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| 2. |
- Merbold, L., et al.
(author)
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Precipitation as driver of carbon fluxes in 11 African ecosystems
- 2009
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In: Biogeosciences. - 1726-4189. ; 6:6, s. 1027-1041
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Journal article (peer-reviewed)abstract
- This study reports carbon and water fluxes between the land surface and atmosphere in eleven different ecosystems types in Sub-Saharan Africa, as measured using eddy covariance (EC) technology in the first two years of the CarboAfrica network operation. The ecosystems for which data were available ranged in mean annual rainfall from 320 mm (Sudan) to 1150 mm (Republic of Congo) and include a spectrum of vegetation types (or land cover) (open savannas, woodlands, croplands and grasslands). Given the shortness of the record, the EC data were analysed across the network rather than longitudinally at sites, in order to understand the driving factors for ecosystem respiration and carbon assimilation, and to reveal the different water use strategies in these highly seasonal environments. Values for maximum net carbon assimilation rates (photosynthesis) ranged from -12.5 mu mol CO2 m(-2) s(-1) in a dry, open Millet cropland (C-4-plants) up to -48 mu mol CO2 m(-2) s(-1) for a tropical moist grassland. Maximum carbon assimilation rates were highly correlated with mean annual rainfall (r(2)=0.74). Maximum photosynthetic uptake rates (Fp(max)) were positively related to satellite-derived f(APAR). Ecosystem respiration was dependent on temperature at all sites, and was additionally dependent on soil water content at sites receiving less than 1000 mm of rain per year. All included ecosystems dominated by C-3-plants, showed a strong decrease in 30-min assimilation rates with increasing water vapour pressure deficit above 2.0 kPa.
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| 3. |
- Rosenberg, M. J. F., et al.
(author)
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The Herschel Comprehensive (U)lirg Emission Survey (Hercules): Co Ladders, Fine Structure Lines, and Neutral Gas Cooling
- 2015
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In: Astrophysical Journal. - : American Astronomical Society. - 1538-4357 .- 0004-637X. ; 801:2
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Journal article (peer-reviewed)abstract
- (Ultra) luminous infrared galaxies ((U)LIRGs) are objects characterized by their extreme infrared (8-1000 mu m) luminosities (L-LIRG > 10(11) L-circle dot and L-ULIRG > 10(12) L-circle dot). The Herschel Comprehensive ULIRG Emission Survey (PI: van derWerf) presents a representative flux-limited sample of 29 (U)LIRGs that spans the full luminosity range of these objects (10(11)L(circle dot)
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| 4. |
- Sjöström, Martin, et al.
(author)
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Evaluation of MODIS gross primary productivity for Africa using eddy covariance data
- 2013
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In: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257. ; 131, s. 275-286
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Journal article (peer-reviewed)abstract
- MOD17A2 provides operational gross primary production (GPP) data globally at 1 km spatial resolution and 8-day temporal resolution. MOD17A2 estimates GPP according to the light use efficiency (LUE) concept assuming a fixed maximum rate of carbon assimilation per unit photosynthetically active radiation absorbed by the vegetation (epsilon(max)). Minimum temperature and vapor pressure deficit derived from meteorological data down-regulate epsilon(max) and constrain carbon assimilation. This data is useful for regional to global studies of the terrestrial carbon budget, climate change and natural resources. In this study we evaluated the MOD17A2 product and its driver data by using in situ measurements of meteorology and eddy covariance GPP for 12 African sites. MOD17A2 agreed well with eddy covariance GPP for wet sites. Overall, seasonality was well captured but MOD17A2 GPP was underestimated for the dry sites located in the Sahel region. Replacing the meteorological driver data derived from coarse resolution reanalysis data with tower measurements reduced MOD17A2 GPP uncertainties, however, the underestimations at the dry sites persisted. Inferred epsilon(max) calculated from tower data was higher than the epsilon(max) prescribed in MOD17A2. This, in addition to uncertainties in fraction of absorbed photosynthetically active radiation (FAPAR) explains some of the underestimations. The results suggest that improved quality of driver data, but primarily a readjustment of the parameters in the biome parameter look-up table (BPLUT) may be needed to better estimate GPP for African ecosystems in MOD17A2. (C) 2013 Elsevier Inc. All rights reserved.
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| 5. |
- Mantlana, K. B., et al.
(author)
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Biomass and leaf-level gas exchange characteristics of three African savanna C-4 grass species under optimum growth conditions
- 2009
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In: African Journal of Ecology. - : Wiley. - 1365-2028 .- 0141-6707. ; 47:4, s. 482-489
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Journal article (peer-reviewed)abstract
- C-4 savanna grass species, Digitaria eriantha, Eragrostis lehmanniana and Panicum repens, were grown under optimum growth conditions with the aim of characterizing their above- and below-ground biomass allocation and the response of their gas exchange to changes in light intensity, CO2 concentration and leaf-to-air vapour pressure deficit gradient (D-l). Digitaria eriantha showed the largest above- and below-ground biomass, high efficiency in carbon gain under light-limiting conditions, high water use efficiency (WUE) and strong stomatal sensitivity to D-l (P = 0.002; r2 = 0.5). Panicum repens had a high aboveground biomass and attained high light saturated photosynthetic rates (A(sat), 47 mu mol m-2 s-1), stomatal conductance, (g(sat), 0.25 mol m-2 s-1) at relatively high WUE. Eragrostis lehmanniana had almost half the biomass of other species, and had similar A(sat) and g(sat) but were attained at lower WUE than the other species. This species also showed the weakest stomatal response to D-l (P = 0.19, r2 = 0. 1). The potential ecological significance of the contrasting patterns of biomass allocation and variations in gas exchange parameters among the species are discussed.Resume On a fait pousser des especes herbeuses de savane de type C4, Digitaria eriantha, Eragrostis lehmanniana et Panicum repens, dans des conditions optimales dans le but de caracteriser l'allocation de leur biomasse aerienne et racinaire et la reponse de leur echange gazeux des changements d'intensite de la lumiere, de concentrations de CO2, et un gradient deficitaire (D-l) de pression de vapeur feuille-air. D. eriantha montrait la plus grande biomasse aerienne et racinaire, une grande efficience de l'assimilation de carbone dans des conditions de luminosite limitee, une grande efficience d'utilisation de l'eau (WUE) et une forte sensibilite des stomates D-l (P = 0,002; r2 = 0,5). P. repens avait une grande biomasse aerienne et atteignait des taux photosynthetiques eleves en lumiere saturee (A(sat), 47 mu mol m-2 s-1), et une conductance stomatique (g(sat) 0.25 mol m-2 s-1) une WUE relativement elevee. E. lehmanniana avait une biomasse qui etait presque la moitie de celle des autres especes et avait un A(sat) et un g(sat) similaires mais qui etaient atteints une WUE plus basse que les autres especes. Cette espece montrait aussi la plus faible reponse stomatique D-l (P = 0,19, r2 = 0,1). L'on discute de la signification ecologique potentielle de ces schemas contrastes d'allocations de biomasse et des variations des parametres des echanges gazeux entre les especes.
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| 6. |
- Mantlana, K. B., et al.
(author)
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Photosynthetic properties of C-4 plants growing in an African savanna/wetland mosaic
- 2008
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In: Journal of Experimental Botany. - : Oxford University Press (OUP). - 0022-0957 .- 1460-2431. ; 59:14, s. 3941-3952
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Journal article (peer-reviewed)abstract
- Photosynthesis rates and photosynthesis-leaf nutrient relationships were analysed in nine tropical grass and sedge species growing in three different ecosystems: a rain-fed grassland, a seasonal floodplain, and a permanent swamp, located along a hydrological gradient in the Okavango Delta, Botswana. These investigations were conducted during the rainy season, at a time of the year when differences in growth conditions between the sites were relatively uniform. At the permanent swamp, the largest variations were found for area-based leaf nitrogen contents, from 20 mmol m(-2) to 140 mmol m(-2), nitrogen use efficiencies (NUE), from 0.2 mmol (C) mol(-1) (N) s(-1) to 2.0 mmol (C) mol(-1) (N) s(-1), and specific leaf areas (SLA), from 50 cm(2) g(-1) to 400 cm(2) g(-1). For the vegetation growing at the rainfed grassland, the highest leaf gas exchange rates, high leaf nutrient levels, a low ratio of intercellular to ambient CO2 concentration, and high carboxylation efficiency were found. Taken together, these observations indicate a very efficient growth strategy that is required for survival and reproduction during the relatively brief period of water availability. The overall lowest values of light-saturated photosynthesis (A(sat)) were observed at the seasonal floodplain; around 25 mu mol m(-2) s(-1) and 30 mu mol m(-2) s(-1). To place these observations into the broader context of functional leaf trait analysis, relationships of photosynthesis rates, specific leaf area, and foliar nutrient levels were plotted, in the same way as was done for previously published 'scaling relationships' that are based largely on C-3 plants, noting the differences in the analyses between this study and the previous study. The within-and across-species variation in both A(sat) and SLA appeared better predicted by foliar phosphorus content (dry mass or area basis) rather than by foliar nitrogen concentrations, possibly because the availability of phosphorus is even more critical than the availability of nitrogen in the studied relatively oligotrophic ecosystems.
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| 7. |
- Arneth, Almut, et al.
(author)
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Water use strategies and ecosystem-atmosphere exchange of CO2 in two highly seasonal environments
- 2006
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In: Biogeosciences. - 1726-4189. ; 3:4, s. 421-437
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Journal article (peer-reviewed)abstract
- We compare assimilation and respiration rates, and water use strategies in four divergent ecosystems located in cold-continental central Siberia and in semi-arid southern Africa. These seemingly unrelated systems have in common a harsh and highly seasonal environment with a very sharp transition between the dormant and the active season, with vegetation facing dry air and soil conditions for at least part of the year. Moreover, the northern high latitudes and the semi-arid tropics will likely experience changes in key environmental parameters (e.g., air temperature and precipitation) in the future; indeed, in some regions marked climate trends have already been observed over the last decade or so. The magnitude of instantaneous or daily assimilation and respiration rates, derived from one to two years of eddy covariance measurements in each of the four ecosystems, was not related to the growth environment. For instance, respiration rates were clearly highest in the two deciduous systems included in the analysis (a Mopane woodland In northern Botswana and a Downy birch forest in Siberia; > 300mmol m(-2) d(-1)), while assimilation rates in the Mopane woodland were relatively similar to a Siberian Scots pine canopy for a large part of the active season (ca. 420 mmol m(-2) d(-1)). Acknowledging the limited number of ecosystems compared here, these data nevertheless demonstrate that factors like vegetation type, canopy phenology or ecosystem age can override larger-scale climate differences in terms of their effects on carbon assimilation and respiration rates. By far the highest rates of assimilation were observed in Downy birch, an early successional species. These were achieved at a rather conservative water use, as indicated by relatively low levels of lambda the marginal water cost of plant carbon gain. Surprisingly, the Mopane woodland growing in the semi-arid environment had significantly higher values of lambda However, its water use strategy included a very plastic response to intermittently dry periods, and values of lambda were much more conservative overall during a rainy season with low precipitation and high air saturation deficits. Our comparison demonstrates that forest ecosystems can respond very dynamically in terms of water use strategy, both on interannual and much shorter time scales. But it remains to be evaluated whether and in which ecosystems this plasticity is mainly due to a short-term stomatal response, or alternatively goes hand in hand with changes in canopy photosynthetic capacity.
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| 8. |
- Mantlana, K B, et al.
(author)
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Seasonal and inter-annual photosynthetic response of representative C-4 species to soil water content and leaf nitrogen concentration across a tropical seasonal floodplain
- 2008
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In: Journal of Tropical Ecology. - 0266-4674. ; 24, s. 201-213
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Journal article (peer-reviewed)abstract
- We examined the seasonal and inter-annual variation of leaf-level photosynthetic characteristics of three C-4 perennial species, Cyperus articulatus, Panicum repens and Imperata cylindrica, and their response to environmental variables, to determine comparative physiological responses of plants representing particular microhabitats within a seasonal tropical floodplain in the Okavango River Delta, Botswana. Five measurement campaigns were carried out over a period of 2 y which covered two early rainy seasons, two late rainy seasons and one dry season. For all three species, light-saturated net photosynthetic rates (A(sat)) and stomatal conductance (9,at) decreased with decreasing soil water content with a seasonal range for A(sat) of approximately 5-45 mu mol m(-2) s(-1), and for g(sat) of 0.03-0.35 mol m-2 s(-1). The species representing the wettest microhabitat (Cyperus) had the highest g(sat) at low leaf-to-air vapour pressure deficits (D-l), the highest ratio of intercellular to ambient CO2 concentration (C-i/C-a), as well as the highest degree of variation in C-l/C-a from season to season. We interpret this as being indicative of its adaptation to a moist growth environment allowing for non-conservative water use strategies as soil moisture is usually abundant. For all three species there was significant variation in photosynthetic fluxes from one year to another that was related to variation in leaf nitrogen and phosphorus. This study shows that when assessing the role of savanna stands in large-scale carbon balance models, the remarkable inter-annual variation in leaf photosynthesis reported in this study should be taken into account.
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| 9. |
- Sjöström, Martin, et al.
(author)
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Exploring the potential of MODIS EVI for modeling gross primary production across African ecosystems
- 2011
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In: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257. ; 115:4, s. 1081-1089
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Journal article (peer-reviewed)abstract
- One of the most frequently applied methods for integrating controls on primary production through satellite data is the light use efficiency (LUE) approach, which links vegetation gross or net primary productivity (GPP or NPP) to remotely-sensed estimates of absorbed photosynthetically active radiation (APAR). Eddy covariance towers provide continuous measurements of carbon flux, presenting an opportunity for evaluation of satellite estimates of GPP. Here we investigate relationships between eddy covariance estimated GPP, environmental variables and the enhanced vegetation index (EVI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) across African savanna ecosystems. EVI was found to correlate well with estimated GPP on a site-by-site basis. Combining EVI with tower-measured PAR and evaporative fraction (EF, a measure of water sufficiency) improved the direct relationship between GPP and EVI at the majority of the sites. The slope of this relationship was strongly related to site peak leaf area index (LAI). These results are promising for the extension of GPP through the use of remote sensing data to a regional or even continental scale.
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