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Diverse water-use strategies help forests cope with drought

In order to better understand how climate change will affect ecosystems, a team of 80 international scientists have recently performed an unprecedented experiment: they forced the world’s only enclosed rainforest (located in the University of Arizona’s Biosphere 2 research facility) through a four-month long controlled drought and recovery.

The researchers noted that, although there was a 70 percent drop in the rainforest’s carbon storage capacities during the drought period, an intricate web of water-use strategies and soil interactions emerged to support the forest’s resilience against severe drought.

“The forest was, in some ways, surprisingly resilient to the drought,” said study lead author Laura Meredith, an assistant professor in the School of Natural Resources and the Environment at the University of Arizona.

“We used stable isotopes to trace the movement of carbon and water through the ecosystem under normal conditions and severe drought, which revealed surprising plant-ecosystem interactions,” Professor Meredith explained. 

“Importantly, individual plants did not all respond to drought in the same way. Some were highly drought sensitive and quickly slowed their critical carbon and water cycling to play it safe, while others were more tolerant of drought and maintained their function even under more risky drought conditions.”

Large, drought-sensitive trees which generally consume the most water, suffered the fastest and most severely from the lack of water. However, they did not immediately tap into the water resources in the deep soil, as researchers expected.

“Instead, they drastically reduced their water consumption and only resorted to their deep-water reserves under very extreme drought,” said another of the project leaders, Christiane Werner, a professor of Ecosystem Physiology at the University of Freiburg. “In this way, they conserved the deep-lying water reserves for as long as possible.”

On the other hand, large, drought-tolerant trees held onto their canopy leaves the longest, providing shade to the undergrowth to protect it against further dehydration.

“Having a diversity of drought responses within the plants helped maintain greater carbon and water cycling functions of the entire ecosystem, both during the fullest extent of drought, as well as for quickly responding to the renewed availability of moisture with the arrival of rain,” Meredith explained.

“Experimental ecosystems, like what we have at Biosphere 2, allow researchers to understand the holistic response of an entire ecosystem to stress. As we work to understand and predict ecosystem function in response to global change, we have to consider plant functional groups and their interactions with soils and the atmosphere, in both observational and modeling studies,” she concluded.

By Andrei Ionescu, Staff Writer

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