An increment borer (pictured here) is used to drill into the tree by hand.
Their rings tell the environmental history of southwest Wisconsin.
Story by Sara Allen with Evan Larson, Ph.D., contributing
The sun is bright and gleaming off the snow–covered hills as my boots crunch across the frozen landscape. I breathe the crisp winter air in deeply as I scan a ridge in search of old oak trees. As I work my way up the slope, their gnarled branches fill my vision. I can't help but smile.
The oaks I'm looking at on this bright winter day are located outside of the village of Rewey, tucked away in the southwest corner of Iowa County amidst the rolling hills of Wisconsin's Driftless Area. I have visited similar sites across 10 of the Driftless Area counties in southwest Wisconsin in a search of the old prairie oaks that dot the landscape like weathered sentinels, their spreading canopies iconic, and their very presence defining Wisconsin's physical and cultural landscape.
These trees have endured tremendous changes over their life spans — the sweeping fires that maintained the open prairies and savannas of the region, the arrival of the plow and the passing of the seasons with their cycles of rain and drought. All the while, these trees have steadfastly recorded their history — told their story — through the annual growth rings they lay on year after year.
I am here to give a voice to their stories. I am a dendrochronologist and my work is the Driftless Oak Project.
There is a lot of conversation about weather patterns, including dramatic swings in moisture conditions; from the floods of 2008, to the extraordinary drought of 2012 and back to the cold wet spring of 2013.
In Wisconsin, these conversations are more than just academic. We live by the land and feel these droughts and floods in our bones and our pocketbooks. Southwest Wisconsin is the state's agricultural heartland and extreme events play a decisive role in local and regional economic stability. Recorded history gives us measurements of rain and temperature over most of the state for nearly 100 years, but as the 1,000–year storms of 2008 illustrated, our recorded history does not capture the full range of weather variability in Wisconsin.
We need a broader perspective. That brings us to dendrochronology, the science of tree rings.
In southwest Wisconsin, warm years with plenty of rain typically lead to good growth and a wide annual growth ring within the trees of the region. Short, cool summers and years of drought tend to result in narrow rings. This relationship is often particularly strong in oak trees.
Oaks are conservative by nature, with slower growth than many other Wisconsin trees during years of plenty, a strategy that provides insurance against growing more than can be supported during years of drought. Their thick, insulating bark enables oaks to withstand the fires that maintained prairies throughout much of southwest Wisconsin prior to European settlement. Matched with their longevity, which can often span centuries, these traits provide sound reasons for why we still find oaks occupying sites that were too dry, or in the past, too fire–prone for most other tree species in Wisconsin.
Perhaps it is this rugged persistence of oaks, the propensity to stand alone on a windswept hill or resolutely keep watch over a field, that lead so many people to revere them. It is also these traits that make oaks exemplary candidates for reconstructing drought patterns.
The Driftless Oak Project is a two–year research program being conducted through the University of Wisconsin–Platteville Tree–Ring, Earth and Environmental Sciences Laboratory (TREES Lab for short) with funding from the University of Wisconsin Water Resources Institute.
With this project we are developing a network of tree ring chronologies, or records of tree growth, across southwest Wisconsin in order to better understand drought patterns. The open–grown bur and white oaks of the Driftless Area are of particular interest. Savanna oaks can often be found growing on south– or southwest–facing slopes, which tend to be warmer and drier than slopes facing other directions. This setting reduces the growing ability of other, less hardy trees, while also enhancing the link between tree growth and moisture availability.
By collecting tree ring samples from these oaks and examining their growth patterns, we are developing exact date records of tree growth that can be compared to records taken using instruments to measure temperature and rainfall. If a strong relationship is identified, the data can then be used to estimate conditions over the life span of the trees.
To date, we have visited 20 sites and collected samples from 175 trees. A regional picture of tree growth is emerging and many of the oldest trees we have cored so far have inner ring dates extending into the early 1700s.
Back on that hill by Rewey, the day is winding down and my arms, as well as those of my colleagues, are exhausted. We have collected our tree ring samples with a tool called an increment borer, similar to a hollow drill bit, that we turn into the trees by hand. Anyone who has worked with oak knows that this is not an easy task! The bur oaks that we sample on this brisk winter day have experienced their share of drought, and the stories they have to tell are tantalizing, if I am reading their weather–beaten trunks and contorted branches correctly.
Later, in the laboratory, I'll find out that these trees have been growing on this site for over 300 years and have seen droughts far worse than any we have recorded using instruments. What are the implications of this work? We are only now getting to a stage where we can begin making inferences about past conditions, but before we do, there are more hills to hike, trees to discover and rings to count.
How do you ask a tree?
One of the first questions we often get from people potentially interested in offering up their oaks for this study is if we are going to hurt their trees. The answer is "no."
We only core during the late fall, winter and early spring to avoid damaging oaks during the oak wilt season. Even in areas with little or no known oak wilt, we simply don't want to bear the slightest risk of opening up some of the most spectacular trees in the state to potential infection.
Once we are inside that seasonal window, the process is simple. Using an increment borer, we collect pencil–sized core samples along two radii of each tree included in the study, typically from opposite sides of the tree and parallel to any slope contours. This allows us to determine an average growth rate for that tree, which can then be compared and combined with the growth patterns of other nearby oaks.
In addition to collecting tree ring samples from living trees, through the scientific principle of cross–dating, we can compare patterns of tree growth from living trees to those of trees that are long dead. This includes stumps, logs or even the furniture and timbers in historic buildings. We have already incorporated records of tree growth from timber samples in historic structures near Shullsburg, Mineral Point and Stitzer, with some of these samples extending into the early 1700s. Samples from long–dead trees will be key to moving further into the past.
We need your help
One of the most interesting parts of this project has been meeting people from all over the state and from all different backgrounds, but who are unified by their shared love of the Driftless Area and take pride in their oaks. As guardians of these oak relics, their pride is justified and, as Aldo Leopold so eloquently states in "A Sand County Almanac," "He who owns a veteran bur oak owns more than a tree. He owns a historical library, and a reserved seat in the theater of evolution."
The response from Wisconsin residents is essential to locating the trees we need for this research and we are once again asking for your help. Although we sampled extensively throughout the region last fall and winter, there are still gaps within our research area that we are hoping to fill.
If you have an oak tree that you think is old, interesting, weathered, unique or just may have a story to tell, please contact Sara Allen at (608) 342–6149 or email, or Evan Larson at (608) 342–6139 or email at the UW–Platteville TREES Lab.
Sara Allen is a research associate at UW-Platteville in the Tree-Ring, Earth and Environmental Sciences (TREES) Laboratory. Evan Larson, Ph.D., is a UW-Platteville associate professor of geography.