Arboretum examines climate change’s impact on maple trees – Harvard Gazette


Gazette: The Arboretum has a diverse collection of maples — including rare and endangered species from around the world. What is the effect of this research on the Arboretum’s collection? What is the effect on United States forests?

Grossman: Our research helps us understand more about the response of maples to what we might call climate stress — the environmental factors that challenge woody plants and that are likely to get even worse as our climate changes. Our findings will help the Arboretum’s managers decide which maples to seek out and plant — species that will be able to survive here in the future. They also will help staff keep the existing maples alive by, for instance, informing irrigation priorities. When we think about forests overall, maples are dominant trees in eastern deciduous forests and important sources of wood, syrup, and other things. Knowing how climate stress affects particular maples species will help foresters, conservationists, and other land managers to prioritize the planting, care, and harvest of natural forests, plantations, and urban woodlands.

maple twigs just clipped from a red maple tree.
Freshly clipped red maple twigs.

Gazette: Can maple species fail?

Grossman: It is maybe best to think about failure in terms of individual trees, and the answer is yes. For instance, all trees have small tubes that extend all the way around their trunks, these are called xylem. Their purpose is to conduct water from the ground to the leaves at the top of the tree, and everywhere in between. During exceptionally warm conditions, if a particular tree’s soil becomes really dry, bubbles form in these tubes. When that happens to a particular xylem tube, it is unusable forever. If most or all of a tree’s xylem gets emptied out — or cavitated — the tree dies. Or with freezing, we could imagine that a particular maple tree has been exposed to warm weather for several weeks. It begins to send out new leaves and flowers because it has received signals that spring has arrived. If a really cold period moves in, this tender, actively growing material might freeze or get dried out. If so, the tree has now lost its investment in a whole cohort of leaves or flowers. If it is a small or already weak tree, it may have trouble replacing them and could starve to death in the coming year. Finally, if we want to think about the ultimate “failure” of a particular species, that would be something like extinction. This is certainly possible, although it often takes a long time for long-lived trees like maples. If humans are not overharvesting a species, it takes a long time for total climate-induced extinction to affect a long-lived woody species.

Kovaleski: Adding to Jake’s example of freezing, which is more easily observed because you could see green tissues on the tree or plant, this can also happen within the buds of the plants before they’ve gone through any visible changes. If the temperatures drop below the cold hardiness level a certain plant has, the buds can be killed and they just won’t grow the following season, without a very clear sign — unless you are scientifically tracking the cold hardiness of things throughout the winter.

Gazette: What does the broader impact of your research mean for scientists working on climate change mitigation around the world?

Grossman: Our research helps demonstrate the consequences of climate change for temperate forests, urban trees, and forestry plantations. Hopefully, if people know more about what is likely to happen, they will be motivated to mitigate climate change. From an adaptation angle, our research can guide management of trees and forests in a rapidly changing climate.

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