Understory contribution to water vapor and CO₂ fluxes from a subtropical shrubland in northwestern Mexico

The net ecosystem production and evapotranspiration (ET) of arid and subtropical ecosystems is poorly understood by the lack of measurements of CO2 and water vapor fluxes. Moreover, the contributions related to the fluxes in the different strata (i. e. understory) have been poorly studied. The present investigation estimates ET and CO2 fluxes of a shrubby understory with the presence of Mimosa distachya from a subtropical shrubland in relation to the net estimates of ecosystem ET and net CO2 fluxes determined with the eddy covariance technique. Instantaneous ET and CO2 exchange measurements were carried in four plots for different time periods during the day (9, 11, 14 and 18 hours) using a static chamber (16.4 m(3)) coupled with a fast response infrared gas analyzer. The variation of the fluxes during diurnal cycles was used to integrate the magnitude of the flux during the daylight hours. There was great variation in the understory fluxes, during July and September where there was more CO2 gain from the atmosphere towards the ecosystem with a net CO2 exchange of -1.35 +/- 1.93 g C m(-2) d(-1) and -1.15 +/- 0.74 g C m(-2) d(-1), respectively, indicating that photosynthesis was higher than respiration in this stratum. In a dry period during August the CO2 flux was -0.85 +/- 0.73 g C m(-2) d(-1). During these periods, ET was 3.63 +/- 0.15 mm d-1 in July; 2.71 +/- 0.08 mm d(-1) in August, and 1.59 +/- 0.5 mm d(-1) in September. Comparing these results with the total ecosystem ET and the net CO2 exchange, it was found that the understory contributes with 17 and 21% to CO2 fluxes and from 25 to 39% in water vapor fluxes during the North American Monsoon.