According to a study published recently in the journal PNAS, climate change has caused forests to alter the way they grow, in that they only take advantage of the fertilizing effect of carbon dioxide (CO₂) to grow faster if they have plenty of water.

Various studies have suggested that water-use efficiency in plants has increased in recent decades, possibly because they can photosynthesize faster and save water owing to more carbon being available in the atmosphere. However, research led by the University of New Hampshire (UNH) in the USA and involving CREAF scientists has found that the relationship is not as direct or straightforward as was thought. It is rather the case that trees “have learned” to react differently to increased CO₂ depending on the availability of water. If water is in abundant supply, trees capture more CO₂ and photosynthesize and grow faster without paying any heed to transpiration. If water is scarce, on the other hand, they opt to conserve it and grow more slowly.

The results of the research were obtained by analysing 12 different species of trees in the USA. “We chose eight of the country’s mature forests and extracted wood cores from the trunks of trees of each species”, explains CREAF ecologist Rossella Guerrieri, the study’s lead author. “We identified the rings corresponding to the last 30 years of growth and carried out a chemical analysis involving isotopes on them”, she continues. “That’s the only procedure that provides an insight into how trees have responded to the increase in atmospheric CO₂ and climate change over the last few decades”, she says.

The plants that photosynthesize most are not always the most efficient

Plants use pores in their leaves called stomata to control gas exchange, their temperature and the water they have absorbed. During photosynthesis, they take in the CO₂ they need to feed and grow through the stomata, but also lose water through them. “To find out whether a plant is efficient or not, we look at how much it grows in relation to the amount of water it uses to do so”, explains UNH researcher Scott Ollinger. “It’s the same principle as working out how many kilometres a car can travel per litre of fuel, but with tons of carbon per litre of water.”

Article referenced:

Guerrieri, R., Belmecheri, S., Ollinger, S.V. et al (2019). Disentangling the role of photosynteshis and stomatal conductance on rising forest water-use efficiency. PNAS, 116 (34).