| 1. |
- Brogaard, Sara, et al.
(author)
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Primary production of Inner Mongolia, China, between 1982 and 1999 estimated by a satellite data-driven light use efficiency model
- 2005
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In: Global and Planetary Change. - : Elsevier BV. - 1872-6364 .- 0921-8181. ; 45:4, s. 313-332
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Journal article (peer-reviewed)abstract
- Declining biological production as a part of an ongoing land degradation process is considered a severe environmental problem in the dry northern and northwestern regions of China. The aim of this study is to develop and adapt a satellite data-driven gross primary production model called Lund University light use efficiency model (LULUE) to temperate conditions in order to map gross primary production (GPP) for the Grasslands of Inner Mongolia Autonomous Region (IMAR), China, from 1982 to 1999. The water stress factor included in the original model has been complemented with two temperature stress factors. In addition, algorithms that allocate the proportions of C3/C4 photosynthetic pathways used by plants and that compute temperature-based C3 maximum efficiency values have been incorporated in the model. The applied light use efficiency (LUE) model is using time series of the Normalized Difference Vegetation Index (NDVI), CLouds from AVHRR (CLAVR) from the 8-km resolution NOAA Pathfinder Land Data Set (PAL). Quasi-daily rainfall and monthly minimum and maximum temperatures, together with soil texture information, are used to compute water limitations to plant growth. The model treats bare soil evaporation and actual transpiration separately, a refinement that is more biophysically realistic, and leads to enhanced precision in our water stress term, especially across vegetation gradients. Based on ground measurements of net primary production (NPP) at one site, the LULUE reproduces the variability of primary production better than CENTURY or NDVI alone. Mean annual GPP between 1982 and 1999 range from about 100 g/m(2) in desert regions in the west to about 4000 g/m(2) in the northeast of IMAR, and the coefficient of variation for GPP is highest near the margins of the deserts in the west where rainfall is erratic. Linear trends fitted through the 18-year time series reveal that the western regions have encountered no change, while a large area in the center of the IMAR shows marked increases in GPP. In the northeast, negative trends in GPP are noted and coincide with rainfall trends. Though the high inter-annual variability in primary production undermines the identification of significant trends, we could not isolate any general decline in grassland primary production.
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| 3. |
- Connolly, John, et al.
(author)
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Using MODIS derived fPAR with ground based flux tower measurements to derive the light use efficiency for two Canadian peatlands
- 2009
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In: Biogeosciences. - : Copernicus GmbH. - 1726-4189. ; 32:6, s. 225-225
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Journal article (peer-reviewed)abstract
- We used satellite remote sensing data; fractionof photosynthetically active radiation absorbed by vegetation(fPAR) from the Moderate Resolution Imaging Spectrora-diometer (MODIS) in combination with tower eddy covari-ance and meteorological measurements to characterise theLight Use Efficiency parameter (ε)variability and the maxi-mumε(εmax)for two contrasting Canadian peatlands. Eight-day MODISfPAR data were acquired for the Mer Bleue(2000 to 2003) and Western Peatland (2004). Flux towereddy covariance and meteorological measurements were in-tegrated to the same eight-day time stamps as the MODISfPAR data. A light use efficiency model: GPP =ε×APAR(where GPP is Gross Primary Productivity and APAR is ab-sorbed photosynthetically active radiation) was used to cal-culateε. Theεmaxvalue for each year (2000 to 2003) at theMer Bleue bog ranged from 0.58 g C MJ−1to 0.78 g C MJ−1and was 0.91 g C MJ−1in 2004, for the Western Peatland.The average growing seasonεfor the Mer Bleue bog forthe four year period was 0.35 g C MJ−1and for the West-ern Peatland in 2004 was 0.57 g C MJ−1. The average snowfree period for the Mer Bleue bog over the four years was 0.27 g C MJ−1and for the Western Peatland in 2004 was0.39 g C MJ−1. Using the light use efficiency method wecalculated theεmaxand the annual variability inεfor twoCanadian peatlands. We determined that temperature was agrowth-limiting factor at both sites Vapour Pressure Deficit(VPD) however was not. MODISfPAR is a useful tool forthe characterization ofεat flux tower sites.
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| 5. |
- Heumann, Ben, et al.
(author)
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AVHRR Derived Phenological Change in the Sahel and Soudan, Africa, 1982 - 2005
- 2007
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In: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257. ; 108:4, s. 385-392
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Journal article (peer-reviewed)abstract
- The Sahel region of Africa has experienced a decrease in rainfall from the early 1960s to mid 1990s. Recent studies have detected an increased in NDVI amplitude and growing season integrated NDVI for the region since 1982. However, these studies have not examined how plant phenology has changed. Phenology examines life cycle events such as bud burst and leaf senescence. Using the software TIMESAT to estimate phenological parameters from the GlMMS AVHRR NDVI dataset, we have found significant positive trends for the length of the growing and end of the growing season for the Soudan and Guinean regions, but significant trends in the Sahel could not be detected. The geographical extent of these trends contrasts with the more northern extent of positive trends of NDVI amplitude and growing season integrated NDVI. Results suggest two types of "greening" trends associated with rainfall change since the drought in the early 1980s. (c) 2006 Elsevier Inc. All rights reserved.
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| 6. |
- Heumann, Ben, et al.
(author)
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AVHRR derived phenological change in the Sahel and Soudan, Africa, 1982–2005
- 2007
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In: Remote Sensing of Environment. - : Elsevier. - 0034-4257. ; 108:4, s. 385-392
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Journal article (peer-reviewed)abstract
- The Sahel region of Africa has experienced a decrease in rainfall from the early 1960s to mid 1990s. Recent studies have detected an increased in NDVI amplitude and growing season integrated NDVI for the region since 1982. However, these studies have not examined how plant phenology has changed. Phenology examines life cycle events such as bud burst and leaf senescence. Using the software TIMESAT to estimate phenological parameters from the GIMMS AVHRR NDVI dataset, we have found significant positive trends for the length of the growing and end of the growing season for the Soudan and Guinean regions, but significant trends in the Sahel could not be detected. The geographical extent of these trends contrasts with the more northern extent of positive trends of NDVI amplitude and growing season integrated NDVI. Results suggest two types of "greening" trends associated with rainfall change since the drought in the early 1980s.
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| 8. |
- Hickler, Thomas, et al.
(author)
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Precipitation controls Sahel greening trend
- 2005
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In: Geophysical Research Letters. - 1944-8007. ; 32:21
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Journal article (peer-reviewed)abstract
- The Sahel region has been identified as a "hot spot'' of global environmental change, but understanding of the roles of different climatic and anthropogenic forcing factors driving change in the region is incomplete. We show that a process-based ecosystem model driven by climatic and atmospheric CO2 data alone closely reproduces the satellite-observed greening trend of the Sahel vegetation and its interannual variability between 1982 and 1998. Changes in precipitation were identified as the primary driver of the aggregated simulated vegetation changes. According to the model, the increasing carbon uptake through vegetation was associated with an increasing relative carbon sink; but integrated over the whole period, the Sahel was predicted to be a net source of carbon.
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| 10. |
- Seaquist, Jonathan, et al.
(author)
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Broad-scale Increase in NPP Quantified for the African Sahel, 1982-1999
- 2006
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In: International Journal of Remote Sensing. - : Informa UK Limited. - 1366-5901 .- 0143-1161. ; 27:22, s. 5115-5122
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Journal article (peer-reviewed)abstract
- In association with a recently discovered greening trend in the Sahel, several interesting new perspectives have appeared in the literature regarding its climate and ecology. In this Letter, satellite data from 1982 to 1999 and a light use efficiency model are used to map net primary production (NPP) increases throughout the Sahel (total area of 1.13 x 10(13) m(2)). A patchy, east-west band of increasing NPP is identified, with several hotspots showing large increases. The total rate of NPP change for the Sahel is estimated to be 51.0 Mt C year(-1) over the 18-year period, yielding an absolute net gain of 918.0 Mt C. This increase is associated with a decrease in the inter-annual variability of NPP for the 1990s compared to the 1980s. These results lay the groundwork for untangling the effects of direct, localised human impact and climate forcing on land cover by conducting model intercomparison experiments, contextualizing the role the Sahel may play in the tropical carbon cycle, and for reducing the uncertainty regarding Sahelian carbon sequestration.
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