Here I intend to share some of the research that I am currently involved with. I am a PhD candidate with the University of Arkansas Tree-Ring Laboratory, studying under Professor David W. Stahle. Prior to receiving 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 the 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.
Reconstructions of seasonal hydroclimate
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 spatial 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, and University of Memphis, the Tree-Ring Laboratory at the University of Arkansas is working on a project designed to produce gridded reconstructions of seasonal hydroclimate variability for the North American continent. The new dataset will partly rely on sub-annual growth measurements (of earlywood and latewood widths) to extract discrete climate information from the trees. I have recently worked on mapping the relationship between these growth variables in previously collected data, as well as producing new measurements of the two.
Persistence of moisture across the Americas
Water availability is becoming an increasingly important aspect of how we view our changing world. Skillful forecasts of precipitation can greatly mitigate risks associated with drought (e.g. agricultural yields) but predictability on seasonal timescales remains difficult for the broader climate science community. Seasonal persistence, the relationship between one season and the next, can provide some modest skill in forecasting if the relationship is time-stable (or at least predictable over time). We have investigated such persistence over the central United States, using both instrumental and proxy data, to understand how summer rainfall relates to prior moisture conditions. Our tree-ring based reconstructions of May soil moisture and summer atmospheric moisture represent some of the strongest of their kind. The analysis indicates that Atlantic sea surface temperatures, specifically the Atlantic Multidecadal Oscillation, drives the magnitude of moisture persistence recorded over the southcentral United States. Similar studies are under way focusing on specific areas in South America.
Dendrochronology in Brazil
The importance of annually-resolved paleoclimatic records from the tropics is perhaps greater than from anywhere else on the planet. Brazil is not only expected to experience some of the greatest changes in the face of anthropogenic warming, but the hydroclimate of the Amazon basin is also strongly connected to the El Niño Southern Oscillation. Despite being the most diverse forest ecosystem on Earth, estimated to contain more than 15,000 different species of trees, Amazonia has produced very little in terms of tree-ring chronologies. We want to change that. I am currently working as a research assistant funded through a National Science Foundation grant given to the Tree-Ring Laboratory at the University of Arkansas, together with collaborators at the Federal University of Lavras, the Brazilian Institute for Amazonian Research at Manaus, and the Argentine Institute of Snow Research, Glaciology, and Environmental Sciences in Mendoza. We have collected, and will continue to collect, tree-ring samples from several regions in Brazil. Over the past two years, we have produced the first precipitation reconstructions based on tree-ring widths for both eastern and western Amazon. We hope to continue this work and have developed records that are tuned to both annual and sub-decadal hydroclimate variability.
Uncertainty in Holocene time-scales
Paleoclimate products do not only come in the form of tree-rings. For several decades, scholars have developed records from the sediment layers of lakes and bogs, from ice-cores, and from many other proxy types. Some of these records have their own dating methods and chronologies associated but many rely on radiocarbon dating to provide accurate ages. One of the main obstacles in comparing proxy records dated by different techniques is the potential asynchrony between time-scales. Together with other researchers, I have compared dates from the Greenland Ice Core Chronology (GICC05) and dates connected to the radiocarbon calibration curve (INTCAL) for the same climatic events during the past 13,000 years. Our results suggest that GICC05 dates are consistently older than their counterparts in INTCAL, and that the offset may be up to 70 years for the early Holocene (8000-11,500 years ago). If this holds true, some of the current ideas on how climate evolved across space in the past may be inaccurate. There are several unanswered questions regarding temporal uncertainty in paleoclimate proxies that still need to be addressed.
Citations based on Google Scholar counts (as of September 18th 2018)
Tree rings and rainfall in the equatorial Amazon
D. Granato-Souza, D.W. Stahle, A.C. Barbosa, S. Feng, M.C.A. Torbenson, G. de Assis Pereira, J. Schöngert, J.P. Barbosa, and D. Griffin
Climate Dynamics, in press <doi>
The climate response of Cedrela fissilis annual ring width in the Rio São Francisco basin, Brazil
G. de Assis Pereira, A.C. Barbosa, M.C.A. Torbenson, D.W Stahle, D. Granato de Souza, R.M. dos Santos, and J.P. Delfino Barbosa (2018)
Tree-Ring Research, v. 74, p. 162-171 <doi>
Tree-ring reconstructed rainfall over the southern Amazon Basin
L. Lopez, D.W. Stahle, R. Villalba, M.C.A. Torbenson, S. Feng, and E.R. Cook (2017)
Geophysical Research Letters , v. 44, p. 7410-7418 <doi>
The relationship between earlywood and latewood ring-growth across North America
M.C.A. Torbenson, D.W. Stahle, J. Villanueva Díaz, E.R. Cook, and D. Griffin (2016)
Tree-Ring Research, v. 72, p. 53-66 <doi>
The Mexican Drought Atlas: Tree-ring reconstructions of the soil moisture balance during the late pre-Hispanic, colonial, and modern eras
D.W. Stahle, E.R. Cook, D.J. Burnette, J. Villanueva Díaz, J. Cerano, J. Burns, D. Griffin, B.I. Cook, R. Acuna, M.C.A. Torbenson, P. Szejner, and I. Howard (2016)
Quaternary Science Reviews, v. 149, p. 34-60 <doi>
The rarity of absent growth rings in Northern Hemisphere forests outside the American Southwest
S. St. George, T.R. Ault, and M.C.A. Torbenson (2013)
Geophysical Research Letters, v. 40, p. 3727-3731 <doi>
ACCEPTED, IN REVIEW & SUBMITTED
Landscape and meteorological factors associated with frost rings in bristlecone pine at Mt. Goliath, Colorado
A. Barbosa, D.W. Stahle, D.J. Burnette, M.C.A. Torbenson, E.R. Cook, M. Bunkers, G. Garfin, and R. Villalba
(in review Journal of Applied Meteorology and Climatology)
Longevity, climate sensitivity, and conservation status of bald cypress at Black River, North Carolina
D.W. Stahle, J.R. Edmondson, I. Howard, C.R. Robbins, R. Griffin, A. Carl, K. Hall, D.K. Stahle, and M.C.A. Torbenson
(submitted Geophysical Research Letters)