The recent research has found that trees grow mostly at night. The forest at dusk has never seemed a still place, and it is not. They have just discovered that trees mainly grow at night time.
To grow, trees form new cells using the carbohydrates they produce through photosynthesis. What is striking about a new study published in New Phytologist is that they give “the spurt” as it gets dark.
An international research team led by Roman Zweifel of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) has come to the surprising conclusion that trees grow mostly at night, and that this trend is largely explained by the level of dryness of the air.
They have carried out the world’s first comprehensive study of radial stem growth, measuring it in the space of hours. The scientists analyzed data recorded over 8 years on 170 trees of 7 common species located at 50 locations across Switzerland. They have collected more than 60 million data points.
Researchers from ETH Zurich and other research institutions in Switzerland and Europe participated in the study. The investigated sites are part of TreeNet, a network in which changes in the stem radius of the trees have been continuously measured using high precision point dendrometers in parallel with information on air dryness (vapor pressure deficit, VPD) and soil (soil water potential) in Swiss forests since 2011.
The data show that the probability of tree growth varies significantly throughout the 24 hours of a day: the stems shrink under the effect of water stress and expand in a range of 1-200 µm per day, and these fluctuations are superimposed at growth rates of 1-5 µm per hour.
Air humidity is key to tree growth
The research team concluded that air humidity plays a key role, allowing growth primarily at night. In their study, during the day, a high VPD severely limited radial stem growth and allowed little growth, except first thing in the morning.
“The biggest surprise for us was that the trees grew even in moderately dry soil conditions when the air was humid enough. By contrast, growth was still very poor when the soil was moist but the air was dry, ”recalls Roman Zweifel, lead author of the WSL.
The reason is the limited water transport capacity of trees: as the air becomes drier, trees temporarily lose more water through perspiration than they absorb through their roots. The entire tree becomes stressed, its water potential decreases and growth stops regardless of the availability of carbohydrates.
Stem growth is more sensitive to air dryness than photosynthesis
Furthermore, the authors 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,” sums up Roman Zweifel. This could explain, for example, why trees in dry environments seem to store carbohydrates but are hardly growing anymore.
While current climate-sensitive forest growth models are based on knowledge of 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 period of 24 hours and consequently only for a limited time throughout the growing season.
As carbon gain (photosynthesis during the day) and carbon consumption (growth at night) are temporarily uncoupled, their sensitivity to climatic factors differs between day and night. These findings can change the way of looking at the impact of climate change on forests, in particular to predict their carbon sink.
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