Trees need light to make sugars and oxygen from carbon dioxide and water via photosynthesis. However, this does not necessarily mean that they grow during the day. Extensive empirical data with a high time resolution, which would allow researchers to pinpoint the hours of the day during which a tree grows, had not been available so far.
Roman Zweifel, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland, and colleagues have conducted a comprehensive field study of seven temperate tree species with a high time resolution, based on 57 million data points collected over a period of up to eight years. The team looked at the radial growth of the tree stems, i.e., their increase in thickness, for 170 trees at 50 different sites. For this, they used high-precision point dendrometers, in which a movable pin allows measurements of the change in thickness. They also measured air and soil conditions such as temperature and humidity.
The team found that all tree species grew mainly at night, with the highest contribution to total stem growth after midnight. The lowest amounts of growth were observed in the middle of the day and the afternoon. According to the researchers, the dryness of the air, or the vapor pressure deficit (VPD), plays a key role in this phenomenon. During daytime, a high VPD severely limits growth, except in the early morning. 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. Then, growth stops regardless of the availability of carbohydrates.
- Why trees grow at night,
Roman Zweifel, Frank Sterck, Sabine Braun, Nina Buchmann, Werner Eugster, Arthur Gessler, Matthias Häni, Richard L. Peters, Lorenz Walthert, Micah Wilhelm, Kasia Ziemińska, Sophia Etzold,
New Phytol. 2021.