A millennial multi-proxy reconstruction of summer PDSI for Southern South America

We present the first spatially explicit field reconstruction of the summer (DJF) Palmer Drought Severity Index (PDSI) for the Southern Hemisphere. Our multi-proxy reconstruction focuses on Southern South America (SSA, south of 20° S) and is based on a novel spectral analogue method that aims at reconstructing low PDSI frequencies independently from higher frequencies. The analysis of past regimes and trends in extreme wet spells and droughts reveals considerable geographical and temporal variations over the last millennium in SSA. Although recent changes are in some cases notorious, most were not exceptional at the scale of the last thousand years. Our reconstruction highlights that low frequency water availability fluctuations in Patagonia were generally in antiphase with the rest of the subcontinent. Providing the fact that modern patterns of changes are transferable to the past, we show that such antiphases within SSA's hydroclimate could be attributed to the spatially contrasted response of summer PDSI to the Antarctic Oscillation (AAO). However, El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) signals are also embedded within the PDSI series during the 20th century. All these ocean-atmospheric forcings acted synergically, but the dominant influence appeared highly compartmentalized through space, highlighting clear AAO- (e.g. South Patagonia) and ENSO- (e.g. the Pampas) dominated regions. Our results therefore emphasize the complexity of water-availability fluctuations in SSA and their important dependence on external ocean-atmospheric forcings

Ecophysiological modeling of photosynthesis and carbon allocation to the tree stem in the boreal forest

A better understanding of the coupling between photosynthesis and carbon allocation in the boreal forest, together with its associated environmental factors and mechanistic rules, is crucial to accurately predict boreal forest carbon stocks and fluxes, which are significant components of the global carbon budget. Here, we adapted the MAIDEN ecophysiological forest model to consider important processes for boreal tree species, such as nonlinear acclimation of photosynthesis to temperature changes, canopy development as a function of previous-year climate variables influencing bud formation and the temperature dependence of carbon partition in summer. We tested these modifications in the eastern Canadian taiga using black spruce (Picea mariana (Mill.) B.S.P.) gross primary production and ring width data. MAIDEN explains 90% of the observed daily gross primary production variability, 73% of the annual ring width variability and 20-30% of its high-frequency component (i.e., when decadal trends are removed). The positive effect on stem growth due to climate warming over the last several decades is well captured by the model. In addition, we illustrate how we improve the model with each introduced model adaptation and compare the model results with those of linear response functions. Our results demonstrate that MAIDEN simulates robust relationships with the most important climate variables (those detected by classical response-function analysis) and is a powerful tool for understanding how environmental factors interact with black spruce ecophysiol-ogy to influence present-day and future boreal forest carbon fluxes.Peer reviewe

Liquid-gas phase transition in hot nuclei studied with INDRA

Thanks to the high detection quality of the INDRA array, signatures related to the dynamics (spinodal decomposition) and thermodynamics (negative microcanonical heat capacity) of a liquid-gas phase transition have been simultaneously studied in multifragmentation events in the Fermi energy domain. The correlation between both types of signals strongly supports the existence of a first order phase transition for hot nuclei.Comment: 9 pages, 2 figures, Invited talk to Nucleus-nucleus 2003 Moscow June 200