Advances in Climate and Extreme-Event Analyses Using Quantitative Wood Anatomy

Abstract

Understanding long-term temperature fluctuations is crucial for projecting future climate change and placing recent trends and extreme events in a long-term context. Paleoclimate reconstructions covering the past 2000 years (the Common Era) provide a high-resolution perspective on Earth's climate that is not available from short instrumental records alone. Tree-ring data are widely used for reconstructing climate and studying forced and internal climate system forcing over this time period. Rising anthropogenic greenhouse gas concentrations have unequivocally caused rising temperatures, but the role and magnitude of internal climate variability particularly at regional scales is still the subject of important debate. Large volcanic eruptions were the most significant short-term forcing on the climate of the Common Era, but different proxies and models disagree on the magnitude and duration of their impacts. By extending the period of climate observations, tree-ring data can enhance the study of trends, variability, and extreme events. Nevertheless, tree-ring proxy data still have limitations for climate reconstructions, and the varying physiological responses of trees complicate the identification and quantification of potential varying climate influences. Persistent discrepancies between paleoclimate model simulations, recent instrumental data, and tree-ring proxy reconstructions indicate that further investigations into how tree-ring data as biological archives reflect climate variability across scales from seasonal to millennial are needed. The objective of this dissertation is to address ways to overcome the limitations of traditional tree-ring data by leveraging new techniques that specifically examine the physiological responses of trees to both internal climate variability and changing radiative forcing. I use quantitative wood anatomy (QWA) to obtain high-resolution measurements at the cellular level, investigate biases in tree-ring data, and identify the impact on climate reconstructions. The work presented in this dissertation uses three specific cases studies comprising different environments, time scales, and climate regimes. First, I investigate discrepancies between historical observations of a warm European summer following the 1783-1784 CE Laki eruption and tree-ring data indicating cold temperatures. Using QWA, I resolved this issue showing that direct defoliation by the volcanic haze and anomalous wood density fluctuation. In the second study I use multiple cell measurements derived from QWA analysis to disentangle complex climate-growth relationships in Rocky Mountain bristlecone pine. In my third study, I use a long series of replicated and high-resolution QWA anatomical MXD measurements over most of the last millennium to improve the accurate capture of low-frequency temperature signals in trees growing at the northern treeline in North America. In all three cases, the use of QWA data is demonstrated to provide novel information and advance knowledge across multiple climate contexts, research questions, and time scales. These studies demonstrate in diverse ways the clear potential of QWA as a critical tool for addressing pressing environmental and climate questions in paleoclimatology.