The three vital signs of terrestrial ecosystems: vegetation growth, water-use efficiency and carbon-use efficiency

CREAF-based CSIC researcher Josep Peñuelas has contributed to an article published in Nature which proposes three key indicators for gauging how well a terrestrial ecosystem is functioning and assessing its state of health or wellbeing.

Vital signs — blood pressure, heart rate, respiratory rate, oxygen saturation and body temperature — provide us with information about our health. They can tell us whether we are suffering from stress of some kind (virus, infection, etc.) or from inflammation, for example. Could similar indicators be identified for the health of terrestrial ecosystems? An international team including CREAF and led by Mirco Migliavacca from the Max Planck Institute for Biogeochemistry in Jena (Germany) set out to answer that very question. The conclusions of their research have been published in an article in the renowned journal Nature, where they put forward three key indicators of how well terrestrial ecosystems are functioning. In order of importance, the first is maximum vegetation productivity and growth capacity; the second is how efficiently plants use water; and the third is how efficiently they use carbon. By monitoring those indicators, according to the article, it is possible to determine an ecosystem’s state of health and predict its capability to adapt to, survive and thrive under changes in climate and environment. “We can explain almost 72% of an ecosystem’s state of health on the basis of just those three main factors,” says Migliavacca. “Bearing in mind that water-use efficiency is the second of those main factors, our results clearly show that water availability is crucial to how well ecosystems work,” adds Markus Reichstein, director of the Biogeochemical Integration Department at the Max Planck Institute for Biogeochemistry and one of the article’s co-authors. “That will be vital for considerations related to the impact of climate change,” he continues.

“Identifying indicators of the wellbeing of terrestrial ecosystems is not a simple task. Ecosystems are complex as far as their structure and their responses to environmental changes are concerned. That makes it necessary to analyse lots of different ecosystems and determine which parameters do react to changes and can provide information as to whether an ecosystem is functioning well, is under stress or, in the worst-case scenario, is in decline.” Josep Peñuelas, CREAF-based CSIC researcher and co-author of the article.

The three vital signs

The maximum primary productivity indicator (which refers to plants’ level of growth activity) reflects an ecosystem’s carbon dioxide absorption capacity. The water-use indicator combines metrics that represent how efficiently an ecosystem uses water, a relationship between the carbon plants absorb and the water they transpire. The carbon-use efficiency indicator reflects how an ecosystem uses carbon, corresponding to carbon respired as a proportion of carbon assimilated.

The three indicators identified depend on how each ecosystem’s vegetation is structured (greenness, nitrogen content in leaves, canopy height and biomass, for example). So, if we want to act on an ecosystem’s health, we need to keep in mind that its structure is of great relevance and can be influenced naturally by disturbances or artificially through forest management. At the same time, efficiency in the use of water and carbon is critically dependent on the climate and, in part, on aridity, highlighting the decisive role climate change will play in the functioning of ecosystems in the future. “Our exploratory analysis is a vital step in the development of indicators of ecosystem health and functioning, and provides an exhaustive evaluation of the response of the world’s ecosystems to changes in climate and environment,” states Markus Reichstein.

Water use and carbon use, two key indicators

The team combined multiple data sources and methods in their research. For example, they brought environmental data from global networks of stations monitoring a host of ecosystems together with satellite observations, mathematical models and statistical and causal discovery methods. They examined carbon dioxide, water vapour and energy flux data from 203 FLUXNET sites around the world. FLUXNET is a network through which numerous research teams collaborate by collecting and sharing data. The sites chosen for the study encompass a wide range of climate zones and vegetation types. For each site, the scientists calculated a set of ecosystem functional properties, as well as average climate and soil water availability variables, and complied information on vegetation characteristics and satellite data on vegetation biomass.

Terrestrial ecosystems provide services that are vital to society, such as the production of biomass (wood and fruit), efficiency in plants’ use of sunlight and water (linked to climate regulation and cooling the planet), water retention and protection against erosion, and, ultimately, food security. Changes in climate and environment and the impact of the activities of humankind pose a constant threat to the provision of such services. The three indicators identified in the article are crucial in that they define which functions are essential to an ecosystem working properly, thus showing us how to respond to such threats.

Article

Migliavacca, M., Musavi, T., Mahecha, M. D., Nelson, J. A., Knauer, J., Baldocchi, D. D., … & Reichstein, M. (2021). The three major axes of terrestrial ecosystem function. Nature, 598(7881), 468-472.