For short-lived spring wildflowers such as wood anemone (quinquefolia anemone) and Dutch pants (dicentra cucullaria), Timing is everything.
These fleeting plants, known as ephemera, grow in temperate forests around the world, budding and flowering in early spring before the trees towering above them sprout. If they leave too early, it will still be winter; but if it appears too late, it is already too shaded under the forest canopy for essential photosynthesis to take place.
Throughout their evolutionary history, these plants 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 flowering 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 out of luck? What if interconnected species respond to changes at different rates, disrupting long-standing ecological relationships?
Phenology and climate change of wildflowers
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, like pollinators leaving at the wrong time for flowers.
Far fewer have observed plant-plant interactions, like the spring ephemera that need time to grow before the trees fill with leaves and block out sunlight.
Our research group investigated the mismatch between understory wildflowers and tree canopies in Concord, Massachusetts, using historical observations recorded by Henry David Thoreau, author of “Walden,” his classic account of forest life. We found that trees in Concord were more sensitive to spring temperatures than wildflowers, and this resulted in early defoliation of trees that reduced available light in the understory.
This discovery was an important first step, but we wanted to know whether these patterns persisted in other temperate forests in North America and the Northern Hemisphere. Our latest study shows that the answer is yes.
Incompatibilities in North America
For this research we used specimens from herbariums, collections of pressed, dried and cataloged plants. The plants we examined were collected from eastern North America over the last 100 years. We evaluated more than 3,000 specimens of pressed plants to trace the time of appearance of leaves on trees and the time of flowering of spring wildflowers.
The large scale of this study was possible because herbaria have digitized millions of photographs of plant specimens and made them available online over the past decade. Before the existence of this resource, researchers had to travel to different museums across the country.
Historical weather records are also available online now. This allows researchers to determine spring temperatures for the year and location where each specimen was collected.
Our new study has allowed us to confirm the results of our work at Concord. We found that as temperatures warm, deciduous trees in eastern North America are advancing faster than native wildflowers.
temperature changes
For example, during the coldest springs with 24-hour average temperatures in March and April of 41 degrees Fahrenheit (5 degrees Celsius), trees shed their leaves 13 days later than native wildflowers. This gave the flowers nearly two weeks of full sun on the forest floor.
However, during the warmest springs, with temperatures averaging 58 F (15 C), the trees shed their leaves just 10 days after the native wildflowers. This gave the wildflowers about 25% less time in full sunlight to photosynthesize.
As spring temperatures get even warmer with climate change, we expect wildflowers to have even shorter periods of sunlight. This can mean a considerable decrease in the energy supply and in the flower’s ability to survive, grow and reproduce.
We also observed that trees and wildflowers in the warmer southern part of their ranges advanced more rapidly through their flowering and flowering period, respectively, than those in cooler northern locations. In these areas, we found greater temporal differences between trees and wildflowers.
This means that the potential for phenological mismatch, where trees are more likely to shade native wildflowers, is greater in the southeastern US than in areas farther north.
Parallels and differences of wildflowers on other continents
For another recent study, we collaborated with colleagues in China and Germany to evaluate more than 5,000 tree and wildflower specimens collected over the past 120 years. We wanted to see if the phenological incompatibilities we documented in North America could also be found in the temperate forests of East Asia and Central Europe.
Our team found a common pattern across all three continents. Trees and wildflowers are active earlier now than in the past, and earlier in warm years and places.
However, in a surprising turn of events, we haven’t seen the pattern of North American trees be any more sensitive than wildflowers on the other two continents. In Europe, wildflowers and treetops seemed to change over time. In Asia, understory wildflowers were moving more than trees, meaning they could receive more light, not less, in a warmer future.
The differences we found between the three regions are mainly due to the variation in the sensitivity of trees to temperature. Trees in eastern North America responded most strongly to temperature changes, while Asian trees responded less strongly.
Inform forest management
Our results raise questions for future research. If spring temperatures are not the main cues that determine the flowering and flowering of trees and wildflowers in East Asia, what are those cues? How does the narrow spring daylight window for wildflowers in eastern North America affect their energy budgets and their ability to survive, grow, and flourish?
Another question is whether there are practical management techniques, such as thinning tree tops or removing invasive plants, that can help wildflowers cope with the current challenges of climate change.
These strategies can help people appreciate and conserve the full range of plants in the forests we depend on and cherish around the world.