For short-lived spring wildflowers like wood anemone (Anemone quinquefolia) and Dutchman’s panties (Dicentra cucullaria), timing is everything.
These short-lived plants, known as mayflies, grow in temperate forests around the world, budding and flowering in early spring before the trees above them bud. If they leave too early, it will still be winter; but if it emerges too late, it will already be too shaded under the canopy for essential photosynthesis to occur.
Throughout their evolutionary history, these plants have discovered the best time for their survival. But climate change is altering spring growing conditions and plant life is changing with it.
There are many examples of plants changing bloom times in response to rising temperatures, such as cherry blossoms opening earlier and earlier each year.
However, when one part of an ecosystem changes, do all the organisms that depend on it successfully change as well? Or will they be unlucky? What if interconnected species react to change at different rates, disrupting long-standing ecological relationships?
Wildflower phenology and climate change
Researchers have been asking these kinds of questions about phenology, the timing of biological events, related to climate change for years. But most studies have focused on plant-animal interactions, such as pollinators coming out at the wrong time for flowers.
Far fewer have looked at plant-plant interactions, such as spring mayflies that need time to grow before trees fill with leaves and block sunlight.
Our research group studied the mismatch between understory wildflowers and canopy trees around Concord, Massachusetts, using historical observations recorded by Henry David Thoreau, the author of “Walden,” his classic account of life in the woods. . We found that Concord trees were more sensitive to spring temperatures than wildflowers, and this led to early tree defoliation that reduced available light in the understory.
This discovery was an important first step, but we wanted to know if these patterns persisted in other temperate forests in North America and the Northern Hemisphere. Our latest study shows that the answer is yes.
Mismatches in North America
For this research, we use herbarium specimens, collections of plants that have been pressed, dried and catalogued. The plants we looked at were harvested in eastern North America within the last 100 years. We evaluated more than 3,000 specimens of pressed plants to trace when tree leaves appeared and when spring wildflowers bloomed.
The large scale of this study was possible because Herbaria has digitized millions of photographs of plant specimens and made them available online over the past decade. Before this resource existed, scholars had to visit numerous museums spread across the country.
Historical weather records are also available online now. This allows researchers to determine spring temperatures for the year and where each specimen was collected.
Our new study has allowed us to confirm the results of our work at Concord. We have found that as temperatures warm, deciduous trees in eastern North America grow faster than native wildflowers.
For example, during cooler springs with average 24-hour temperatures in March and April of 41 degrees Fahrenheit (5 degrees Celsius), trees dropped their leaves 13 days later than native wildflowers. This gave the flowers nearly two weeks of full sun on the forest floor.
However, during warmer springs, with average temperatures of 58 F (15 C), the trees dropped their leaves only 10 days after the native wildflowers. This gave the wildflowers about 25% less time in full sun for photosynthesis.
As spring temperatures get even warmer with climate change, we expect wildflowers to have even shorter periods of full sun. This can mean a considerable decrease in energy supply and the ability of flowers to survive, grow and reproduce.
We also observed that trees and wildflowers in the warmer southern part of their range advanced their flowering and flowering times faster, respectively, than those in cooler northern regions. In these areas, we found larger temporal differences between trees and wildflowers.
This means that the potential for phenological mismatch, where trees are more likely to shade out native wildflowers, is higher in the southeastern United States than in regions further north.
Parallels and Differences of Wildflowers on Other Continents
For another recent study, we collaborated with colleagues in China and Germany to assess over 5,000 tree and wildflower specimens collected over the past 120 years. We wanted to see if the phenological shifts we documented in North America could also be found in the temperate forests of eastern Asia and central Europe.
Our team found a common pattern across all three continents. Trees and wildflowers are active earlier than in the past, and earlier in warmer years and locations.
However, in a surprising twist, we didn’t see the pattern of North American trees being any more susceptible than wildflowers from the other two continents. In Europe, wildflowers and canopy trees seemed to change together over time. In Asia, understory wildflowers moved more than trees, meaning they could receive more light, not less, in a warmer future.
The differences we found between the three regions were mainly due to variation in tree sensitivity to temperature. Eastern North American trees reacted more strongly to temperature changes, while Asian trees reacted less strongly.
Inform forest management
Our results raise questions for future research. If spring temperatures are not the main clues determining flowering and flowering times for trees and wildflowers in East Asia, what are those clues? How does the shrinking window of spring sunlight for wildflowers in eastern North America affect their energy budgets and their ability to survive, grow and thrive?
Another question is whether there are any practical management techniques, such as thinning canopy trees or removing invasive plants, that can help wildflowers cope with the current challenges of climate change.
Such strategies could help people appreciate and conserve the full range of plants in the forests we depend on and cherish around the world.