How robust are ecosystems? Three key indicators hold the clues

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The Hainich station, operated by the Bioclimatology Group, plays a particular ro

The Hainich station, operated by the Bioclimatology Group, plays a particular role since it takes measurements at a site which is both an unmanaged forest and also one of the oldest forests Photo: Alexander Knohl

Göttingen University research team involved in global study on conditions and capacity to adapt Ecosystems provide a wide range of services to people. These services depend on basic ecosystem functions, which are shaped by natural conditions like climate, the mix of species and by human intervention. A research team including the University of Göttingen has identified three key indicators that describe the functioning of terrestrial ecosystems: the capacity to maximise primary productivity; the efficiency of using water; and the efficiency of using carbon. Monitoring these three indicators will make it possible to assess how adaptable an ecosystem is to climate and environmental changes and how it can evolve under certain conditions.

Ecosystems on the land surfaces of the Earth support multiple functions and services that are critical for society, like biomass production, the vegetation’s efficiency in using sunlight and water, water retention and climate regulation, and ultimately food security. Climate and environmental changes as well as the impact of people are continuously threatening the ability to provide these functions. To understand how terrestrial ecosystems will respond to this threat, it is crucial to know which functions are essential to obtain a good representation of the ecosystems’ overall well-being and functioning. This is particularly difficult since ecosystems are rather complex regarding their structure and their responses to environmental changes.

A large international network of researchers, led by the Max Planck Institute for Biogeochemistry in Germany, tackled this question by combining multiple data streams and methods. The scientists used environmental data from global networks of ecosystem stations, combined with satellite observations, mathematical models, and statistical and causal discovery methods.  Among the stations is the Hainich flux tower, which is operated by the Bioclimatology group of the University of Göttingen as part of the European Integrated Carbon Observation System (ICOS). "The Hainich site plays a particular role within the network, since it is both an unmanaged forest and also one of the oldest forests studied," says Professor Alexander Knohl, Head of the Bioclimatology group at the University of Göttingen and co-author.

The result is strikingly simple: "We were able to identify three key indicators that allow scientists to summarise how ecosystems function: the maximum achieved productivity, the efficiency of using water, and the efficiency of using carbon," says Dr Migliavacca, first author.

The maximum productivity indicator reflects the capacity of the given ecosystem to uptake CO2. The water use indicator is a combination of metrics representing the ecosystem water use efficiency, which is the carbon taken up per water transpired by plants. The carbon use efficiency indicator reflects the use of carbon by an ecosystem, which represents the carbon respired versus carbon taken up. The surprising results made the team reflect on how complex ecosystems are ultimately driven by a small set of major factors just like was found, for instance, for leaf photosynthesis based on a handful of leaf traits.

"Using only these three major factors, we can explain almost 72 percent of the variability within ecosystem functions", Migliavacca adds. "With water-use efficiency being the second major factor, our results emphasize the importance of water availability for the performance of ecosystems. This will be crucial for climate change impact considerations," says Professor Markus Reichstein, director of the department Biogeochemical Integration at MPI for Biogeochemistry and co-author.

The researchers inspected the exchange rates of carbon dioxide, water vapour, and energy at 203 world-wide monitoring stations that belong to the FLUXNET network, a collaborative network of multiple research teams and field sites that collect and share their data. The selected sites cover a large variety of climate zones and vegetation types. For each site, they calculated a set of the ecosystems’ functional properties, and, in addition, included calculations on average climate and soil water availability variables as well as vegetation characteristics and satellite data on vegetation biomass.

The three identified functional indicators critically depend on the structure of vegetation, meaning the vegetation greenness, nitrogen content of leaves, vegetation height, and biomass. This result underlines the importance of ecosystem structure, which can be shaped by disturbances and forest management in controlling ecosystem functions. At the same time, the water and carbon use efficiency also critically depend on climate and partly on aridity, which points at the critical role of climate change for future ecosystem functioning. "Our exploratory analysis serves as a crucial step towards developing indicators for ecosystem functioning and ecosystem health," considers Professor Markus Reichstein, "adding to a comprehensive assessment of the world’s ecosystems response to climate and environmental changes."

Original publication: Mirco Migliavacca et al. The three major axes of terrestrial ecosystem function (2021) Nature DOI 10.1038/s41586-021-03939-9:


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