Der Einsatz der Fernerkundung zur Bestimmung des Stickstoffgehaltes von Vegetation - ein Beitrag zur IGBP/BAHC Forschung in Deutschland
|Fachgebiet: Fernerkundung, Geländemessungen, Geo-Informationssysteme, Pflanzenphysiologie, Spektroskopie, Vegetationsgeographie
Gefördert durch: Deutsche Forschungsgemeinschaft DFG
Förderkennzeichen: DFG 6-1 bis DFG 6-5
Projektleitung: Prof. Dr. Wolfram Mauser
Projektwissenschaftler: , Oppelt, N.
Laufzeit: 07/1998 - 03/2002
The aim of this project was to test the potential of AVIS for the purpose of environmental monitoring, especially of the chlorophyll and nitrogen status of plants. The land cover types under investigation were grassland, maize and winter wheat in a test area located in the northern Bavarian foothills, Germany (48° 6' N, 11° 17' E). Within this scope, a total of 26 AVIS flights were carried out during the vegetation periods of the years 1999 to 2001. The hyperspectral data were preprocessed before analysis, including dark current and flat field correction, resampling as well as atmospheric correction and calibrated to reflectances.
Several stands were chosen as test fields, including three fields each of maize and wheat in 1999 and three fields of maize and six fields of wheat in 2000 and 2001. During all years, four meadows belonging to the same plant community (Arrhenatherion elatioris) were investigated. Ground truth campaigns were carried out during the vegetation periods (April October) including weekly measurements of plant parameters, such as height, dry and wet biomass, phenological stage, chlorophyll and nitrogen content, as well as a photographic documentation for each field.
The chlorophyll and nitrogen measurements, which were derived from the sampling on ground, are available in contents per area [g/m²] and in contents per mass ([mg/g] for chlorophyll and [%DM] for nitrogen). The former can be used to evaluate the photosynthetic capacity or productivity of a canopy, which is an important input parameter for hydrological or vegetation models; the latter is an indicator for plant physiological status or level of stress, which is a valuable source of information for optimising field management.
Investigations of the chlorophyll-nitrogen relationship based on ground measurements lead to the conclusion that the chlorophyll and nitrogen contents per mass of the investigated land covers are decoupled when a compensation point for effective photosynthesis is exceeded. Beyond this limit the nitrogen in the plants is no longer incorporated into chlorophylls, but mainly into proteins, alkaloids and nucleic acids, whereby the proteins especially are used for internal storage of nitrogen. Therefore it must be considered that the plant nitrogen can only be derived via the chlorophyll content if a significant correlation between both parameters exists, i.e. below the compensation point.
The derivation of the chlorophyll and nitrogen content of the plant leaves on a mean field basis was conducted using three hyperspectral approaches, namely the hyperspectral NDVI (hNDVI), the Optimised Soil Adjusted Vegetation Index OSAVI as well as the relatively unknown Chlorophyll Absorption Integral CAI. The multispectral NDVITM was simulated as an established reference.
The results of the derivation of both chlorophyll and nitrogen content of plants with the investigated approaches depend strongly on a priori knowledge of the canopies monitored. In general, the use of contents per area rather than contents per mass has been found more suitable for the investigated remote sensing applications.
A significant correlation between any index and the chlorophyll or nitrogen content for the whole sample size could not be derived. The optimal spectral approach for derivation is species-dependent, but also dependent on the cultivar. The chlorophyll and nitrogen level of the plants under observation as well as their temperature sensitivity mainly caused this dependence. The NDVITM, hNDVI and OSAVI became insensitive for high chlorophyll content above about 1g/m² (1.5mg/g) chlorophyll a and 0.2g/m² (0.4mg/g) chlorophyll b, respectively. A saturation of the indices was also found for nitrogen content above 2.5g/m². The saturation limit of nitrogen in percentage of dry matter was observed at about 4%.
The CAI is not affected by saturation as much as the other spectral approaches, leading to higher coefficients of determination, especially for contents per area. The CAI becomes insensitive at chlorophyll contents per area above 2g/m². The results lead to the assumption that the flattening and narrowing of the chlorophyll absorption feature at 680nm most probably causes the saturation of the NDVITM, hNDVI and OSAVI. The ratios are directly affected by an increase in reflectance in the red wavelength region. The high correlations between the CAI and contents per area can be ascribed to the fact that the CAI is based on an integrated measurement over an area and therefore is less affected by an increase of reflectance in the red wavelengths. The CAI becomes insensitive at the point where the narrowing of the absorption feature leads to a shift of the red edge position towards the blue wavelength region. This saturation limit lies at approximately 2g chlorophyll per m².
In contrary, the chlorophyll content per mass, which indicates the plant's physiological status or level of stress, could be estimated more accurately using spectral indices such as hNDVI and OSAVI, especially for wheat. The low correlations derived for maize are caused by its higher temperature dependence, leading to daily variations in the chlorophyll content per mass.
The chlorophyll and nitrogen contents of the grassland canopies could not be derived with the spectral approaches investigated. When the meadows were investigated separately, correlations could only be found between the CAI and the chlorophyll content per area for the most intensely utilised meadow (four to five cuts), which on the one hand is characterised by the highest level of fertilisation, but on the other hand is affected by the highest nutrient offtake. The low potential of the investigated indices can be mainly assigned to the fact that the chlorophyll and nitrogen values of the meadows often exceeded the saturation limits of the applied indices.