Dendrochronological investigations of living and relict yew trees from eastern England

Abstract

Understanding how the Earth’s climate system has varied in the past is essential for contextualizing its current state and predicting future climate changes. However, for most of the planet, instrumental measurements of temperature and precipitation began only in the early 20th century and thus do not reveal a long-term climate variability. The development of palaeoclimate reconstructions is therefore needed. Tree rings have provided annually resolved and absolutely dated reconstructions of temperature or hydroclimate variability over past centuries to millennia, but as wood decays with time, their number declines drastically. In this PhD, I benefit from hundreds of living and subfossil yew (Taxus baccata L.) trees from eastern England and reconstruct climate and environmental conditions for pre-industrial and mid-Holocene periods. First, I apply dendrochronological methods to develop a tree-ring width (TRW) chronology spanning the past three centuries from a network of over 150 modern yew trees across eastern England. Analysis of climate-growth relationships reveals an unexpectedly strong signal that enables reconstructing interannual to multi-decadal spring-summer precipitation and mid-summer drought variability for western Europe. Second, I analyse hundreds of exceptionally well-preserved subfossil yew wood remains that were excavated by generations of farmers from near sea-level peat-rich sediments in the Fenland region of eastern England. Using dendrochronological and radiocarbon dating, I develop two floating TRW chronologies for the mid-Holocene. Stable oxygen (δ18O) isotopes are then used for establishing an absolute date via cross-dating against a continuous oak δ18O chronology from the same region. I then develop an eco-physiological model based on dual isotopic signal from stable oxygen and carbon isotopes (δ18O and δ13C) to reconstruct hydroclimatic changes on interannual to centennial timescales between 5220 and 4150 years ago. Juxtaposing the pre-industrial and mid-Holocene hydroclimate reconstructions allows to learn from the past about the present and vice versa. This PhD reveals a shift towards unusually wet conditions around 4,700 and 4,200 years ago, when extensive yew woodlands declined and later suddenly disappeared from eastern England. Engaging with independent palynological, limnological, and archaeological proxy evidence, I hypothesize that the later pluvial was likely driven by increased storminess in the North Atlantic, a sea-level rise in the North Sea, and a prolonged negative phase of the North Atlantic Oscillation, which jointly resulted in severe waterlogging of the region. The temporal coincidence of the shift with the highly-debated 4.2 ka climate event is recognised and critically discussed. While the 4.2 ka anomaly is commonly associated with extreme droughts in Asia that contributed to a collapse of several civilisations, the newly developed palaeoclimate record sheds new light onto the nature and potential origin of these climate changes.