Ultrasound reveals trees’ drought-survival secrets

The tissues of living trees may hold the secrets of why some can recover after drought and others die. But those tissues are challenging to assess in mature forests. After all, 90-year-old trees can’t travel to the lab to get an imaging scan. So most studies of the impacts of drought on plants are done in the lab and on younger trees — or by gouging cores out of mature trees.

Barbara Beikircher, an ecophysiologist at the University of Innsbruck in Austria, and colleagues came up with a different approach: They brought the lab to the trees.

In the Kranzberg Forest outside Munich, the team outfitted stands of mature spruce and beech trees with rugged, waterproof ultrasound sensors. Some of the stands had been covered by roofs to block the summer rain, creating artificial drought conditions.

Five years of monitoring revealed that beeches (Fagus sylvatica) are more drought-resilient than spruces (Picea abies), the team reported in the December Plant Biology. Delving into the underlying mechanisms explained this difference.

Drought-stressed trees produced more ultrasound signals than trees exposed to summer rains. Those 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 — evaporation from pores in leaves drives water up the trunk (SN: 9/6/22). But if there’s insufficient water in the soil, this upward pull can generate embolisms that clog vessels. In the experiments, spruces pinged much more than beeches, suggesting they had far more embolisms.

That’s despite the fact that beeches appear to be less conservative with their water management, at least aboveground. Trees can prevent embolisms by closing the pores on their leaves, but there’s a trade-off. Doing so cuts off the supply of the carbon dioxide that drives photosynthesis, which makes the carbohydrates and sugars that trees need to live and grow. In dry conditions, trees face an impossible choice “between starving and dying of thirst,” Beikircher says.

Beeches suffered fewer embolisms than the spruce, even though they kept their pores open longer than the conifers did. Perhaps that’s because beeches have roots that extend into deeper, wetter soil as well as more robust water reserves, Beikircher says. Another set of experiments after the researchers relieved the drought suggests that’s the case.

At the end of the experiment, the team drenched the soil. 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.

But when Beikircher measured the trees’ resistance to an electrical current, an indication of 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. It’s unclear whether spruces can replenish their reserves after prolonged drought or how long that might take.

Species that can withstand drought conditions and recover more quickly may become more populous in future forests as climate change causes droughts to become more frequent and intense (SN: 3/10/22). 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. 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, she says.