Assessment and preservation of climate signals in multi-proxy tree-ring chronologies from Europe

Abstract

Tree rings provide precise annual dating of past climatic and ecological events, offering unique insights into the timing, frequency, and magnitude of natural environmental changes and their impact on human history. However, statistical age-trend removal, heteroscedasticity, and non-stationary climate-growth responses frequently challenge tree-ring width (TRW) and maximum latewood density (MXD) measurements in capturing climate information beyond multicenten-nial to millennial timescales. Novel proxies, including stable carbon (δ13C), oxygen (δ18O), and hydrogen (δ2H) isotopes, as well as xylem anatomical traits, show great promise in addressing these challenges. Yet, long and well-replicated isotopic and wood anatomical records remain rare and are currently limited to a few sites. This dissertation addresses the aforementioned topic by (I) analyzing non-stationary TRW and MXD responses to temperature and hydroclimate conditions; (II) introducing a methodological advancement that enables faster and more resource-efficient development of wood anatomical data; (III) evaluating the skill of annually resolved and non-pooled δ13C in improving relict wood dating and preserving climate signals in early chronology periods; (IV) examining the presence of age trends in cellulose and lignin methoxy δ13C, δ18O, and δ2H; and (V) assessing the paleoclimatic skill of TRW, MXD, δ13C, δ18O, δ2H, radial lumen diameter (Drad), and cell wall thickness (CWT) through a comprehensive evaluation of covariances and climate signals. For this, a new southwest German TRW network, an 850-year-long Pinus nigra MXD chronology from Corsica (France), and new isotopic and wood anatomical data of Bosnian pines (Pinus heldreichii) from Mt. Smolikas (Greece) are established within the framework of this thesis. Key findings include the detection of drought-related, non-stationary TRW and MXD responses to temperature in central and southern European forests, the strength of non-pooled cellulose δ13C measurements in enhancing the crossdating precision of relict pine stems and preserving climatic signals in early chronology periods, and the advantage of isotopic and anatomical data over TRW for reconstructing past Eastern Mediterranean precipitation variability at inter-annual to multicentennial timescales. The results of this thesis pave the way for a better understanding of late-Holocene hydroclimate dynamics in the Mediterranean and underscore the importance of integrating isotopic, particularly cellulose δ13C, and wood anatomical data into existing tree-ring networks to improve paleoclimate reconstructions and future climate projections.