Water flows down, so how can even the tallest trees draw it up to the top of the canopy?
Tall trees can experience water limitations even when soil water is readily available. Why is this? We can think of the water conducting tissues in a tree (the xylem) like a straw, and the transpiration (water loss) through the leaves like the suction power to pull water up the straw. The longer the straw (xylem), the more suction power (transpiration) that is needed, and there is a limit on how long the straw could be based on the maximum suction power. Tall trees must maintain high transpiration rates to keep water flowing up from the roots, which means that the upper canopy of tall trees is often water-stressed, and may have difficulty retaining water. So how do tall trees retain water in their upper canopy?
In a recent article in Tree Physiology, Wakana Azuma and colleagues applied infrared micro-spectroscopy (a technique used by chemists) to figure out how the upper canopy of trees is adapted to water limitations. Infrared micro-spectroscopy uses infrared light to cause vibrations in chemical bonds, which then emit infrared light as the vibrations dissipate. The emitted light is specific to certain types of chemical bonds, which allows this technique to look at which chemicals are where.
In tall trees, there often is not enough water transport capacity to maintain transpiration in the upper canopy during the day, which is important for photosynthesis and survival. To meet the demand for water in their canopies, many tall trees therefore must rely on some form of stored water (replenished at night) during the day. Where and how tall trees store water in the upper canopy is important for understanding the mechanisms that trees have evolved to overcome water stress due to height, and for understanding possible adaptations that plants can employ to deal with drought. Looking at leaves as high as 51 m above the ground in Cryptomeria japonica, Azuma and colleagues found that leaves in the upper canopy had higher amounts of structural and dissolved carbohydrates, and higher water content than lower canopy positions. They conclude that upper canopy leaves use these carbohydrates to retain water despite the water stress conditions imposed by height. The application of a technique used primarily by chemists to water retention in leaves by Azuma and colleagues represents the first major step towards understanding the mechanisms used by tall trees to survive water-limited conditions.
Azuma, W., Nakashima, S., Yamakita, E., Ishii, H. R., Kuroda, K., & Ryan, M. (2017). Water retained in tall Cryptomeria japonica leaves as studied by infrared micro-spectroscopy. Tree Physiology, 1–12. https://doi.org/10.1093/treephys/tpx085