•Seasonal trajectories of nine bands and six indices from Sentinel-2 were analysed.•Seasonal change values were better for early detection than absolute reflectance.•Tasselled Cap Wetness and ...SWIR-based NDVI were most successful.•Separation of green-attack trees was best in autumn before next beetle generation.
In the past decade, massive outbreaks of bark beetles (Ips spp.) have caused large-scale decline of coniferous-dominated, prevailingly managed forests of Central Europe. Timely detection of newly infested trees is important for minimizing economic losses and effectively planning forest management activities to stop or at least slow outbreaks. With the advancement of Copernicus services, a pair of Sentinel-2 satellites provides a unique remote sensing data source of multi-temporal observations in high spatial resolution on the scale of individual forest stands (although not allowing for individual tree detection). This study investigates the potential for using seasonal trajectories of Sentinel-2 bands and selected vegetation indices in early detection of bark beetle infestation (so–called green-attack stage detection) in Norway spruce monoculture forests in the Czech Republic. Spectral trajectories of nine bands and six vegetation indices were constructed for the 2018 vegetation season using 14 satellite observations from April to November to distinguish four infestation classes. We used a random forest algorithm to classify healthy (i.e., stands not infested) and infested trees with various trajectories of decay. The seasonal trajectories of vegetation indices separated the infestation classes better than did the individual bands. Among the most promising vegetation indices we identified the tasselled cap wetness (TCW) component and normalized difference index constructed from near and shortwave infrared bands. Analysing the inter-annual change of the indices was more promising for early detection than is single-date classification. It achieved 96% classification accuracy on day of year 291 for the tested data set.The algorithm for early detection of tree infestation based on the assessment of seasonal changes in TCW was applied on the time series of Sentinel-2 observations from 2019 and its outputs were verified using field observations of forest conditions conducted on 80 spruce forest plots (located in spruce monoculture stands). The overall accuracy of 78% was achieved for the separation of healthy and green-attack classes. Our study highlights the great potential of multi-temporal remote sensing and the use of shortwave infrared wavelengths for early detection of spruce forest decline caused by bark beetle infestation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The issue of aircraft tracking using passive sensors with poor measurement precision and a variable update rate is investigated. First, the need for a time‐dependent Markov chain transition matrix ...for the Interacting Multiple Model (IMM) is thoroughly evaluated using simulation and real data from an experimental multilateration system. Three different methods for computing the time‐dependent matrix are compared, showing their superiority for highly aperiodic sensors. Second, the Variable Structure Interacting Multiple Model (VS‐IMM) for passive sensors with poor geometry is proposed, in which the mode set update is based on local dilution of precision or distance root mean square error. The performance of the estimator is compared with the IMM using real data. The VS‐IMM results in the best performance for areas of both high and low measurement precision.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
•We simulated albedo of boreal zone forests using a radiative transfer model.•Species composition had a strong impact on forest albedo.•Diurnal courses of albedo were related to forest density.•The ...albedos decreased with increasing stand biomass, LAI, and canopy cover.
The relationship between albedo and forest areas is complex. Little is known about the driving factors of albedo in the boreal zone. Using a radiative transfer model and an extensive forest inventory database, we simulated albedo of forest stands composed of the most abundant tree species of Fennoscandia – Scots pine, Norway spruce and Silver birch. The physically-based radiative transfer model allowed us to uncouple the driving factors of the forest albedo. We analyzed separately how biomass, canopy cover, and species composition influence the shortwave albedo of a boreal forest. The albedos differed significantly between species and increased with solar zenith angle. The lowest values were observed for spruce stands, followed by pine stands and the highest values were observed for birch stands. Diurnal courses of albedo were tightly related to forest density as quantified by biomass or canopy cover. The albedos generally decreased with increasing stand biomass and canopy cover whereas the sensitivity to solar angle increased as the stands became denser. The sharpest decrease in albedo was observed at low biomass values, after which the albedo remained relative stable. The strength of the relationships was weaker for larger solar zenith angles.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this study, Sentinel-2 data were used for the retrieval of three key biophysical parameters of crops: leaf area index (LAI), leaf chlorophyll content (LCC), and leaf water content (LWC) for ...dominant crop types in the Czech Republic, including winter wheat (Triticum aestivum), spring barley (Hordeum vulgare), winter rapeseed (Brassica napus subsp. napus), alfalfa (Medicago sativa), sugar beet (Beta vulgaris), and corn (Zea mays subsp. Mays) in different stages of crop development. Artificial neural networks were applied in combination with an approach using look-up tables that is based on PROSAIL simulations to retrieve the biophysical properties tailored for each crop type. Crop-specific PROSAIL model optimization and validation were based upon a large dataset of in situ measurements collected in 2017 and 2018 in lowland of Central Bohemia region. For LCC and LAI, respectively, low relative root mean square error (rRMSE; 25%, 37%) was achieved. Additionally, a relatively strong correlation with in situ measurements (r = 0.80) was obtained for LAI. On the contrary, the results of the LWC parameter retrieval proved to be unsatisfactory. We have developed a generic tool for biophysical monitoring of agricultural crops based on the interpretation of Sentinel-2 satellite data by inversion of the radiation transfer model. The resulting crop condition maps can serve as precision agriculture inputs for selective fertilizer and irrigation application as well as for yield potential assessment.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The aqueous-phase chemistry of reactive chlorine species initiated in saline solution by the gas phase discharge plasma above the liquid surface was investigated. A micro-atmospheric pressure 13.56 ...MHz plasma jet operating with a 0.6% O2 in He was used to promote reactions of plasma-generated oxygen atoms with chloride anions at the gas-liquid interface. Physiological NaCl solution and phosphate buffered saline (PBS) were used. Three chlorine oxoacids or their conjugate bases were detected in the plasma treated saline: hypochlorites HOCl/OCl−, chlorites ClO2−, and chlorates ClO3−, being formed in the concentration ratio 4.5:1:1, respectively. In addition, chlorine dioxide ClO2 was formed in minor amounts. The final pH value of the plasma treated saline was typically 10.2. The total hypochlorite production, HOCl + OCl−, was directly proportional to the initial NaCl concentration, supporting the idea of a direct reaction of Cl− with O atoms. Subsequent post-discharge chemical processes in the bulk plasma treated saline solution were found to lead to the disproportionation of hypochlorite into different oxidation states of chlorine with formation of ClO3− as the final product. The reactivity of O atoms in NaCl solution was studied using taurine and phenol as chemical probes. Experiments with 50 mM taurine revealed a very high initial formation rate of OCl− in PBS (up to 4 M s−1). Chlorinated products of phenol (2- and 4-chlorophenol) in addition to the hydroxylated products (hydroquinone and catechol) demonstrated chlorination of phenol by hypochlorite. The rate of Cl− oxidation by O atoms was estimated to be 2-3 orders slower than the reaction of O atoms with phenol.
In this study, we examine leaf reflectance as the main optical property used in remote sensing of vegetation. The total leaf reflectance consists of two main components: a diffuse component, ...originating from the leaf interior, and a component reflected directly from the leaf surface. The latter contains specular (mirror-like) reflectance (SR) and surface particle scattering, driven by the surface roughness. Our study aimed to (1) reveal the effects of key leaf structural traits on SR in 400–2500 nm, and (2) compare the performance of PLSR models of leaf biophysical properties based on the total reflectance and SR removal reflectance. Four Arabidopsis thaliana structural surface mutants and six Hieracium species differing in trichome properties were studied. PCA did not reveal any systematic effect of trichome density, length, and morphology on SR. Therefore, the results do not support the hypothesis that leaves with denser and longer trichomes have lower SR and higher total reflectance than the smooth leaves. SR removal did not remarkably improve PLSR models of biophysical traits (up to 2% of RMSE). Thus, in herbaceous dorsiventral leaves with relatively sparse trichomes of various morphology and without apparent waxy surface, we cannot confirm that SR removal significantly improves biophysical trait prediction.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Foliage spectra form an important input to physically-based forest reflectance models. However, little is known about geographical variability of coniferous needle spectra. In this research note, we ...present an assessment of the geographical variability of Norway spruce ( (L.) H. Karst.) needle albedo, reflectance, and transmittance spectra across three study sites covering latitudes of 49â62°N in Europe. All spectra were measured and processed using exactly the same methodology and parameters, which guarantees reliable conclusions about geographical variability. Small geographical variability in Norway spruce needle spectra was observed, when compared to variability observed between previous measurement campaigns (employing slightly varying measurement and processing parameters), or to variability between plant functional types (broadleaved vs. coniferous). Our results suggest that variability of needle spectra is not a major factor introducing geographical variability to forest reflectance. The results also highlight the importance of harmonizing measurement protocols when collecting needle spectral libraries. Furthermore, the data collected for this study can be useful in studies where accurate information on spectral differences between broadleaved and coniferous tree foliage is needed.
Picea abies
In this study, an approach for the harmonized calculation of the Leaf Area Indices (LAIs) for agronomic crops from Sentinel-2 MSI and Landsat OLI multispectral satellite data is proposed in order to ...obtain a dense seasonal trajectory. It was developed and tested on dominant crops grown in the Czech Republic, including winter wheat, spring barley, winter rapeseed, alfalfa, sugar beet, and corn. The two-step procedure harmonizing Sentinel-2 MSI and Landsat OLI spectral data began with deriving NDVI, MSAVI, and NDWI_1610 vegetation indices (VIs) as proxy indicators of green biomass and foliage water content, the parameters contributing most to a stand’s spectral response. Second, a simple linear transformation was applied to the resulting VI values. The regression model itself was built on an artificial neural network, then trained on PROSAIL simulations data. The LAI estimates were validated using an extensive dataset of in situ measurements collected during 2017 and 2018 in the lowlands of the Central Bohemia Region. Very strong agreement was observed between LAI estimates from both Sentinel-2 MSI and Landsat OLI data and independent ground-based measurements (r between 0.7 and 0.98). Very good results were also achieved in the mutual comparison of Sentinel-2 and Landsat-based LAI datasets (rRMSE < 20%, r between 0.75 and 0.99). Using data from all currently available Sentinel-2 (A/B) and Landsat (8/9) satellites, a dense harmonized LAI time series can be created with high potential for use in precision agriculture.
Mountain forests are more prone to environmental predispositions (EPs) than submountain ones. While remote sensing of mountain forests enables instantaneous damage mapping, the investigation of the ...causes requires field data. However, a local field or regionally modeled environmental characteristics influence remote data evaluation differently. This study focused on the evaluation of EPs effects damaging mountain forests between various spatial resolutions during environmental change. The evaluation was divided into managed and natural forests in the Hrubý Jeseník Mts. (Czech Republic; 240–1491 m a.s.l.; 50.082°N, 17.231°E). Damage was assessed through the discrimination analysis of the normalised difference vegetation index (NDVI) by MODIS VI during alternating drought and flood periods 2003–2014. The local environmental influence was assessed using the discrimination function (DF) separability of forest damage in the training sets. The regional influence was assessed through map algebra estimated via the DF and a forest decline spatial model based on EPs from differences between risk growth conditions and biomass fuzzy sets. Management, EPs and soil influenced forest NDVI at different levels. The management afflicted the NDVI more than the EPs. The EPs afflicted the NDVI more than the soil groups. Strong winters and droughts had a greater influence on the NDVI than the flood events, with the winter of 2005/2006 inverting the DF direction, and the 2003 drought increasing differences in managed forest biomass and decreasing differences in natural forest biomasses. More than 50% of declining managed forests in the training sets occurred on Leptosols, Podzols and Histosols. On a regional scale, the soil influence was eliminated by multiple predispositions. The EPs influenced 96% of natural forest and 65% of managed forest, though managed forest damage was more evident. The mountain forest NDVI decline was dependent on both management and risk predispositions.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Boreal forests exhibit strong seasonal dynamics in their reflectance spectra during the short, snow-free growing period. This short communication paper reports an analysis of the seasonality of ...boreal forest spectra from the end of snowmelt until the time of maximal leaf area. We apply a forest reflectance model (FRT) to estimate the seasonal contribution of understory vegetation to forest reflectance from a time series of three Earth Observing 1 (EO-1) Hyperion images acquired in May, June and July. The reflectance simulations are based on detailed seasonal series of leaf area index and understory spectra measurements carried out in ten stands at the Hyytiälä Forestry Field Station in Finland. Our results show that the contribution of understory to boreal forest reflectance is high in the visible domain, but it drops at the red edge and stays relatively low and constant in near infrared (NIR). Throughout the growing season, the contribution of the understory remains approximately the same in the NIR domain, whereas larger changes can be observed in the visible domain.
•Contribution of understory to forest reflectance estimated from Hyperion data.•A radiative transfer model used as a tool.•Contribution of understory higher in the VIS than in the NIR range.•Seasonal changes in understory contribution the largest in VIS wavelengths.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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