Seasonal variation in carbon dioxide exchange over a Mediterranean annual grassland in California

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 CO 2 (F c , 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 F c over the course of a year we decomposed net ecosystem CO 2 exchange into its constituent components, ecosystem respiration (R eco ) and gross primary production (GPP). Daytime R eco was extrapolated from the relationship between temperature and nighttime F c under high turbulent conditions. Then, GPP was estimated by subtracting daytime values of F c from daytime estimates of R eco .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, R eco was an exponential function of soil temperature, but with season-dependent values of Q 10 . The temperature-dependent respiration model failed immediately after rain events, when large pulses of R eco 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, R eco , 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 R eco 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.

How plant functional-type, weather, seasonal drought, and soil physical properties alter water and energy fluxes of an oak–grass savanna and an annual grassland

“…Valentini et al (1995) reported that the slope of the G c versus A c rh/C a relationship ranged between 1.3 and 0.288 and decreased as soil moisture decreases; note, we converted their published data to express G c in units of mol m −2 s −1 . But the study of Valentini et al was on a low productivity serpentine soil and the photosynthetic rates were about one-third of what we measured (Xu and Baldocchi, 2004). In the other relevant study, Wever et al (2002) reported significant interannual variability of the stomatal conductance index for a perennial grassland-slopes ranged between 6 and 15-and they observed higher values slopes for the drier years.…”

“…Maximum and minimum air temperatures are rather mild during the winter. Maximum temperatures during the grass's growing season, the winter and spring, were rarely above 15 • C and minimum temperatures never dropped below −5 • C. But frost and freezing were frequent during the winter and their occurrence stymied growth of the grass and stomatal opening (Xu and Baldocchi, 2004). During the summer, after the grass died, maximum air temperature often exceeded 40 • C in the afternoon and dropped below 10 • C at night.…”

“…A switch in temporal trend of albedo occurred around day 100, which coincided with the period of minimum albedo, for both the woodland and grassland. The switch was prompted by the reproductive heading of the grasses and was reinforced by their sequential senescence (Xu and Baldocchi, 2004). And during the summer the dead grass was 'golden' and highly reflective.…”

“…Here we report on a comparative study of water and energy fluxes over an oak-grass savanna and an annual grassland; information on carbon dioxide exchange measurements over the grassland is discussed in a companion report (Xu and Baldocchi, 2004). These two ecosystems are located in northern California and coexist in the same climate.…”

Efficiency of chlorophyll in gross primary productivity: A proof of concept and application in crops

“…Daytime net ecosystem exchange (NEE) values were computed by integrating the hourly CO 2 flux collected during a day when incoming photosynthetically active radiation (PAR in ) exceeded 1 μmol m −2 s −1 . Daytime estimates of ecosystem respiration (Re) were obtained from the nighttime CO 2 exchange-temperature relationship (e.g., Xu and Baldocchi, 2003). The GPP was then obtained by subtracting Re from NEE.…”