Oak Trees and Climate in the Midwest

Post by Kelly Heilman, a graduate student with Jason McLachlan at the University of Notre Dame

If trees could talk…

Every year, oak trees in the Midwest awaken in spring, to spread their leaves to grow through the summer, and settle down for a long winter’s rest in the fall. As we watch this seasonal cycle, trees warn us of the coming of winter as they shed their leaves, putting on a glorious show of red, gold and orange. At times, trees tell us to slow down by inviting us into their expansive shade to rest, read a book, and listen the birds sing in the distance. However, to many dendrochronologists, trees talk about something else–their records of the past. The patterns of each season’s tree growth are recorded in the annual rings of trees, and are protected like a memory beneath the tree’s bark. These annual variations in tree rings provide researchers with information on the responses of trees to climate variations, as well as to other stresses that trees face, such as competition with their neighbors for light, and fire disturbances. All this makes tree ring records particularly useful tools for looking into the past. So, in a way trees can talk . . . but it takes some effort for us to listen to them.

Trees in the temperate zone, such as this Bur Oak, record annual rings of growth, allowing us to count their rings to get tree age, and correlate growth with climate records. Pictured above is a 5-mm wide core sample from a tree at Bonanza prairie SNA (Scientific and Natural Area) in Minnesota, with the bark pictured on the left and the center of the tree on the right. (Click on image for a larger view.)

Looking at the past responses of ecosystems to climate variations through the lens of tree rings can provide a better understanding of how these ecosystems might respond to future changes. My research focuses broadly on the savanna-forest boundary in the Midwest, both on the environmental conditions that form this boundary and how environmental changes impact these savannas and forests. Over the last century, humans have altered the landscape in the Midwest, through large scale agriculture, land-use change, fire suppression, changes in CO2 and climate shifts (Goring et al. 2016, Rhemtulla et al. 2007). Many of these changes have likely affected the growth, survival, and climate sensitivity of trees, which could impact the trajectory of forests in the future. Therefore, I set out to quantify how savanna and forest trees functioned both in the past, and on the modern landscape. Using annual growth increments recorded in tree rings, my objective was to quantify how both modern and past ecosystems functioned (in terms of how much carbon they uptake and store in their annual growth rings), and to determine if and how tree growth patterns vary across temperature, precipitation, and soil gradients.

Thoughts from the field (don’t forget your bug spray and sunscreen):

To view the annual rings of tree growth, we collected tree core samples from sites across the historic savanna-forest boundary in the Midwest from several Minnesota Scientific and Natural Areas (SNAs), Minnesota State Parks, State Parks in Iowa and Missouri, as well as several sites within McHenry County Conservation District in Illinois (see map). In my sampling design, I aimed to capture the growth responses of young and old trees in both savannas and forests, across the wet-to-dry climate gradient in the Midwest. We targeted several Oak species, including Bur Oak (Quercus macrocarpa), White Oak (Quercus alba), Red Oak (Quercus rubra), and Chinkapin Oak (Quercus muehlenbergii), but also sampled several eastern forest species as well. With the help of fellow Paleonistas (Ann Raiho, Monika Shea) and field technicians Evan Welsh and Santi Thompson, we sampled tree cores at 23 different sites during the summers of 2015 and 2016. Coring trees can be monotonous, physically difficult, and relaxing all at the same time. Lucky for us, we got to go to some beautiful places across the Midwest.

Map of all the sites where we collected tree core samples from during 2015 and 2016. Background color represents mean annual precipitation (MAP) of the region obtained from PRISM climate data. Tree cores were collected from savannas and forests that occur along the historic prairie-forest boundary in the Midwest. (Click on image for a larger view.)

Coring a large Bur Oak tree in a savanna at Maplewood State Park, Minnesota.

Bur Oak acorn from a young tree located in St. Croix savanna SNA in Minnesota

Working out in the field gave us opportunities to see some awesome ecosystems and sunsets. Looking out onto to prairie from a savanna at Glacial Lakes State Park, MN, the tallest plants were not trees, but tallgrass prairie plants, such as the Big Bluestem, or “turkey foot” (Andropodon gerardii) pictured here.

An open savanna and prairie complex at Mound Prairie SNA, in Minnesota.

The Oak savanna canopy is sparse compared to a closed forest, letting in plenty of light for understory grasses and forbs to grow.

After driving a couple thousand miles total, spending over 30 nights in a tent in 2015 and 2016, and battling what seemed like an infinite amount of mosquitoes and ticks, we headed back to the lab to measure the width of each annual tree ring, and determine how climate affects Midwestern oak tree growth.

Back at the Lab:

Once we returned from the field, the cores were glued to wooden mounts, sanded, counted and measured. This work was done with the help of several awesome undergraduate students over the last two years, including: Jacklyn Cooney, Clare Buntrock, Santi Thompson, and Da Som Kim. Once the cores were measured and cross-dated with each other (using common “marker” years of extremely low growth, such as the 1934 Dust bowl drought, to double check our measurements), we have a temporal record of growth fluctuations for each site.

What climate factor affects oak tree growth in savannas and forests?

Tree growth responds strongly to the most limiting factor to their growth.

For example, in water limited regions of Southwestern North America, tree growth is highly correlated with interannual precipitation and drought, often making tree ring records from these regions good candidates for precipitation reconstructions (Charney et al. 2016, Peterson 2014). However, in many Eastern North American forests, water availability for growth is not a huge limiting factor, and tree growth is more sensitive to summer temperatures, drought, and light availability (Peterson 2014, Charney et al. 2016).  The savanna-forest boundary in the Midwest is located between these Eastern closed forests and the more water limited prairies to the West. Therefore, tree species that occur along this boundary are often thought to exist at the edge of their theoretical and climatic range boundaries, and theoretically could respond strongly to moisture stress or temperature stress. The historic transition from open prairie to savanna to forests occurred at a range of precipitation and temperature climatic envelopes; this transition zone in Minnesota had low mean annual precipitation (300-600 mm/year), and much higher mean annual precipitation in Indiana & Illinois (700-1200). Therefore, I originally hypothesized that these western savannas and forests may respond more strongly to low precipitation and drought than Eastern savannas and forests.

Contrary to my original hypotheses, oak tree ring growth is not primarily controlled by precipitation in oak trees near the savanna-forest boundary. Rather, tree growth at most sites is strongly linked to summer drought severity and summer temperatures. The negative impacts of drought on tree ring growth are likely mediated by temperature-induced drought stress, as suggested by the strong negative correlations with minimum and maximum June and July temperatures at almost all sites. While growth at some sites is mildly correlated to late summer precipitation, these places tend to have sandy soils, suggesting that future decreases in precipitation could have larger negative consequences for tree growth on sites with sandy soils. Interestingly, despite low moisture availability, sites with the lowest mean annual rainfall were only weakly correlated with monthly precipitation, suggesting that perhaps these systems are relying heavily on deeper groundwater sources for water. However, sites with low rainfall, such as Bonanza Prairie SNA in Minnesota, do have strong sensitivity to drought indices (Palmer Drought Severity Index, PDSI) and temperature indicating that high temperature drought stress, rather than water stress due to low precipitation is more important in this ecosystem.

Correlations with monthly climate indicate that Oak trees at most sites are most sensitive to summer drought index and summer temperatures, but few are strongly sensitive to monthly precipitation. Red colored sites have lower mean annual precipitation, and blue sites have higher mean annual precipitation. A). Tree growth at all sites is most strongly correlated to summer drought (Palmer Drought Severity Index is positive in non-drought periods and negative in drought periods). B). July precipitation is only weakly correlated with growth at some sites. C). Tree growth is somewhat negatively correlated with summer maximum temperatures. (Click on image for a larger view.)

Have growth sensitivities changed over time?

The climate-growth relationship is often assumed to be constant for the purposes of climate reconstructions. However, recent dendroecological studies recognize that growth-climate relationships may change due to shifts in climate seasonality, changes in tree size class, tree competition, and possibly due to increases in atmospheric CO2 (Voelker et al. 2006). In theory, higher levels of CO2 in the atmosphere can enhance tree growth by increasing CO2 available for photosynthesis in the leaf, without changing stomatal conductance (gs, the amount of water that moves through the stomata). This results in an increase in plant Water Use Efficiency (WUE), or the amount of carbon taken up per unit of water used, which could help reduce the impacts of drought on trees (McCarroll and Loader 2004). While the effect of atmospheric CO2 on tree growth is still largely debated, past researchers found that Bur Oak (Quercus macrocarpa) trees in Western Minnesota have become less sensitive to drought since the beginning of the 20th century, and mortality due to drought has decreased  as well (Wyckoff and Bowers 2009). Additionally, Voelker et al. (2006) found that the positive effects on growth that may result from increased atmospheric CO2 likely decline with tree age. My sampling effort has extended the spatial range of oak sampling in the Midwest, allowing us to test whether a change in the growth–drought relationship over the 20th century is regional and if it has occurred in different oak species and site conditions.

With our data across the Midwest, we find preliminary evidence supporting a change in growth sensitivity to climate. Trees across the Midwest were less sensitive to drought after 1950, and younger trees established under high CO2 were also less sensitive to drought than older trees.

These results are consistent with the previous work in Minnesota (Wyckoff and Bowers 2009), and with a positive enhancement of CO2. In two of the three closed forest sites sampled, we find no difference in the growth-drought sensitivity over time, suggesting that savanna trees, but not forest trees have become less susceptible to drought in the region. While the stand structure (open savanna or closed forest) may help explain where we see shifts in growth-climate sensitivity, species sampled may also play a role, as well as the mean annual precipitation and temperature. To specifically test whether CO2 enhancement is driving the decreased drought sensitivity, I am currently working on a project that tests to see if the composition of carbon isotopes recorded within annual tree rings have changed. The ratio of heavy to light carbon isotopes can be used to quantify plant Water Use Efficiency, which will increase over time if CO2 has a net positive effect on tree growth.

Up Next…

This project is still ongoing and there are several questions that I am still exploring. I am continuing to work on a formal analysis of the tree ring growth data, and look more at species-specific sensitivities to climate, since the preliminary analyses focus on site specific responses.

If growth and sensitivity of growth to climate changes over time, I want to know if it is due to the effects of CO2, or some other factor affecting forest growth. This next year, I will be spending a lot of time in the lab quantifying stable carbon isotopes, to determine if plant WUE increases result in the decrease in drought sensitivity over time.


Charney, N. D., et al. (2016). Observed forest sensitivity to climate implies large changes in 21st century North American forest growth. Ecology Letters, 19(9), 1119–1128. https://doi.org/10.1111/ele.12650

Goring, S. J., et al. (2016). Novel and Lost Forests in the Upper Midwestern United States, from New Estimates of Settlement-Era Composition, Stem Density, and Biomass. PLOS ONE, 11(12), e0151935. https://doi.org/10.1371/journal.pone.0151935

McCarroll, D., & Loader, N. J. (2004). Stable isotopes in tree rings. Quaternary Science Reviews, 23(7–8), 771–801. https://doi.org/10.1016/j.quascirev.2003.06.017

Peterson, D. L. (2014). Climate Change and United Steates Forests. In Climate Change and United States Forests. Springer. Retrieved from http://www.springer.com/us/book/9789400775145

Rhemtulla, J. M., et al. (2007). Regional land-cover conversion in the U.S. upper Midwest: magnitude of change and limited recovery (1850–1935–1993). Landscape Ecology, 22(1), 57–75. https://doi.org/10.1007/s10980-007-9117-3

Voelker, S. L., et al. (2006). Historical CO2 Growth Enhancement Declines with Age in Quercus and Pinus. Ecological Monographs, 76(4), 549–564.

Wyckoff, P. H., & Bowers, R. (2010). Response of the prairie–forest border to climate change: impacts of increasing drought may be mitigated by increasing CO2. Journal of Ecology, 98(1), 197–208. https://doi.org/10.1111/j.1365-2745.2009.01602.x

A Living Forest

Post by Dave Moore, Associate Professor at the School of Natural Resources and the Environment, University of Arizona

Reposted from http://djpmoore.tumblr.com/

Just a couple more photos from our walk in the woods with Prof Kerry Woods.

Since settlement of the US, Eastern Hemlock has been lost from many of the forests. Hemlock, once established, is a fantastic competitor and maintains it’s own dark, moist micro-climate beneath it’s branches. The effectively excludes other species from the location but allows the shade tolerant Hemlock seedlings to thrive.


This particular Hemlock is growing out of the darkness reaching out for light in a gap caused by a blowdown. The tree is taking advantage of a temporary resource that will likely disappear in the next few years.


Dr. Kerry Woods explains the gap dynamics of the Huron Mountain Wildlife Foundation to our group. Kerry is standing waist deep in Sugar Maple seedlings – the trees are competing with each other to close the gap and make it to the canopy – but most of them will not survive.


Yellow Birch needs to start out on nurse logs on the forest floor. This is the reason you often find this tree growing in straight lines in a natural forest.


Rachel @rachelgallerys and Kelly (ND)


Kelly and Ann (from ND)


Evan (ND) and our driver

Huron Mountain Wildlife

Post by Dave Moore, Associate Professor at the School of Natural Resources and the Environment, University of Arizona
Reposted from http://djpmoore.tumblr.com/


We’ve spent a few days walking in the woods in Michigan and Wisconsin. Over the last few years the PalEON team have been trying to work out how to challenge and improve terrestrial biosphere models using long term records of vegetation in the North East and Upper Midwest of the US. It’s a diverse team of scientists who use empirical measurements, statistics and modeling approaches to explore how plants and climate have changed in tandem over the last 1-2 thousand years.

This trip was a great opportunity to get away from the models and data and stick our noses firmly in the dirt, leaves and clouds of mosquitoes of the Upper Midwest.


On our first day we had the pleasure of staying with Dr Kerry Woods at the Huron Mountain Wildlife Foundation. Kerry directs the Foundation’s research efforts which range from biodiversity studies to population genetics to community dynamics, aquatic biology and climate. The station is set in old growth forest not far from Marquette, MI.


Kerry gave us a tour of the forest he’s been watching and studying for years. Kerry read the forest to us like a story – history, life history strategies, windstorms and mysteries. It was a pleasure.

Kerry4 Kerry5

Edge of the Prairie

Posted by Jody Peters, PalEON Program Manager

Prior to major land use changes, the tall grass prairie was a widespread ecotone in North America extending east into Minnesota, Illinois and Indiana.  Caitlin Broderick, a

Figure 1. Study area. Townships within the Yellow River watershed.

University of Notre Dame undergraduate working in the McLachlan lab, wanted to learn more about the edge of the prairie this spring semester.  She could have listened to NPR’s Garrison Keillor describe life in Lake Wobegone on the edge of the prairie in Minnesota.  But given that Notre Dame has been compiling historical records of trees from Indiana and Illinois, she decided to look at bit closer to home and focused on townships that are within the Yellow River watershed.  Coincidentally, this watershed is split almost in two by what is currently US 31 and what was historically known in the Public Land Survey (PLS) notes as the Michigan Road Lands. (Fig 1).

The following are some of the results Caitlin presented at the University of Notre Dame College of Science Joint Annual Spring Meeting.

Prairie_Fig2Currently, the Yellow River region is dominated by agriculture and deciduous forests (Fig 2).  However, from a 1935 depiction of the extent of the prairie prior to Euro-American settlement, prairie ecosystems extended into northwest Indiana including into the Yellow River region (Fig 3; Transeau 1935).  Caitlin explored what the edge of the prairie in the Yellow River region looked like during 1829-1837 using historical forest data obtained from PLS.Prairie_Fig3The survey notes provide identification of 1-2 trees, their diameters and distances from corner posts set every mile in each township (n=30 townships; 1055 corners). For this study, trees at each corner were categorized as “Oak” (only oaks were present), “Other” (22 non-oak tree taxa which were dominated by beech, ash, maple and elms), or “Oak + Other” (a combination of oak and non-oak trees).  There were also corners that were categorized as “Water” (in lakes, rivers, creeks, etc.), “No Tree” (had a post set in a mound but with no trees nearby), or “No Data” (corners with no information provided in the notes).

Caitlin used Arc GIS to map the tree composition at each corner (Fig 4).  Much to our surprise, she did not see many corners with no trees in the areas of Transeau’s prairie in the Yellow River region.  However, there was a striking pattern in the distribution of the trees, with the majority of the trees to the west of the Michigan Road Lands being oaks and those to the east being other hardwoods.


In addition to examining tree composition, Caitlin wanted to look at the structure of the trees and the physical environment across the region. To better define the two groups of trees (Oaks to the west and Other hardwoods to the east), Caitlin created buffers around individual corners and dissolved the buffers with matching tree classifications to create two groups of trees (Fig 5A). Comparing the trees in the two groups, Caitlin found that there was no difference in tree diameter (mean (cm)± se; Oak: 40.9± 0.5; Other: 40.2± 0.6). However, the trees in the Oak group were significantly further from the corner posts compared to the Other group (mean (m)± se; Oak: 36.2± 4.3; Other: 13.6± 2.4; p<0.001).  Given that there was a difference in the tree composition and distance from the corners, Caitlin looked at climatic and physical features that could help explain this difference between the oaks of the west and the other hardwoods in the east.  There was no ecological difference in either temperature or precipitation (mean± se; Temperature (ºC): Oak: 9.88± 0.003; Other: 9.89± 0.003; Precipitation (mm): Oak: 1010± 0.59; Other: 1007± 0.74).  However, although northern Indiana is quite flat, as with the tree composition, there was a striking difference in the pattern of elevation change across the Yellow River region that mirrored the change in the tree distribution. Trees in the Oak group were at significantly lower elevation compared to trees in the Other group (mean (m)± se; Oak: 225.5± 0.7, Other: 251.0± 0.6; p<0.01; Fig 5B).


IPrairie_Fig6n addition to exploring what the historical prairie-forest boundary looked like, Caitlin compared the National Commodity Crop Productivity Index (NCCPI) to see if there is a lasting impact on the current crop production by the physical factors that contributed to the differences in historical tree composition and structure.  The NCCPI models the ability of soils, landscapes, and climates to foster non-irrigated commodity crop productivity (Dobos et al. 2012). Caitlin found there was in fact, a difference between NCCPI of the two groups with crop productivity being significantly lower in the western area historically dominated by low density Oak tree communities (Fig 6).

Typically when people think of prairies, they think of open expanses of grass.  However, over the spring semester Caitlin found that in the Yellow River region of northern Indiana, the prairie described by Transeau actually looked more like a low density oak community compared to the closed forests further east.  While at first glance we can no longer see the prairie’s edge in the current vegetation of northern Indiana (Fig 2), when Caitlin looked closer, the factors that once controlled the boundary between the closed forests of the east and the open savannas moving west may still be at work limiting crop production in what was once savanna compared to production in the historically wooded uplands (although what these factors specifically are is still an open question).

As a fun follow-up to Caitlin’s research, nine of us took a lab canoe trip down the Yellow River. Although we didn’t see the same striking pattern of oaks to the west and other hardwoods to the east, as we drove the 50 minutes south on US 31 to get to the River, we did see small stands of hardwoods intermingled with a number of lone oaks standing in the middle of corn and soybean fields.  On our way to the ice cream store after our canoe trip, we also saw some of the lasting impacts of the oak savannas as we drove through Burr Oak, Indiana.  According to McDonald (1908), the village plat was filed in 1882 and was “nearly in the center of what is known as the “Burr Oak Flats”, which is as beautiful and productive a region as can be found anywhere.”

The trip down the Yellow River was a wonderful way to get outside on a beautiful spring day and experience firsthand the watershed/ecosystem that Caitlin has spent this spring studying in the lab.  The river was an easy enough paddle that novice canoers would be comfortable, but had enough downed trees to provide excitement for the more experienced canoer.  If you are ever in the area, connect with the Little Bit Canoe Rental. They do a wonderful job providing canoes and transport. Here are a couple of pictures of the fun!





Dobos, R.R., H.R. Sinclair, Jr., M.P. Robotham. 2012. User Guide for the National Commodity Crop Productivity Index (NCCPI), Version 2.0. Access pdf at the bottom of the page here.

McDonald, D. 1908. A Twentieth Century History of Marshall County, Indiana, Volume 1. Lewis Publishing Company, pg 134. Online access here.

Transeau, E. N. 1935. The Prairie Peninsula. Ecology 16(3): 423-437.

In a New Light

Post by Neil Pederson, Senior Ecologist at Harvard Forest
Reposted from The BroadLeaf Papers

We all love the colors of autumn. Fall brings to mind the vivid reds, oranges, yellows, and deep purples of September and October. By November in the Northeast, the leaves are gone and the sky often tilts into various shades of pale grey. The weather can be bone-chilling in a damp kind of way. It can be a bad time to be in the field. November in New England was the closest I’ve ever been to hypothermia. I now relish fieldwork in November, however, because of its light. November Light has helped me see things in a new way.

Filtered Light. Photo: N. Pederson

Filtered Light. Photo: N. Pederson

The first time I experienced the long-lasting glow of November Light was late in my dissertation field campaign. I recently had some great luck with a collection from an old-growth forest and wanted to see if I could squeeze out a few more diamonds before I called it a dissertation (and work would be lit by fluorescent light).

Kevin was my most reliable volunteer field assistant. I could call at a moment’s notice to see if he wanted to hit the field. He always said yes.

We bolted to southeast Pennsylvania and the weather was on our side. Blue skies and warm temperatures. We scoured this tiny patch of old forest to see if I had missed much on a prior trip. Soon after a brief lunch, it became apparent that we had done about all that was possible in that forest and we were slipping into lazy. So, we leaned back, chatted, and stared at the vernal roof.

At some point I kept checking the time on my GPS. My eyes kept telling me it was getting late. In reality, it was just approaching mid-afternoon. It dawned on me that angle of the Earth in that part of the Northern Hemisphere was delivering us an ever-lasting gobstopper dose of diffuse light. It felt like “sunset” above the Arctic Circle during summer. The light was low and hitting at all kinds of slanted angles. Colors glowed. It was glorious.

At the same time, it dawned on us that we were south of the last glaciation. Elk, woolly mammoth, and other megafauna likely used the game trails we were using that day more than a 100,000 years ago. More glory.

Bronzed Canopy. Photo: N. Pederson

Bronzed Canopy. Photo: N. Pederson

Just this past week we rolled up our field tapes for the last time during the 2014 PalEON season. It was a glorious feeling. Putting in ecological plots for tree-ring analysis is long and rather repetitive work. It is exhausting in a deeply different way than to reconstruct climate from tree rings. It was nice to know we had done a ton of work this year and that we were done. I imagine farmers get these feelings this time of year, too. As I dropped a coiled field tape into a backpack, it was instantly satisfying. We were putting our loyal field equipment down for a long winter’s rest.

Dan Bishop and Javier Martin Fernandez sampling in November Light. Photo: N. Pederson

Dan Bishop and Javier Martin Fernandez sampling in November Light. Photo: N. Pederson

We scheduled this last field campaign more than a month in advance. It is risky scheduling that far in advance in central New England this late in the year. But, after a Nor’Easter and a cold snap, the atmosphere shifted in our favor.

Blue skies. Brilliant Fagaceae colors. Stark contrast of a wide range of brightly-lit yellow leaves with the dark bark of red oak… and black oak?

While installing ‘nests’ around an older plot, or as a rather poetic colleague termed it, installing ‘doughnuts’, I ‘discovered’ a new species. Of course, black oak (Quercus velutina) was always there. It was just not talked about as much and, being hard to identify and often hybridizing with northern red oak Quercus rubra), it is often put in the red oak category. But, there it was, right in front our eyes.

Black oak reaching for the upper canopy among towering northern red oaks. Photo: N. Pederson

Black oak reaching for the upper canopy among towering northern red oaks. Photo: N. Pederson

Black oak bark. Photo: N. Pederson

Black oak bark. Photo: N. Pederson

Perhaps it was the November Light that made it ‘appear’? Maybe it was the showering of diffuse, angled light that made black oak jump out of the forest. Whatever it was, I now saw black oak everywhere. It wasn’t, of course. It was often red oak borrowing some of the velutinous traits of its sleeker, rarer cousin.

The glorious nature of November returned this past week and I saw these forests in a new light.

Canopy dominant red oak. Photo: N. Pederson

Canopy dominant red oak. Photo: N. Pederson

Classic red oak bark, bronzed. Photo: N. Pederson

Classic red oak bark, bronzed. Photo: N. Pederson

Oh there are some conifers in this forest (Pinus strobus). Photo: N. Pederson

Oh there are some conifers in this forest (Pinus strobus). Photo: N. Pederson

Glowing American beech (Fagus grandifolia). Photo: N. Pederson

Glowing American beech (Fagus grandifolia). Photo: N. Pederson

American beech. Photo: N. Pederson

American beech. Photo: N. Pederson

It comes in bronze, too. Photo: N. Pederson

It comes in bronze, too. Photo: N. Pederson

Fading Light. Photo: N. Pederson

Fading Light. Photo: N. Pederson

Late November Light. Photo: N. Pederson

Late November Light. Photo: N. Pederson

Underwater In New England

Post by Bryan Shuman, Associate Professor of Geology & Geophysics at the University of Wyoming

To evaluate how forests have responded to climate change in the past, we need to reconstruct the climate history. Fortunately, in terms of moisture, lakes provide a geological gauge of precipitation (P) minus evapotranspiration (ET). As effective moisture (P-ET) changes, the water tables and lake surfaces rise and fall in elevation. When this happens, sands and other materials that typically accumulate near the shore of a lake are either moved deeper into the lake during low water or shift out from the lake’s center as water levels rise. Ongoing work in New England is building on existing datasets to provide a detailed picture of the multi-century trends in effective moisture. Here are a few highlights of recent progress.

First “the fun part” was fieldwork that I conducted while on sabbatical in New England. The work included a cold but fun day on the ice of Twin Pond in central Vermont with Laurie Grigg and students from Norwich University (pictured).

Coring at Twin Ponds

Coring at Twin Ponds

This trip was a follow up to a previous trip that coincided with Hurricane Sandy’s visit to New England in 2012. As the result of both trips, we now have a series of three cores that record shoreline fluctuations at the pond. Because the sediment contains both carbonate minerals and organic compounds, we have also been able to examine the ratios of oxygen and hydrogen isotopes in the sediment and provide some constraints on the temperature history too.

Ice makes coring easy (its stable), but the swimming was not as good as in the summer when I worked in southern New England with Wyatt Oswald (Emerson College), Elaine Doughty (Harvard Forest), and one of Harvard Forest’s REU students, Maria Orbay-Cerrato. Over several days, we collected new cores that record the Holocene water-level changes at West Side Pond in Goshen, Connecticut, and Green Pond, near Montague, Massachusetts. Floating on a pair of canoes, we enjoyed the early summer sun, told jokes, ate delightful snacks brought from home by Wyatt, and strained our muscles to pull about 5 cores out of each lake. Near shore, the cores from both lakes contained alternating layers of sand and mud consistent with fluctuating water levels. In the lake center at West Side Pond, we also obtained two overlapping cores about 14 m long, which promise to provide a detailed pollen record. Both lakes proved to be excellent swimming holes too!

Second, on a more earnest note, the existing geological records of lake-level change from Massachusetts have been synthesized in a recent (2014) paper in Geophysical Research Letters by Paige Newby et al. The figure shown here summarizes the results and compares the reconstructions with the pollen-inferred deviation from modern annual precipitation levels from a paper by University of Wyoming graduate student, Jeremiah Marsicek, last year (2013) in Quaternary Science Reviews.

Figure 4 from Newby et al. 2014

Figure 4 from Newby et al. 2014

All of the records show a long-term rise in effective moisture since >7000 years ago as well as meaningful multi-century deviations. By accounting for the age uncertainties from the reconstructions, we were able to show that a series of 100-800 year long droughts at 4200-3900, 2900-2100, and 1300-1200 years before AD 1950 affected lake levels (blue curves with reconstruction uncertainty shown) on Cape Cod (Deep Pond), the coastal Plymouth area (New Long Pond), and the inland Berkshire Hills (Davis Pond) – as well as the forest composition as recorded by the pollen from Deep Pond (red line). Interestingly, an earlier drought in the Berkshires at 5700-4900 years ago was out of phase with high water recorded in the eastern lakes. This difference is one of the motivations for the new work in Vermont, Connecticut and central Massachusetts, as well as other ongoing work with Connor Nolan in central Maine: what are the spatial patterns of drought?

Maine Fieldwork Part 2: The Bog

Post by Bob Booth, Associate Professor at Lehigh University; Steve Jackson, Center Director for the U.S. Department of the Interior’s Southwest Climate Science Center; Connor Nolan, Steve’s PhD Student at University of Arizona, and Melissa Berke, Assistant Professor at University of Notre Dame

Read about Maine Fieldwork Part 1.

Maine Fieldwork Part 2
Our adventures in bog coring, lobster consumption, dehydration, lake scouting, dipteran-slapping, and driving (lots of driving) began on July 6 when Bob Booth, Steve Jackson, Melissa Berke, and Connor Nolan rendezvoused in Portland, Maine, and drove to Bangor, our home base for coring at Caribou Bog. A testate-amoeba record of water-table depth from the bog will be compared to a lake-level record from Giles Pond (cored by Connor and Bryan Shuman back in November). These two sites are the new paleoclimate proxies for our Howland Forest HIPS (Highly Integrated Proxy Site). We also plan to use these records to better understand how lakes and peatlands respond to and record climate variation.

bog CaribouMap

Caribou Bog is a huge (~2200 hectares) ombrotrophic bog that has been the subject of many past investigations. We targeted a part of the bog that had been worked on in the 1980s by Feng Sheng Hu and Ron Davis. Coring took two full days (check out the video below to really appreciate the dipterans and the team’s jumping abilities). On the first day, we surveyed the bog with probbog - MBe rods to select a coring site. Then we hauled all of the heavy coring gear from the car, down a logging trail into the forest, through the “moat”, and then across the lumpy bog. Every part of the walk from the van to the bog and back was challenging, each for different reasons. The trail was hot and infested with deerflies and mosquitoes, the forest had no trail and low clearance and forced us to wrestle with young trees, the moat provided ample opportunity for losing boots and called for some gymnastic moves while carrying large and heavy stuff, and finally walking the 300 meters across the bog was like being on a demented stairmaster as we sunk a foot or two into the bog with every step.

bog flower - MB
After three trips to haul all of our gear, we cored the bog, collecting the upper peat (~3-4 meters) with a modified piston corer and the overlaps and deeper sections with a Russian corer. Although we thought we had ample drinking water the first day, we didn’t, and we chose not to drink the brown bog-water. Once we returned to the van, we headed straight to the nearest rural convenience store (only 3 miles away) and restored electrolytes and fluids.

We completed the coring on the second day, and dragged everything back to the van in three trips.  After dropping Bob off to meet his family in Portland, the rest of us enjoyed a seafood extravaganza at Fore Street restaurant in downtown Portland.  

portland head light

Portland Head Light

Lobster Feast

Lobster Feast

The cores went to Lehigh with Bob, but will eventually be analyzed by Connor. We will count testate amoebae and pollen in the core to get records of paleohydrology and paleovegetation spanning the past 2000 years.  Stay tuned!

Watch on YouTube: Caribou Bog 2014

Hu, F. S., & Davis, R. B. (1995). Postglacial development of a Maine bog and paleoenvironmental implications. Canadian Journal of Botany, 73(4), 638–649.


Self thin you must


Post by Dave Moore, Professor at The University of Arizona
This post also appeared on the Paleonproject Tumblr

We spent a lot of time last week in Tucson discussing sampling protocols for PalEON’s tree ring effort that will happen this summer. The trouble is that trees (like other plants) will self thin over time and when we collect tree cores to recreate aboveground biomass increment we have to be careful about how far back in time we push our claims. Bonus points if you can explain the photo in ecological terms! I stole it from Rachel Gallery’s Ecology class notes.

Neil Pederson and Amy Hessl will be taking the lead in the North East while Ross Alexander working with Dave Moore and Val Trouet (LTRR) will push our sampling into the Midwest and beyond the PalEON project domain westwards. This is a neat collaboration between the PalEON project and another project funded by the DOE. Francesc Montane and Yao Liu who recently joined my lab will be helping to integrate these data into the Community Land Model. Also Mike Dietze‘s group will be using the ED model to interpret the results.

Because we want to integrate these data into land surface models we need to have a robust statistical framework so we had some equally robust discussions about statistical considerations with Chris Paciorek and Jason McLachlan and other members of the PalEON team.

The Invasion of the Zombie Maples

Post by Ana Camila Gonzalez, Undergraduate Researcher with Neil Pederson and the Tree Ring Laboratory at Columbia’s Lamont-Doherty Earth Observatory

As an undergraduate student interning at the Tree Ring Lab at Lamont-Doherty Earth Observatory, my involvement with PalEON has been rather localized to the data production side of things. My knowledge on the dynamics of climate and the models involved in forecasting future climate change is obviously limited as a second-year student. My knowledge on how frustrating it can be to cross-date the rings in Maple trees, however, is more extensive.

This past summer I was able to join the Tree Ring Lab on a fieldwork trip to Harvard Forest in Petersham, MA. My main task was to map each plot where we cored, recording the species of each tree cored, its distance to the plot center, its DBH, its canopy position, its compass orientation, and any defining characteristics (the tree was rotten, hollow, had two stems, etc.). The forest was beautiful, but it became more beautiful every time I wrote down the letters QURU (Quercus rubra)I had plenty of experience with oaks, and knew that they did not often create false or missing rings and are thus a fairly easy species to cross-date. I shuddered a little every time I had to write down BEAL (Betula alleghaniensis), however, since I had looked at a few yellow birches before and knew the rings were sometimes impossible to see let alone cross-date. I had no reaction to the letters ACRU (Acer rubrum), however, since I had never looked at a red maple core before. I was happy that it was a tree I could easily identify, and so I didn’t mind that the letters kept coming up. Had I known what was to come, I would’ve found a way to prevent anyone from putting a borer to a red maple.

At first, the maples seemed to be my friends. The rings were sensitive enough that multiple marker years helped me figure out where the missing rings where, what was false and what was real. I morbidly became a fan of the gypsy moth outbreak of 1981, because in many cases (but not all) it produced a distinct white ring that marked that year very clearly. This was definitely challenging, as the trees also seemed to be locally sensitive (a narrow ring in one tree might not at all be present in another) but all in all it seemed to be going well.

And then came the Zombie Maples.

Fig (a) Anatomy of a White Ring: Above is a core collected in 2003. It was alive. The white ring in the center of the image is 1981, the year of the regional gypsy moth outbreak in New York and New England.

That white ring you’re seeing above is the characteristic 1981 ring from a Zombie Maple cored in 2003. After that ring we can only see four rings – but this tree is alive, which means that there should be 13 rings after 1990 (Fig b). This means approximately 10 are missing.

Fig (b) Anatomy of a Zombie Maple: Above is a core collected in 2003. It was alive. The 1990 ring is marked in the image just right of center. There should be 13 rings between 1990 and the bark. You can only see four. Is it Alive? Is it Dead? Eek! It is a Zombie!!

This kind of suppression in the last two decades was present in multiple cores, and it made many perfectly alive trees seem like they should have been dead. Nine rings missing in a little over one millimeter. We see even more severe cases in our new collection: 15 rings where there should be 30 rings in about 2 millimeters – how is this tree supporting itself?

Cross-dating these cores took a lot longer than planned, and at times I was tempted to pretend my box of maples went missing, but afterwards I felt I was a much stronger cross-dater, and I’m realizing more and more that this really matters. If you’re going to base a model off of data that involves ring-width measurements from particular years, you better make sure you have the right years. What if we didn’t know the gypsy moth outbreak occurred in 1981, and somebody counting the rings back on the Zombie maple core above was led to believe it occurred in 1996? Our understanding of the trigger for this event would be incorrect because we would be looking for evidence from the wrong decade.

In a way, the Maples are still my friends. They were almost like the English teacher in high school who graded harshly who you didn’t appreciate until you realized how much better your writing had become.

PalEON Goes Into the Field

Post by Connor Nolan, University of Arizona Graduate Student

On Sunday November 3, Bryan Shuman and I (Connor Nolan) packed up a rental van with coring gear and hit the road for the 5.5 hour drive from Woods Hole, MA to Bangor, ME. 

Our aim was to do identify and core a lake for lake-level reconstruction near-ish to the Howland Experimental Forest flux site. We can survey lakes for evidence of past lake level changes using ground penetrating radar. The first day we had adventures in learning to navigate Maine’s back roads and we surveyed 2 lakes – Crystal Lake and Pickerel Pond. Both were beautiful sites, but the past lake level story was not very clear. 

Day 2 included surveys of 3 more lakes — Peep Lake, Salmon Pond, and Giles Pond — and an excursion through the largest industrial blueberry farm in North America (an interesting looking site called Rocky Lake is on their property, we did not survey it this trip due to big no trespassing signs on the property…). Just as we were starting to wonder if we would find the right lake on this trip we surveyed Giles Pond and it turned out to be the one! We arrived in the perfect light to take a great picture.Day 3 we cored Giles Pond along a transect. We ended up with 4 cores in all from this trip with lots of sand layers (a good thing for this kind of work!!). The Younger Dryas has a very distinctive lithology in this region, a light gray clay, and we have this lithology in some of our cores so we should have a record that goes back nearly 15,000 years! 

It was my first lake coring experience and it was a lot of fun! The cores are currently at Woods Hole Oceanographic Institute with Bryan. I will make a return trip there before long to do some initial analyses and then ship the cores back to Arizona for initial dating and more work!