Ultrasound reveals secrets of tree survival during drought

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Scientists have turned the forest into a laboratory to understand how some species cope with periodic dry spells.

The tissues of living trees may hold secrets to why some can recover from drought while others die. But these tissues are difficult to assess in mature forests. After all, 90-year-old trees can’t go to the lab to get an image. So most studies of the effects of drought on plants are done in the laboratory and on young trees—or by core removal from mature trees.

Barbara Beikkircher, an ecophysiologist at the University of Innsbruck in Austria, and her colleagues proposed a different approach: They moved the laboratory to the trees.

In the Krantzberg Forest near Munich, the team equipped mature spruce and beech stands with robust, waterproof ultrasonic sensors. Some stands were covered with roofs to prevent summer rain, creating artificial drought conditions.

Researchers fitted stands of mature spruces and beeches with ultrasonic sensors and electrical probes to understand how these species cope with long dry periods.INNSBROOK UNIVERSITY

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

Trees affected by drought produced more ultrasonic signals than trees exposed to summer rains. These weak acoustic waves bounced off air bubbles, called emboli, deep within the trees’ vascular system. Surface tension keeps water moving through the tree’s thousands of tiny vessels—evaporation from leaf pores lifts water up the trunk. But if there isn’t enough water in the soil, this upward thrust can cause an embolism that clogs the blood vessels. During the experiments, the spruces rang much more than the beeches, indicating that they had many more emboli.

And this despite the fact that beeches are less conservative in terms of water resources management, at least above ground. Trees can prevent embolism by closing the pores in their leaves, but there is a trade-off. This cuts off the carbon dioxide that stimulates photosynthesis, which produces the carbohydrates and sugars that trees need to live and grow. In arid conditions, trees face an impossible choice “between starvation and dying of thirst,” Beikircher says.

Beechs were less affected by embolism than spruces, even though their pores remained open longer than conifers. Perhaps that’s because beeches have roots that reach deeper, wetter soil, as well as more reliable water supplies, Beikircher says. Another set of experiments, after the researchers got rid of the drought, shows that this is the case.

At the end of the experiment, the team moistened the soil. All trees recovered well in most measures, with rates of photosynthesis in pre-dried trees reaching those of trees in controls and water-filled emboli.

But when Beikircher measured the trees’ resistance to electric current, an indicator of moisture levels deep in the trunks, the spruces’ water reserves were still depleted. One rainy season was not enough for these trees to fully recover. It is not clear whether the spruces will be able to rebuild their stocks after the prolonged drought, or how long it may take.

Species that can withstand drought conditions and recover more quickly may become more abundant in future forests as climate change causes more frequent and intense droughts. This means that the composition of the trees that make up the world’s temperate forests may change as the climate warms, with uncertain consequences for other plants and animals in those ecosystems.

Beikircher plans to test whether a more diverse forest can help drought-sensitive species like spruce survive. Deep-rooted beeches interspersed with spruce can help increase topsoil moisture by directing water to spruce roots, she says.

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