Sap Rising – How does it work?

Now is the time, just before leaf burst when the sap begins to rise in trees.

This is famously known about with maple trees to make syrup. In our country birch trees can also be carefully tapped and the liquid used as a refreshing drink or used to make birch wine, but how does that sap rise? The main method we know about for drawing liquid up a tree (transpiration) can’t really work as there are no leaves!

I thought this would be a quick and easy article to write, but it turns out sap rise is still a debated phenomenon, with no completely satisfactory answer and scientists still unsure which is the main mechanism!

Here are a few probable causes. As with many things in nature perhaps there is an undiscovered factor waiting to be revealed! :

Root pressure

Just before the leaves come out, the trees convert starch stored over winter back to sugar, this increase of sugar in tree roots causes water to flow in from the surrounding soil (think water potential and osmosis), increasing the pressure in the roots and forcing liquid up the tree. This seems to be the likely explanation for species like birch which can develop considerable root pressure.

With trees such as Maple there has been a lot more research – driven by a concern about maple syrup production being effected by climate change….

Stem Pressure in Maples

For positive stem pressure to develop in Maples experiments show that temperatures need to change from freeze to thaw a number of times. Many theories exist including compression and expansion of gases within tissues as they freeze and thaw (maple xylem cells have a higher proportion of gas than other hardwood trees),  freezing and thawing of sap and increase of sugars. With the most recent theories combining these processes along with increased root water uptake and lower freezing point due to increased sugars in cells.

Its a complicated subject and if you really want to get your teeth into it this paper has lots of information: ‘ Muiltscale model of a freeze – thaw process for tree sap education’ Isabell Graf, Maurizio Ceseri, John M Stockie’ 

Trees are so precisely engineered it is a wonder! Good to remember as you plant those little saplings in the ground

Ali Murfitt

Site and Community Officer.