Researchers Examine Threats to Wildflower Diversity
By Sara Kuebbing, Jessica Poteet, Cheyenne Moore, and Mason Heberling
Although the old saying reminds us to see the forest for the trees, some of the most exciting plants are not trees at all. While trees might take up more space in forests, over 80% of the plant diversity in forests are less than a few feet tall, living near the forest’s floor. The Barking Slopes Conservation Area, along the Allegheny River on the outskirts of Plum Borough near New Kensington, PA is a great example of how incredibly diverse a forest in this region can be, even beyond its trees. The site was a coal mine for many decades until the late 1970s but now the forest’s slopes and ridges are home to a huge array of beautiful native wildflowers, such as trillium, wild ginger, delphinium, hepatica, toothwort, violets, and so many more. Researchers and community scientists have documented over 76 native forest wildflower species at the site, including rare species found in very few other parts of the state. Many of these species are documented on the free and open community science platform, iNaturalist, in the Barking Slopes project page (https://www.inaturalist.org/projects/barking-slopes).
Although the Allegheny Land Trust currently protects the vast wildflower diversity at Barking Slopes from development, there are other significant ecological threats to these wildflowers that are harder to manage. For example, many forests in our region are home to overabundant white-tailed deer populations, who love to eat the flowers and foliage of many of these charismatic wildflowers. Luckily, deer haven’t made Barking Slopes a cafeteria stop–possibly deer find the steep slopes an unappealing place for a meal—making Barking Slopes a special refuge for local wildflower diversity.
While deer and development are not currently problem for wildflowers at Barking Slopes, human actions that land trusts cannot control may still lead to future decline of plant species. As with many other forests worldwide, the wildflowers and trees at Barking Slopes are experiencing warmer winters and springs than in previous decades because human combustion of fossil fuels is increasing carbon dioxide in our atmosphere and changing the climate. Humans have also transported many different shrub species from different continents to decorate our gardens. Some of these introduced, ornamental shrubs have spread from people’s yards into the forests and changed the patterns and processes of the forest ecosystems. Together, changing climates and these new, nonnative invasive shrubs represent two of the most dominant human-caused ecological threats to biodiversity around the globe.
Because Barking Slope is currently home to so many wildflowers, the site is an ideal place for researchers to study how forests may change in the future in response to changing climates and plant invasions. With a grant from the National Science Foundation starting in 2020, Dr. Sara Kuebbing, an Assistant Professor at the University of Pittsburgh and Dr. Mason Heberling, Assistant Curator of Botany at Carnegie Museum of Natural History are leading a team of student researchers in a multi-year project to study how changing climates and invading plants might affect forest wildflower diversity at Barking Slopes.
Warming climates means shorter winters and earlier springs. Researchers around the world have tracked how plants and animals are changing the timing of seasonal events, like the flowering of cherry blossoms and daffodils, the arrival of the first migratory robin, or the first cicada’s song are occurring earlier than they were decades ago. However, recently, Heberling and colleagues discovered another disturbing aspect of this trend. By comparing historical records—including Henry David Thoreau’s journals of spring flowering plants—to contemporary observations of flowering and leaf-out times for the same species, the researchers surprisingly found that wildflowers were leafing out about one week earlier than they had in the past, but trees were leafing out two weeks earlier than they were 160 years ago.
This ‘mismatch’ in the way trees and wildflowers respond to warming temperatures could have large consequences for some forest wildflower species. Many wildflowers emerge from their winter dormancy before the trees are flush with leaves. These wildflowers rely on this short period of time when lots of light reaches the forest floor so they can photosynthesize and generate enough energy to reproduce before the tree canopy fills in and makes the forest floor dark. If trees are responding faster than wildflowers to warming spring temperatures, wildflowers may have smaller periods of time to capitalize on the high light in the early spring. Heberling estimates that the reduction in light from earlier tree leaf out could reduce the energy wildflowers can gather each spring by nearly half in the next 50 years if climate warming continues at its current pace.
Then there are the invasive shrubs. Hailing from different continents with different climates, many of these shrubs also leaf out very early each spring, casting deeper shade on the wildflowers growing beneath them. At Barking Slopes, invasive shrubs such as privet, honeysuckle, and five-finger aralia might be further reducing the early spring light that reaches the forest floor. Over the past year, Kuebbing and Heberling’s field team have monitored the timing of leaf out and the differences in light levels beneath native and invasive shrubs at Barking Slopes, as well as the health of wildflowers growing underneath. The researchers have already found that invasive shrubs reduce understory light earlier in the spring and cast deeper shade than native shrubs, and that these nonnative shrubs—like the trees–may be more responsive to a warming climate than their native counterparts, causing them to leaf out and cast shade earlier in the season.
But not all the invasive shrubs at Barking Slopes are early spring risers. Some of the most prevalent invasive shrubs at the site are knotweeds, shrubs that grow along the roads, slopes, and trailsides in dense patches of tall, reddish-brown bamboo-like stalks with bright green, heart-shaped leaves. Knotweed, unlike some of the other invasive shrubs, doesn’t start poking out of the ground with sizeable leaves until late May, after the trillium has flowered. Although slow to get started growing each spring, by July knotweeds can tower over hikers’ heads and cast deep shade to the forest floor. Any summer or fall blooming wildflowers, like broad-leaved goldenrod or white wood aster, that might not be bothered too much by the early spring leaf-out of trees and shrubs, will certainly have trouble capturing enough light if they are growing within or below a dense knotweed patch.
Invasive knotweeds are found all around Pittsburgh, across the United States, and even in Europe but Barking Slopes presents a unique opportunity for research on the impacts of knotweed on wildflowers. Unlike many areas, Barking Slopes is home to three different types of knotweed. Japanese knotweed and giant knotweed came to this area in the early 1900s as part of the horticulture trade. These two plants, both originally only found in east Asia, never lived at the same sites in their native habitats. However, after their introduction to North American gardens, they were able to hybridize and create a new type of knotweed, Bohemian knotweed. Because Barking Slopes is home to all three types of knotweed, it provides is a great opportunity to study ecological similarities and differences between each. This summer, Cheyenne Moore, a graduate student in Kuebbing’s lab at Pitt, will study the knotweed, the native plants growing within the knotweed patches, and the surrounding environment to understand how these three shrubs are impacting native plants and the soil.
This summer, when you’re out hiking at Barking Slopes, you might see some field researchers peering underneath the patches of invasive shrubs or laying on the forest floor measuring the height of small herbs. You should wave, say hi, and ask them about their work. They will tell you how, by investigating the changing light availability for wildflowers and unique variety of invasive knotweed at Barking Slopes, they are gathering vital information to promote forest diversity throughout the northeastern US.
About the Authors:
Sara Kuebbing is an Assistant Professor in the Department of Biological Sciences at the University of Pittsburgh.
Cheyenne Moore is a first-year graduate student studying plant conservation and invasion biology in the Department of Biological Sciences at the University of Pittsburgh.
Jessica Poteet is a research technician in the Department of Biological Sciences at the University of Pittsburgh.
Mason Heberling is an Assistant Curator of Botany at Carnegie Museum of Natural History.
