So it transpires…
5th Aug 2021
Did you know that trees don't just drink water from the ground, but send about 10% of it back into the air?
After roots absorb water from the soil, it's pulled up through the tree to its leaves, where some is heated by sunshine and escapes as vapour through tiny pores called stomata. This process, transpiration, is critical for the tree. Firstly, water is vital for photosynthesis - the chemical reaction where water, carbon dioxide and light are turned into energy for the plant. But transpiration also keeps trees cool, so in summer it ramps up a notch.
About 10% of water taken up by trees is released into the air through transpiration. This amount will vary depending on species, climate and size of tree. A mature oak can transpire as much as 150,000 litres a year! In this way, amongst others, trees affect microclimate (e.g. nearby air humidity and temperature) and there are even sophisticated modelling tools for investigating the phenomenon, as urban trees are a tool for mitigating the effects of climate change in cities.
Water also acts a vehicle for nutrients. So a dry tree is not only thirsty, but hungry too. Meanwhile, in very hot weather, leaf stomata will close to conserve water and this can slow down growth as the whole transpiration process stalls. It's a fine balancing act for a tree, juggling its need to stay cool while not losing too much water. A tree may take the drastic measure of dropping leaves if water is scarce.
Transpiration is affected by amount of sunlight, temperature, air humidity and wind, but the ability of trees to close their stomata means they can regulate water use, up to a point. Most species can't just completely stop. So if you want your new trees to thrive, best not to let them get thirsty!
Tree vector created by brgfx - www.freepik.com