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 resulted in large-scale tree death in forests.’ Dr. Jaideep Joshi of the Plant-FATE project, which is researching plant traits to protect forests from climate change, made the statement.
According to a study on the potential for global forest cover to mitigate climate change, planting billions of trees is a relatively inexpensive way to address the climate crisis.
However, as the drought worsens, forests all over the world are at risk. Between 1987 and 2016, droughts wiped out 500 000 hectares of forest in Europe.
Joshi oversaw the Horizon-funded Plant-FATE project, which pioneered the prediction of the effects of drought on trees of all kinds.
Tree resilience
Current models have a significant limitation in that they rarely consider trees’ ability to adapt to dry conditions and how resilience varies between species. As a result, there are inconsistencies in forecasting how forests will respond to future climate scenarios.
‘That is where the most uncertainty currently exists,’ Dr. Joshi said. ‘You have this entire ecosystem of mixed species, and we attempted to bring it all together in a simple but comprehensive modeling framework.’
A model is a tool for simulating outcomes, and he believes his team’s model will be especially useful in planning tree-planting programs. This is because it can predict different species’ carbon capture and storage potential over the next 50-100 years, when climate conditions will be different than they are now.
‘It could help choose which species to plant or where to plant them,’ said Dr. Joshi. ‘It’s the most promising conservation application for our model.’
The Plant-FATE researchers incorporated trees’ ability to adapt to changing climates and examined a variety of timescales in their model.
Trees exposed to drought, for example, may shed their leaves to conserve water (because water evaporates through pores on the surface of leaves) in what is known as a ‘false autumn’ in shorter timeframes of weeks to months.
New wood
However, trees can grow new wood with different properties that are better suited to dry conditions over longer timescales.
Dr. Joshi and his team also considered scale. Some responses, for example, occur in specific parts of a tree such as roots and leaves, whereas others occur at the species level.
Dr. Joshi and his colleagues used data from an Amazon rainforest site with 400 species in a 5 000 square-meter area to test their full model. They discovered that their model’s predictions closely matched what actually happened at the site. According to Dr. Joshi, a researcher at the International Institute for Applied Systems Analysis in Laxenburg, Austria, this is the first time a vegetation model has been performed realistically over different timescales while using very few parameters.
‘It allows you to predict forest performance under unknown conditions,’ he says. ‘This makes predicting the response of global forests to future climate scenarios much more useful.’
Tall trees
While tall trees are often thought to be more vulnerable to drought than shorter trees, much remains unknown about why and to what extent a tree’s height affects its drought resistance.
The Horizon-funded DISTRESS project, led by Dr. Laura Fernández de Ua, is investigating how a tree’s ability to transport water changes with height and how this may influence drought responses.
She and her colleagues have the potential to upend some conventional wisdom in the field.
‘We see differences between individual trees and also between species types,’ said Dr. Fernández de Ua, a post-doctoral fellow at Barcelona’s public research center CREAF.
Unsurprisingly, research confirms that water has a more difficult time reaching the heights of taller trees. Gravity is a fundamental impediment even under normal circumstances.
Air bubbles
It is more difficult for trees to extract water from dry soil and draw it upwards during a drought. This raises the possibility of water-transporting conduits sucking in air bubbles, which can obstruct flow (similar to embolisms in human blood vessels). If there are any bubbles, parts of the tree may be deprived of 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.
Well rooted
Furthermore, larger trees have more roots that reach deeper into the ground, allowing them to access water even when levels in the upper ground are low.
They also have thicker trunks, allowing larger trees to store more carbohydrates and water.
All of this, according to Dr. Fernández de Ua, demonstrates that, contrary to popular belief, tall trees have a fighting chance when temperatures soar and water becomes scarce for extended periods of time.
‘They can adapt and overcome their limitations,’ she explained. ‘We need to be more open to how they might respond to drought.’ Trees would not grow tall if it was not worth it to be tall.’
