There are some interesting and helpful [usually] web links available.
Several are listed below with some info re myths from one included.
Hope this helps.
E. Lee Hadden
Professor of Biology
Wingate, NC 28174
North American Maple Syrup Producers Manual
Bulletin 856 Chapter 6 . Maple Sap Production - Tapping, Collection and
Storage Physiology of Sap Flow
Understanding Sap Flow Timothy Wilmot
University of Vermont Proctor Maple Research Center
Common myths about sap flow in maple: [from link listed above]
1. Sap flow does not start until the soil thaws out. Actually, forest
soils are rarely frozen at any depth if there has been snow cover for most
of the winter. Snow is an excellent insulator. In addition, loose and
decaying leaves on the forest floor provide a good layer of insulation to
the roots beneath them.
2. Sap flow in maple occurs because of root pressure. There is some flow
caused by root pressure in most trees, but the pressures generated this way
are nowhere near as high as the pressure that comes from the branches, as
described above. Root pressure does not occur until the soil starts warming,
which is usually after the maple sap season has ended.
3. How sap flows from maple is still a mystery to scientists. There are
some aspects of the mechanism that are still in debate, but there is general
agreement about the way positive pressure is generated, and how that
pressure is responsible for sap flow.
How to Explain Sap Flow
Michigan Maple Syrup Association [with assorted links listed]
From: owner-plant-ed at hgmp.mrc.ac.uk [mailto:owner-plant-ed at hgmp.mrc.ac.uk]
On Behalf Of Kathleen Archer
Sent: Tuesday, April 12, 2005 11:01 AM
To: plant-ed at net.bio.net
Subject: rising tree sap
Dear Plant Ed Folks,
I have tapped my maple tree in the yard to collect sap for maple syrup, and
in doing this I noticed a couple of things.
First, the volume of sap is astonishing - the fluid practically gushes out.
Second, the flow rate is closely linked to sunshine. On cloudy days, the
sap flow is strikingly reduced, as is the flow at night.
The second observation calls into question something I have been told by
others, namely that the driving force of upward sap flow is root
pressure. I don't see how this can be. Root pressure is what causes
guttation droplets to form on the leaves of plants, and this is most
typically seen in the early morning. That suggests to me that root
pressure should be operating at night. Further, I don't see how cloudy
days or sunshine could affect root pressure.
Naturally, since I am lecturing now about water transport in xylem, and
phloem transport, students are questioning me about maple sap. I'm
guessing we may not know as much about how this works as we would
like. But if anyone has good solid knowledge of what governs the flow
upward of tree sap in the spring, I would be grateful if you would share it
with the group.
By the way, tapping a single tree for several weeks yielded 4 - 5 cups of
truly delicious maple syrup!