We analyzed the eddy covariance measurements of momentum, mass, and energy taken daily throughout five consecutive seasonal courses (i.e. 840 d after planting) of a pineapple [Ananas comosus (L.) Merr. cv. Red Spanish] field growing in the Orinoco lowlands. This field provides an opportunity for micrometeorological studies because of the flat and windy site; the seasonal weather including ENSO effects and the Crassulacean Acid Metabolism (CAM) physiology of the crop were additional attributes. Soil CO2 flux was quantified and added to the net ecosystem exchange in order to obtain the canopy flux (FC). The canopy CO2 flux partially followed the four phases of CAM sensu Osmond (1978). The daily pattern of gaseous exchange in pineapple showed a continuum spectrum in which a major proportion of CO2 uptake occurring during the daytime was common and in which the CAM expression was related to day and nocturnal CO2 uptake. However, the benefits of CO2 uptake at low water cost were constrained by the limited nocturnal CO2 uptake. Seasonal and ontogenetic changes affected the energy exchange as well as the partitioning of available energy into sensible (QH) and latent (QLE) heat. When the hourly net radiation (QRn) reached its maximum value, latent heat flux (QLE) to available energy throughout the vegetative and reproductive stages was 0.65, 0.05, 0.30, 0.11, and 0.33 for the 1997 wet season, 1997/98 dry season, 1998 wet season, 1998/99 dry season, and 1999 wet season, respectively. Throughout the growth period, we found the pivotal role of surface conductance (gs) in both QLE and FC. Furthermore, the canopy responded to environmental changes. During the wet seasons the gs was strongly influenced by humidity mole fraction deficit and was usually lower than aerodynamic conductance, whereas during the dry seasons, soil water deficit limited evapotranspiration and production rates. For the fully canopy cover, the hourly trend of marginal water cost of pineapple carbon gain in the dry seasons indicated that gs became sufficiently efficient to reduce the amount of water transported per unit of carbon gain. In the wet season, the coupling of CO2 uptake and stomatal conductance was more effective in maintaining a higher proportionality between QLE and gs. and J. San-José, R. Montes, N. Nikonova.
In the Orinoco lowlands, savannas have been often replaced by pastures composed of the C4 grass, Brachiaria decumbens Stapf. We addressed following questions: (1) How does the replacement of the native vegetation affect CO2 exchange on seasonal and annual scales? (2) How do biophysical constraints change when the landscape is transformed? To assess how these changes affect carbon exchange, we determined simultaneously the CO2 fluxes by eddy covariance, and the soil CO2 efflux by a chamber-based system in B. decumbens and herbaceous savanna stands. Measurements covered a one-year period from the beginning of the dry season (November 2008) to the end of the wet season (November 2009). During the wet season, the net ecosystem CO2 exchange reached maximum values of 23 and 10 μmol(CO2) m-2 s-1 in the B. decumbens field and in the herbaceous savanna stand, respectively. The soil CO2 efflux for both stands followed a temperature variation during the dry and wet seasons, when the soil water content (SWC) increased above 0.087 m3 m-3 in the latter case. Bursts of CO2 emissions were evident when the dry soil experienced rehydration. The carbon source/sink dynamics over the two canopies differed markedly. Annual measurements of the net ecosystem production indicated that the B. decumbens field constituted a strong carbon sink of 216 g(C) m-2 y-1. By contrast, the herbaceous savanna stand was found to be only a weak sink [36 g(C) m-2 y-1]. About 53% of the gross primary production was lost as the ecosystem respiration. Carbon uptake was limited by SWC in the herbaceous savanna stand as evident from the pattern of water-use efficiency (WUE). At the B. decumbens stand, WUE was relatively insensitive to SWC. Although these results were specific to the studied site, the effect of land use changes and the physiological response of the studied stands might be applicable to other savannas., J. San José, R. Montes, N. Nikonova, J. Grace, C. Buendía., and Obsahuje bibliografii