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F. Albrecht, W. Dorigo, A. Gruber, W. Wagner, W. Kainz:
"Studying the Role of Soil Moisture on Tree Growth: Linking 32 Years of Satellite Soil Moisture with Tree Rings";
Talk: GV2M: Global Vegetation Monitoring and Modeling, Avignon; 2014-02-03 - 2014-02-07.

Gaining an insight into past climate variations is essential to put current climate change into a historic context. Climatic proxy data such as tree rings have become inevitable for climate change research, serving as a substitute for hydrological variables in regions lacking instrumental records. Tree rings reflect the annual fluctuations in temperature and precipitation. Tree ring based analyses are a potential approach to study the role of hydrological parameters as major drivers of tree growth. Long-term remote sensing datasets of hydrological parameters have the potential to bridge spatial and temporal limitations that are inherent to in situ measurements, which are traditionally used for this purpose. Our study uses a recently developed merged microwave based surface soil moisture dataset (ECV_SM) to investigated the role of soil moisture on tree growth. The ECV_SM dataset, which is being distributed through ESA´s Climate Change Initiative for soil moisture covers the period 1979 to 2010 at a spatial resolution of 0.25°. Tree ring data is obtained from the International Tree ring Data Bank (ITRDB), which contains more than 3000 sites globally. Around 700 of these sites are overlapping with the period covered by the ECV_SM dataset. First analyses were performed for Mongolia where we assumed that water is the limiting factor in summer, while temperature is governing the length of the growth period. By applying simple correlation methods we investigated the relationship between tree growth (as reflected by tree ring width index) and soil moisture satellite observations. In terms of correlation coefficient there is a strong response of tree growth to soil moisture conditions from June to September of the prior as well as the current years. This might indicate that moisture conditions during the summer months are essential for tree growth. Further, the analysis revealed the strongest relationship for September of the previous year (R=0.54) and June of the current year (R=0.57). Hence filling up of the soil water storage before the start of the growing season might be vital for tree growth in the subsequent growth period, too. Our preliminary results align with a recent study examining the relationship between PDSI and tree growth for far-western Mongolia. However, to confirm the moisture signal in tree rings we will further consider other remotely sensed datasets like precipitation and snow water equivalent. Although our preliminary results are only based on one site, we are confident that tree ring data serve as an appropriate tool to study the role of soil moisture on tree growth. Our preliminary results are promising, but further in-depth research is needed to confirm the reliability of such an approach.