The goal of this research is a realistic assessment of how atmospheric CO2 concentrations affect carbon capture by forests. Environmental factors, such as temperature and water availability, and tree age greatly influence how tree growth and forest carbon capture are influenced by atmospheric CO2. Tree rings in ancient and modern wood will be used as an archive of tree growth at known points in time, and when combined with independent estimates of climate and CO2 levels over the past 14,000 years, will provide information on how tree growth responds to atmospheric CO2. The tree core samples will be collected for different tree ages and in different environments. Models that predict carbon capture over a broad range of forest age and environmental conditions will be developed from information on several isotope indicators found in the tree rings of modern and ancient wood combined with measurements of photosynthesis in living trees.
To date, the influence of CO2 enrichment on forest growth has only been studied in young trees exposed to artificially elevated levels of CO2 in a limited number of costly short-term field studies. The broad applicability of these studies remains uncertain because forest carbon capture depends strongly on tree age and its interactions with site-specific growth environments. This new research provides a unique opportunity to test and enhance understanding of how forest ecosystems respond to climate change. The results will have important implications for policy decisions concerning carbon emissions, land-use and forest management to optimize carbon capture.
Tree rings contain a wealth of information about the history of tree growth over decades to many centuries in some cases. As such, chronologies of variation in tree-ring widths have been used to infer relationships between tree growth and variation in aspects of climate such as precipitation and temperature. Although many decades of tree ring research have provided valuable insights concerning these relationships, tree-rings contain additional information that can be decoded using a broader range of analytical approaches. For example, in many tree physiological processes such as photosynthesis and transpiration alter the natural abundance of stable isotopes of important elements such as carbon, oxygen and hydrogen. This isotopic fractionation is recorded in the wood contained in tree rings and can be used to better understand specific relationships between the environment and tree growth and physiology. We developed a novel combination of approaches to use the information contained in tree rings of ancient and modern wood to develop proxies for variations in important climate variables as well as impacts of forest pathogens. By integrating standard ring width analyses with structural, physiological and stable isotope approaches, this project has significantly expanded the toolbox of techniques that can be used to decipher the wealth of information contained in tree rings. The project’s findings will contribute to more reliable tree-ring-based reconstructions of ancient and modern climates. Our results also contribute to a better understanding of impacts of climatic variables and forest pathogens on tree physiology and growth. A postdoctoral researcher and two graduate students received training in the course of conducting this project. Major findings An approach was developed that allowed use of dual stable hydrogen and oxygen isotopes in the cellulose of tree rings to disentangle past variations in temperature from those in humidity. This has traditionally been difficult because temperature and humidity vary together with humidity decreasing as temperature increases. A combination of physiological measurements and analyses of stable carbon isotopes in tree rings showed that the main drivers of growth, photosynthesis and carbon isotope composition in bur oak differ across its range. Growth is more light-limited in the cool, humid northern portion of its range and by the dryness of the air in the central and western portion of its range. This switch in the constraints on tree growth has implications for understanding climatic limitations on growth of other tree species with broad geographic distributions. Trends in wood anatomy of modern bur oaks were used to reconstruct spring temperature conditions during the last glaciation based on trends in wood anatomy of ancient oak wood. Variation in the impact of important foliar fungal pathogen of Douglas-fir in the coastal Pacific Northwest generates a stable carbon isotope signal in tree rings that can be used to track infection history and to identify the major components of climate associated with variation in the severity of Swiss needle cast symptoms. This could be an important tool for plant pathologists and forest managers. Development of dwarf mistletoe infection in western hemlock alters the stable carbon and oxygen isotope composition of its tree rings. The ability to track the development of increasing dwarf mistletoe impacts is important for understanding the ecological role of this forest pathogen.
