Forests help to mitigate global warming, but they are also endangered by it. Many tree species struggled this summer as much of Europe was hit by heat waves and severe drought—the worst in 500 years, according to experts.
Even olive trees, which are known for their resistance to drought, have suffered. Spain is the world’s leading producer of olive oil, but many Spanish farmers anticipate a 50% decrease in olive oil harvests this year. Horizon researchers are racing against the clock to learn more about how trees respond to drought as part of the fight against climate change.
Carbon sinks
Existing forests already remove roughly one-third of human-caused greenhouse gas emissions. A global afforestation program could achieve the same result for nearly one-third of the discharges that remain in the atmosphere.
‘Over the last decade or so, there have been a number of events where severe drought has caused large-scale death of trees in forests,’ said Dr. Jaideep Joshi of the Plant-FATE project, which is researching plant traits to protect forests from climate change.
It could help make the right choices of which species to plant or where to plant them. It’s our model’s most promising conservation application.
Dr. Jaideep Joshi
Planting billions of trees is a relatively inexpensive way to tackle the climate crisis, according to a study about the potential for global forest cover to mitigate climate change. But as 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 project, which broke new ground when it comes to predicting the impact of drought on trees of all kinds.
Tree resilience
A major limitation of current models is that they rarely consider trees’ ability to adapt to dry conditions and how resilience may differ between species. That leads to inconsistencies when projecting how forests will respond to future climate scenarios.
‘That is where the largest uncertainty currently lies,’ said Dr. Joshi. ‘You have this whole ecosystem of mixed species—we have tried to bring this all together in a simple but comprehensive modelling framework.’
A model acts as a tool for simulating outcomes and he believes his team’s model will be particularly useful when it comes to planning tree-planting programmes. That’s because it can signal the carbon capture and storage potential of different species over the next 50-100 years, when climate conditions will be different to what they are today.
‘It could help make the right choices of which species to plant or where to plant them,’ said Dr. Joshi. ‘It’s our model’s most promising conservation application.’
In their model, the Plant-FATE researchers incorporated trees’ ability to adjust to changing climate and looked at a range of timescales. In shorter timeframes of weeks to months, for example, 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 autumn’.
New wood
But over longer timescales, trees can grow new wood with different properties better suited to dry conditions. Dr. Joshi and his team also took scale into account. For example, some responses occur in specific parts of a tree as roots and leaves, while others take place at the level of an entire species.
To test their full model, Dr. Joshi and his colleagues used data from an Amazon rainforest site containing about 400 species in a 5 000 square-metre area. They found that their model’s predictions closely matched what happened in real life at the site.
It marks the first time that a vegetation model has performed realistically over different timescales while using very few parameters, according to Dr. Joshi, a researcher at the International Institute for Applied Systems Analysis in Laxenburg, Austria.
‘It gives you the capacity 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.’
Tall trees
While tall trees are often considered to be more vulnerable to dry conditions than shorter ones, much remains to be discovered about why and to what extent a tree’s height affects its resistance to drought.
Dr. Laura Fernández de Uña leads the Horizon-funded DISTRESS project, which is examining how a tree’s ability to transport water changes with height and how that might influence drought responses.
She and her colleagues stand to shake up some conventional wisdom in the field. ‘We see certain differences between individual trees and also between species types,’ said Dr. Fernández de Uña, a post-doctoral fellow at public research centre CREAF in Barcelona, Spain. Unsurprisingly, research confirms that it’s harder for water to reach the heights of taller trees. Even in normal conditions, gravity is a basic impediment.
Air bubbles
During a drought, it is harder for trees to extract water from dry soil and draw it upwards. This increases the risk of water-transporting conduits sucking in air bubbles, which can block the flow (similar to embolisms in human blood vessels). If any bubbles occur, parts of a tree can be denied water and die.
In addition, tall trees in a forest are subjected to more heat, wind, and humidity. ‘The canopy conditions are drier than for a smaller tree in the understory,’ Dr. Fernández de Ua explained. ‘All of this is detrimental to tall trees during a drought.’
Nonetheless, previous research indicates that tall trees can adapt to and even cope with heat and water stress better than small trees. They can, for example, expand their water-conducting pipes to allow more flow up their long trunks.
