Petr Lukeš

Optical properties of leaves and needles for boreal tree species in Europe

Reliable information on the optical properties of leaves and needles is needed for parameterization of radiative transfer models and interpretation of remotely sensed data. The optical properties also convey information about the structure and biochemical constituents of the leaf or needle tissues, and can be linked to the photosynthetic processes of plants. Currently, very little is known about the optical properties of tree species in the European boreal zone. To bridge this gap, we measured directional-hemispherical reflectance and transmittance factors of the three most common tree species in this zone: Scots pine, Norway spruce and Silver birch. The measurements covered a wide spectral range from 350 to 2500 nm with a high spectral resolution of 3–10 nm. To explore the driving factors of the observed leaf-level optical properties, supplementary measurements of structural and biochemical traits of leaves and needles were made. The results showed that the transmittance of pine and spruce needles is clearly lower than reflectance, whereas for birch reflectance and transmittance were similar. In conifers, exposed needles had higher albedo than the shaded needles. Also, the spectra of needles were more dependent on canopy position than the spectra of birch leaves. The relationships between narrowband reflectances and chlorophyll and nitrogen percentage concentrations and specific leaf area were similar for all species, but the strongest correlations were observed for birch.

Multidecadal analysis of forest growth and albedo in boreal Finland

Abstract It is well known that forests serve as carbon sinks. However, the balancing effect of afforestation and increased forest density on global warming due to carbon storage may be lost by low albedo (thus high absorption) of the forests. In the last 30 years, there has been a steady increase in the growing stock of Finnish forests by nearly a quarter while the area of the forests has remained virtually unchanged. Such increase in forest density together with the availability of detailed forest inventories provided by the Multi-Source National Forest Inventory (MS-NFI) in high spatial resolution makes Finland an ideal candidate for exploring the effects of increased forest density on satellite derived estimates of bio-geochemical products e.g. albedo (directional-hemispherical reflectance, DHR), fraction of photosynthetically active radiation absorbed by canopies (fAPAR), leaf area index (LAI) and normalized difference vegetation index (NDVI) in both current and long-term perspective. In this study, we first used MODIS-based vegetation satellite products for Finnish forests to study their seasonal patterns and interrelations. Next, the peak growing season observations are linked to the MS-NFI database to yield the generic relationships between forest density and the satellite-derived vegetation indicators. Finally, long-term GIMMS3g datasets between 1982 and 2011 (2008 for DHR) are analyzed and interpreted using forest inventory data. The vegetation peak growing season NIR DHR and VIS DHR showed weak to moderate negative correlation with fAPAR, whereas there was no correlation between NIR DHR and fAPAR. Next, we show that the spectral albedos in the near-infrared region (NIR DHR) showed weak negative correlation with forest biomass, basal area or canopy cover whereas, as expected, the spectral albedo in the visible region (VIS DHR) correlated negatively with these measures of forest density. Interestingly, the increase in forest density (biomass per ha) of Finnish forests during the last 30 years was not accompanied by trends in the indicators of vegetation ‘greenness’ and photosynthetic productivity (fAPAR, LAI and NDVI) or in forest albedo (DHR). Even though there were small increases in both DHR, fAPAR, LAI and NDVI and from the start to the end of the study period (1982–2011), the pattern and magnitude of change in these variables did not follow the development of forest biomass in the different NFI inventories.

Estimation of Spruce Needle-Leaf Chlorophyll Content Based on DART and PARAS Canopy Reflectance Models

Needle-leaf chlorophyll content (Cab) of a Norway spruce stand was estimated from CHRIS-PROBA images using the canopy reflectance simulated by the PROSPECT model coupled with two canopy reflectance models: 1) discrete anisotropic radiative transfer model (DART); and 2) PARAS. The DART model uses a detailed description of the forest scene, whereas PARAS is based on the photon recollision probability theory and uses a simplified forest structural description. Subsequently, statistically significant empirical functions between the optical indices ANCB_670-720 and ANMB_670-720 and the needle-leaf Cab content were established and then applied to CHRIS-PROBA data. The Cab estimating regressions using ANMB670_720 were more robust than using ANCB_670-720 since the latter was more sensitive to LAI, especially in case of PARAS. Comparison between Cab estimates showed strong linear correlations between PARAS and DART retrievals, with a nearly perfect one-to-one fit when using ANMB_670-720 (slope = 1.1, offset = 11 μg · cm^-2). Further comparison with Cab estimated from an AISA Eagle image of the same stand showed better results for PARAS (RMSE = 2.7 μg · cm^-2 for ANCB_670-720; RMSE = 9.5 μg · cm^-2 for ANMB670_720) than for DART (RMSE = 7.5 μg · cm^-2 for ANCB_670-720; RMSE = 23 μg · cm^-2 for ANMB_670-720). Although these results show the potential for simpler models like PARAS in estimating needle-leaf Cab from satellite imaging spectroscopy data, further analyses regarding parameterization of radiative transfer models are recommended.