The lack of information on the ways seasonal drought modifies the CO2 exchange between Neotropical rainforest ecosystems and the atmosphere and the resulting carbon balance hinders our ability to precisely predict how these ecosystems will respond as global environmental changes force them to face increasingly contrasting conditions in the future. To address this issue, seasonal variations in daily net ecosystem productivity (NEPd) and two main components of this productivity, daily total ecosystem respiration (REd) and daily gross ecosystem productivity (GEPd), were estimated over 2 years at a flux tower site in French Guiana, South America (511605400N, 5215404400W). We compared seasonal variations between wet and dry periods and between dry periods of contrasting levels of intensity (i.e. mild vs. severe) during equivalent 93-day periods. During the wet periods, the ecosystem was almost in balance with the atmosphere (storage of 9.0 gCm_2). Seasonal dry periods, regardless of their severity, are associated with higher incident radiation and lower REd combined with reduced soil respiration associated with low soil water availability. During the mild dry period, as is normally the case in this region, the amount of carbon stored in the ecosystem was 32.7 gCm_2. Severe drought conditions resulted in even lower REd, whereas the photosynthetic activity was only moderately reduced and no change in canopy structure was observed. Thus, the severe dry period was characterized by greater carbon storage (64.6 gCm_2), emphasizing that environmental conditions, such as during a severe drought, modify the CO2 exchange between Neotropical rainforest ecosystems and the atmosphere and potentially the resulting carbon balance

Comparisons were made among Douglas-fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C 3 ) grassland for ecosystem-level water-use efficiency (WUE). WUE was defined as the ratio of photosynthetic CO 2 assimilation rate and evapotranspiration (ET) rate. The ET data measured by eddy covariance were screened so that they overwhelmingly represented transpiration. The three sites used in this comparison spanned a range of vegetation (plant functional) types and environmental conditions within western Canada. When compared in the relative order Douglas-fir (located on Vancouver Island, BC), aspen (northern Saskatchewan), grassland (southern Alberta), the sites demonstrated a progressive decline in precipitation and a general increase in maximum air temperature and atmospheric saturation deficit (D max ) during the mid-summer. The average ( AE SD) WUE at the grassland site was 2.6 AE 0.7 mmol mol À1 , which was much lower than the average values observed for the two other sites (aspen: 5.4 AE 2.3, Douglasfir: 8.1 AE 2.4). The differences in WUE among sites were primarily because of variation in ET. The highest maximum ET rates were approximately 5, 3.2 and 2.7 mm day À1 for the grassland, aspen and Douglas-fir sites, respectively. There was a strong negative correlation between WUE and D max for all sites. We also made seasonal measurements of the carbon isotope ratio of ecosystem respired CO 2 (d R ) in order to test for the expected correlation between shifts in environmental conditions and changes to the ecosystemintegrated ratio of leaf intercellular to ambient CO 2 concentration (c i /c a ). There was a consistent increase in d R values in the grassland, aspen forest and Douglas-fir forest associated with a seasonal reduction in soil moisture. Comparisons were made between WUE measured using eddy covariance with that calculated based on D and d R measurements. There was excellent agreement between WUE values calculated using the two techniques. Our d R measurements indicated that c i /c a values were quite similar among the Douglas-fir, aspen and grassland sites, despite large variation in environmental conditions among sites. This implied that the shorter-lived grass species had relatively high c i /c a values for the D of their habitat. By contrast, the longer-lived Douglas-fir trees were more conservative in water-use with lower c i /c a values relative to their habitat D. This illustrates the interaction between biological and environmental characteristics influencing ecosystem-level WUE. The strong correlation we observed between the two independent measurements of WUE, indicates that the stable isotope composition of respired CO 2 is a useful ecosystem-scale tool to help study constraints to photosynthesis and acclimation of ecosystems to environmental stress.