University of Göttingen: Strategies of the forest during extreme drought

How do tropical rainforests and their plants react to extreme drought? Understanding these processes is crucial to making forests more resilient to the increasing risk of drought due to climate change, and will improve the scientists’ ability to refine climate models. A research team led by the University of Freiburg has now conducted the most extensive experiment to date on this theme. They exposed an artificial rainforest to drought for nine-and-a-half weeks and observed which strategies different plants use to combat the drought, and how they interact with other plants, the soil and the atmosphere. The University of Göttingen performed the analysis of the carbon stocks and dynamics in the soil. The results of the study were published in the journal Science.

Eighty scientists took part in the experiment at the US research facility Biosphere 2. Their observations included that the carbon storage of the forest system decreased by about 70 percent. The team from Göttingen University investigated how this affects carbon storage and turnover in the soil. “Our Postdoc and PhD students studied the altered interactions of roots, soil and microorganisms under drought stress,” says Professor Michaela Dippold from the Biogeochemistry of Agroecosystems research group.

The result: the various plants react extremely differently to drought, even below the surface. Some increase their proportion of fine roots, others form new roots deep in the soil, whilst other plants primarily increase root secretions deep down in the still moist subsoil, to ensure water and nutrient uptake from there. Such changes in the root, and also the soil directly surrounding the root (i.e. the rhizosphere) influences the soil microbiome and all processes of the soil carbon cycle.

Overall, a picture emerged of the complex interaction between trees and plants with different resistance to drought. This interaction was crucial for maintaining the stability of the entire forest system as long as possible during the drought event. The researchers identified four plant types with different reactions to the drought generated in the framework of the experiment: drought-tolerant and drought-sensitive; and then subsets within these two categories – large, canopy-forming trees or species that don’t reach the top layers of the forest.

“One of the most surprising reactions we observed was between the large, drought-tolerant and drought-sensitive trees,” explains Professor Christiane Werner from the University of Freiburg. The sensitive ones are the ones that generally consume the most water, especially from the topsoil. Since this also dried out the fastest, they suffered from water shortage earlier and also most intensely. It was expected that they would immediately tap the water resources in the deep soil to maintain their high consumption.

“But instead,” says Werner, “they drastically curbed their water consumption and only resorted to their deep water reserves under very extreme drought. In this way, they conserved the low-lying water reserves for as long as possible, supporting drought-tolerant trees.” These maintained their leaf canopy longer because they generally have a lower water flux through their biomass, which in turn resulted in retaining moisture longer for the growth of the undergrowth. Thus, the undergrowth was conserved, which helped prevent the topsoil drying out. This water reserve in the topsoils is highly important as the drought-sensitive trees heavily rely on it. This meant the water remained longer in the entire system due to the complex interaction among different functional types of rainforest plants and the system remained stable for longer.

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