Some hypotheses about why my birch taps have not produced for two days in a row.
OK. So there was one drop on the bottom of the bucket, but it had dried before I got there. What gives?
Hypothesis #1: It's too early. Carl thought that the season for birches might not have started yet. Peterson's suggested range for tapping maples goes from late January to early April. For birches they say late March and April.
Hypothesis #2: It's the weather. Lucy came over this evening and I told her of my dry birch taps. She said that her maple taps had stopped running over the last two days as well. She'd heard that it was caused by the air pressure. Whoa!! What a cool thought! It made sense to me. The sap flows through big long tubes in the tree's trunk (xylem and phloem). Sap moves up through a process called capillary action. Pressure is an important variable here. So even though the weather has been textbook for sugar (with warm, sunny days and freezing nights), the maples and everything else have stopped flowing. That's so cool. I've got to learn more about this one.
I was fascinated by Lucy’s suggestion that air pressure plays a role in moving fluids up the trees. So, I went looking for info to find out. Trying to find out the relationship between air pressure and capillary action within the xylem of the tree is a good example of how much time one can spend on the internet exploring the multitude of caverns stuffed with literature, both old and new, and how one can become lost in these tendrils of accumulated literature. Just go to http://www.1911encyclopedia.org/Capillary_action and see what I mean! Still, I was fascinated by the question, so I continued to browse around for a bit. Here’s what I found in my brief journey.
ReplyDeleteEvidently, the issue of determining the effects of air pressure on capillary action was explored may years ago and was pretty much dismissed by 1679. An interesting ( I need to get a life!) and exhaustive history of investigations into capillary action is well treated at:
http://www.escholarship.org/editions/view?docId=ft587006gh&chunk.id=d0e2947&toc.depth=1&toc.id=d0e2792&brand=eschol
It should be pointed out that the capillary action – air pressure issue does not seem to be completely settled, however, as it has been rather consistently reported that capillary action can pull water and dissolved substances up a tree about 32 feet before gravity and pressure within the tubes. Obviously something else must be at work ---- air pressure?
It has been found that as we rise up the xylem of a tree, the pressure inside the xylem decreases. As we rise up the tree, the diameter of xylem also decreases. So, capillary action seems to in fact overcome gravity by outwitting it with a decrease of internal pressure and smaller capillaries to take advantage of the adhesion/cohesion relationship! So cool……Evidently, the game is up in xylem, however, at about 427 feet, the highest calculated growth for any tree.
A short and simple but satisfying overview can be found at:
http://www.davidlnelson.md/Cazadero/Trees&CapillaryAction.htm
How does this relate to your birch trees and the tiny drop sitting lonely at the bottom of your bucket? Not sure, but am dying to know if he wound up evaporating away into atmospheric nirvana before being joined by other not-so-sugary brethren who share its surface tension in such a stressful world! I’m rooting for a whole gaggle of birch sap drops.