Expedition to Door County

Last June, I headed up to Door County, Wisconsin, with Kay Havens, our director of plant science and conservation,  for a 31-day trip to undertake our annual fieldwork. “A month at the beach!” you say, thinking it such a treat! Well, yes and no.

Four undergraduate students in our REU program joined us to track literal life and death events in two plant populations on the dunes of Lake Michigan. The dunes can be more than 20 degrees Fahrenheit hotter than ambient temperatures, and we work in the interdunal swales, where no lovely breezes off the lake can reach us. It is often well over 95 degrees in the dunes, even if it’s a balmy 75 degrees in Sturgeon Bay. But, no matter—we are on a mission! On days with the hot sun both beating down and reflecting up from the sand, we observed, measured, and recorded the births, deaths, and reproductive successes of one of our favorite plants: the threatened pitcher’s thistle (Cirsium pitcheri). 

PHOTO: Pitcher's thistle (Cirsium pitcheri)
Pitcher’s thistle (Cirsium pitcheri)

We find every seedling we can, and place a flag next to it to help us keep track of the ones we’ve counted. We don’t want to miss a single one. Each seedling is a measure of successful reproduction for this monocarpic perennial. Monocarps—plants that only flower once before they die, are completely dependent upon producing as many successful offspring as they can, all in the quest to ensure that they just replace themselves. When all plants successfully replace themselves, a population is stable.

Just to replace yourself is a monumental undertaking for a plant that flowers once and then dies. Especially for pitcher’s thistle. The dunes are a harsh environment for a tiny baby plant. Many of them die—exposed to the heat, and without enough water to sustain them. We estimate that fewer than one in ten seeds germinate and survive each year, and in some years, only a small percent of those survive the winter to become a juvenile plant the next year. That means that each flowering plant must produce many seeds to replace itself. The good news? Generally, if a seedling survives to the juvenile stage, it has a much increased chance of survival to make it to the next stage—a vegetative plant—and the vast majority of those go on to reproduce at some point.

PHOTO: Kay Havens, ready to record data at Ship Canal Nature Preserve, owned by the Door County Land Trust.
Kay Havens is ready to record data at Ship Canal Nature Preserve, owned by the Door County Land Trust.

However, seed germination and seedling survivorship and growth depend upon two things: where you come from and where you live. To look at this, we took 100 seeds from each of our two study populations and grew them in “seed baskets” in our study garden at the Chicago Botanic Garden. We also grew the same number in seed baskets at their respective home sites. Regardless of population, they germinated and grew very readily in our study garden. But there were very stark differences at our study sites in Door County: seed germination was 39% at one site, but only 9% at the other.

PHOTO: Pitcher’s thistle seedlings sprouted in one of our seed baskets at the Ship Canal Nature Preserve.
Pitcher’s thistle seedlings sprouted in one of our seed baskets at the Ship Canal Nature Preserve. The pair of yellow-green “leaves” opposite each other are actually cotyledons, or seed leaves, and are the first photosynthetic organs to emerge from the seed during germination.
PHOTO: These are Pitcher's thistle seedlings that have grown very large under the favorable conditions of the test garden on the south side of the Plant Science Center.
These are pitcher’s thistle seedlings that have grown very large under the favorable conditions of the test garden on the south side of the Plant Science Center. In just one growing season, they have grown as large as plants three to four years old that grow under natural conditions.

Why the difference? Well, our first site is definitely more hospitable! Even we are happier to work here. It’s not nearly as hot, and the dune structure is more flat, so the breeze off the lake makes things more pleasant—for plants and people alike! And it appears to this observer’s eye that there’s more water available close to the surface here. This year, there are two large patches in the dune that have been perpetually damp. In contrast, our second population is literally high and dry, making life hard for the little pitcher’s thistle seedlings. How does this affect the prospects of these two populations overall? Stay tuned! We’ll let you know when we have finished our analysis of the long-term trends at these two very different sites.

One plant, two places—offering a fascinating glimpse of a life of contrasts.

©2016 Chicago Botanic Garden and my.chicagobotanic.org

Weevil Warriors

A bracelet of pink-and-cream blooms borders Lake Michigan at this time of year. Growing up from barren, sandy areas, Pitcher’s thistle (Cirsium pitcheri) is like an oasis for nearby insects looking for nectar.

Helpful pollinators visit the plants often, but not nearly enough to offset the damage of the predatory visitors, according to Garden scientists, who hope to tip the balance in order to save the federally threatened plant species. It all began in 1997, when Kay Havens, Ph.D., joined a team of researchers to reintroduce Pitcher’s thistle to Illinois Beach State Park. But she wondered why a reintroduction was needed for a normally sturdy group of plants.

PHOTO: Adult weevils on Pitcher’s thistle plants in Door County, Wisconsin
Adult weevils on Pitcher’s thistle plants in Door County, Wisconsin

“Thistles as a group are typically pretty successful, even weedy, and it’s unusual to have a native thistle that is so rare,” said Dr. Havens, Medard and Elizabeth Welch director of plant science and conservation at the Garden.

Along with her Garden colleague, Pati Vitt, Ph.D., she found the plant is especially susceptible to a species of weevil (Larinus planus) whose larvae feast on emerging seeds. Unfortunately, if a blooming thistle loses its seeds before they are dispersed, new plants cannot be started.

“We were the first to document the weevil in this threatened thistle,” said Havens. “If we don’t find a way to control it, the plant is further threatened with extinction.” This discovery was followed by the identification of a second, equally destructive weevil (Rhinocyllus conicus) in Indiana Dunes State Park.

“The weevils have become very widespread in Pitcher’s thistle and they cut the seed output by about half,” said Havens. Both insect species, she explained, are biocontrol weevils that were intentionally scattered in the area for years in an effort to control the invasive Canada thistle (Cirsium arvense). Now, the damaging insects are off the list of solutions for Canada thistle.

As we talked, it was clear that this rescue team is not willing to accept defeat from a 7-millimeter bug. For years, they have been hard at work gathering information to help them arm the thistle against its adversary, and theorizing potential solutions.

Pitcher’s thistle lives four to eight years, and only blooms once, the conservation scientists told me. And they are also working to better understand the remarkable plant that has a custom set of strategies, like all plant species, to survive and reproduce.

“Every species represents a unique solution to an entire suite of environmental problems,” said Dr. Vitt, Susan and Roger Stone curator of the Dixon National Tallgrass Prairie Seed Bank at the Garden.

These solutions, she explained, could solve large-scale problems for people or other species in ways we don’t yet know. “The species has intrinsic value because it has these unique solutions that evolved over hundreds of thousands of years,” she added.

PHOTO: The research site in Wisconsin with flags marking study plants
The research site in Wisconsin with flags marking study plants

This summer, Havens and Vitt spent weeks in Door County, Wisconsin, observing the interactions of the weevil with Pitcher’s thistle, as well as those of its suite of pollinator species.

Watch a video to learn how Havens became interested in plants as a child, and why she says we couldn’t survive without them today.

Together with their research team, they recorded detailed notes about the frequency and time of visits by the helpful pollinators, like bees, and the dreaded weevils. In total, they monitored 27 visiting insect species.

“We want to find ways to protect this plant from weevils without affecting its pollinators,” said Havens. Could a certain floral scent do the trick? Could a natural insecticide be the answer? Each solution must be carefully tested and put to trial first.

For now, they are busy trying to understand the life cycle of the weevil as it relates to the thistle—from the time an adult lays its eggs in the seedhead to the time the larvae emerge from the flower and eat the seeds that could have been the next generation of plants.

As I left Havens and Vitt in the Daniel F. and Ada L. Rice Plant Conservation Science Center, they were sorting through their samples of seedheads and weevils, collected in the field, and musing over the potential solutions planted by their research that are just beginning to grow.

©2013 Chicago Botanic Garden and my.chicagobotanic.org