Life at the Limits: The Nature of People and Plants

"In a frighteningly short period of time, humans have altered all ecosystems and changed the trajectory of life on Earth forever."

Is it possible for life to exist in the most extreme of environmental conditions? Humans have been interested in this question for generations. We love reading stories and watching movies about life at the limits, pushing boundaries on barren planets or in different worlds. Could humans one day live on Mars or a yet undiscovered Earth-like planet? And are we interested in answering these questions because we are simply curious or because we have inflicted so much damage on the Earth that it’s time to look for a new home?

It’s true that our planet looks vastly different than it used to even 200 years ago. In a frighteningly short period of time, humans have altered all ecosystems and changed the trajectory of life on Earth forever. The authors of this article live in the state of Illinois, which is known as “the prairie state,” even though today less than one-tenth of one percent (that’s less than 0.01%) of the tallgrass prairie that once covered the land is still intact.

"Endangered Prairie Habitat Creation In North America," Global Impact

Instead, we primarily have farms and cities. Humans have altered all our ecosystems to the point of unrecognition from their original state. For the tens of thousands of species that also call Illinois home, how will they be able to cope with even more habitat loss? If far-away planets and the pristine wilderness of pre-colonial times are both impossible homes right now, what other options exist?

One possible underused habitat happens to be right over our heads most of the time: roofs. Modern green roofs are now designed to incorporate a layer of soil and plants on top of buildings. Usually, they are built to trap rainwater and help provide insulation which reduces heating and cooling costs of the building and prolongs the life of the roof. Green roof soil is engineered to be lightweight, non-absorbent, and low in nutrients so that excess water doesn’t stress the roof and weeds don’t take over. Green roof soil is therefore the opposite of the soft, moist soil you would find in a garden and makes green roofs just about the last place most plant species will grow.

Oxalis violacea (Violet wood sorrel) at the Elmhurst University green roof (2021)

In fact, most green roofs are planted only with one type of succulent plant: stonecrops (from the plant genus Sedum). Stonecrops have evolved to live in extreme environments like cliffs and rock faces, where nutrients and water are scarce. But what about plant species that used to call our cities home or their homes are threatened? If native plants could also survive on green roofs and provide ecosystem services (the benefits nature provides humans such as clean air, crops, raw materials, energy, etc.), they would have the added benefit of supporting wildlife as the basis of the food chain and provide homes for local organisms such as fungi, insects, birds, and even small mammals and reptiles.

“If native plants on green roofs were grown with helpful neighbors and beneficial fungi, would it be enough to overcome the harsh environmental conditions?”

Unfortunately, it turns out that green roofs might as well be Mars, as far as many native plants are concerned. Research has shown that native flowers and grasses typically don’t make it past the first year or two on a green roof. Why not? Previous research suggests that some causes for these issues might be that shallow, quick-draining soil results in a lack of water and nutrients for the plants. Because specific soil is necessary to support the structural integrity of the building, this can’t be changed. If native plants are going to survive on green roofs, solutions must to found to address these limitations.

Two of the possible solutions that we are looking into in the GREEN Lab (Green Roof, Ecology and ENvironmental science) at Elmhurst University is related to teamwork. First, we think that the native plants might be able to get help from soil-dwelling fungi. In prairies, fungi and plants sometimes form a symbiotic relationship in which the fungi live in the roots of a plant and provide it with nutrients from the soil while the plant feeds the fungi with carbohydrates that it’s made through photosynthesis. Plants in prairies live very closely to other species that grow, flower, and produce seeds at different times and using different methods; an idea know as niche complementarity. By using different strategies from their neighbors, plants can help each other out in the same way that people can perform different jobs in their communities. If native plants on green roofs were grown with helpful neighbors and beneficial fungi, would it be enough to overcome the harsh environmental conditions?

Image of polyculture trays of different native plants at the Elmhurst University green roof (2021)

To answer this question about teamwork, we chose eight flowering plant species native to Illinois for an experiment. We set up a green roof made of experimental trays –- the same type of tray system that is commonly used on green roofs planted with stonecrops. We planted some native species by themselves and others with a combination of different neighbors (polyculture). In half of the trays, we added beneficial soil fungi. We then spent last summer measuring plant survival, growth and reproduction.

It has only been one year, but we are already seeing a trend that looks like plants really need to have specific drought-tolerant traits to survive on green roofs. For most species, it doesn’t look like adding beneficial soil fungi or planting in facilitative groups is enough to overcome the harsh environments of the green roofs (although, some species showed-

Elmhurst University students setting up green roof.

Left: Norbaya Durr (2021)

improved regeneration of plant vegetation post experiencing drought). Some species, like wild onion and cactus (yes, there is a cactus species native to Illinois!) have the traits to live on the roofs but others like wood sorrel (Oxalis violacea; you might mistake these for delicate clovers) and Lobelia don’t fare so well. As the weather starts to get warmer, we are looking forward to getting back to our experimental green roof to see which species made it through winter and continue to collect data throughout another growing season.

While more work is needed to determine how best to help native plants survive on green roofs, we are beginning to uncover some of the mysteries. We hope to be as successful and confident as Matt Damon’s character was in the sci-fi movie, The Martian, when he said, “Mars will come to fear my botany powers.”

Growing native plants on green roofs will be a small but powerful step in supporting ecosystems in which people, plants, and other organisms can live together in the place that we all call home. It’s also important to know that you don’t have to be a scientist or do science to support native plants and their habitats. We encourage you to get involved with your local native plant community garden, botanical societies and organizations.

Opuntia humifusa (Eastern Prickly Pear) on a green roof at Elmhurst University (2021)

We thank Elmhurst University and the Illinois Native Plant Society for their financial support of this research.