Frequency-dependent signals in multi-centennial oak vessel data

This work evaluates whether vessel size data contain a coherent spring precipitation signal over different frequency domains, and how this climatic information varies from the signal encoded in radial growth. The study was conducted at a forest site dominated by old oak trees (Quercus petraea (Mattuschka) Liebl.) located in the southern part of the Swiss Alps. Measurements of earlywood vessel size of 27 cores from 12 oak trees were performed in order to build an unprecedented multi-centennial vessel size chronology. Age-independent long-term growth changes and climatic responses in the high-, mid- and low-frequency domains of differently composed chronologies are explored to test the stability of the signals. Results are compared with analyses of tree-ring width measurements from the same material to gain a deeper understanding of physiological processes. We find that mean vessel size data, in contrast to tree-ring width, carry a frequency-dependent signal. Although the signal of both parameters is similar in the low-frequency domain, the climate response in the high-frequency domain differs: vessel size responds negatively to wet spring conditions, whereas ring width mainly responds positively to water availability during the summer. We hypothesize that the processes responsible for signal registration in the specific frequencies are different. The differing high-frequency signals appear to be directly linked to dissimilar responses to weather conditions driving ring and vessel formation. In contrast, the long-term response is related to conditions that affect tree vigour and, due to a positive feedback on growth, is indirectly reflected in the vessels and ring width characteristics. Due to the change in the frequency related response observed in this pioneering study, it appears that the mean earlywood vessel size of oak is not an adequate proxy for long-term reconstruction of spring precipitation.