Productivity, Respiration, and Light-Response Parameters of World Grassland and Agroecosystems Derived From Flux-Tower Measurements

Abstract

Grasslands and agroecosystems occupy one-third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of CO₂ exchange measurements to quantify gross primary productivity, respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration with the use of the light-response method (Gilmanov, T. G., D. A. Johnson, and N. Z. Saliendra. 2003. Growing season CO₂ fluxes in a sagebrushsteppe ecosystem in Idaho: Bowen ratio/energy balance measurements and modeling. Basic and Applied Ecology 4:167-183) from the RANGEFLUX and WORLDGRASSAGRIFLUX data sets supplemented by 46 sites from the FLUXNET La Thuile data set partitioned with the use of the temperature-response method (Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, M. Falk, T. Gilmanov, A. Granier, T. Grunwald, K. Havrankova, D. Janous, A. Knohl, T. Laurela, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J.M. Ourcival, D. Perrin, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. Seufert, F. Vaccari, T. Vesala, and D. Yakir. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11: 1.424-1439). Maximum values of the quantum yield (alpha = 75 mmol.mol(-1)), photosynthetic capacity (A(max) = 3.4 mg CO₂ . m(-2).s-1), gross photosynthesis (P-g,P-max = 1.16 g CO₂ . m(-2).d(-1)), and ecological light-use efficiency (epsilon(ecol) = 59 mmol . mol(-1)) of managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of nonforest ecosystems for uptake of atmospheric CO₂. Maximum values of gross primary production (8 600 g CO₂ . m(-2).yr(-1)), total ecosystem respiration (7 900 g CO₂ . m(-2).yr(-1)), and net CO₂ exchange (2 400 g CO₂ . m(-2).yr(-1)) were observed for intensively managed grasslands and high-yield crops, and are comparable to or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the nonforest sites were apparent sinks for atmospheric CO₂, with mean net uptake of 700 g CO₂ . m(-2).yr(-1) for intensive grasslands and 933 g CO₂ . m(-2).d(-1) for croplands. However, part of these apparent sinks is accumulated in crops and forage, which are carbon pools that are harvested, transported, and decomposed off site. Therefore, although agricultural fields may be predominantly sinks for atmospheric CO₂, this does not imply that they are necessarily increasing their carbon stock

Assessing MODIS GPP in Non-Forested Biomes in Water Limited Areas Using EC Tower Data

“…Their inclusion in the models agrees with the fact that radiation, temperature and water have been reported by several authors as the most important factors influencing ecosystem productivity and respiration (e.g., [9,46]). …”

“…The existence of a strong correlation between annual GPP (GPPec and GPPm) and precipitation in water-limited grasslands and in shrublands (Table 3 and Figure 3a,b) agrees with the results obtained by [9]. It shows the ability of GPPec and GPPm to capture the relationship between photosynthetic activity and one limiting factor like precipitation, and it makes possible the use of this variable to improve the estimation of GPPec in non-forested biomes on an annual basis.…”

“…A linear regression was adjusted when data were normal, and the r 2 coefficient of adjustment and the standard error of the estimates (SEE) were calculated. In addition to the relationship between GPPec and GPPm, the relationships between the latter variables and precipitation were tested, in order to see if the results were similar to those obtained in previous works regarding non-forested [9] and forested sites [6]. Moreover, the relative error term was calculated for each site and year as in Equation (1):…”

“…Though water content or water potential of topsoil horizons are the most desirable predictors in GPP modelling [9], such data is hard to summarise on a yearly basis and it is also not available for all the towers, making precipitation a more adequate variable to be considered. The existence of a strong correlation between annual GPP (GPPec and GPPm) and precipitation in water-limited grasslands and in shrublands (Table 3 and Figure 3a,b) agrees with the results obtained by [9].…”

“…Although shrublands, savannas and grasslands account for 37% of the world's terrestrial area, many studies have been conducted to estimate how forests contribute to the global carbon cycle [1][2][3][4][5][6][7][8], but not many monitor the implications of grasslands/shrublands on the carbon cycle at a large scale [9,10]. Monitoring these non-forested ecosystems in dry areas is important since they are very sensitive to changes in water availability [11] and because they have been suggested as potential candidate areas for major carbon storage efforts [12].…”