Elemental variability in tree-rings as indicator for climate change : a case study on beech and oak trees at the Laacher See, Germany

Sammanfattning: The aim of this study was to contribute to the comprehension of the connection of element uptake in trees at the Laacher See in Germany and climatic parameters on different time scales. Understanding the relationship of certain elements in trees and temperature might enable the assignment of extraordinary high peaks in the elemental concentration, which cannot be explained by temperature, to another process as CO2 degassing of the mofettes, which are relics of the volcanic past of the lake. Beech and oak trees at the Laacher See are not growing close to their climatic distribution limit. The relationship between tree-ring width and temperature might be not distinct enough and therefore insufficient to reconstruct climate back in time based on typical dendrochronological methods. Increasing temperatures affect the production of biomass and the release of elements in the soil. These elements are taken up by roots of trees and are implemented in yearly forming tree-rings. Using elements in tree-rings is discussed to be an alternative method for the reconstruction of climate back in time. However, there is an ongoing debate about dendrochemical methods as it is still not clear how much the resolution of the results is affected by translocation of elements between tree-rings. In this study, tree-cores of living beech and oak trees, sampled 2020, have been measured using an energy dispersive x-ray fluorescence technique (ED-XRF), which is a non-destructive method for multi-element analysis. Eleven elements (Ca, Co, Cu, Cr, Ge, Fe,K, Mn, Ni, Sc, Zn) and three elemental ratios (K/Ca, Ca/Mn, Fe/Mn) have been found to show significant positive or negative correlations in the time period from 1901 to 2018 between their concentration in tree-rings and temperature. Since Cu, Ge and K also show significant correlations with temperature in 10-year periods, higher mobility between tree-rings is assumed for those elements. Low mobility is suggested for the elements Ca and Mn as their concentration in the bark and in the wood shows little correlation. Low mobility indicated by significant correlations between elements and yearly temperature or a lack ofcorrelation between the concentration in the bark and in the wood, makes elements more suitable as indicators for temperature. However, the explanation of element concentration in tree-rings is complex, as elements are influencing each other. Antagonistic behaviour has especially been observed for the elements Ca, Co, Ge, Fe, K, Mn, Ni and Sc. These elements are showing significant correlations with temperature as well, which reveals the dependency of elemental concentrations on more than one variable. Furthermore, acidic soil at the study site might be caused by elevated CO2 concentrations, which originate from mofettes. Increasing concentrations of Fe and Ca in tree-rings give indication for decreasing soil pH. Acidification is a contrary process to temperature increase, as it generally reduces the microbial activity and therefore the availability of nutrients.