Why trees grow at night
Last updated: 22 Jun, 2021
Trees form new cells – and grow – by using carbohydrates they produce through photosynthesis. Research recently published in New Phytologist shows, however, that the primary factor limiting growth isn't the supply of carbohydrates, but the water tension in the tree – the so-called water potential.
The international research team led by Roman Zweifel at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL has come to the surprising conclusion that trees grow primarily at night, and that this trend is largely explained by the level of air dryness. In the world’s first comprehensive study of radial stem growth with an hourly data resolution, the scientists analysed data recorded over up to eight years on 170 trees of seven common species located at 50 sites all over Switzerland.
Point dendrometers mounted on tree stems continuously measure stem radii with a resolution of micrometers. The readings contain information about stem growth and tree water relations (Photo: Roman Zweifel).
Humidity is key to tree growth
During day time, high VPD severely limited radial stem growth and allowed only little growth, except in the early morning. 'The biggest surprise to us was that trees grew even under moderately dry soil conditions when the air was humid enough. Conversely, growth remained very low when the soil was moist but the air was dry,' recalls lead author Roman Zweifel.
The reason for this is the limited water transport capacity of the trees: as soon as the air becomes drier, trees temporarily lose more water through transpiration than they absorb through their roots. The entire tree comes under tension, its water potential decreases, and growth stops regardless of the availability of carbohydrates.
Stem growth is more sensitive to air dryness than photosynthesis
The researchers found that the level of air dryness that strongly reduces stem growth is significantly lower than that known to close the tree's stomata and thus to stop photosynthesis. 'In other words, trees stop growing before photosynthesis is inhibited,' summarises Roman Zweifel. This could explain, for example, why trees in dry environments appear to store carbohydrates but hardly grow anymore.
While current climate-sensitive forest growth models are based on knowledge about annual or monthly averages, the high-resolution scale of this study shows that trees only grow during a narrow time window of a few hours within the 24-hour period and consequently only during a limited time throughout the growing season.
As carbon gain (photosynthesis during the day) and carbon consumption (growth during the night) are temporally decoupled, their sensitivity to climatic factors differs between day and night. These findings may change the way we look at the impact of climate change on forests, in particular for predicting the carbon sink of forests.
Read the paper: Zweifel, R. et al. (2021) Why trees grow at night. New Phytologist. doi: 10.1111/nph.17552
Edited from the original press release.