View Full Version : How Does Sap Rise in Spring?

07-24-2013, 07:12 AM
How does sap rise in deciduous trees in Spring?

There are three ways to transport fluid in a tree:

1. by Osmosis (https://en.wikipedia.org/wiki/Osmosis) where a weak solution will flow into a stronger solution via a semi-permeable
membrane. Trees do this by storing strong solutions of sugars in the roots.

2. by Capillary Action (https://en.wikipedia.org/wiki/Capillary_action) which occurs because of intermolecular forces between the liquid and
solid surrounding surfaces. If the diameter of a tube is sufficiently small, then the combination of
surface tension (which is caused by cohesion within the liquid) and adhesive forces between the
liquid and container act to lift the liquid.


3. by Transpiration ( https://en.wikipedia.org/wiki/Transpiration) which is the process of water movement through a plant and its evaporation from
aerial parts especially from leaves but also from stems and flowers. Mass flow of liquid water from
the roots to the leaves is driven in part by capillary action, but primarily driven by water potential
differences. In taller plants and trees, the force of gravity can only be overcome by the decrease in
hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of
stomata into the atmosphere.

So now we have a tree with no leaves and little to no sap which eliminates Capillary Action and
Transpiration. This only leaves Osmosis, but there is always a concentration of sugars in the roots
so what stops sap rising due to osmosis in winter?

One could say that the sap rises due to a rise in temperature, but looking at an article on Ground-
Coupled Heat Exchangers ( https://en.wikipedia.org/wiki/Ground-coupled_heat_exchanger) it states:

They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for
residential, agricultural or industrial uses... underground where the ambient earth temperature is
typically 10 to 23 C (50-73 F ) all year round

So now I’m baffled as to how the sap rises in Spring?

Anyone have an answer?


07-25-2013, 09:52 AM
I gather by the lack of replies that few are really sure what is going on in spring...

I have continued the search for answers and below are some, but still not quite definitive...

A British investigator found that in the fall the center of a tree is very wet, and the outer regions are
comparatively dry, while in the spring this condition is reversed. He concluded that if we desire to
make our language conform with the fact we should not say that the sap is up in the spring and
down in the fall, but that it is out (near the bark) in the spring and in (toward the center) in the fall.
Analysis shows that pieces of wood cut from trees in the winter sometimes have a moisture content
just as high or even higher than pieces cut in the spring or early summer.

Source: (http://www.4information.com/trivia/tree-rise-spring/)

Have any of you ever seen a tree shoot sap like a geyser out of a damaged limb? Or where a limb is cut off?
Last night I went to dinner with a group of friends from all over Springfield and we were
talking about how the trees are waking up to find themselves so damaged and we all
noticed that the trees are literally squirting sap in some places where their wounds are
worst...I mean like small fountains.

Source: (http://www.city-data.com/forum/garden/53891-spring-tree-sap-rising.html)

In general xylem does remain full of sap during winter months and water conduction can begin again
in spring with little effort because of the cohesion tension theory that was mentioned.
From what i understand the mechanism is a combination of osmotic root pressure and increased
soil water pressure caused by higher soil water volume during late winter and early spring.
There will be some bulk movement as trees are not totally dormant during winter, at this time they
consume stored energy instead of energy manufactured by leaves.

It may be worth reading this whole thread just to see how vague and misunderstood the rising of sap is.

Source (http://www.physicsforums.com/showthread.php?t=581943)

This is perhaps the best answer I have found so far, but still leaves a lot of questions.

What does happen is linked directly to this specific time of year, when nights fall below freezing and
days warm above it. Take away either of those conditions, and sap does not rise. What happens is
this. First, in the fall, sugars are transported down into the stems and converted to starch. In the
spring, during the warm days, living cells convert that starch into sugar. They also generate carbon
dioxide gas. This gas diffuses into the xylem. As the temperature cools, the gas dissolves, lowering
the pressure and pulling the sugary water from the living cells into xylem. This water is replaced
from adjacent cells, which form a conveyor belt for water down to the roots. As night comes and
the temperature drops further, water freezes along the inside walls of the xylem and in between its
cells. The remaining gas is compressed and locked in this ice. With morning, things warm, the gases
expand and force the now liquid sap out of the trunk or stem and into the tap. As the day cools in
the afternoon, the process repeats itself. For more details, go here (http://www.employees.csbsju.edu/ssaupe/biol327/Lab/maple/maple-sap.htm ) or here (http://www.massmaple.org/sap.php ). The process stops when
the temperature remains above freezing and the buds begin to open.

Source: (http://www.r4r.ca/en/step-outside/nature-guides-archive/winter-2011-12/early-march-2012)