| 1. |
- Schubert, Per, et al.
(författare)
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Modeling GPP in the Nordic forest landscape with MODIS time series data-Comparison with the MODIS GPP product
- 2012
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Ingår i: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257 .- 1879-0704. ; 126, s. 136-147
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Tidskriftsartikel (refereegranskat)abstract
- Satellite sensor-derived data are suitable for regional estimations of several important biophysical variables. Data with a finer spatial resolution should improve regional estimations of GPP (gross primary productivity), since they better capture the variation in a heterogeneous landscape. The main objective of this study was to investigate if MODIS 500 m reflectance data can be used to drive empirical models for regional estimations of GPP in Nordic forests. The performance of the proposed models was compared with the MODIS 1 km GPP product. Linear regression analyses were made on 8-day averages of eddy covariance GPP from three deciduous and ten coniferous sites in relation to MODIS 8-day composite data and 8-day averages of modeled incoming PPFD (photosynthetic photon flux density). Time series of EVI2 (two-band enhanced vegetation index) were calculated from MODIS 500 m reflectance data and smoothed by a curve fitting procedure. For most sites, GPP was fairly strongly to strongly related to the product of EVI2 and PPFD (Deciduous: R-2=0.45-0.86, Coniferous: R-2=0.49-0.90). Similar strengths were found between GPP and the product of EVI2 and MODIS 1 km daytime LST (land surface temperature) (R-2=0.55-0.81, 0.57-0.77) and between GPP and EVI2, PPFD and daytime LST in multiple linear regressions (R-2=0.73-0.89,0.65-0.93). One year of data was collected from all coniferous sites to derive a general empirical model for GPP versus (1) the product of EVI2 and PPFD (R-2=0.70), (2) the product of EVI2 and daytime LST (R-2=0.62) and (3) EVI2, PPFD and daytime LST (R-2=0.72). These three models were then validated at six sites for the remaining years by linearly relating eddy covariance GPP to modeled GPP, which resulted in fairly strong to strong relationships for most sites (R-2=0.49-0.91, RMSE=0.63-1.22 gC m(-2) day(-1), R-2=0.53-0.73, RMSE=0.90-1.43 gC m(-2) day(-1) R-2=0.56-0.87, RMSE=0.79-1.11 gC m(-2) day(-1)). In comparison, similar validation strengths were found for the latest collection 5.1 of the MODIS 1 km GPP product (R-2=0.59-0.88, RMSE=0.80-1.16 gC m(-2) day(-1)). The main conclusion is that the suggested empirical models driven by MODIS 500 m reflectance data can be used for regional estimations of Nordic forest GPP, while preserving a finer resolution than the MODIS 1 km GPP product. (C) 2012 Elsevier Inc. All rights reserved.
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| 2. |
- Wu, J., et al.
(författare)
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Modelling the decadal trend of ecosystem carbon fluxes demonstrates the important role of functional changes in a temperate deciduous forest
- 2013
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Ingår i: Ecological Modelling. - : Elsevier. - 0304-3800 .- 1872-7026. ; 260, s. 50-61
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Tidskriftsartikel (refereegranskat)abstract
- Temperate forests are globally important carbon sinks and stocks. Trends in net ecosystem exchange have been observed in a Danish beech forest and this trend cannot be entirely attributed to changing climatic drivers. This study sought to clarify the mechanisms responsible for the observed trend, using a dynamic ecosystem model (CoupModel) and model data fusion with multiple constraints and model experiments. Experiments with different validation datasets showed that a multiple constraints model data fusion approach that included the annual tree growth, the seasonal canopy development, the latent and sensible heat fluxes and the CO2 fluxes decreased the parameter uncertainty considerably compared to using CO2 fluxes as validation data alone. The fitted model was able to simulate the observed carbon fluxes well (R-2 = 0.8, mean error = 0.1 g C m(-2) d(-1)) but did not reproduce the decadal (1997-2009) trend in carbon uptake when global parameter estimates were used. Annual parameter estimates were able to reproduce the decadal scale trend; the yearly fitted posterior parameters (e.g. the light use efficiency) indicated a role for changes in the ecosystem functional properties. A possible role for nitrogen demand during mast years is supported by the inter-annual variability in the estimated parameters. The inter-annual variability of photosynthesis parameters was fundamental to the simulation of the trend in carbon fluxes in the investigated beech forest and this demonstrates the importance of functional change in carbon balance.
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| 3. |
- Yi, Chuixiang, et al.
(författare)
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Climate control of terrestrial carbon exchange across biomes and continents
- 2010
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 degrees N). The sensitivity of NEE to mean annual temperature breaks down at similar to 16 degrees C (a threshold value of mean annual temperature), above which no further increase of CO2 uptake with temperature was observed and dryness influence overrules temperature influence.
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