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1 2004

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Badania górskich zbiorowisk roślinnych z użyciem technik hiperspektralnych

Zagajewski B., Sobczak M., Wrzesień M.

Przegląd Geofizyczny

2004

Z. 3-4

115-129

Mountain plant species have very specific environmental adaptations (like for example increased carotenoid content) to protect them from harsh conditions: e.g. excessive sun radiation, high amplitudes of temperature, strong winds etc. Those adaptations result from plant physiology that is chemical and physical properties of the green, vegetative matter. Laboratory and field analyses of spectral properties of plants have shown, that identification of plants and vegetation communities in mountains is possible. Spectral signature of a plant is distinctive and variable with wavelength, its characteristic absorption features being a direct result of its physiological process. Photosynthesis is based on conversion of radiation absorbed in blue and red parts of electromagnetic spectrum to energy. Chlorophyll a is the main factor in photosynthesis while chlorophyll b plays a secondary function, supporting chlorophyll a in light absorption. Carotenoids' role is to protect chlorophyll from photooxidation and thylakoid membranes from destruction resulting from excess sun radiation. High content of carotenoids is noted for plants subjected to extensive sun radiation (e.g. alpine species). Quantity of carotenoids increases also with plant senescence. Other pigments such as xanthophyslls and anthocyanins can also contribute to absorption of visible radiation. In addition to spectral characteristics of vegetation, there exists a wide range of supporting indices used in vegetation research, like: NDVI, SR, WDVI, SAVI, MSAVI, NLI and NLI2, AVI, PRI. The principal of a vegetation index is to define a simple relationship between the reflectance measured by a sensor in particular wavelengths and parameter directly characterising a plant (e.g. condition of a photosynthesising apparatus, efficiency of evapotranspiration process) or vegetation stand (biomass or canopy structure). Other commonly used biophysical vegetation characteristics which can be directly derived from remote sensing measurements include, among others: LAI (Leaf Area Index), fAPAR (fraction of Absorbed Photosynthetically Active Radiation) and plant-air temperature difference. In this paper methodology of vegetation monitoring using field remote sensing techniques is presented. This is the first stage of the assessment of the potential of hyperspectral data for analysis and monitoring of mountain environments with a special focus on vegetation mapping and condition investigation. The research aims advanced field measurements, laboratory analysis of pigments (chlorophyll a, b and carotenoids), dry/fresh biomass and large scale, hyperspectral imagery (DAIS 7915, ROSIS). The study was conducted in Tatra National Park ("High Tatras"). Field remote sensing measurements were carried out on July and August 2002. The year has been exceptionally good for vegetation development and the state of the researched vegetation was good. No vegetation in poor condition or under stress has been detected. Four sets of measurements characterising different aspects of vegetation and its habitat condition were carried out at both sites: spectrometric measurements; survey of Leaf Area Index (LAI); measurements of Accumulated Photosynthetic Active Radiation (APAR); plant heat and water balance assessment; fluorescence; biomass, water content in leaves, absorption of photosynthetic plant pigments; sun radiation and GPS measurements and detailed land-use and vegetation mapping. Results of field campaign can be outlined: the qualitative and quantitative analysis of photosynthetising pigments showed significant differences between analysed species; field radiometric measurements confirmed the results achieved in the laboratory analysis of leaf pigment content; spectral signatures of researched communities are characteristic for plants in good condition; LAI index measured for all researched communities oscillates around an optimal value; productivity defined as an APAR/ PAR0 ratio for all researched communities was very high; temperature differences between plant and air temperature for all plant communities were negative, which indicates good performance of the process of evapotranspiration of the plant species building the communities. The applied methods of field measurements and laboratory analysis show a potential of remote sensing techniques for research and mapping of vegetation in mountainous environments. The next stages of research (analysis of the hyperspectral data) should show proper recognition and state of alpine plants and communities.