University of Freiburg: The diversity of plants’ water usage strategies makes forests resilient to extreme drought

Precisely how does a forest system and the individual plants that make it up react to extreme drought? Understanding the processes involved is crucial to making forests more resilient in the increasingly dry climate that will result from climate change, as well as to refining climate models. A research team headed by Prof. Dr. Christiane Werner from the University of Freiburg has conducted the most extensive experiment to date into this subject using stable isotopes as markers. To do this they exposed an artificial rainforest to a drought lasting 9.5 weeks and observed the specific strategies of different plants to cope with the dryness and how they interacted with other plants, the soil and the atmosphere. The whole process revealed a complex interplay of different drought-resistant trees and plants, which was crucial to maintaining the stability of the entire forest system for as long as possible. In addition, the experiment yielded further clues to how drought affects the CO2 storage of forests and how gas emissions from drought-stressed plants can influence the atmosphere and the climate. Coordinated with Dr. Nemiah Ladd (University of Freiburg) and Dr. Laura Meredith (University of Arizona/USA), the experiment took place in the US Research Center Biosphere 2. The international and interdisciplinary team involved 80 scientists. The researchers published their findings in the journal Science.

With their experiment the researchers identified four types of plant with different reactions to the imposed drought: drought-tolerant and drought-sensitive – and in both these categories there were large, crown-forming trees and types of undergrowth.

“We observed one of the most astonishing reactions between the large drought-sensitive and drought-tolerant trees,” explains Christiane Werner. Sensitive trees are the ones that generally draw the most water, especially from the topsoil. As topsoil is also fastest to dry out, they began to suffer sooner and had the most intensive water deficiency. It had been assumed that they would switch to drawing water resources from deep in the earth, to maintain their high consumption rate. “But instead,” Werner says, “they restricted their water consumption drastically and only drew on their deep reservoirs in very extreme drought. So they preserved the deep reservoirs for as long as possible, even for drought-tolerant trees.” And those trees in turn because of their naturally lower water flow retained their leaf canopy for longer, which then preserved the moisture in the undergrowth for longer. The protection this gives the undergrowth helps to counter drying out the topsoil, on which drought-sensitive trees massively depend. So this complex interaction retains the water in the entire system for longer and thereby keeps it stable for longer.

“This reveals,” Werner says, “that plants can develop different and at the same time complementary hydraulic strategies in a forest system – and with this interaction boost the resilience of the entire forest to drought. By learning more precise details we can make a substantial contribution to helping forests be more resilient to climate-related drought.”

The researchers studied the flows of H20, CO2 and volatile organic compounds (VOCs), such as isoprene and monoterpene, to obtain their results. To do this, they fed labeled 13CO2 and 2H2O into Biosphere 2 and then tracked how these substances were distributed by the trees, plants and soil over the course of the experiment. In this way the scientists observed, among other things: the intensity of water consumption and flow rate in the plants, the regions of the soil from which they drew water and at what times, as well as how and where CO2 and VOCs were stored in the plants and soil or emitted into the atmosphere. This was the first time such a tracer experiment has been carried out in an entire forest, something that is only possible within the contained system in Biosphere 2.

When looking at the storage and emission of CO2 and VOCs, the researchers noted among other things that the forest system’s carbon storage reduced by about 70 percent, and as drought stress increased the plants emitted more VOCs, which can lead to the formation of ozone and other gases through interactions in the atmosphere. In addition there was a cascade of emissions of various VOCs, such as isoprene, monoterpene and hexanal, reflecting the increasing drought stress. Monoterpene in particular can in turn promote the condensation of clouds and thus lead to rain, probably as another protective mechanism against drought.

“All these findings are also important to climate research,” says Christiane Werner. “What water usage strategies plants deploy against drought and how this involves them in interactions with other plants, with the soil and the atmosphere – all this can make modeling studies on climate change more precise in future,” says Christiane Werner.

The international and interdisciplinary research team includes many specialists such as hydrologists, ecophysiologists, microbiologists, ecologists and atmospheric researchers. “This breadth of expertise has enabled among other things that we have a better understanding of the changes in processes at the microscopic level, such as molecular processes in cells and microbes right up to ecosystem/atmosphere exchange,” says Werner. The research is part of her ERC Consolidator project.

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