Maple sugaring and the science of sap flow - University of Vermont

Maple sugaring and the science of sap flow

Photo by: Paula Murakami

What makes this possible?

The Science of Sap Flow and Maple Syrup

Production

1

Basic physiology of maple sugaring

Maple syrup production possible - 2 unique

properties of maple trees:

Ability to generate positive pressure in xylem sap

(Allows sap to flow)

The high sugar concentration in spring xylem sap

Basic physiology of maple sugaring

Must also have the right weather

conditions for sap to flow!

Freezing nights followed by warm days

Phloem Photosynthate (sugar) from source to sink

Xylem Water from soil to

leaves

(usually not sugars!)

2

Xylem Sap sugar

Air temperature

Nov 1

Jan 1

Mar 1

May 1

Vessel

Fiber cells

Sugar maple cross section

Ray cells containing starch

The sap flow mechanism in maple:

Freezing and thawing temperatures are required

3

Freezing causes liquid to expand

Why doesn't sap exude during a freeze and shrink during a thaw?

Vessel

Sugar maple wood section

Fiber cells air filled in

maple

Ray cells containing starch

Temperature: Above freezing for hours or days

Vessel with sap

Air-filled fibers

4

Vessel with sap

Air-filled fibers

Ice crystal growth (frost) occurs inside the air-filled fibers as the branches freeze. The water is supplied by the sap in the vessels.

Vessel with sap

Temperature: Falling from above freezing to

just below freezing

Air-filled fibers

Generates (-) pressure

Temperature: Below freezing for several hours

or days.

Vessel with sap

Air-filled fibers

5

Vessel with sap

Temperature: Rising from below to above freezing.

Air-filled fibers

Generates (+) pressure!

Temperature (oC)and Pressure (psi)

30

30

25

25

20 SSaapppprreessssuurree202-15t0p-o3psp-0sis5iippssii

20

15

Pressure

15

10

10

5

5

0

-5 March 20

0

Branch temperature -5

March 21

March 22

Temperature and pressure in the spring in untapped trees

But why does sap flow when we put a hole in the tree?

6

Sap pressure = 15 psi

Air pressure = 0 psi Sap

Atmospheric pressure lower than inside the tree ~ sap flows out of the wound!

Figure by: Tim Wilmot

Maple syrup production

Sap collection

Photos by: Brian Stowe

7

Sap collection

Trees are tapped with cordless drills and plastic spouts are most common

Photos by: PMRC

Sap collection

Photos by: PMRC

Sap collection

Network of 5/16" plastic tubing

connects to larger `mainlines'

8

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