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Another Reason to Battle Buckthorn

Enhancing carbon storage through woodland restoration

Dan Larkin —  August 21, 2013 — 1 Comment

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 my.chicagobotanic.org

In late October 2012 when I was driving down a country road in rural northwest Illinois, I spotted some bright sky-blue asters blooming near the corner of a woodlot. I was traveling between nature preserves in this area of the state collecting seeds for the Dixon National Tallgrass Prairie Seed Bank, so my mind was already tuned in for interesting native flora that might produce a collection for the seed bank. It was impossible to make a positive I.D. traveling at 60 m.p.h., but the color of that patch of blue was intriguing enough to warrant turning around to go back for a closer look.

As I had hoped, the attractive blue flowers belonged to a fine native species called Short’s aster (Symphiotrichum shortii), which inhabits high quality woodlands. This being the case, I thought it prudent to take a peek into the adjoining woodland to see what else might be growing there. To my surprise, within 30 feet of my entry point I stumbled across a large patch of goldenseal (Hydrastis canadensis)—a once common but now rare plant of Illinois woodlands. Its rarity is attributed to its past popularity as a medicinal plant, which led to its overharvest. Along with goldenseal, other quality woodland plants such as bellwort (Uvularia grandiflora) and baneberry (Actaea sp.) were also present.

PHOTO: golden seal (Hydrastis canadensis) in bloom.

This native woodland perennial produces two large, broad, palmately divided leaves atop a 1-foot-tall hairy stem. A small cluster of greenish-white flowers are produced at the base of one of the leaves in spring when the leaves are expanding, and they mature into a cluster of bright red fruits by midsummer. The knotty, bright yellow root of goldenseal has been harvested by humans for centuries for a variety of medicinal uses. Its popularity has led to extreme harvesting pressure culminating in drastically reduced natural populations.

I did not have time to explore the woodland further that day, so I made a note of where it was located and planned on returning to the site soon to explore it further. However, I had one question that I needed to answer before this was to happen: Who owned that woodland? I was confident that it wasn’t a nature preserve, because I had lists and maps of all of the protected preserves in the area. My guess was that it was privately owned. When I left the site, I recalled that there was a home situated just off the road near the middle of the woodland, so I thought that would be a good starting point.

PHOTO: Hydrastis canadensis leaves (with one ripe fruit).

Hydrastis canadensis leaves (with one ripe fruit)

As luck would have it, the owners of the home also owned the entire woodland. I spoke to the owner about the goldenseal I found in the corner of his property and the possibility of making a collection for the seed bank. It turns out that he already knew about the goldenseal population from a conversation that he had had with a local forester years ago. The forester thought that there was a good chance it had been planted there to be harvested for its medicinal value at a later date. This was a likely scenario, considering the size of the population. The owners agreed to allow me to collect its seeds as well as seeds from any other species that I sought for the seed bank. The only problem was that for the seed bank we are primarily interested in preserving the seeds of natural populations, not introduced ones. Seeds from natural populations represent individuals that are ideally suited to that environment by natural selection across generations, and are therefore of more value to those seeking genetically adapted seeds from a particular area.

PHOTO: Hydrastis canadensis fruit.

The raspberry-like fruit of goldenseal is considered inedible, but the roots have many medicinal properties.

So here lies the dilemma: large populations of goldenseal are rare, because of overharvesting. I rarely see this plant in woodlands, and when I do, it is always in small numbers. If this population was cultivated for the medicinal value of its roots, there is a good chance that it does not represent a natural population. I did not collect seeds of goldenseal that fall—the seeds had ripened and dropped much earlier in the summer. Any seed collection for the seed bank could not occur until the following year. This gave me plenty of time to contemplate whether or not to make the collection.

Since my first visit to the woodland, I have made several seed collections of many quality native woodland plants for the seed bank, including a collection of the Short’s aster that led me to the goldenseal discovery. During those collections I have become more familiar with the woods and have discovered additional colonies of goldenseal—some quite distant from the original population. Could these additional colonies represent multiple plantings? Maybe, but the sizes of the additional colonies are quite a bit smaller than the original. Perhaps they represent offspring from the original colony. If that is the case, this may be an indication that this particular woodland is an ideal habitat for goldenseal—even if it is not the original habitat. Or, there is a chance that this is a remnant population that for some reason survived overharvesting forays years ago.

PHOTO: A field of goldenseal in fruit.

A population of goldenseal on the property; one of several colonies where seeds were harvested

I completed a collection of the goldenseal population (estimated at more than 500 plants) for the seed bank on July 24, 2013. A notation in our database notes will read: “Population may not be natural.”


©2013 Chicago Botanic Garden and my.chicagobotanic.org

Battling Buckthorn

Garden and local partners join forces to fight plant invader

Amy Spungen —  July 6, 2013 — 2 Comments
PHOTO: Ecologist Joan O'Shaughnessey

Garden ecologist Joan O’Shaughnessey wields her weapons of choice.

Our weapons: saws and loppers.

Before our might, the foliaged foe fell along a stretch of the Green Bay Trail near the Braeside Metra station.

That’s where a group of 50 of us gathered to vanquish thickets of invasive buckthorn (Rhamnus sp.) on Wednesday, June 19. The removal was initiated by the City of Highland Park and the Park District of Highland Park in partnership with the Chicago Botanic Garden. Ravinia Festival also provided volunteers and space for parking and picnicking afterward.

“This was an ideal place to demonstrate what buckthorn control could accomplish through taking back our beautiful natural surroundings, even in very busy settings,” said Rebecca Grill, natural areas manager for the Park District. The area links two major cultural institutions serving the community, the Garden and Ravinia. It is also near a planned bike path for cyclists and pedestrians that eventually will extend to the Garden, connecting the North Branch and Green Bay Trails. 

PHOTO: the Park District team.

The Park District team included (left to right) Rachel Cutler, Dianna Juarez, Dan Malartsik, Liz Ettelson, Rebecca Grill, Ted Baker, Steve Meyer, and Chris Wilsman.

 

PHOTO: Sophia Siskel and helpers.

Sophia Siskel, the Garden’s president and CEO, brought her sons to help.

From 9 to 11 a.m. we sawed, chopped, cut, tugged, pulled, dragged, and stacked so much buckthorn that by the time a break was called, some piles were as high as me (5 feet), extending in an impressive line down the trail. Joggers, walkers, and cyclists made their way past the activity, occasionally cheering on the workers, including Garden President and CEO Sophia Siskel and her sons.

Why has buckthorn become such a problem in the Midwest? It’s the story of a plant species imported for perceived benefits that runs amok, crowding out less aggressive native plants and altering the landscape. Buckthorn arrived in the United States in the mid-1800s, brought from Europe as an ornamental plant admired for its thick, long-lasting foliage and fast growth. Native birds relished the fruits of the tall shrub and helped to disperse them. Once scattered, the seeds could remain dormant for as long as six years. In contrast to native plants, buckthorn supports almost no native invertebrates, like butterflies and moths, many of which are either food for native animals or serve as important pollinators. Soon buckthorn expanded far beyond its original boundaries of home landscapes and farms—where it was used as a windscreen—crowding out native plants, changing nutrients in the soil, and threatening native habitats. 

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Participants work to clear buckthorn near the Braeside Metra station and Ravinia Festival.

Buckthorn on Green Bay Trail

The cut buckthorn was dragged out of the way of cyclists and pedestrians using the Green Bay Trail.

How much buckthorn did we eliminate? Rebecca estimated that our group cleared 1,500 feet of trail—more than a quarter-mile. At 2:30 p.m.,  she and her crew were still feeding the cut plants into a wood chipper, making sure the chances of reseeding are minimal. They also selectively applied herbicide to the remaining stumps. In the future, they plan to monitor the area for the return of native wildflowers, seeding if necessary.

I live near the trail and run along it often. Already I have enjoyed seeing how the sunlight filters into this new clearing, reaching areas that will soon flourish with returning “natives.”

Rebecca Grill, PDHP, and Bob Kirschner, director of Aquatic Plant and Urban Lake Studies

Rebecca Grill of the Park District and Bob Kirschner of the Garden pause before tackling the buckthorn.

Learn more about invasive species on the Garden’s website. For information about future buckthorn workdays, contact Liz Ettelson at the Park District of Highland Park  (eettelson@pdhp.org). 


©2013 Chicago Botanic Garden and my.chicagobotanic.org