New Study Identifies Climate-Ready Trees Around the World

Botanists from Trinity College Dublin have learned that “penny-pinching” evergreen species such as Christmas favorites, ivy, and holly are more climate-ready in the face of warming temperatures when compared to deciduous “big-spending” water consumers such as oak and birch.

Dr Soh with part of the research team in Seaqaqa, Fiji. The research took them all over the globe. Image Credit: Dr Wuu-Kuang Soh.

They are more likely to flourish in the near future — with this pattern set to be observed more often in cooler climates, such as in Ireland.

According to theory, growing global carbon dioxide (CO2) concentrations in the earth’s atmosphere will make the world’s trees grow in a more water-efficient way. However, to date, few studies have verified these predictions.

The Trinity team’s study reveals that atmospheric CO2 rise in the last 25 years has already had an obvious effect on the water use of forests, making them more water-wise. But not all tree species act in the same manner.

Jennifer McElwain, Professor of Botany at Trinity, guided the research that has just been reported in the top international journal Science Advances.

Remarkably, we found that with rising CO2, evergreen trees and shrubs are more efficient in using water than deciduous plants in cooler climate locations, but there is no evidence for such a pattern in parts of the world with warmer climates. As well as offering a fresh insight our results allow us to make some concrete predictions on how future modification of our atmosphere and climate will impact the world’s forests.

Dr Wuu Kuang Soh, Study Lead Author and Research Fellow in Botany, Trinity College Dublin

Dr Wuu Kuang Soh continued, “Our results suggest that evergreen trees and shrubs will benefit more than deciduous plants in a higher CO2 future, particularly in parts of the globe that have generally cooler climates, like Ireland and in other countries in the temperate latitudes.”

Gardeners are very mindful that water is a valuable commodity for all plants. Water is vital for healthy growth and development, and plants will soon wither and perish without it. However, in the wild, where plants do not get daily water from a caring gardener, species vie for it and all other vital resources like nutrients, light, and space.

The team’s results thus suggest that the evergreen woody species—as they are more water-wise — will have a different ecological benefit over their deciduous neighbors if atmospheric CO2 concentrations increase continuously. During periods of water shortage — such as in droughts that are growing in frequency and duration because of climate change — this predicted benefit should be even greater.

This recently discovered effect of current global climate change on evergreen and deciduous plants becomes more evident as one travels from the warmer to cooler zones of the earth.

To uncover this trend, the researchers set out on a five-year research program sponsored by Science Foundation Ireland. They examined museum leaf specimens gathered in the late 1980s and compared them with leaves from the same species gathered 25 to 30 years later from the same geographic areas.

Moreover, the researchers carried out fieldwork expeditions to examine trees and shrubs from the warm tropical and desert landscapes of Puerto Rico, Fiji, and Arizona to the cold temperate and boreal forests of Alaska. This enabled them to monitor woody plant reactions to anthropogenic climate change spanning a period of time 30 years and across space.

The findings have a wider significance as the results can presently be used to feed into climate models to reflect as much as possible the condition of real plant and vegetation responses to climate change on the ground.

For cooler climates like Ireland’s we should be really looking at how rising CO2 affects our forest ecosystems in the context of evergreen and deciduous tree types. For example, evergreen climbers like ivy, and trees like Holly—two of our iconic Christmas plants—may outcompete deciduous trees such as birch and oak if water becomes an even more precious recourse in the future.

Jennifer McElwain, Professor of Botany, Trinity College Dublin

McElwain added, “This is perhaps something that future gardeners, foresters and urban landscape architects should consider.”

The reason for the detected differences in the evergreen and deciduous plant responses to climate change lies in their leaf texture. The leaves of evergreens are generally thicker and sturdier than deciduous plants in cooler climates, while they are mostly similar in texture between the two groups in the warmer climates,” stated Dr Wuu Kuang Soh.

Because leaf texture affects plant sensitivity to rising CO2 and because the evergreen and deciduous leaf habits are an important hallmark that define forest type, their differential behavior to rising CO2 will have a profound impact on the land carbon and water cycles today and in the very near future.

Dr Wuu Kuang Soh, Study Lead Author and Research Fellow in Botany, Trinity College Dublin

One way of assessing the prediction that evergreen woody species may have a competitive edge in the future is to look back through geological history at periods when the CO2 concentration in the atmosphere was a lot higher compared to the existing levels.

Fifty million years ago, in the Eocene, palaeo-botanists uncovered far more evergreen leaves than deciduous leaves, especially across much of ancient Europe. It has thus far baffled researchers as to why this was the scenario. However, with the new results, the researchers believe it can now be concluded that there were more evergreen species in the Earth’s warmer past since evergreens are more water-wise in conditions of high CO2.

The following step is to explore these patterns at a species level as the botanists intend to use their phenomenal 30-year data set to find the most climate-ready trees for future forests.

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