No abstract

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

Summary• Photosynthetic carbon (C) isotope discrimination (D A ) labels photosynthates (d A ) and atmospheric CO 2 (d a ) with variable C isotope compositions during fluctuating environmental conditions. In this context, the C isotope composition of respired CO 2 within ecosystems is often hypothesized to vary temporally with D A .• We investigated the relationship between D A and the C isotope signals from stem (d W ), soil (d S ) and ecosystem (d E ) respired CO 2 to environmental fluctuations, using novel tuneable diode laser absorption spectrometer instrumentation in a mature maritime pine forest.• Broad seasonal changes in D A were reflected in d W, d S and d E . However, respired CO 2 signals had smaller short-term variations than D A and were offset and delayed by 2-10 d, indicating fractionation and isotopic mixing in a large C pool. Variations in d S did not follow D A at all times, especially during rainy periods and when there is a strong demand for C allocation above ground.• It is likely that future isotope-enabled vegetation models will need to develop transfer functions that can account for these phenomena in order to interpret and predict the isotopic impact of biosphere gas exchange on the C isotope composition of atmospheric CO 2 .

Ocellulose were different between years and individuals, and mostly captured by the model, suggesting that the single-substrate hypothesis is a good approximation for tree ring studies on Pinus pinaster, at least for the environmental conditions covered by this study. A sensitivity analysis revealed that the model was mostly affected by five isotopic discrimination factors and two leaf gas-exchange parameters. Modelled early wood signals were also very sensitive to the date when cell wall thickening begins (twt). Our model could therefore be used to reconstruct twt time series and improve our understanding of how climate influences this key parameter of xylogenesis.