Zoe Panchen Sheds Surprising Light on Plant Behaviour with Climate Change Implications
Despite conventional wisdom among gardeners, foresters and botanists that all woody plants open their lead buds, or “leaf out” at about the same time each spring, a new study proves otherwise. Carleton University PhD candidate Zoe Panchen was the lead author on a research paper that found a surprisingly wide span of as much as three months in leaf-out times.
Observations of 1,597 woody plants in eight botanical gardens in Canada, the United States, Germany and China over two spring seasons suggest that differences between species in leaf-out times could impact the length of the growing season and the activities of birds, insects and other animals. Leaf-out times therefore must be factored into climate change model predictions.
“As species distribution and abundance shift due to climate change, interspecific differences in leaf-out timing may affect ecosystem processes such as carbon, water and nutrient cycling,” reported the study in the journal New Phytologist. “Our open-access leaf-out data provide a critical framework for monitoring and modelling such changes going forward.”
While previous researchers observed the leaf-out process for limited numbers of species in a single location, this study obtained observations of the same species from gardens around the world. Notably, the order of leafing out among the species was almost the same in different gardens and with different climates, suggesting that leaf-out time is a fixed character of a species, like the shape of its leaves or flowers; some species tend to leaf out early and others late.
“We expected to see that deciduous species would leaf out earlier than evergreen species, but it was interesting to see that there were groups of deciduous species, such as the oaks and beeches, that were quite late to leaf out,” said Panchen. “There are definitely distinct evolutionary groups of woody species that are early or late to leaf out.”
“Prior to this study, no one would have suspected that there was so much difference in the leafing out times of different species,” says Boston University Prof. Richard Primack, who recruited colleagues from around the world for the study. “At the Arnold Arboretum in Boston, some gooseberry and honeysuckle shrubs start leafing out mid-March and early April, and evergreen rhododendrons and pine trees don’t start leafing out until two to three months later in late May or even June. These differences are quite striking.”
The study showed that shrubs leaf out on average 10 days before trees and deciduous plants leaf out on average 17 days before evergreens. And certain groups of plants, such as honeysuckles, willows, lilacs and apples, tended to leaf out early while other groups, such as oak, beeches, honey locusts, and grapes, tended to leaf out late. As a result, forests will have flushes of new leaves over an extended period. This phenomenon has implications for insect survival and carbon dioxide absorption by forests.
“Leaf-out phenology affects a wide variety of ecosystem processes and ecological interactions and will take on added significance as leaf-out times increasingly shift in response to warming temperatures associated with climate change,” the study said. “There is, however, relatively little information available on the factors affecting species differences in leaf-out phenology.”
Panchen was responsible for observing plants in Ottawa and has some interesting conclusions about how local plants behave.
“The Ottawa leaf out was by far the latest, but it was also the shortest in duration,” she said. “It was like the trees hurried to catch up with their cousins further south.”
The study conclude that as the climate warms, trees will tend to leaf out earlier in the spring, perhaps extending the growing season and affecting animal behavior. But this pattern will become complicated if the overall tree composition changes. In eastern North America for example, maple and birch trees, which leaf out early, may be replaced gradually by more heat-tolerant oak trees, which tend to leaf out later in the spring.
The team also included Richard Primack (Boston University), Birgit Nordt and Albert Dieter-Stevens (Berlin Botanical Garden), Elizabeth Ellwood (Florida State University), Susanne Renner (University of Munich), Charles Willis and Charles Davis (Harvard University), Robert Fahey (Morton Arboretum), Alan Whittemore (U.S. National Arboretum) and Yanjun Du (Chinese Academy of Science, Beijing). Their next project will extend their observations to autumn to determine if there are major differences among species when trees change color and drop their leaves at the end of the growing season.
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Chris Cline
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Carleton University
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