How can trees be able to transport hundreds of liters of water all the way up to the crown and out in the leaves?
The most popular theory is the ‘hydraulic limitation hypothesis’ (Ryan & Yoder, 1997), which suggests that as trees grow taller, it becomes more difficult to supply water to their leaves. This hydraulic limitation results in reduced transpiration and less photo-synthesis, causing reduced growth. It is known that in larger/taller trees water supply is limited by two factors — distance and gravity. Larger trees have a longer pathway of transporting tissue, this tissue is known as xylem. This is known as hydraulic resistance and makes it more strenuous for the water to move around in the tree. So now not only is the pathway long, but the water also has to beat gravity. Increased force is necessary to pull the water up to the highest leaves. Additional difficulty comes from the trees growing upright. Fast - growing/short trees frequently have shorter life-spans. They achieve their expeditious growth by having broader, wider xylem vessels, this increasing their hydraulic efficiency. At the same time this provides space for embolisms. An embolism makes it unable for a xylem vessel to transport water. Taller, slow-growing trees are longer-lived, this is caused by them having multiple xylem vessels, rather than having only a few wider ones like the fast-growing trees. Short-growing trees also have a more secured hydraulic system, preventing the forming of air-locks.