Drought and Tree Species: Ultrasound Sensors Show How Trees Cope in the Face of Climate Change

Researchers have found that the tissues of mature trees hold the key to understanding why some trees can recover after a drought, while others die. But the challenge lies in assessing these tissues in mature forests as older trees cannot be taken to the lab for imaging scans. Hence, most studies on the effects of drought on plants are done on younger trees or by gouging cores out of mature ones. However, Barbara Beikircher, an ecophysiologist at the University of Innsbruck in Austria, and her colleagues have found a unique way to overcome this challenge.

They brought the lab to the trees by outfitting stands of mature spruce and beech trees with rugged, waterproof ultrasound sensors. The Kranzberg Forest outside Munich was chosen for this experiment, and some of the stands were covered with roofs to block the summer rain, creating artificial drought conditions. After five years of monitoring, the team found that beeches are more drought-resilient than spruces. Beikircher and her colleagues delved deeper into the underlying mechanisms that explained this difference.

In their experiment, drought-stressed trees produced more ultrasound signals than trees exposed to summer rains. These faint acoustic waves were bouncing off air bubbles called embolisms deep within the trees’ vasculature. Surface tension keeps water moving through a tree’s thousands of tiny vessels. Still, if there’s insufficient water in the soil, this upward pull can generate embolisms that clog the vessels. The team discovered that spruces had far more embolisms than beeches, which pinged much less than the spruces.

Beeches appeared to be less conservative with their water management, at least aboveground. They kept their pores open longer than the conifers, preventing embolisms from occurring. However, there’s a trade-off since trees can prevent embolisms by closing the pores on their leaves, but doing so cuts off the supply of the carbon dioxide that drives photosynthesis. In dry conditions, trees face an impossible choice “between starving and dying of thirst,” Beikircher says.

Beikircher hypothesized that beeches suffered fewer embolisms than spruce because they have roots that extend into deeper, wetter soil as well as more robust water reserves. Another set of experiments after the researchers relieved the drought suggested that her hypothesis was accurate.

At the end of the experiment, the team drenched the soil, and all the trees recovered well by most measures. Rates of photosynthesis in the previously parched trees caught up to the rates of trees in the control groups, and embolisms filled with water. However, when Beikircher measured the trees’ resistance to an electrical current, indicating moisture levels deep within trunks, the spruces’ water reserves were still depleted. One season of rain was not enough to help these trees fully recover.

As climate change causes droughts to become more frequent and intense, species that can withstand drought conditions and recover more quickly may become more populous in future forests. That means the compositions of the trees that make up the world’s temperate forests could change as the climate warms, with uncertain consequences for the other plants and animals in these ecosystems. Beikircher plans to test whether a more diverse forest could help drought-sensitive species like the spruce survive. She suggests deep-rooted beeches interspersed with spruces might help increase moisture in the soil’s upper levels by wicking water up to where spruce roots are.

According to Beikircher, “Our research provides new insight into how different species of trees can adapt to drought conditions. With climate change leading to more frequent and severe droughts, it is crucial to understand how tree species can cope with and recover from such conditions. The study by Beikircher and colleagues provides valuable insights into the mechanisms underlying tree resilience to drought and offers potential solutions for forest management in the face of climate change.

By using ultrasound sensors to monitor the responses of mature trees to artificial drought conditions, the researchers were able to investigate the impact of drought on the trees’ vascular systems. The ultrasound signals produced by the trees revealed the presence of air bubbles, or embolisms, that can clog the tiny vessels responsible for transporting water from the roots to the leaves.

The results of the study showed that beech trees are more resilient to drought than spruce trees, despite the fact that they appear to be less conservative with their water management. While both species close the pores on their leaves to prevent embolisms, beech trees are able to keep their pores open longer than spruce trees, allowing them to continue photosynthesis and produce the carbohydrates and sugars needed to survive and grow.

One possible explanation for the greater resilience of beech trees is their ability to access deeper, wetter soil and maintain more robust water reserves. The study also suggests that a more diverse forest, with deep-rooted beeches interspersed with spruce trees, could help increase moisture in the soil’s upper levels by wicking water up to where the spruce roots are.

These findings have important implications for forest management strategies in the face of climate change. As droughts become more frequent and severe, understanding the mechanisms that allow some tree species to cope and recover could help inform decisions about which species to plant and where. A more diverse forest, with a mix of species that have different water requirements and rooting depths, could help mitigate the impacts of drought and promote forest resilience in the face of a changing climate.

In addition, the study highlights the importance of using innovative techniques to study the responses of mature trees to environmental stressors. By bringing the lab to the trees, Beikircher and her colleagues were able to investigate the complex interactions between tree physiology and the environment in a way that would not have been possible with traditional methods.

As Beikircher notes, “It’s important to understand how the trees function in their natural environment…We need to be innovative in our methods to learn more about these ecosystems.”

Overall, the study by Beikircher and colleagues offers valuable insights into the mechanisms underlying tree resilience to drought and provides potential solutions for forest management in the face of climate change. By continuing to use innovative methods to study these complex ecosystems, we can better understand how to promote forest resilience and ensure the survival of these vital ecosystems for generations to come.