The resilience of nature revealed in the drought experiment in Biosphere 2



A group of 80 international researchers wanted to conduct an unprecedented experiment in which they forced the world’s only enclosed rainforest, located in Biosphere 2 of the University of Arizona, through a controlled drought and recovery for a period of four months.

The “rainforest under glass” of Biosphere 2 receives rain after a 2-month drought as part of the B2 FOREST experiment. (Image source: Rosemary Brandt).

This unique experiment would help paint a better picture of how climate change is affecting earth’s ecosystems around the world.

Their findings, recently reported in the journal science, found a roughly 70% decrease in rainforest carbon storage – an indication of concerns about the ability of forests to capture and store carbon dioxide from the air as climate change advances. However, a complex web of water use strategies and soil interactions has been discovered to support forest stability during extreme drought.

“In a way, the forest was surprisingly drought resilient,” said Laura Meredith, one of three leaders on the project and assistant professor in the School of Natural Resources and the Environment at the College of Agriculture and Life Sciences.

The glass-enclosed rainforest in Biosphere 2, which is home to 90 plant species on an area the size of seven tennis courts, enabled the scientists to mimic a total drought in the ecosystem.

The experiment called Water, Atmosphere and Life Dynamics – or WALD, which means “forest” in German – aimed to collect all possible data during the drought and rewetting process.

Roughly 3.2 km of Teflon tubing and over 133 sensors were spread across the roughly 3 hectare rainforest to instantly take measurements on everything from carbon pools in the atmosphere and vegetation to deep-sea soil processes and microbiome.

We used stable isotopes to track the movement of carbon and water through the ecosystem under normal conditions and severe drought, revealing surprising interactions between plants and ecosystems.

Laura Meredith, Project Leader and Assistant Professor, School of Natural Resources and Environment, College of Agriculture and Life Sciences, University of Arizona

“It is important that not all plants respond to drought in the same way. Some were very drought sensitive and quickly slowed down their critical carbon and water cycles to be on the safe side, while others were more drought tolerant and were able to maintain their function even under more risky drought conditions “, Meredith added.

In their experiment, the team classified the reactions of the plants according to their drought tolerance and sensitivity to drought, both in undergrowth species and in large tree canopies.

We observed one of the most amazing reactions between the large, drought tolerant and drought sensitive trees.

Christiane Werner, project leader and professor for ecosystem physiology, University of Freiburg

Large, drought-sensitive trees typically swallow most of the water, especially from topsoil. Since the topsoil was the first to dry out during the drought, these trees suffered the fastest and most severely from a lack of water, explains Werner. The scientists expected that drought-sensitive trees would immediately draw from the water resources in the deep ground.

Instead, they drastically reduced their water consumption and only resorted to their deep-sea reserves in extreme drought. In this way, they have preserved the deep water reserves for as long as possible.

Christiane Werner, project leader and professor for ecosystem physiology, University of Freiburg

Large, drought-tolerant trees tended to hold onto their leaves the longest, provided lasting shade, and spared the undergrowth from additional desiccation of the topsoil.

The diversity of drought responses within plants helped maintain greater carbon and water cycle functions throughout the ecosystem, both during the greatest extent of drought and to respond quickly to the re-availability of moisture when rain hits.

Laura Meredith, Project Leader and Assistant Professor, School of Natural Resources and Environment, College of Agriculture and Life Sciences, University of Arizona

While carbon storage in the forest system decreased significantly under increasing drought stress, plants emitted more volatile organic compounds or VOCs, which are part of the communication and signal transmission between plants and soil microbes. VOCs are primarily critical to how stressful plants are.

According to the results, there has been a stream of emissions of a variety of VOCs, including hexanal, isoprene, and monoterpenes, the latter of which can aid in cloud condensation and rain formation, and act as protection against drought.

While plants were strong emitters of VOCs into the atmosphere, soil microbial life picked up some of these compounds, reducing the maximum amount that would be released into the atmosphere over a tropical rainforest.

“This balancing role of the soil microbes in the VOC emissions of plants persisted even in severe drought, which suggests that we need to better consider the role of microbial activity in atmospheric processes.” said Meredith.

The WALD research team is continuing to work on the data from the experiment and is currently studying the smallest microbial life in the ecosystem. The team is focused on defining mechanisms of the carbon and water cycle on these small scales by mapping the genomic and metabolomic profiles of soil and root microbiomes.

“Experimental ecosystems such as we have at Biosphere 2 enable researchers to understand the holistic response of an entire ecosystem to stress.” said Meredith. “As we work to understand and predict ecosystem functions in response to global change, we need to consider functional groups of plants and their interactions with soils and the atmosphere in both observational and model studies.”

Journal reference:

Werner, C., et al. (2021) Ecosystem fluxes during drought and recovery in an experimental forest. science.




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