Here I intend to share some of the research that I am currently involved with. As of July 1st 2021, I am a research associate with the tree-ring lab of the Institute of Geography at Johannes Gutenberg-Universität-Mainz. I was previously with the Department of Civil Engineering at the Ohio State University, working in the Stagge Hydrology Lab. I received my Ph.D. in Geoscienes from the University of Arkansas (UARK Tree-Ring Laboratory). Prior to obtaining my master's degree in Geography from the University of Minnesota (at the Center for Dendrochronology), I studied for a Bachelor of Science in Archaeology and Palaeoecology at Queen's University Belfast, Northern Ireland. My research interests mainly revolve around tree growth and past and current climate variability.
 
One of the first scientific discoveries we make as kids is that trees form rings over time. These rings are annual features in most tree species found outside of the tropics, and how well a tree grows (and therefore the how wide a ring becomes) is dependent on several factors including climate. Simplified, growth is limited by soil moisture in arid regions and by temperature at cold sites. The shared variability in year-to-year growth across a stand of trees, or trees from a whole region, is the basis of our science - dendrochronology. Tree rings have been used to date archaeological artifacts with unrivaled precision, and have provided a greater understanding of many of our planet's ecosystems. The relationship between tree-growth and climate has also allowed for the reconstructions of past changes in temperature and precipitation regimes. This information is crucial because our observational record of 100 years or so (at best) does not encompass the full variability of Earth's climate. By using tree rings as a proxy, we can extend the climate records hundreds, if not thousands, of years back in time.

An underlying assumption in paleoclimate reconstruction exercises is a time-stable relationship between predictor (e.g., tree rings) and predictand/target (e.g., precipitation or temperature). Because the relationship between climate and tree growth is undoubtedly dictated (at least in part) by Leibig's law of minimum, it is likely that the limiting factor has changed over time. The "divergence problem" in dendroclimatology refers - somewhat simplified - to the disconnect of temperature as the main driver of tree-growth at high altitude/latitude sites, as general temperatures increase. Although many studies on this subject have been presented, the full ramifications on paleoclimate reconstructions (and possible solutions) are still not clear. As part of the international MONOSTAR team, lead by Dr. Jan Esper, I hope to help make progress in this important area of long-term climatology. These questions tie into larger paleoclimate themes of noise, uncertainty estimates, and the potential of non-linear signals - which has often been points of focus in my previous research. One of the tools available to us for understanding tree growth and its relation to external (climatic) factors is tree-growth models. My work at Mainz will revolve around applying such models to new data produced by the MONOSTAR project.

One of the most important paleoclimate products ever to have been produced is the North American Drought Atlas (NADA) by Cook et al. (1999). Using a vast network of tree-ring chronologies, the NADA provides spatially gridded reconstructions of estimated summer soil moisture across the continent for the past 300 years. However, due to regional climatology, some tree-ring records in North America are tuned to other climatic signals than that of summer soil moisture. In collaboration with researchers at Lamont-Doherty Earth Observatory of Columbia University, NASA Goddard Institute for Space Studies, University of Minnesota, and University of Memphis, the Tree-Ring Laboratory at the University of Arkansas has produced a new gridded reconstruction network of seasonal precipitation variability for the North American continent (NASPA, Stahle et al., 2020). The new dataset partly relies on sub-annual growth measurements (of earlywood and latewood widths) to extract discrete climate information from the trees. Early in this project, I worked on mapping the relationship between these growth variables in previously collected data, as well as producing new measurements of the two (Torbenson et al., 2016). More recently, I have published results from the analyses of drought relief and reversals recorded by the NASPA reconstructions (Torbenson et al., 2021).