Seasonal variation in CO2 exchange over a Mediterranean open grassland in California

Ecosystem Science Division, Department of Environmental Science, Policy and Management, 151 Hilgard Hall, University of California at Berkeley, Berkeley, CA 94720, USA
Agricultural and Forest Meteorology (Impact Factor: 3.76). 05/2004; 123(1-2):79-96. DOI: 10.1016/j.agrformet.2003.10.004


Understanding how environmental variables affect the processes that regulate the carbon flux over grassland is critical for large-scale modeling research, since grasslands comprise almost one-third of the earth’s natural vegetation. To address this issue, fluxes of CO2 (Fc, flux toward the surface is negative) were measured over a Mediterranean, annual grassland in California, USA for 2 years with the eddy covariance method.To interpret the biotic and abiotic factors that modulate Fc over the course of a year we decomposed net ecosystem CO2 exchange into its constituent components, ecosystem respiration (Reco) and gross primary production (GPP). Daytime Reco was extrapolated from the relationship between temperature and nighttime Fc under high turbulent conditions. Then, GPP was estimated by subtracting daytime values of Fc from daytime estimates of Reco.Results show that most of carbon exchange, both photosynthesis and respiration, was limited to the wet season (typically from October to mid-May). Seasonal variations in GPP followed closely to changes in leaf area index, which in turn was governed by soil moisture, available sunlight and the timing of the last frost. In general, Reco was an exponential function of soil temperature, but with season-dependent values of Q10. The temperature-dependent respiration model failed immediately after rain events, when large pulses of Reco were observed. Respiration pulses were especially notable during the dry season when the grass was dead and were the consequence of quickly stimulated microbial activity.Integrated values of GPP, Reco, and net ecosystem exchange (NEE) were 867, 735, and −132 g C m−2, respectively, for the 2000–2001 season, and 729, 758, and 29 g C m−2 for the 2001–2002 season. Thus, the grassland was a moderate carbon sink during the first season and a weak carbon source during the second season. In contrast to a well-accepted view that annual production of grass is linearly correlated to precipitation, the large difference in GPP between the two seasons were not caused by the annual precipitation. Instead, a shorter growing season, due to late start of the rainy season, was mainly responsible for the lower GPP in the second season. Furthermore, relatively higher Reco during the non-growing season occurred after a late spring rain. Thus, for this Mediterranean grassland, the timing of rain events had more impact than the total amount of precipitation on ecosystem GPP and NEE. This is because its growing season is in the cool and wet season when carbon uptake and respiration are usually limited by low temperature and sometimes frost, not by soil moisture.

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    • "When mean windspeed (U) was below the threshold value (U = 2.5 m s −1 corresponding to a friction velocity of approximately 0.25 m s −1 ), data were filled in using night CO 2 exchange-temperature relationships from windier conditions. The daytime estimates of ecosystem respiration (Re) were determined from the temperature-adjusted nighttime CO 2 exchange (Xu and Baldocchi, 2004). The GPP was obtained from the difference between NEP and Re (sign convention: GPP and NEP are positive during C uptake by the vegetation and Re is negative). "
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    ABSTRACT: Vegetation productivity metrics, such as gross primary production (GPP) may be determined from the efficiency with which light is converted into photosynthates, or light use efficiency (ε). Therefore, accurate measurements and modeling of ε is important for estimating GPP in each ecosystem. Previous studies have quantified the impacts of biophysical parameters on light use efficiency based GPP models. Here we enhance previous models utilizing four scalars for light quality (i.e., cloudiness), temperature, water stress, and phenology for data collected from both maize and soybean crops at three Nebraska AmeriFlux sites between 2001 and 2012 (maize: 26 field-years; soybean: 10 field-years). The cloudiness scalar was based on the ratio of incident photosynthetically active radiation (PARin) to potential (i.e., clear sky) PARpot. The water stress and phenology scalars were based on vapor pressure deficit and green leaf area index, respectively. Our analysis determined that each parameter significantly improved the estimation of GPP (AIC range: 2503-2740; likelihood ratio test: p-value<0.0003, df=5-8). Daily GPP data from 2001 to 2008 calibrated the coefficients for the model with reasonable amount of error and bias (RMSE=2.2gCm-2d-1; MNB=4.7%). Daily GPP data from 2009 to 2012 tested the model with similar accuracy (RMSE=2.6gCm-2d-1; MNB=1.7%). Modeled GPP was generally within 10% of measured growing season totals in each year from 2009 to 2012. Cumulatively, over the same four years, the sum of error and the sum of absolute error between the measured and modeled GPP, which provide measures of long-term bias, was ±5% and 2-9%, respectively, among the three sites.
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    • "During the nighttime (incoming solar radiation <10 W m −2 ), the relationship between R eco (i.e., NEE) and soil temperature at a depth of 0.05 m (T s , °C) under high turbulence conditions (u * >0.1 m s −1 ) can be given by (Xu and Baldocchi 2004) R eco ¼ b 0 exp bT s ð Þ ð2Þ Q 10 ¼ exp 10b ð Þ ð3Þ where b 0 and b are the regression parameters. Q 10 is the temperature sensitivity coefficient of R eco . "
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    • "For example, in natural grasslands, such as central Mongolian grass steppe, it was reported that annual net ecosystem carbon exchange rate is partitioned into 179 gÁm –2 carbon per year of photosynthesis and 138 gÁm –2 carbon per year of respiration, suggesting that unmanaged grassland is a weak carbon sink (Li et al., 2005). Maximal seasonal photosynthetic carbon gains and respiratory carbon loss were 10.1 and 6.5 gÁm –2 carbon per year, respectively, across two growing seasons for a Mediterranean annual grassland in California, and significantly higher levels of respiratory carbon loss occurred as a result of increasing air temperature during late June (Xu and Baldocchi, 2004). For managed turfgrass systems, previous studies have focused on quantifying organic carbon sequestration or long-term carbon storage in soil and organic matter (Bandaranayake et al., 2003; Milesi et al., 2005; Qian and Follett, 2002; Qian et al., 2003, 2010; Townsend-Small and Czimczik, 2010). "
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