Contribucián del estrato arbustivo a los flujos de agua y CO₂de un matorral subtropical en el Noroeste de México

The net ecosystem production and evapotranspiration (ET) of arid and subtropical ecosystems is poorly understood by the lack of measurements of CO₂and 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 CO₂fluxes 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 CO₂fluxes determined with the eddy covariance technique. Instantaneous ET and CO₂exchange 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 m3) 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 CO₂gain from the atmosphere towards the ecosystem with a net CO₂exchange of -1.35 ± 1.93 g C m^-2d^-1and -1.15 ± 0.74 g C m^-2d^-1, respectively, indicating that photosynthesis was higher than respiration in this stratum. In a dry period during August the CO₂flux was -0.85 ± 0.73 g C m^-2d^-1. During these periods ET was 3.63 ± 0.15 mm d^-1in July, 2.71 ± 0.08 mm d^-1in August and 1.59 ± 0.5 mm d^-1in September. Comparing these results with the total ecosystem ET and the net CO₂exchange, it was found that the understory contributes with 17 and 21% to CO₂fluxes and from 25 to 39% in water vapor fluxes during the North American Monsoon.