Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Remote sensing methodology

Spectral reflectance in waveband 400-950 nm was measured from nadir at height of 1m above the canopy with narrow angle (12 degrees) receiver during sunny days around midday. For spatial averaging receiver was scanned over the plot and at least 90 reflectance spectra per plot was recorded. A calibrated grey standard was used as a reference surface. Vegetation indices (NDVI, PRI) and their statistics were calculated. In order to get information about spectral properties of canopy background litter layer and bare soil outside the plots were measured. Measurements were made with clear weather between 10 AM and 3 PM.

Reflectance measurements showed that differences in reflectance indices between treatments were considerably smaller than differences between sites. In general, higher NDVI as an indicator of biomass was accompanied with higher efficiencies of photosynthetic machinery measured with both, reflectance and fluorescence methods. Photochemical efficiency index PRI as well as NDVI differed significantly between heat and drought treatments in all sites showing that realised photon efficiency of photosynthetic machinery was higher under the heat treatment than under the draught treatment.

Comparison of NDVI values with green biomass data obtained from pin-point measurements revealed expected relationship between these two parameters with strong saturation at high biomass conditions. Consequently, change in NDVI values can be interpreted as changes in green biomass with some limitations at high biomass conditions as in Wales where second order effects (e.g. intensive flowering of Calluna) may overshadow small changes in biomass.

Gross sites comparison showed that the general effect of warming treatment is positive on both NDVI and green biomass whereas drought treatment had negative effect. Due to variability pattern and better averaging ability of reflectance measurements if compared to pin-pointing the treatment effect was statistically significant only in case of NDVI showing that in general, the treatment effects were tiny and can be revealed only with methods that allow better spatial averaging.

The study showed that PRI had no tendency to saturate at high biomass conditions as NDVI making PRI more valuable tool for assessing vegetation status in such conditions. However, if to exclude data from Wales then the treatment effects on values of NDVI and on PRI are well correlated with each other showing that the treatment effect on the amount of green biomass is the most likely a result of changed photochemical efficiency of photosynthetic apparatus.

Our study revealed great potential of reflectance measurements in monitoring climate change related alterations in European shrublands. Reflectance is easier to measure and has better averaging ability than direct measurements of biomass or physiological status of vegetation.

Informations connexes

Reported by

University of Tartu
Uelikooli 18
50090 Tartu
See on map