Publications (14)0 Total impact
-
Conference Proceeding: Physically-based retrievals of Norway spruce canopy variables from very high spatial resolution hyperspectral data
[show abstract] [hide abstract]
ABSTRACT: This study was conducted to answer two research questions: (1) what is the spatial variability of the leaf optical properties between 400-1600 nm (hemispherical-directional reflectance, transmittance, absorption) within young Norway spruce crowns, and (2) how to design a suitable physically-based approach retrieving the total chlorophyll content of a complex coniferous canopy from very high spatial resolution (0.4 m) hyperspectral data? It was proved that sun-exposed needles of current age-class statistically differ (alpha-level = 0.01) from rest of the needles in reflectance between 510-760 nm. Last four age-classes of sun-exposed needles were also found to be significantly different from almost all age-classes of sun-shaded needles in transmittance from 760-1350 nm. An operational estimation of chlorophyll a+b content (C<sub>ab</sub>) from an airborne AISA Eagle hyperspectral image was proposed by means of a PROSPECT-DART inversion employing an artificial neural network (ANN). A spatial pattern of estimated C<sub>ab</sub> was successfully validated against the C<sub>ab</sub> map produced by a vegetation index ANCB<sub>650-720</sub>. Coefficients of determination (R<sup>2</sup>) between ground measured and retrieved C<sub>ab</sub> were 0.81 and 0.83, respectively, with root mean square errors (RMSE) of 2.72 mug cm<sup>-2</sup> for ANN and 3.27 mug cm<sup>-2</sup> for ANCB<sub>650-720</sub>.Geoscience and Remote Sensing Symposium, 2007. IGARSS 2007. IEEE International; 08/2007 -
Article: Chlorophyll concentration of leaves and canopies estimated from spectroscopic measurements using the radiative transfer models
-
Article: Physically-based retrievals of Norway spruce canopy variables from very high spatial resolution hyperspectral data
-
Article: Influence of forest canopy structure simulated using the Discrete Anisotropic Radiative Transfer (DART) model on the retrieval of spruce stand LAI
-
Article: A neural network inversion of the DART model to retrieve Norway spruce LAI at a very high spatial resolution
-
Article: Parameterization of high resolution radiative transfer for Norway spruce forest stand ensuring quality of retrievable biophysical variables
-
Article: Spectral differences of the functional crown parts and status of Norway spruce trees studied using remote sensing
Ekologia-Bratislava 22 (2003) 1. -
Article: Influence of woody elements of a Norway spruce canopy on nadir reflectance simulated by the DART model at very high spatial resolution
[show abstract] [hide abstract]
ABSTRACT: A detailed sensitivity analysis investigating the effect of woody elements introduced into the Discrete Anisotropic Radiative Transfer (DART) model on the nadir bidirectional reflectance factor (BRF) for a simulated Norway spruce canopy was performed at a very high spatial resolution (modelling resolution 0.2 m, output pixel size 0.4 m). We used such a high resolution to be able to parameterize DART in an appropriate way and subsequently to gain detailed understanding of the influence of woody elements contributing to the radiative transfer within heterogeneous canopies. Three scenarios were studied by modelling the Norway spruce canopy as being composed of i) leaves, ii) leaves, trunks and first order branches, and finally iii) leaves, trunks, first order branches and small woody twigs simulated using mixed cells (i.e. cells approximated as composition of leaves and/or twigs turbid medium, and large woody constituents). The simulation of each scenario was performed for 10 different canopy closures (CC = 50¿95%, in steps of 5%), 25 leaf area index (LAI = 3.0¿15.0 m2 m¿ 2, in steps of 0.5 m2 m¿ 2), and in four spectral bands (centred at 559, 671, 727, and 783 nm, with a FWHM of 10 nm). The influence of woody elements was evaluated separately for both, sunlit and shaded parts of the simulated forest canopy, respectively. The DART results were verified by quantifying the simulated nadir BRF of each scenario with measured Airborne Imaging Spectroradiometer (AISA) Eagle data (pixel size of 0.4 m). These imaging spectrometer data were acquired over the same Norway spruce stand that was used to parameterise the DART model.Remote Sensing of Environment 112 (2008) 1. -
Article: Applicability of the PROSPECT model for Norway spruce needles
[show abstract] [hide abstract]
ABSTRACT: The potential applicability of the leaf radiative transfer model PROSPECT (version 3.01) was tested for Norway spruce (Picea abies (L.) Karst.) needles collected from stress resistant and resilient trees. Direct comparison of the measured and simulated leaf optical properties between 450¿1000 nm revealed the requirement to recalibrate the PROSPECT chlorophyll and dry matter specific absorption coefficients kab(¿) and km(¿). The subsequent validation of the modified PROSPECT (version 3.01.S) showed close agreement with the spectral measurements of all three needle age¿classes tested; the root mean square error (RMSE) of all reflectance (¿) values within the interval of 450¿1000 nm was equal to 1.74%, for transmittance (¿) it was 1.53% and for absorbance (¿) it was 2.91%. The total chlorophyll concentration, dry matter content, and leaf water content were simultaneously retrieved by a constrained inversion of the original PROSPECT 3.01 and the adjusted PROSPECT 3.01.S. The chlorophyll concentration estimated by inversion of both model versions was similar, but the inversion accuracy of the dry matter and water content was significantly improved. Decreases in RMSE from 0.0079 g cm¿2 to 0.0019 g cm¿2 for dry matter and from 0.0019 cm to 0.0006 cm for leaf water content proved the improved performance of the recalibrated PROSPECT version 3.01.S.International Journal of Remote Sensing 27 (2006) 24/20. -
Article: ANMB650-725 : A new optical index for chlorophyll estimation of a forest canopy from hyperspectral images
-
Article: Towards remote sensing of physiological processes: Up-scaling the reflectance and fluorescence signals of vegetation
-
Article: Variability in leaf optical properties of Norway spruce crowns
-
Article: A new hyperspectral index for chlorophyll estimation of a forest canopy: Area under curve normalised to maximal band depth between 650-725 nm
[show abstract] [hide abstract]
ABSTRACT: Total chlorophyll (Cab) content of a forest canopy is used as indicator for the current state of a forest stand, and also as an input for various physiological vegetation models (i.e. models of photosynthesis, evapo-transpiration, etc.). Recent hyperspectral remote sensing allows retrieving the Cab concentration of vegetation using the appropriate optical indices, and/or by means of biochemical information, scaled up from leaf to canopy level within radiative transfer (RT) models. Plenty of chlorophyll optical indices can be found in the literature for the leaf level, nevertheless, only some of them were proposed for a complex vegetation canopy like a forest stand. A robust chlorophyll optical index at the canopy level should be driven by the Cab concentration without negative influence of other factors represented by soil background or understory, canopy closure, canopy structure (e.g. leaf area index (LAI), clumping of leaves), etc. A new optical index named Area under curve Normalised to Maximal Band depth between 650-725 nm (ANMB650-725) is proposed to estimate the chlorophyll content of a Norway spruce (Picea abies, /L./ Karst.) crown. This index was designed to exploit modifications of a vegetation reflectance signature invoked within the red-edge wavelengths mainly by the changes in leaf chlorophyll content. ANMB650-725 is based on the reflectance continuum removal of the chlorophyll absorption feature between wavelengths of 650-725 nm. Suitability of the index and sensitivity on disturbing factors was tested using a 3D Discrete Anisotropic Radiative Transfer (DART) model coupled with a leaf radiative transfer model PROSPECT adjusted for spruce needles. The results of the ANMB650-725 abilities within a coniferous forest canopy were compared with the performance of the chlorophyll indices ratio TCARI/OSAVI. Test results, carried out with the DART model simulating hyperspectral data with 0.9 m pixel size, showed a strong linear regression of the ANMB650-725 on spruce crown Cab concentration (R2=0.9798) and its quite strong resistance against varying canopy structural features such as LAI and canopy closure. The root mean square error (RMSE) between real and the ANMB650-725 estimated Cab concentrations was only 9.53 mg/cm2 while the RMSE generated from prediction of the TCARI/OSAVI was two times higher (18.83 mg/cm2). Chlorophyll retrieval using the ANMB650-725 index remained stable also after introduction of two reflectance signal disturbing features: a) 20% of the spectral information of epiphytic lichen (Pseudevernia sp.) regularly distributed within the spruce canopy, and b) simulation of the sensor noise (computed for a signal to noise ratio equal to 5). RMSE of predicted Cab concentration after the introduction of lichens appeared to be 10.51 mg/cm2 and the combined influence of lichen presence and sensor noise in the image caused an increase of the RMSE to 12.13 mg/cm2EARSeL eProceedings 5 (2006) 2. -
Article: Influence of woody elements of a Norway spruce canopy on nadir reflectance simulated by the DART model at very high spatial resolution
[show abstract] [hide abstract]
ABSTRACT: A detailed sensitivity analysis investigating the effect of woody elements introduced into the Discrete Anisotropic Radiative Transfer (DART) model on the nadir bidirectional reflectance factor (BRF) for a simulated Norway spruce canopy was performed at a very high spatial resolution (modelling resolution 0.2 m, output pixel size 0.4 m). We used such a high resolution to be able to parameterize DART in an appropriate way and subsequently to gain detailed understanding of the influence of woody elements contributing to the radiative transfer within heterogeneous canopies. Three scenarios were studied by modelling the Norway spruce canopy as being composed of i) leaves, ii) leaves, trunks and first order branches, and finally iii) leaves, trunks, first order branches and small woody twigs simulated using mixed cells (i.e. cells approximated as composition of leaves and/or twigs turbid medium, and large woody constituents). The simulation of each scenario was performed for 10 different canopy closures (CC=50–95%, in steps of 5%), 25 leaf area index (LAI=3.0–15.0 m2 m−2, in steps of 0.5 m2 m−2), and in four spectral bands (centred at 559, 671, 727, and 783 nm, with a FWHM of 10 nm). The influence of woody elements was evaluated separately for both, sunlit and shaded parts of the simulated forest canopy, respectively. The DART results were verified by quantifying the simulated nadir BRF of each scenario with measured Airborne Imaging Spectroradiometer (AISA) Eagle data (pixel size of 0.4 m). These imaging spectrometer data were acquired over the same Norway spruce stand that was used to parameterise the DART model. The Norway spruce canopy modelled using the DART model consisted of foliage as well as foliage including robust woody constituents (i.e. trunks and branches). All results showed similar nadir BRF for the simulated wavelengths. The incorporation of small woody parts in DART caused the canopy reflectance to decrease about 4% in the near-infrared (NIR), 2% in the red edge (RE) and less than 1% in the green band. The canopy BRF of the red band increased by about 2%. Subsequently, the sensitivity on accounting for woody elements for two spectral vegetation indices, the normalized difference vegetation index (NDVI) and the angular vegetation index (AVI), was evaluated. Finally, we conclude on the importance of including woody elements in radiative transfer based approaches and discuss the applicability of the vegetation indices as well as the physically based inversion approaches to retrieve the forest canopy LAI at very high spatial resolution.
