Another Reason to Battle Buckthorn

As plant enthusiasts, we often focus on how plants are affected by their environments. Their growth is affected by weather, water, nutrients, etc. But the plant-environment relationship is a two-way street, and plants can have a strong influence on the habitats they live in. We might experience this by walking in a forest and feeling ground beneath our feet that is spongy from the buildup of slowly decaying leaves that accumulated over decades or centuries. The trees in such a forest have “engineered” their environments, changing the very ground beneath them in ways that are beneficial to other plants, to animals, and to ecosystem functioning.

Unfortunately, we can also experience negative engineering effects of plants on their environments when we visit natural areas throughout the Chicago region and beyond that have been heavily invaded by the nonnative common or European buckthorn (Rhamnus cathartica). Buckthorn was brought to the Midwest for ornamental use—it’s great at making hedges—but escaped from human control and is now one of our region’s worst invasive species. As its hedge-producing credentials suggest, buckthorn is good at forming dense, shady thickets. These thickets have been shown to harm native plants and wildlife and to change physical, chemical, and soil conditions where they occur. Where there once may have been an open oak woodland underlain by a thick carpet of grasses, sedges, and wildflowers, we now often see an uninviting tangle of shrubs with little other than bare soil beneath them.

This contrast, between an environment that looks like this

Buckthorn removal has opened this area up and ongoing management restored a robust understory of native vegetation. Photo: Jim Steffen
An open-canopied patch of woodland with robust growth of native vegetation in the understory (Photo: Jim Steffen)


…and one that looks like this

Buckthorn-dominated area; there is almost no vegetation beneath it. Photo: Jim Steffen
Buckthorn-dominated area showing a lack of understory vegetation (Photo: Jim Steffen)

…got me and my colleague Jim Steffen wondering whether the ability of woodlands to perform carbon-storage ecosystem services could be impaired by buckthorn invasion. If so, could restoring impacted habitats back to native woodland vegetation return these services? Some background: Ecosystem services are essentially benefits nature provides to humans (e.g., clean air and drinking water, food, and other resources). Sequestering carbon—removing it from the atmosphere where it contributes to global climate change and instead locking it away harmlessly for potentially hundreds of years—is one such service.

Steffen’s more than two decades of work removing buckthorn from the Garden’s Mary Mix McDonald Woods and restoring native plant species had created the natural, long-term experiment we needed to answer these questions. We had access to areas still dominated by buckthorn for use as control treatments and patches of restored woodland of various ages that had previously been dominated by buckthorn (this is called a “restoration chronosequence”). And we had human capital to put to work: Rachel (Hesselink) Gentile and Chad Zirbel, who participated in the Garden’s Research Experiences for Undergraduates program (funded by the National Science Foundation) in 2009 and 2010, respectively. Gentile and Zirbel, in turn, mentored College First high school students Alan Lane and Kassandra Altantulkhuur.

So why did we think that buckthorn’s engineering of its environment might reduce carbon storage? Why not increase it? Mainly because of all the missed opportunities for carbon sequestration represented by the vegetation that buckthorn displaces: countless individuals of hundreds of species of understory plants, plants that make their living by drawing in carbon dioxide from the atmosphere. During photosynthesis, this captured CO2 is transformed into plant biomass, which occurs largely underground, in a complex tangle of fine, deeply penetrating roots that interact with the soil to produce tough, carbon-based compounds that are very slow to cycle back to the atmosphere. Not incidentally, all of these living understory plants and their deceased brethren (dead plant material is called “litter”) could help stabilize the soil, so that captured carbon would be less likely to wash away in thunderstorms and spring thaws.

What did we find? That buckthorn-dominated areas were indeed bad at storing carbon and that restored areas got better as they got older. Restored patches had lower rates of erosion and higher concentrations of persistent organic carbon in their soil. As restored areas matured, their vegetation continued to rebound, with plant diversity and litter biomass increasing over time. This maturation of the plant community was matched by steady increases in soil-carbon accumulation.

But wait, what about those thickets we see? Surely all that wood must be good for storing carbon? Not so much. We calculated wood biomass by measuring hundreds of trees. Even though restoration involved cutting down a lot of buckthorn, it actually led to a net increase in wood biomass, an increase that was also positively correlated with restoration age. Buckthorn’s thickets may look impressive, but its trunks and branches are puny compared to most trees, there is (almost) “no there there.” We think that taking out buckthorn may have freed native trees that can really get big, like white and red oaks, to better reach their growth potential.

In addition to the well-recognized benefits to biodiversity from active, long-term woodland restoration, our work and that of other scientists shows that there can also be tangible benefits to society. You can learn more about this research in our manuscript recently published in the journal Restoration Ecology. And you can expect to see further interesting work from Gentile (now pursuing a Ph.D. in ecology at Notre Dame), Zirbel (pursuing his Ph.D. in ecology at Michigan State), Lane (an undergraduate at DePaul University), and Altantulkhuur (attending the University of Illinois at Chicago).

©2013 Chicago Botanic Garden and

Fossil Hunting in Mongolia

I just returned from two weeks in Mongolia searching for fossil flowers. Why go halfway around the world to look for fossils of flowering plants when there are plenty of fossil flowers closer to home? Easy—because nobody has really looked there before.

PHOTO: A smattering of light blue tents are dwarfed by the surrounding grassland and sky that stretches on forever.
Camping on the Mongolian steppe. it doesn’t get any better than this!

A little background first. Mongolia is a large country in Central Asia with a population very close in size to the city of Chicago (Mongolia: 2.89 million; Chicago: 2.71 million). But that is where the similarities end. More than half of the population (roughly 1.7 million) lives in one large, sprawling city, the capital, Ulaanbaatar (also spelled Ulan Bator, or “UB” for short). Most of the country is rural with very few people; it ranges from mountains and forests mainly in the north and west, to steppe in the center, and desert in the south; and it is breathtakingly beautiful. The elevation is high, the climate is continental, and with Siberia to the north you can imagine what winters are like. Mongolia is a landlocked country with Russia to the north and China to the south. Inner Mongolia is an Autonomous Region within China and it borders the country of Mongolia to the south. The Gobi Desert is shared by Mongolia and Inner Mongolia.

PHOTO: Pat Herendeen poses on the vast steppe, binoculars in hand.
Looking for the fossil site
PHOTO: A herd of bactrian camel rest on the steppe.
It might look like central Montana, but those aren’t buffalo!

My trip to Mongolia in late June and early July was my fourth time conducting fieldwork there. We have been working in the Mongolian steppes, which resemble the shortgrass prairie of Montana and North Dakota, so much that it is easy to get confused—until a herd of Bactrian camels makes an appearance, or perhaps a ger, the typical Mongolian house, known elsewhere as a yurt. The Mongolian steppe supports the pastoral lifestyle of many Mongols, who raise goats, sheep, horses, yak, cattle, and camels on the lush grass growth that formerly supported the many horses of Genghis Khan’s army. Mongolians eat a lot of meat, and they consume a great diversity of dairy products made from their livestock. Being a vegetarian in Mongolia would not be easy!

About those fossil plants… Mongolia is well known for its fossils. Paleontologists have been exploring and collecting dinosaurs in Mongolia for many years, perhaps most famously Roy Chapman Andrews in the 1920s. Mongolia has an abundance of Jurassic and Cretaceous rocks, which have yielded numerous dinosaurs, especially in the Gobi Desert.

Dinosaurs are great and all, but you can’t build an ecosystem on dinosaurs! What did they eat? There had to be lots of plants there to support all of those herbivorous and carnivorous dinosaurs. That is what we went looking for: the plants. In particular, we are interested in finding fossils of flowering plants, or angiosperms. The flowering plans first appear in the early Cretaceous, and then diversify rapidly through the middle Cretaceous, such that by the late Cretaceous they are the dominant group of plants in almost all ecosystems. My collaborators and I are interested in understanding that early diversification of the flowering plants. I will explain more about fossil flowers in another post in the near future.

PHOTO: Two Mongolian ger, semi-permanent, circular tent structures, complete with welcome mats at the doors.
Our lunch stop was in a traditional Mongolian ger on the way to the field site.
PHOTO: Pat Herendeen examines fossil evidence from an exposed sub-terranian rock face.
Checking out the exposure at Tevshiin Govi, a famous dinosaur locality that also contains preserved fossil plants.
PHOTO: Five researchers dig through a mound of lignite, collecting fossil samples.
Lignite at the Tevshiin Govi mine site

In our fieldwork we have visited about ten different places in Mongolia so far. Some are natural exposures of bedrock on bluffs and hillsides, and others are coal mines. Coal is, after all, plant material that has been deposited in a peat swamp and then compressed and metamorphosed. Mongolia has a great abundance of coal deposits, which are being mined to support domestic and foreign energy needs. Some of these deposits in Mongolia have been only minimally compressed and metamorphosed, and they are called “lignite” rather than “coal.” Lignite deposits are great for studying fossil plants because the plant material is less compressed than in coal and can be disaggregated and studied using light microscopy and other methods.

Breakfast time!
Breakfast time!

Our field sites are mainly south of Ulaanbaatar in the gently rolling hills and plains of the steppe, where we camp under the vast Mongolian sky. Our field group consists of ten people and three four-wheel drive vehicles. Everything we need for seven or so days in the field must be brought with us, including food, water, tents and other camping supplies, tools for digging, boxes, newspaper for wrapping fossils, and Ziploc bags for rock samples, which we disaggregate back in the lab to isolate small seeds, cones, flowers, and fruits (called “mesofossils”). As the trip progresses, fitting everything in the vehicles becomes more challenging as the rock samples take up more and more room; eventually we must strap some supplies on the roof of the vehicles to make room.

We have camped in some amazing places. The grasslands seem to go on forever with no trees or bushes in sight. In some places there are ephemeral lakes that draw hundreds or thousands of animals to drink. Tempting as it may be, it is best not to set up camp too close to the water! Then there are the less picturesque locations—coal and lignite mines can be very messy and unpleasant environments when it comes to camping. When we are working at a mine we try to camp upwind from the mine, and far enough away that it is not too noisy or dusty when the wind shifts.

PHOTO: Pat Herendeen sits in a lignite trench on the steppe recording fossil finds. Two plastic baggies of fossils are nearby.
Collecting samples at Khuren Dukh
Fossil fern leaves
Fossil fern leaves

The first task at a new fossil site is to scout the locality to look for the kinds of rocks that we need. For mesofossils we normally have best luck with clay and siltstone with fragments of organic material. In the field we often can’t tell whether the sample will be good or not; we only know that for sure once we get it back to the lab and process it. If 50 percent of the samples are good we feel lucky! We collect one to several gallon-size Ziploc bags of rock from as many sample points as possible, and then photograph the sample locations and take GIS coordinates for future reference. Sometimes we try to relocate samples that turn out to be really good, and photographs and GIS data are critical for this. We also try to split blocks of claystone and siltstone to look for compression fossils (“macrofossils”). If we are lucky we will find a nice assemblage of fossil leaves to complement the mesofossil assemblage.

When we are working in lignite mines we search for sections of lignite where the plant material is not too compressed, remove samples, and box them up to be shipped home for processing and study. The macrofossils require extra care because they are very delicate, and the roads are rough! Each macrofossil is wrapped individually in newspaper. When there is a part and counterpart for a particular fossil (the two sides of a block that have been split open), these must be wrapped separately but kept together; otherwise, someone not paying attention might think they are two separate fossils. All of the wrapped macrofossils are carefully packed into boxes, with generous padding of crumpled newspaper on the bottom, sides, and top.  Macrofossils must be packed vertically on edge in the boxes (something most novice collectors don’t realize); if they are packed horizontally, they are likely to break.

PHOTO: The research van drives along, a cloud of dust in its wake.
A typical, unimproved dirt road on the steppe
PHOTO: Camp with the Shivee Ovoo coal mine in the background.
Camp with the Shivee Ovoo coal mine in the background.

Yes, I have lots of rules for field work! But it is for a reason. All of this material must be securely packed into the vehicles and carried along for the rest of the trip. Did I mention that the roads are rough? Careless packing of the collections can result in great damage by the time we get back to UB. These trips are expensive, and the last thing we want is to have the fruits of our labor destroyed in transit!

At the end of a day of field work we are often quite a sight, especially if we were working in a lignite mine! So it is back to camp to get cleaned up. But with no running water, cleaning up consists of wiping down with baby wipes (unscented ones for me!). Although it can get pretty hot during the day, especially in June and July, evenings cool off reliably and are very pleasant. Our camp cook has prepared a nice meal, and if it is not too windy we dine outside and watch the sun set over the horizon.

A little advice- take the tent down before pulling out the last stake!
A little advice- take the tent down before pulling out the last stake!

After a couple days at a site we break camp and move on to the next locality. Not a bad way to spend a couple weeks of the summer!

A special thank you to Masamichi Takahashi, Peter Crane, and Andrew Leslie for some of the images used in this post.

©2013 Chicago Botanic Garden and

Through the Lens of a Plant Detective

On a cool August 2011 morning in Ulaanbaatar, the capital of Mongolia, Patrick Herendeen, Ph.D., of the Chicago Botanic Garden began a mission. Gathering with a team of international researchers, he set out into the vast countryside in search of rare plant fossils dating back to the time of dinosaurs, a time called the early Cretaceous period. Through this ongoing research project, the team is working to solve mysteries of plant evolution and inform science around the globe.

PHOTO: Dr. Herendeen collects lignite samples at Khuren Dukh, an unusual site with both plant and animal (dinosaur) fossils.

After completing fieldwork on expeditions in 2011 and 2012, Dr. Herendeen, a senior scientist at the Garden, is poised to make major strides in his research this year. He told me more about his work on a recent tour of his laboratory at the Garden.

During fieldwork in the grassland steppes of Mongolia, just north of the Gobi Desert and often at an elevation of nearly 5,000 feet, he picks through lignite coal in active strip mines and looks for fossils of everything from plant parts to pollen. “Mongolia is an area where there has been very little research done on fossil plants,” said Herendeen. Yet, “there is a lot of material there of an age that is very interesting for understanding the evolution of flowering plants.”

Sometimes, he finds leaf imprints in shales between layers of coal, and will spend a day or two in the field splitting rocks to prepare them for study. More often, he places promising chunks of lignite, a flaky black form of coal, into gallon-sized plastic bags and brings them back to his lab at the Garden for closer study.

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Learn more about Dr. Herendeen’s work and watch a video interview.

In the Garden’s Daniel F. and Ada L. Rice Plant Conservation Science Center, Herendeen empties the bags of samples and gently breaks up the lignite pieces to uncover fossils. He often finds 20 or more types of reproductive structures, stems, leaves, and pieces of wood from multiple plants in just one sample. Some pieces are so tiny that they can only be seen through a microscope. He must then determine which parts go together in order to create a picture of what each plant looked like in the days of the dinosaur. “It’s like collecting the litter off the forest floor and then trying to sort out which parts are from the same plant,” he said. Already, he and his collaborators have identified more than a dozen types of extinct plants.

The majority of plants found so far are gymnosperms, mostly members of the pine family and related conifers. These discoveries have yielded some answers as to how plants evolved over the past hundred million or so years—such as how major events like volcanic eruptions, disease, or climate change may have impacted changes in their structure and in plant communities. “You can’t understand the history of those habitats without understanding the history of the plants that make them up,” said Herendeen. Although many answers have been found, many questions have also been opened.

Discover more: Attend a lecture on the gingko tree by Dr. Herendeen’s research colleague, Sir Peter Crane, and the unveiling of his forthcoming book, Ginkgo: The Tree That Time Forgot.

A particularly interesting discovery, according to Herendeen, was a seed plant without a clear connection to any group of plants known today. Its physical structure is unique, and its story is yet unknown. “It doesn’t often happen that we find something that we don’t know what major group of plants it belongs to, so it is quite intriguing,” he said.

Although his findings have been rich so far, Herendeen is still searching for what he considers to be the ultimate treasure. He is particularly interested in the origin and evolution of flowering plants, also known as angiosperms. Fossils of angiosperm pollen grains are present in the Mongolian material, but so far no fossil flowers or other parts of flowering plants have been discovered. He hopes to find better angiosperm fossils when he returns to Mongolia this summer to help document their early existence in central Asia.

The plants Herendeen is researching are the ancestors of many that can be seen at the Garden today, including those in the Dwarf Conifer Garden. Gingko trees, also gymnosperms, may be found in the Elizabeth Hubert Malott Japanese Garden and on the north side of the meadow on Evening Island. Cycads are yet another group of gymnosperms that may be represented in the Mongolian fossil assemblages. Visitors can see a cycad tree with pods, standing about 10 feet tall, in the tropical greenhouse at the Regenstein Center.

©2013 Chicago Botanic Garden and