Twentieth century changes of tree ring isotopes in Southeastern Brazilian forests: How do climate conditions influence growth and water use efficiency (WUE) and thus enforce tree migration

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Following IPCC, there is strong evidence that the global temperature has risen by 1°C in less than 150 years while the atmospheric CO2 concentration increased by more than 30%. This partly man-made effect raises the important question to what extent these anthropogenic activities interfere with the ecophysiological requirements of tropical and subtropical forests. To understand the effects of changing environmental forcing on ecosystems and in particular on trees requires a basic knowledge about the decadal to centennial variability of indigenously growing trees. Consequently, it is of utmost importance to understand the long term behavior of living trees by creating highly resolved regional and temporal environmental data-sets over longer periods of time. Therefore, recording their temporal behavior by measuring the respective parameters (tree ring increment, stable isotopes etc.) reveals either directly or through tree physiological reactions much of the wellbeing of trees in their environment.The objective of our study is to quantify the natural long term reactions of trees with regard to water use efficiency (WUE) and transpiration on a regional scale. The emphasis is concentrated on a subtropical transect across the State of São Paulo, Brazil. This should pave the way for elucidating the tree development of the past and for predicting tree dynamics in particular tree growth in a climatically changing world. In order to achieve these goals growth rates from all four sites could successfully be calculated. Altogether, more than 40.000 tree rings were counted and measured and 7.000 out of 10.000 samples for isotope analysis are already packed. Next steps will include the development of isotope chronologies, reanalyzes of climate-data and the calculation of the WUE.Climate data sets (precipitation and river gauges) for more than 120 stations in the State of São Paulo were obtained from the Brazilian water agency. Additionally daily micrometeorological data (2003-2004) measured at the sites Caetetus and Carlos Botelho have been provided. For the site Vassununga even eddy covariance flux tower data are available for interpretation of the intra-annual isotope analyses of 2 trees (relative humidity). This exercise should additionally lead to the exciting conclusion of how trustworthy the predominantly used iWUE can replace the genuine WUE. The usage of the iWUE is a stopgap because the relevant meteorological data (rh) are mostly non-existent.As mentioned above almost all studies about WUE consider only the iWUE. This does not permit to separate the contribution of the assimilation rate A from that one of the stomatal conductance gs in the alteration of the WUE. The joint measurement of the stable carbon and oxygen isotopes in the same tree ring provides a promising start to solve this problem. This allows understanding the tree reactions to climate, climate variability (e.g. ENSO) and respectively CO2 driven changes in tree physiology.