High soil carbon efflux rates in several ecosystems in southern Sweden

Soil C effluxes were measured at five forest stands with different vegetation and a meadow in southeastern Sweden (57¡5«N, 16¡7«E). Exponential regressions of soil respiration against air and soil temperatures were used to model soil respiration at forests stands. For the meadow, a light response curve with gross primary production (GPP) against PAR and a cubic regression with GPP against air temperature were used to model GPP. Soil water content limited soil respiration in all ecosystems but spruce where the limitation appeared only at high soil water content. In the forest ecosystems, the forest floor vegetation was scarce and its C uptake had no significant effect on soil C effluxes. Annual soil respiration in all sites was between 2.05 and 4.34 kg CO2 m–2 yr–1, which is large as compared with that reported in many other studies. Annual GPP of meadow was between 1.81 and 1.99 kg CO2 m–2 yr–1, which gives a NEE between 1.39 and 2.41 kg CO2 m–2 yr–1, i.e. a significant loss of C

Applicability of leaf area index products for boreal regions of Sweden

Leaf area index (LAI) of boreal ecosystems were estimated with optical instruments at the Laxemar and the Forsmark investigation areas in Sweden. The aim was to study the relationship between LAI and the normalized difference vegetation index (NDVI) from Landsat-5 and SPOT and evaluate the applicability of the MODIS (Moderate Resolution Imaging Spectroradiometer) LAI product for small boreal regions. Relationships between ground-estimated LAI and NDVI were significant for coniferous, deciduous and mixed forest sites in Laxemar. For Forsmark, effective LAI was correlated to NDVI for all sites. LAI estimated from NDVI was also used for evaluating accuracy of the MODIS LAI product. The comparison showed no correlation between MODIS LAI and NDVI-based LAI in Forsmark whereas there was in Laxemar. MODIS LAI was on average 2.28 higher than NDVI-based LAI and it also showed larger scatter. Scale issues were the main explanation to high MODIS LAI, since the heterogeneous landscapes with open areas (given a value of zero in the NDVI estimates) was seen as forest in the large pixels of the MODIS LAI product. Therefore, we do not recommend using the MODIS LAI product in small boreal regional landscapes, such as the Forsmark and Laxemar investigation areas

Snow cover and snow albedo changes in the central Andes of Chile and Argentina from daily MODIS observations (2000-2016)

The variables of snow cover extent (SCE), snow cover duration (SCD), and snow albedo (SAL) are primary factors determining the surface energy balance and hydrological response of the cryosphere, influencing snow pack and glacier mass-balance, melt, and runoff conditions. This study examines spatiotemporal patterns and trends in SCE, SCD, and SAL (2000–2016; 16 years) for central Chilean and Argentinean Andes using the MODIS MOD10A1 C6 daily snow product. Observed changes in these variables are analyzed in relation to climatic variability by using ground truth observations (meteorological data from the El Yeso Embalse and Valle Nevado weather stations) and the Multivariate El Niño index (MEI) data. We identified significant downward trends in both SCE and SAL, especially during the onset and offset of snow seasons. SCE and SAL showed high inter-annual variability which correlate significantly with MEI applied with a one-month time-lag. SCE and SCD decreased by an average of ~13 ± 2% and 43 ± 20 days respectively, over the study period. Analysis of spatial pattern of SCE indicates a slightly greater reduction on the eastern side (~14 ± 2%) of the Andes Cordillera compared to the western side (~12 ± 3%). The downward SCE, SAL, and SCD trends identified in this study are likely to have adverse impacts on downstream water resource availability to agricultural and densely populated regions in central Chile and Argentina

Evaluating water controls on vegetation growth in the semi-arid sahel using field and earth observation data

Water loss is a crucial factor for vegetation in the semi-arid Sahel region of Africa. Global satellite-driven estimates of plant CO2 uptake (gross primary productivity, GPP) have been found to not accurately account for Sahelian conditions, particularly the impact of canopy water stress. Here, we identify the main biophysical limitations that induce canopy water stress in Sahelian vegetation and evaluate the relationships between field data and Earth observation-derived spectral products for up-scaling GPP. We find that plant-available water and vapor pressure deficit together control the GPP of Sahelian vegetation through their impact on the greening and browning phases. Our results show that a multiple linear regression (MLR) GPP model that combines the enhanced vegetation index, land surface temperature, and the short-wave infrared reflectance (Band 7, 2105-2155 nm) of the moderate-resolution imaging spectroradiometer satellite sensor was able to explain between 88% and 96% of the variability of eddy covariance flux tower GPP at three Sahelian sites (overall = 89%). The MLR GPP model presented here is potentially scalable at a relatively high spatial and temporal resolution. Given the scarcity of field data on CO2 fluxes in the Sahel, this scalability is important due to the low number of flux towers in the region

Temperature and Tree Size Explain the Mean Time to Fall of Dead Standing Trees across Large Scales

Dead standing trees (DSTs) generally decompose slower than wood in contact with the forest floor. In many regions, DSTs are being created at an increasing rate due to accelerating tree mortality caused by climate change. Therefore, factors determining DST fall are crucial for predicting dead wood turnover time but remain poorly constrained. Here, we conduct a re-analysis of published DST fall data to provide standardized information on the mean time to fall (MTF) of DSTs across biomes. We used multiple linear regression to test covariates considered important for DST fall, while controlling for mortality and management effects. DSTs of species killed by fire, insects and other causes stood on average for 48, 13 and 19 years, but MTF calculations were sensitive to how tree size was accounted for. Species’ MTFs differed significantly between DSTs killed by fire and other causes, between coniferous and broadleaved plant functional types (PFTs) and between managed and unmanaged sites, but management did not explain MTFs when we distinguished by mortality cause. Mean annual temperature (MAT) negatively affected MTFs, whereas larger tree size or being coniferous caused DSTs to stand longer. The most important explanatory variables were MAT and tree size, with minor contributions of management and plant functional type depending on mortality cause. Our results provide a basis to improve the representation of dead wood decomposition in carbon cycle assessments

Woody plant cover estimation in drylands from Earth Observation based seasonal metrics

peer reviewedFrom in situ measured woody cover we develop a phenology driven model to estimate the canopy cover of woody species in the Sahelian drylands at 1 km scale. The model estimates the total canopy cover of all woody phanerophytes and the concept is based on the significant difference in phenophases of dryland trees, shrubs and bushes as compared to that of the herbaceous plants. Whereas annual herbaceous plants are only green during the rainy season and senescence occurs shortly after flowering towards the last rains, most woody plants remain photosynthetically active over large parts of the year. We use Moderate Resolution Imaging Spectroradiometer (MODIS) and Satellite pour l'Observation de la Terre (SPOT) — VEGETATION (VGT) Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) time series and test 10 metrics representing the annual FAPAR dynamics for their ability to reproduce in situ woody cover at 43 sites (163 observations between 1993 and 2013) in the Sahel. Both multi-year field data and satellite metrics are averaged to produce a steady map. Multiple regression models using the integral of FAPAR from the onset of the dry season to the onset of the rainy season, the start date of the growing season and the rate of decrease of the FAPAR curve achieve a cross validated r2/RMSE (in % woody cover) of 0.73/3.0 (MODIS) and 0.70/3.2 (VGT). The extrapolation to Sahel scale shows agreement between VGT and MODIS at an almost nine times higher woody cover than in the global tree cover product MOD44B which only captures trees of a certain minimum size. The derived woody cover map of the Sahel is made publicly available and represents an improvement of existing products and a contribution for future studies of drylands quantifying carbon stocks, climate change assessment, as well as parametrization of vegetation dynamic models