Forests help to combat global warming, but are also threatened by it. Many tree species struggled last summer as much of Europe was hit by heatwaves and a severe drought – thought to be the worst in 500 years.
Even olive trees, known for their ability to withstand dry conditions, have suffered. Spain is the world’s largest producer of olive oil, but many Spanish farmers expect their olive oil harvests to drop by as much as 50% this year.
In this context, Horizon researchers are rushing to understand more about how trees respond to drought as part of the fight against climate change.
Existing forests already remove about a third of the greenhouse gas emissions caused by human activities. A global afforestation program could do the same for nearly a third of the discharges left in the atmosphere.
“If you look at the last decade or so, there have been a number of events where severe drought has led to the death of trees in forests on a large scale.” said Dr. Jaideep Joshi of the Plant-FATE project, which studies plant traits to protect forests from climate change.
Planting billions of trees is a relatively inexpensive way to tackle the climate crisis, according to a study on… the potential for global forest cover to mitigate climate change.
But as the drought spreads, forests worldwide are at risk. In Europe, 500,000 hectares of forest were wiped out as a result of drought between 1987 and 2016.
Joshi led the Horizon-funded Plant-FATE projectwhich was groundbreaking when it comes to predicting the impact of drought on all kinds of trees.
A major limitation of current models is that they rarely take into account the ability of trees to adapt to arid conditions and how resilience can differ between species. That leads to inconsistencies in projecting how forests will respond to future climate scenarios.
‘That is where the greatest uncertainty lies at the moment’, says Dr Joshi. “You’ve got this whole ecosystem of mixed species — we’ve tried to bring it all together in a simple but comprehensive modeling framework.”
A model acts as a tool to simulate results and he believes his team’s model will be especially helpful in planning tree planting programs. That’s because it can signal the carbon uptake and storage potential of different species in the next 50-100 years, when climate conditions will be different from now.
“It could help make the right choices of which species to plant or where to plant them,” said Dr. Joshi. ‘It is the most promising conservation application of our model.’
In their model, the Plant-FATE researchers took the ability of trees to adapt to changing climate and looked at a range of time scales.
In shorter time frames, from weeks to months, trees exposed to drought may shed their leaves to conserve water (because water evaporates through pores on the surface of leaves) in what’s known as a “false fall.”
But over longer periods of time, trees can grow new wood with other properties better suited to dry conditions.
dr. Joshi and his team also took scale into account. For example, some reactions occur in specific parts of a tree such as roots and leaves, while others occur at the level of an entire species.
To test their full model, Dr. Joshi and his colleagues collected data from an Amazon rainforest site with about 400 species in an area of 5,000 square meters. They found that their model’s predictions closely matched what happened in real life at the site.
It is the first time that a vegetation model has performed realistically over different time scales and with very few parameters, according to Dr. Joshi, a researcher at the International Institute for Applied Systems Analysis in Laxenburg, Austria.
“It gives you the ability to predict forest performance for unknown conditions,” he said. “That makes it much more useful to predict the response of global forests to future climate scenarios.”
While tall trees are often considered more vulnerable to arid conditions than shorter ones, much remains to be discovered about why and to what extent a tree’s height affects drought resistance.
dr. Laura Fernández de Uña leads the Horizon-funded DISTRESS projectwho studies how a tree’s ability to transport water changes with height and how that can affect drought responses.
She and her colleagues are ready to shake up some conventional wisdom in the field.
“We see certain differences between individual trees and also between species species,” said Dr Fernández de Uña, a postdoctoral researcher at the CREAF Public Research Center in Barcelona, Spain.
Unsurprisingly, research confirms that it is more difficult for water to reach the heights of taller trees. Even under normal circumstances, gravity is a fundamental impediment.
During a drought, it is more difficult for trees to extract water from dry soil and pull it up. This increases the risk of water-carrying pipes aspirating air bubbles, which can block the flow (similar to emboli in human blood vessels). If air bubbles form, parts of a tree can no longer receive water and die.
In addition, tall trees in a forest are exposed to more heat and wind and less humidity. “The conditions in the canopy itself are drier than for a smaller tree in the undergrowth,” says Dr Fernández de Uña. ‘All of this is negative for tall trees during drought.’
Nevertheless, past research indicates that tall trees can adapt to heat and water stress or even cope better than small trees. For example, they can expand their water-conducting pipes to get more flow in their long trunks.
In addition, larger trees have more roots that reach greater depths, allowing access to water even when topsoil levels are low.
They also tend to have thicker trunks, allowing larger trees to store more carbohydrates and water.
for dr. Fernández de Uña all this shows that – contrary to popular belief – tall trees have a fighting chance when temperatures rise and water becomes scarce for extended periods of time.
“They can adapt and overcome their limitations,” she said. “We need to be more open to how they might respond to drought. If it weren’t worth it to be tall, trees wouldn’t grow tall.’
The research in this article was funded through the EU’s Marie Skłodowska-Curie Actions (MSCA). The article was originally published in Horizonthe EU Research and Innovation Magazine.
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