Thawing sweets

Here's a thing that I didn't even know was a thing to wonder about:

You know the sugar maple, which produces the raw material for maple syrup by dripping sap into a bucket in the spring. Well, it turns out that it's not only a case of the sugar maple's sap being particularly sweet and thus well suited for this use. The sugar maple, along with a couple of other trees, are the only ones which drip their sap out in a way that can be usefully collected, and it is also particularly sweet.

The question, or rather questions, are:

Why only a few types of tree?

Why does this only happen during spring thaw, in certain temperature conditions?

How does this happen at all?

Some mathematicians from SFU on the west coast decided to calculate this east coast phenomenon.

The model they built matches the observed behaviour of the tree sap better than previous models. It includes a lot of factors, including semi-permeable membranes, heat transfer, embolisms, and crystals.

It turns out that the embolisms are, in a way, key to the whole system.

All trees have to deal with heat transfer and ice crystals as winter warms to spring and things start to thaw. The special feature of these few types of tree is that they have air bubbles in their fibres, which (being air) can compress, where the water that most trees have can't. When the water around those bubbles freezes in winter, the air bubbles are compressed because water's volume increases when it turns to ice.

So according to this study, when the weather cycles between freezing nights and warm days, the ice melts and re-freezes, in such a way that the air bubbles (at the centre of the fibres) can use their pressure to push the melted ice (at the wall of the fibres—heat coming in to the fibre from outside, of course, so there is a layer of ice between air and water) through the semi-permeable membrane that trees have between fibres and sap vessels. The extra water entering the sap vessels makes the sap start to flow, because that extra volume has to go somewhere.

The model does the calculations for heat transfer from the day warming up, the mass transfer of water going through the semi-permeable membrane, the volume change of ice melting to water, and the associated pressure change of the air bubble responding to all those changes, and found the same sap pressure that is measured at the tree.

And really, that's how math is supposed to work—describing reality. Too bad my short description above isn't nearly as accurate.

It does make me wonder one thing, however: if we weren't draining the sap out, what use does the tree get out of this early-season pumping mechanism that other types of trees don't have or need?