Sap flow in spring

Bleeding sap in sugar maple
Sap flow in a freshly cut sugar maple, Acer saccharum.

Have you noticed any cut branches lately?  Freshly pruned or broken branches at this time of year often ‘bleed’ sap, and that sap may taste very sweet.

Just before trees leaf out, they convert a large amount of starch to sugar.  The high concentration of sugar in the xylem sap causes a flow of water from the soil into the tree by osmosis.  As water flows into the tree down an energy gradient, a positive pressure develops within the tree.  If you cut into the wood, sap will come out because of the pressure.  This is the only time of the year (except in certain diseases) that trees have a positive internal pressure.  It is also the only time of the year that cutting into the tree will cause ‘bleeding’ of sap (again, in the absence of disease).

The picture shows a sugar maple tree in Lexington, Kentucky that was pruned a few days ago.  Notice the drop of water at the base of the cut.  This branch was bleeding at a rate of about 5 ml/minute.  The sap was noticeably sweet to the taste.  This is not always the case as the sugar concentration varies quite a bit. This branch has been bleeding sap for about 5 days.  It is harmless to the tree, since the volume of sap involved is quite small.  Some older texts suggest that it is harmful to prune when sap is bleeding, but I don’t think there is any evidence for this.

This bleeding is the basis of the maple syrup industry.  There have been several online discussions lately about making syrup from other species, such as hickory, walnut and birch.  Not all trees develop a high enough sap pressure to bleed.

For the maple syrup industry, this is make-or-break time. The window of opportunity for gathering maple sap is very small, but the reward is sweet. There is even a song to sing while gathering sap:

Oh, bubble, bubble, bubble goes the syrup in the pan.
Making sweeter music try to top it if you can.
See the golden billows; watch their ebb and flow.
Sweetest joys indeed we sugar makers know.

Maple syrup production is usually a northern industry, centered in the northeastern states and provinces.  There is a good reason for this:  the highest sap pressure and the longest duration takes place where soils are cold, nights are cold and days are warm.  The further south you go, the shorter the sap flow season.  Climate change  threatens to dramatically reduce maple syrup production.   The top maple syrup producers in 2010 were:

Province of Quebec, Canada: 7, 989,000 gallons harvested.

State of Vermont, USA: 890,000 gallons harvested.

Province of Ontario, Canada: 400,000 gallons harvested. *

State of New York, USA: 312,000 gallons harvested.

State of Maine, USA: 310,000 gallons harvested.

Province of New Brunswick, Canada: 300,000 gallons harvested.*

State of Wisconsin, USA: 117,000 gallons harvested.

State of New Hampshire, USA: 87,000 gallons harvested.

State of Michigan, USA: 82, 000 gallons harvested.

State of Ohio, USA: 65,000 gallons harvested.

State of Pennsylvania, USA: 54,000 gallons harvested.

State of Massachusetts, USA: 29,000 gallons harvested.

Province of Nova Scotia, Canada: 22, 000 gallons harvested. *

State of Connecticut, USA: 9,000 gallons harvested.

Source: Maple Syrup World