Mait Lang

Determination of needle area indices of coniferous forest canopies in the NOPEX region by ground-based optical measurements and satellite images

Abstract Eight coniferous ‘ground truth’ stands were studied in the NOPEX region. Needle area indices (LAI) of the stands were estimated via regressions on the tree parameters and stand density measured on the respective sample plots. Reflectances of the ground truth stands were simulated by a forest reflectance model and related to the canopy LAI. In all short-wave spectral bands of Landsat TM and SPOT, decreasing non-linear trends of stand reflectance along with an increase in LAI were observed. The trends were confirmed by an analysis of satellite images. With the logarithmic regression of stand LAI on Landsat TM or SPOT DNs, the ground truth information can be extended to the rest of coniferous forests in the NOPEX region with approximate relative error in LAI 30–40%. The best spectral bands appeared to be TM5, SPOT XS2, SPOT XS1, TM7 and TM3. Canopy LAI can be successfully estimated from the coefficients of penetration of direct and diffuse radiation to the forest floor. This has been demonstrated by an analysis of the results of ground-based measurements of canopy transmission of the ground truth stands by means of a four-channel field radiometer.

Forest reflectance modeling: theoretical aspects and applications.

Abstract Forest reflectance models are able to contribute to the interpretation of satellite images over forested areas. A multipurpose forest reflectance model, its basic ideas, model input and output options are briefly described. The possibility to apply the model in forest remote sensing is demonstrated through the following simulation examples: the age dependence of forest reflectance, the seasonal course of forest reflectance, defoliation effects on forest reflectance, sensitivity of chlorophyll and water indices with respect to leaf chlorophyll and water content, respectively, and uncertainties in these indices introduced by the variation in stand parameters. When used together with the atmospheric radiative transfer and sensor calibration models, the forest reflectance model is applicable in satellite-imagery-aided forest ecology and health assessment, forest inventory and management.

Database of optical and structural data for the validation of radiative transfer models

Recent advances in airborne and spaceborne scanner technologies have been providing vast amounts of multispectral and multi-angular remote sensing data of different spatial and radiometric resolution over the Earth’s vegetation. Remote sensing of forests has been one of the major interest in the remote sensing of environment, because forests impact climate, provide different kind of resources for the economy, are related to biodiversity and, on the other hand, are threatened by several anthropogenic factors and disturbances.

Creating a regional MODIS satellite-driven net primary production dataset for european forests

Net primary production (NPP) is an important ecological metric for studying forest ecosystems and their carbon sequestration, for assessing the potential supply of food or timber and quantifying the impacts of climate change on ecosystems. The global MODIS NPP dataset using the MOD17 algorithm provides valuable information for monitoring NPP at 1-km resolution. Since coarse-resolution global climate data are used, the global dataset may contain uncertainties for Europe. We used a 1-km daily gridded European climate data set with the MOD17 algorithm to create the regional NPP dataset MODIS EURO. For evaluation of this new dataset, we compare MODIS EURO with terrestrial driven NPP from analyzing and harmonizing forest inventory data (NFI) from 196,434 plots in 12 European countries as well as the global MODIS NPP dataset for the years 2000 to 2012. Comparing these three NPP datasets, we found that the global MODIS NPP dataset differs from NFI NPP by 26%, while MODIS EURO only differs by 7%. MODIS EURO also agrees with NFI NPP across scales (from continental, regional to country) and gradients (elevation, location, tree age, dominant species, etc.). The agreement is particularly good for elevation, dominant species or tree height. This suggests that using improved climate data allows the MOD17 algorithm to provide realistic NPP estimates for Europe. Local discrepancies between MODIS EURO and NFI NPP can be related to differences in stand density due to forest management and the national carbon estimation methods. With this study, we provide a consistent, temporally continuous and spatially explicit productivity dataset for the years 2000 to 2012 on a 1-km resolution, which can be used to assess climate change impacts on ecosystems or the potential biomass supply of the European forests for an increasing bio-based economy. MODIS EURO data are made freely available at ftp://palantir.boku.ac.at/Public/MODIS_EURO.

Hyperspectral reflectance of boreo-nemoral forests in a dry and normal summer

The visible-near infrared (VNIR) reflectance of several mature boreo-nemoral stands in Estonia was measured in two concurrent years which differed significantly in precipitation. Custom-designed VNIR spectrometer systems, UAVSpec and UAVSpec2, mounted onboard a helicopter, were used for the measurements. We show that broadleaved and coniferous forests react to changes in water supply differently: the relative change of the NIR reflectance was equally about 10–15% in all stands, while the change of visible reflectance of coniferous stands exceeded that of broadleaved stands. The smallest change was observed for broadleaved stands in the red reflectance, i.e. chlorophyll absorption bands. Based on our results, we conclude that the change of stand reflectance caused by weather conditions, such as drought, should be taken into account in the estimation of the optical properties of the atmosphere, needed in the atmospheric correction of optical satellite images. A widely used correction method directly benefiting from our results for boreo-nemoral forests is the dark object method.

Mixotrophy in Pyroleae (Ericaceae) from Estonian boreal forests does not vary with light or tissue age

Background and Aims In temperate forests, some green plants, namely pyroloids (Pyroleae, Ericaceae) and some orchids, independently evolved a mode of nutrition mixing photosynthates and carbon gained from their mycorrhizal fungi (mixotrophy). Fungal carbon is more enriched in 13C than photosynthates, allowing estimation of the proportion of carbon acquired heterotrophically from fungi in plant biomass. Based on 13C enrichment, mixotrophic orchids have previously been shown to increase shoot autotrophy level over the growth season and with environmental light availability. But little is known about the plasticity of use of photosynthetic versus fungal carbon in pyroloids. Methods Plasticity of mixotrophy with leaf age or light level (estimated from canopy openness) was investigated in pyroloids from three Estonian boreal forests. Bulk leaf 13C enrichment of five pyroloid species was compared with that of control autotrophic plants along temporal series (over one growth season) and environmental light gradients (n=405 samples). Key Results Mixotrophic 13C enrichment was detected at studied sites for Pyrola chlorantha and Orthilia secunda (except at one site for the latter), but not for Chimaphila umbellata, Pyrola rotundifolia and Moneses uniflora. Enrichment with 13C did not vary over the growth season or between leaves from current and previous years. Finally, although one co-occurring mixotrophic orchid showed 13C depletion with increasing light availability, as expected for mixotrophs, all pyroloids responded identically to autotrophic control plants along light gradients. Conclusions A phylogenetic trend previously observed is further supported: mixotrophy is rarely supported by 13C enrichment in the Chimaphila + Moneses clade, whereas it is frequent in the Pyrola + Orthilia clade. Moreover, pyroloid mixotrophy does not respond plastically to ageing or to light level. This contrasts with the usual view of a convergent evolution with orchids, and casts doubt on the way pyroloids use the carbon gained from their mycorrhizal fungi, especially to replace photosynthetic carbon.

Metsa lehepinnaindeksi kaardistamine lennukilidari, satelliidipiltide ja maapealsete mõõtmiste abil Järvselja VALERI testalal / Leaf area index mapping with airborne lidar, satellite images and ground measurements in Järvselja VALERI test site

Abstract The aim of this study was to compile the leaf area index (LAI) map of a 3×3 km VALERI test site in Järvselja, Estonia. Canopy transmittance measurements of LAI by LAI-2000 Plant Canopy Analyzers and digital cameras supplied with fisheye converters were carried out on 42 elementary sampling units at ground level and at breast height level. The vegetation LAI was estimated as a sum of the tree canopy true green LAI obtained from the inversion of canopy gap fraction data and of the ground vegetation effective LAI. Red channel from SPOT-4 HRV-IR image and airborne lidar data based canopy transmittance were used for up-scaling. The LAI map was compared with standard LAI products from Terra MODIS and ENVISAT MERIS. We found that lidar data based LAI estimate on up-scaled map saturated at high values (LAI > 4.5) compared to the LAI estimates based on SPOT-4 HRV-IR red channel. Validation of MODIS LAI product revealed substantial underestimates of LAI compared to the up-scaled field measurements and rather large random noise. ENVISAT MERIS LAI product was more similar to up-scaled field measurements; however, rather large unexpected random variations exist in its time series.

Estimation of canopy cover in dense mixed-species forests using airborne lidar data

ABSTRACT Airborne laser scanning (ALS) data and digital hemispherical photos (DHP) from 93 sample plots in Laeva test site, Estonia, were used to study effects of phenology and scan angle on the ALS-based canopy cover (CCALS) estimates. The relative share of first returns (P1/A) for 6185 forest stands was analysed. The CCALS was calculated using different height thresholds and echoes, and was compared with the CC estimates based on DHP (CCDHP) and crown model (CCRCrown). The first of many echoes-based canopy cover estimate (CCALS,1.3_1) saturated at values greater than 80%. The strongest correlation of CCDHP was found with CCALS,1.3_A using all echoes and a 1.3 m height break (R2 = 0.81, RMSE = 11.8%). Correcting the estimate for view nadir angle did not improve the correlation of CCALS,1.3_A with CCDHP. The CCRCrown had a weak correlation (R2 < 0.25) with CCALS and with CCDHP. The P1/A was not influenced by tree species composition, but by phenology, stand relative density and forest height; however, CCALS was not dependent on stand height. Foliage phenology had a substantial effect on CCALS and CCDHP. In dense mixed-species forests, we recommend to use all returns for canopy cover estimation.

Estimating the beyond-shoot foliage clumping at two contrasting points in the growing season using a variety of field-based methods

Key messageIn a comparison of six methods of estimating foliage clumping, logarithmic methods were found to be the most suitable for validating remote sensing-based clumping maps.AbstractThe clumping index is a measure describing the spatial aggregation of foliage elements and has an important effect on radiation transfer in canopies. Quantifying the clumping index from field-based measurements has shown to be complicated, as values presented in literature vary substantially between methods. The goal of this work was to improve on previous comparison of beyond-shoot clumping quantification methods over Järvselja RAMI test sites by including additional methods as well as emphasizing the temporal variability of the clumping index by including two phenologically contrasting moments in the growing season. We show that while apparent clumping, digital cover photography-based methods and clumping from gap size distribution coincide well, they result in the maximum clumping value that should be considered in any case, whereas logarithmically based methods might provide more accurate estimates of clumping. The results are of great importance for the correct assessment of foliage clumping from space, as well as for current attempts to assess clumping using airborne and terrestrial laser scanning.

Estimation of change in forest height growth

Abstract Forest height increment rate is related to the forest growth conditions. Data bases of previous forest inventories contain information about forest heightage relationship on large number of forest stands while repeated measurements of permanent sample plots provide an excellent reference for comparison. Repeated airborne laser scanning of forest stands is an additional source for the estimation of change in forest structure. In this study, height growth of middle-aged and older forest stands for about 10 year period was compared to an algebraic difference model on permanent sample plots (66) and for a sample of forest stands with repeated airborne laser scanning data (61). The model was based on a large dataset of forest inventory records from the period of 1984–1993. Statistically significant increased forest height growth was found in permanent sample plots based on tree height measurements (9 cm yr−1) as well in stands with repeated laser scanning data (4.5 cm yr−1) in South-East Estonia compared to the algebraic difference model. The difference between the two data sets was explained by their mean age and site class, but the increased forest height growth compared to the old forest inventory data indicates improved growth conditions of forests in the test area. The results hint also that empirical data-based forest growth models need to be updated to avoid biased growth estimates.