Tree-ring climate reconstruction – continuation

The aim of the proposed project is a climate reconstruction with special emphasis on temperature. For the analysis, dendrochronological methods will be applied. In a previous project, a 3.5thousand-year spruce and larch chronology for ring width and ring density which originates from Schwarzer See - an area situated in the northern part of the Alps in Austria - was set up. On the basis of this scale, dendroclimatic analyses of the annual growth- ring width and density were performed. The proposed project represents the next stage of research. The planned investigation of stable carbon isotopes will allow comparing, verifying and expanding the results that have already been obtained. Isotopic measurements have several advantages which may prove to be very significant during the research planned. Their application will provide the possibility to preserve an intact climatic signal in the low frequency domain. This is connected to the fact that isotopic sequences - in contrast to ring-width and density series - do not require standardization. Moreover, for isotopic studies it is not necessary to use many samples to ensure the representative average series. Therefore, the elimination of problems resulting from the limited simple size at the beginning of the chronology is expected. During the isotopic analyses, the influence of anthropopression could also be corrected. Anthropogenic factors are impossible to estimate in the course of the growth-ring width or density research. The results of previous studies unequivocally indicate that, for the trees from the Alpine area, there exists a strong relationship between temperature, precipitation, air humidity and the carbon isotope content in annual growth rings. These studies also demonstrate high correlation with temperature, independent from the altitude of sites. Owing to this fact, the samples from the areas situated below the upper timber line, as in the case of the used chronology, are characterised by a strong climatic signal. The application of isotopic measurements during the planned research will also be the source of additional information. Simultaneous analyses of annual growth-ring width, density and isotopic composition offer the possibility of much more precise reconstructions of weather conditions obtaining higher correlations for more climatic factors and extending the results to a longer period of the growing season. There are numerous favourable factors that predispose the analysed chronology for dendroclimatic research, especially for the reconstruction of temperature. The first of these is the small territorial range of the area where the trees had grown. Next is the mountainous character of the region, where the trees come from - although the sampling was done at quite low elevation, the site is located almost at the timberline and is characterized by cold climate conditions and high snow cover. Other beneficial facts are the long-term meteorological records existing in this area dating back to the eighteenth century and the multi-centennial length of the chronology.

The aim of the presented project was the reconstruction of the past temperature in the area situated in the northern part of Limestone Alps. The research was conducted on the basis of tree-ring chronology. This chronology was created by means of a wood which originated from tree trunks deposited on the bottom of mountain lake Schwarzersee and from the present-day trees growing around this lake. This project is based on the analyses of stable carbon isotopes in wood. The ratio of stable carbon isotopes depends among others, on weather conditions that exist during the formation of particular growth-rings. In high-mountain areas a significant role in this process is played mainly by the temperature factor. In the period of high temperatures the stomata in leaves are closed, preventing the penetration of carbon dioxide into the plant. Under these conditions, a higher amount of heavier fractions of carbon dioxide is used during photosynthesis, and these fractions enrich a newly created growth-ring. The situation is reversed when the temperatures are low and the stomata are opened. In the circumstances, photosynthesis uses preferentially the lighter fractions of carbon dioxide molecules. In this way, the weather changes are reflected in proportions of stable carbon isotopes contained in particular growth-rings. During the project measurements of isotopic ratio were conducted separately for each annual growth-ring. Next, the obtained results were compared with meteorological data available for the area. Using the calculated correlation coefficients and determined parameters of the linear regression equation, the relationship which exists between the temperature and the stable carbon isotope content in wood was defined. This relationship was then extrapolated on time interval which is devoid of meteorological record. Thereby, the changes occurring from year to year in weather conditions were estimated for the period of time specified by the range of chronology. In the case of Schwarzersee dendrochronological scale this was nearly two thousand years. The newly created stable carbon isotope chronology covers the period 150-2000 AD. Tracking the thermal conditions of the environment over many centuries is important in the view of the intensive contemporary climate changes, as it allows placing these changes in the context of fluctuations that existed in the past. It also enables to some degree to forecast the trends of climate variability that can be expected in the future. Such prediction is important for planning and managing the economy, both locally as well as in global scale.