Sap flows, water loss and keeping your trees alive

16th Jun 2022

Did you know that globally, vegetation on land draws some 45,000 cubic kilometres of water from the earth each year, only to send it out into the air? That’s right, plants aren’t just drinking water, they’re losing it to the atmosphere as they have to open tiny pores (‘stomata’) to allow carbon dioxide in, for photosynthesis to occur. Allowing water to evaporate also keeps plants cool when temperatures rise.

Understanding the process of transpiration and working out how much it contributes to earth systems has received a fair amount of attention from scientists in recent years as it’s important in future climate predictions. At the level of individual leaves, researchers can do clever things in the laboratory to measure what a leaf is doing in terms of transpiration and gas exchange. It’s a bit trickier trying to measure rates of a whole plant, especially in the field rather than in a lab, but plant water use has been studied via sap flow (water moving through a plant’s sapwood) since the 1930s.

Readings from sap flow sensors, which indicate water use, give an indication of how much is lost through transpiration, but of course it's not a straightforward, linear relationship as it all depends on things like temperature and atmospheric moisture… Nevertheless, scientists around the world have recently pooled their data on sap flows to produce the SAPFLUXNET global database. This database allows further research into things like the long-term effects of drought and how the hydraulic system of a plant recovers after drought stress. In combination with other large datasets on tree growth, it can allow number crunchers to investigate the link between growth and water use, and also tree carbon assimilation (which is actually quite hard to measure despite the widely held idea that we know lots about it!)

Dealing with drought

Regulating transpiration is key for plants to cope with both heat and drought. It's a fine balancing act, though. If it's getting hotter than the plant's comfort zone, they need to speed up transpiration to avoid heat stress. Conversely, if it's very dry, they can close stomata, slow down transpiration and pretty much halt growth in order to survive. Different types of plant can cope with different levels of heat and drought – we all know that cacti are adapted to cope very well, while your average native British plant, used to plenty of rain and not-too-high temperatures, is less able.

What does this mean for your newly planted trees? In short, try not to put them under stress if you want to see optimal growth! Your average tree in a 15L pot, once planted, will need a bucket of water (around 10 litres) every 2-3 days to get it well established. If it’s really hot, give it some more, or it will start closing down to conserve moisture.

Monitor the leaves to see if they are getting droopy or dull. We don’t have an exact science for how much you should give them to thrive – there are too many factors involved – but use your own judgement to make sure they don’t reach the point of crispy-leaf stress.

And MULCH! We can’t sing the praises enough of a good layer of moisture-retaining mulch. It keeps weeds down and locks in more of that water around the tree roots than bare soil exposed to the drying sun. It’s a layer of insulation. Bark chippings are the most commonly available material.

Careful not to overwater. A plant can have too much of a good thing. If the base of the tree is like a lake, you need to hold off.

Take care of your newly planted tree, and it will take care of you by flourishing in years to come, with a healthy, well-watered canopy and a resilient hydraulic system!


Photo by Liza on Unsplash


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