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

ABSTRACT 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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Available from: Dennis Baldocchi, Sep 28, 2015
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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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    ABSTRACT: Turfgrass growth and physiological activities are sensitive to temperatures and are affected bymowing height. Increasing temperatures associated with global climate changemay limit photosynthetic capacity of established turfgrass stands.The objective of this studywas to determine the effects of mowing height on carbon exchange of a turfgrass system and consequential effects on turfgrass growth in response to temperature variations across the growing season in kentucky bluegrass (Poa pratensis cv. Baron) stands.Mature (8 years old) turfgrass was mowed at 7.6 cm [high mowing height (HM)] or 3.8 cm [low mowing height (LM)] during 2012 and 2013. Both LM and HM plots displayed significant decline in turf quality (TQ), shoot biomass, and canopy photosynthetic rate (Pn) with increasing air temperature above 23–24 8C in both years and the decline was more pronounced for LMplots. Turf plots were carbon emitters when total respiration rate of shoots, roots, and soil (Rtotal) exceeded canopy Pn under high temperatures during July–September but maintained net carbon gain during cooler seasons (May and June) due to greater Pn to Rtotal ratio (Pn:Rtotal). Lowering mowing height accelerated carbon loss by reducing canopy Pn, particularly under high temperatures. Our results suggested that whether mature turfgrass stands fix or emit carbon is heavily dependent on interaction between seasonal temperatures and mowing height gauging whole-stand photosynthetic capacity. Furthermore, increasing mowing height during summer months may offset the deleterious effects of high temperature by maintaining positive carbon balance within the turfgrass system.
    Journal of the American Society for Horticultural Science. American Society for Horticultural Science 08/2015; 140(4):317. · 1.28 Impact Factor
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    • "The second approach derives information on ecosystem respiration from the better-mixed conditions of daytime carbon flux measurements. In one situation, the estimate of ecosystem respiration is produced by extrapolation of the response curve between canopy photosynthesis and light to its zero intercept (Falge et al., 2002; Xu and Baldocchi, 2004). Strength of this method revolves around the fact that it infers night respiration from daytime measurements , when turbulent mixing is greater. "
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    ABSTRACT: It is necessary to partition eddy covariance measurements of carbon dioxide exchange into its offsetting gross fluxes, canopy photosynthesis, and ecosystem respiration, to understand the biophysical controls on the net fluxes. And independent estimates of canopy photosynthesis (G) and ecosystem respiration (R) are needed to validate and parametrize carbon cycle models that are coupled with climate and ecosystem dynamics models. Yet there is a concern that carbon flux partitioning methods may suffer from spurious correlation because derived values of canopy photosynthesis and ecosystem respiration both contain common information on net carbon fluxes at annual time scales.
    Agricultural and Forest Meteorology 07/2015; 207:117-126. DOI:10.1016/j.agrformet.2015.03.010 · 3.76 Impact Factor
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    • "As regards productivity indices, a number of studies compare GPP and biomass or LAI, some of them reporting correlations similar to ours (Hirota et al. 2010) and some reporting a stronger relationship (e.g. Flanagan et al. (2002) and Xu and Baldocchi (2004)). The relatively lower relationship between GPP and GB may reflect the higher spatial and temporal variability of the latter compared to greenness indices. "
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    ABSTRACT: The increasingly important effect of climate change and extremes on alpine phenology highlights the need to establish accurate monitoring methods to track inter-annual variation (IAV) and long-term trends in plant phenology. We evaluated four different indices of phenological development (two for plant productivity, i.e., green biomass and leaf area index; two for plant greenness, i.e., greenness from visual inspection and from digital images) from a 5-year monitoring of ecosystem phenology, here defined as the seasonal development of the grassland canopy, in a subalpine grassland site (NW Alps). Our aim was to establish an effective observation strategy that enables the detection of shifts in grassland phenology in response to climate trends and meteorological extremes. The seasonal development of the vegetation at this site appears strongly controlled by snowmelt mostly in its first stages and to a lesser extent in the overall development trajectory. All indices were able to detect an anomalous beginning of the growing season in 2011 due to an exceptionally early snowmelt, whereas only some of them revealed a later beginning of the growing season in 2013 due to a late snowmelt. A method is developed to derive the number of samples that maximise the trade-off between sampling effort and accuracy in IAV detection in the context of long-term phenology monitoring programmes. Results show that spring phenology requires a smaller number of samples than autumn phenology to track a given target of IAV. Additionally, productivity indices (leaf area index and green biomass) have a higher sampling requirement than greenness derived from visual estimation and from the analysis of digital images. Of the latter two, the analysis of digital images stands out as the more effective, rapid and objective method to detect IAV in vegetation development.
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