegetation and plant growth models and discusses their suitability for inclusion in SVAT schemes and hydrological models, and (3) illustrates the manner in which the coupling of vegetation models and SVAT schemes/hydrological models may be accomplished.

How the environment, canopy structure and canopy physiological functioning influence carbon, water and energy fluxes of a temperate broad-leaved deciduous forest--an assessment with the biophysical model CANOAK.

Abstract: This paper focuses on how canopy structure, its physiological functioning and the environment interact to control and drive the exchange of carbon dioxide (CO2) and water vapor between a temperate forest stand and the atmosphere. First, we present an overview of how temporal and spatial variations in canopy structure (e.g., leaf area index, species, leaf inclination angles, leaf clumping) and physiological functioning (e.g., maximal stomatal conductance, photosynthetic capacity) modulate CO2 and water vapor fluxes. Then, with the biophysical model CANOAK, we quantify the effects of leaf dimension and thickness, vertical variations in leaf area and photosynthetic capacity, leaf clumping, leaf inclination angles, photosynthetic capacity, stomatal conductance and weather on the annual sums of CO2, water vapor and sensible heat exchange. Finally, we discuss how much detail is needed in a model to predict fluxes of CO2 and water vapor with acceptable fidelity.

Relationship between Growth Traits and Spectral Vegetation Indices in Durum Wheat

Abstract: Future wheat yield improvements may be gained by increasing total dry matter (TDM) production. Vegetation indices (VI) based on spectral reflectance ratios have been proposed as an appropriate method to assess TDM and leaf area index (LAI) in wheat. This study was undertaken to determine whether VI could accurately identify TDM and LAI in durum wheat [Triticum turgidum var. durum (Desf.) Bowden [= T. turgidum subsp. durum (Desf.) Husn.]) and serve as indirect selection criteria in breeding programs. Total dry matter and LAI were determined from destructive sampling from booting to milk-grain in seven field experiments conducted under Mediterranean conditions. Each experiment included one of two sets of 20 or 25 genotypes. Field reflectance values were collected using a portable field spectroradiometer. Two VI, the normalized difference vegetation index (NDVI) and the simple ratio (SR), were derived from spectral measurements and their predictive value for TDM and LAI was evaluated. The best stages for growth trait appraisal were Stages 65 and 75 of the Zadoks scale. The power of VI for assessing TDM was lower than their predictive value for LAI. The suitability of VI for the assessment of growth traits depended on the range of variability existing within the experimental data. Vegetation indices accurately tracked changes in LAI when data were analyzed across a broad range of different growth stages, environments, and genotypes. However, their value as indirect genotypic selection criteria for TDM or LAI was limited, since they lacked predictive ability for specific environment/ growth stage combinations.

Estimating leaf nitrogen concentration in ryegrass ( Lolium spp.) pasture using the chlorophyll red-edge: theoretical modelling and experimental observations

Abstract: Chlorophyll red-edge descriptors have been used to estimate leaf nitro- gen concentration in ryegrass ( L olium spp.) pasture. Two-layer model calculations have been used to predict the ine uence of chlorophyll content and Leaf Area Index (LAI) on the shape and location of the peaks observed in the derivative spectra of a ryegrass canopy. The complex structure of the resulting derivative spectra pre- cluded extracting red-edge wavelengths by e tting inverted Gaussian curves to ree ectance proe les. Fitting a combination of three sigmoid curves to the calculated ree ectance spectra provided a better representation of subsequent derivative spectra. The derivative spectra in the vicinity of the chlorophyll red-edge is predicted to contain two peaks ( ~705 and ~725nm), which on increasing the canopy LAI is generally found to shift to longer wavelengths. However, for a canopy containing leaves of low chlorophyll content and LAI >5, the wavelength of the e rst peak becomes insensitive to changes in LAI. The same phenomenon is predicted for high-chlorophyll leaves of LAI >10. The role of multiple scattering, primarily due to increased leaf transmittance at higher wavelengths, has also been verie ed. In subsequent experiments, the predicted shape of the derivative spectra was observed and the use of three sigmoid curves to better represent this shape verie ed. Changes in the descriptors used to describe the chlorophyll red-edge were observed to explain 60% and 65% of the variance of leaf nitrogen concentration and total leaf nitrogen content, respectively. The resulting regression equation was found to predict leaf nitrogen concentration, in the range of 2- 5.5%, with a standard error of prediction (SEP) of 0.4%. The confounding ine uence of canopy biomass on the red-edge determination of leaf nitrogen concentration was found to be signie cantly less at higher canopy biomass, cone rming both theoretical predictions and the potential of using the chlorophyll red-edge as a biomass-independent means of estimating leaf chlorophyll, and hence nitrogen, concentration in high-LAI ryegrass pastures.

The effect of three rootstocks on water use, canopy conductance and hydraulic parameters of apple trees and predicting canopy from hydraulic conductance

Abstract: This study investigated the effect of the dwarfing M9, semi-dwarfing MM106 and local Hashabi rootstocks on the water use, canopy conductance (Gc) and hydraulic conductance (k) of apple orchards with the same scion, Golden Delicious. The average summer leaf area index (LAI) was 2·4, 2·7 and 1·7 for M9, MM106 and Hashabi, respectively. Irrigation in 1997 was less than water use until June, and excessive afterwards. In 1998, irrigation was doubled, and was excessive throughout the season. Sap flow (J) in June–August 1998 totalled 476, 682 and 606 mm (or 0·60, 0·86 and 0·76 of class A pan evaporation) for M9, MM106 and Hashabi, respectively. Maximum sap velocity in the three rootstocks (approximately 70 cm h−1) occurred in the outer 30–60% of the stem, and its decrease with depth was greater in M9 than in the other rootstocks. Midday Gc during both summers was least for M9, intermediate for Hashabi and greatest for MM106. The k value of M9 and MM106 for the soil to stem, stem to leaves and soil to leaves pathways were determined from daily courses of water potential of leaves, Ψl, stem, Ψstem and J. Specific k (ks, i.e. relative to stem sapwood area) did not significantly differ between the two rootstocks for soil to stem and soil to leaf pathways, but leaf specific k (kl) was greater for MM106 soil to stem (71% greater) and soil to leaf (63%) pathways, respectively. The inverse slopes of the relationships between midday canopy resistance (Rc) and vapour pressure deficit (D) for MM106 was 1·75 of that for M9, and the ratio of their Huber values, i.e. the ratio of sapwood to leaf area, was 1·6. These findings indicate that differences in water use are attributable to differences in kl, and not to differences in wood properties (ks). Application of a model relating Rc to orchard area specific k (kg) showed that the slope of the relationships between midday Rc and D for the 1998 data could be predicted using common values of ks (0·134 kg m−2 s−1 MPa−1) and midday Ψl (−1·34 MPa) for the three rootstocks. The implications of these findings, and the similarities in the differences between rootstocks of Gc, kl, kg and Huber values, are discussed with respect to rootstock water use and irrigation.

Application of Geostatistics to Characterize Leaf Area Index (LAI) from Flux Tower to Landscape Scales Using a Cyclic Sampling Design

Abstract: Accurate characterization of leaf area index (LAI) is required to quantify the exchange of energy, water, and carbon between terrestrial ecosystems and the atmosphere. The objective of this study was to use a cyclic sampling design to compare the spatial patterns of LAI of the dominant terrestrial ecosystems that comprised the area around the 447-m WLEF television tower, equipped with an eddy flux system, near Park Falls, Wisconsin, USA. A second objective was to compare the efficiency of cyclic, random, and uniform sampling designs in terms of the precision of spatial information derived per unit sampling effort. The vegetation surrounding the tower was comprised (more than 80%) of four major forest cover types: forested wetlands, upland aspen forests, upland northern hardwood forests, and upland pine forests, and a fifth, nonforested cover type, grass (open meadow). LAI differed significantly among the five cover types and averaged 3.45, 3.57, 3.82, 3.99, and 1.14 for northern hardwoods, aspen, forested wetlands, upland conifers, and grass, respectively. The cyclic sampling design maximized information about the variance of vegetation characteristics of the heterogeneous landscape and decreased by 60% the number of plots needed to obtain the same confidence interval width using a random sampling design. The range of spatial autocorrelation for LAI was 147 m, but it was decreased to 117 m when vegetation cover was included as a covariate. The cyclic sampling design has several important advantages over other sampling designs. The cyclic sampling design increased the sampling efficiency by optimizing the placement of plots so they were distributed more efficiently for geostatistical analyses such as semi-variograms, correlograms, and spatial regression and can incorporate covariates (for example, vegetation cover, soil properties, and so on) that may explain the sources of spatial patterns. The cyclic sampling design was used to derive a spatial map of LAI and the average LAI for the 3 × 2 km area centered on the flux tower was 3.51 ± 0.89 (with a minimum of 0 and a maximum of 6.35). Airborne and satellite reflectance data have also been used to characterize LAI, but in this region, and many other forests of the world, remotely sensed vegetation indexes saturate in forests with an LAI greater than 3–5. The cyclic sampling design also provides a general ecological sampling approach that can be used at multiple scales.

Leaf area renewal, root retention and carbohydrate reserves in a clonal tree species following above-ground disturbance

Abstract: Summary 1 Removal of the above-ground portion of clonally regenerating trees results in a massive imbalance in the ratio of root to leaf area. We investigated the resilience of clones following above-ground disturbance in terms of root carbohydrates, leaf area renewal and root retention. 2 In a 2 × 2 factorial experiment, 40 Populus tremuloides saplings were cut at times of high (late fall) or low (spring after leaf flush) root carbohydrate reserves. Leaf area renewal was manipulated by allowing either only one or all root suckers to re-grow. 3 Root starch concentrations were 10 times higher at the time of cutting in the fall compared with the spring, whereas sugar concentrations were only 10% higher. When saplings were cut in fall and all suckers were allowed to develop (FA treatment), suckers were taller, and had more biomass and leaf area and higher leaf area ratio than all other treatments. In particular, leaf area and leaf area ratio recovered within a year to near pre-treatment levels compared with at least a 50% reduction in the other treatments. 4 At the end of the first season after cutting FA saplings also had the greatest root mass and lowest amount of dead root mass, and root starch concentrations of these saplings had returned to pre-treatment values, compared with a 20% recovery for spring-cut saplings with a single sucker. Root mass and root starch concentrations were correlated with leaf mass in all treatments. 5 Adequate carbohydrate reserves at the time of disturbance and rapid redevelopment of leaf area by extensive sprouting seem critical for resilience of the clone and allow for the retention of the clonal root system and a rapid rebuilding of carbohydrate reserves. Clones with poor sucker and leaf area development showed extensive root mortality and reduced carbohydrate reserves and their prospects for growth in the following year were comparatively poor.

Global mapping of vegetation parameters from POLDER multiangular measurements for studies of surface-atmosphere interactions: A pragmatic method and its validation

Abstract: [1] This paper presents a pragmatic method to produce global maps of vegetation parameters, which offer essential data for weather forecast and climate modeling. The crucial variables are leaf area index (LAI), fractional vegetation cover (FVC), and fraction of absorbed photosynthetically active radiation (fAPAR). The approach relies on the use of spectral and directional vegetation indices simulated by a bidirectional reflectance model and calibrated against sets of satellite data. The model belongs to the kernel-driven category, and the coefficients obtained, as the result of linear inversion, are the basis of the proposed method. The strategy presented relies upon the existence of suitable angular measurements to derive each biophysical parameter. An application is shown with the global POLDER/ADEOS-I database. Special attention is given here to the future production of LAI and fAPAR since the albedo is a product already disseminated by the POLDER production center. Terrestrial ecosystems show a high level of aggregation and, in practice, only effective LAI can be measured. Therefore a correction factor, namely the clumping index, must be applied to help resolve the scaling issue. Clumping corrections are performed biome by biome, using empirical equations where it appears that LAI assessments for boreal and tropical forests would otherwise be significantly inaccurate. However, the effect of clumping will be less on FVC and fAPAR. The relevance of the proposed method is demonstrated through a comparison of POLDER-derived LAI values with a varied set of ground LAI measurements, including their coherence with the corresponding fAPAR.

Reliability of the estimation of vegetation characteristics by inversion of three canopy reflectance models on airborne POLDER data

Abstract: The Bidirectional Reflectance Distribution Function (BRDF) of several plant canopies was extensively sampled by the POLDER air- borne instrument in the Alpilles-ReSeDA campaign. The 16 flights carried out over the Alpilles test site from January to October 1997 cover all the plant growth stages. Estimation of biophysical variables was undertaken by inversion of three one-dimensional radiative transfer models, SAIL, KUUSK and IAPI, coupled with the PROSPECT leaf optical properties model, in order to fully utilize both the directional and the spectral information of the images. This study mainly focuses on the capability of model inversion to retrieve the leaf area index (LAI) of wheat, maize, sunflower and alfalfa crops, for which ground validation was available. In order to evaluate the quality of inversions and to map the LAI or the chlorophyll content Cab, the associated estimation errors are determined for these two biophysical variables.

Age-related effects on leaf area/sapwood area relationships, canopy transpiration and carbon gain of Norway spruce stands (Picea abies) in the Fichtelgebirge, Germany.

Abstract: Stand age is an important structural determinant of canopy transpiration (E(c)) and carbon gain. Another more functional parameter of forest structure is the leaf area/sapwood area relationship, A(L)/A(S), which changes with site conditions and has been used to estimate leaf area index of forest canopies. The interpretation of age-related changes in A(L)/A(S) and the question of how A(L)/A(S) is related to forest functions are of current interest because they may help to explain forest canopy fluxes and growth. We conducted studies in mature stands of Picea abies (L.) Karst. varying in age from 40 to 140 years, in tree density from 1680 to 320 trees ha(-1), and in tree height from 15 to 30 m. Structural parameters were measured by biomass harvests of individual trees and stand biometry. We estimated E(c) from scaled-up xylem sap flux of trees, and canopy-level fluxes were predicted by a three-dimensional microclimate and gas exchange model (STANDFLUX). In contrast to pine species, A(L)/A(S) of P. abies increased with stand age from 0.26 to 0.48 m(2) cm(-2). Agreement between E(c) derived from scaled-up sap flux and modeled canopy transpiration was obtained with the same parameterization of needle physiology independent of stand age. Reduced light interception per leaf area and, as a consequence, reductions in net canopy photosynthesis (A(c)), canopy conductance (g(c)) and E(c) were predicted by the model in the older stands. Seasonal water-use efficiency (WUE = A(c)/E(c)), derived from scaled-up sap flux and stem growth as well as from model simulation, declined with increasing A(L)/A(S) and stand age. Based on the different behavior of age-related A(L)/A(S) in Norway spruce stands compared with other tree species, we conclude that WUE rather than A(L)/A(S) could represent a common age-related property of all species. We also conclude that, in addition to hydraulic limitations reducing carbon gain in old stands, a functional change in A(L)/A(S) that is related to reduced light interception per leaf area provides another potential explanation for reduced carbon gain in old stands of P. abies, even when hydraulic constraints increase in response to changes in canopy architecture and aging.

Using Satellite and Field Data with Crop Growth Modeling to Monitor and Estimate Corn Yield in Mexico

Abstract: The large-scale monitoring and estimation of crop yield is essential for food security in Mexico. This study developed and validated a method of monitoring and estimating corn (Zea mays L.) yield by means of satellite and ground-based data. In autumn-winter 1999 and spring-summer 2000, eight locations under irrigated and nonirrigated conditions in corn valleys of Mexico were localized by Global Positioning Systems (GPS) and were sampled every 15 d. Photosynthetic active radiation (PAR), leaf area index (LAI), crop development stage (DVS), planting dates, and grain yield data were gathered from the field. The normalized difference vegetation index (NDVI) was derived from NOAA-Advanced Very High Resolution Radiometer (AVHRR) images. A growth model was developed to integrate satellite and ground data. Net primary productivity (NPP) was estimated using PAR and NDVI. Dry weight increase (kg ha(-1) d(-1)) was determined considering NPP and the partitioning factor. Results indicated that the model accounts for 89% of the variability in yields under irrigated conditions and 76% under nonirrigated conditions. The methodology seems advantageous in large-scale monitoring and assessment of corn yield.

Leaf area dynamics of a boreal black spruce fire chronosequence.

Abstract: Specific leaf area (SLA) and leaf area index (LAI) were estimated using site-specific allometric equations for a boreal black spruce (Picea mariana (Mill.) BSP) fire chronosequence in northern Manitoba, Canada. Stands ranged from 3 to 131 years in age and had soils that were categorized as well or poorly drained. The goals of the study were to: (i) measure SLA for the dominant tree and understory species of boreal black spruce-dominated stands, and examine the effect of various biophysical conditions on SLA; and (ii) examine leaf area dynamics of both understory and overstory for well- and poorly drained stands in the chronosequence. Overall, average SLA values for black spruce (n = 215), jack pine (Pinus banksiana Lamb., n = 72) and trembling aspen (Populus tremuloides Michx., n = 27) were 5.82 +/- 1.91, 5.76 +/- 1.91 and 17.42 +/- 2.21 m2 x kg-1, respectively. Foliage age, stand age, vertical position in the canopy and soil drainage had significant effects on SLA. Black spruce dominated overstory LAI in the older stands. Well-drained stands had significantly higher overstory LAI (P 40%) of total leaf area in all stands except the oldest.

Correction to the plant canopy gap-size analysis theory used by the Tracing Radiation and Architecture of Canopies instrument

Abstract: A plant canopy gap-size analyzer, the Tracing Radiation and Architecture of Canopies (TRAC), developed by Chen and Cihlar [Appl Opt. 34, 6211(1995)] and commercialized and by 3rd Wave Engineering (Nepean, Canada), has been used around the world to quantify the fraction of photosynthetically activeradiation absorbed by plant canopies, the leaf area index (LAI), and canopy architectural parameters. The TRAC is walked under a canopy along transects to measure sunflecks that are converted into a gap-size distribution. A numerical gap-removal technique is performed to remove gaps that are not theoretically possible in a random canopy. The resulting reduced gap-size distribution is used to quantify the heterogeneity of the canopy and to improve LAI measurements. It is explicitly shown here that the original derivation of the clumping index was missing a normalization factor. For a very clumped canopy with a large gap faction, the resulting LAI can be more than 100% smaller than previously estimated. A test case is used to demonstrate that the new clumping index derivation allows a more accurate change of LAI to be measured.

Analysis of a multiyear global vegetation leaf area index data set

Abstract: [1] The analysis of a global data set of monthly leaf area index (LAI), derived from satellite observations of normalized difference vegetation index (NDVI) for the period July 1981 to September 1994, is discussed in this paper. Validation of this retroactive, coarse resolution (8 km) global multiyear data set is a challenging task because repetitive ground measurements from all representative vegetation types are not available. Therefore the magnitudes and interannual variations in the derived LAI fields were assessed as follows. First, the use of a NDVI-based algorithm, as opposed to a more physically based approach, is estimated to result in relative errors in LAI of about 10–20%, which is comparable to the mean uncertainty of AVHRR NDVI data. Second, the satellite LAI values compared reasonably well to ground measurements from three field campaigns. Third, comparison with an existing multiyear LAI data set showed qualitative agreement with regards to interannual variability, although the LAI values of the earlier data were consistently larger than those derived here. Fourth, interannual variations in LAI were evaluated through correlations with climate data sets, e.g., sea surface temperatures and precipitation in tropical semiarid regions known for ENSO impacts, temperature dependence of vegetation growth, and therefore LAI, in the northern latitudes. The general consistency between these independent data sets imbues confidence in the LAI data set, at least for use in large-scale modeling studies. Finally, improvements in near-surface climate simulation are documented in a companion article when satellite LAI values were used in a global climate model. The data set is available to the community via our Web server (http://cybele.bu.edu).

Leaf senescence and resorption as mechanisms of maximizing photosynthetic production during canopy development at N limitation

Abstract: Summary 1A canopy N distribution model optimizing photosynthesis was combined with mechanisms for senescence and N resorption to predict canopy leaf area index (LAI) development as a function of canopy nutrient content, Nc 2Shedding of leaves at the bottom of the canopy was initiated when it increased canopy photosynthesis, through resorption and redistribution of N from lost leaves The amount of N resorbed was modelled as a fraction (Rf) of the N in the leaf prior to senescence 3For a fixed Nc, the LAI at which leaf shedding was initiated was calculated for different Rfs for canopies of Amaranthus cruentus, Glycine max, Oryza sativa and Sorghum bicolor 4Predicted LAIs exceeded optimal LAIs estimated without leaf shedding and N resorption LAI increased with increasing Rf For all four species, the model closely predicted the observed LAIs with Rf = 0·7 5Area-based resorption efficiency increased and litter N concentration decreased with increasing LAI and average canopy N concentration (mol N m−2 leaf) A reduction of Rf, eg in response to increased soil N availability, decreases resorption efficiency 6The model provides a mechanistic basis for interpreting plant nutrient–resorption–LAI relations