NPR confuses me

[bchncamaro]

http://www.npr.org/blogs/thesalt/2014/02/02/270204651/sap-discovery-could-turn-syrup-making-upside-down
Maybe I'm missing something here but after reading and listening to this article I'm confused. The article suggests that researchers have just now discovered that sap comes from the roots and not the top. Take a look and let me know what you think.

[happy thoughts]

The article is misleading as is several other articles I've seen taken from the same "Modern Farmer" article. It seems going to the real source (UVM) is too much for modern journalists so bad info keeps getting regurgitated. It's like a bad case of the "telephone" game imho.

[BreezyHill]

The sugars of a plant are stored in the roots. The moisture of the ground is absorbed into the roots and used to carry the sugar to the buds to start the green up of spring. This is a simple version of a very complex proceeder.

Smile..Its all good...you learn something new every day...if you listen that is.

Ben

[BlueberryHill]

Don't question yourself. You know more than NPR when it comes to Maple. They probably know more about Darfur than you do, or the next up and coming Cellist. But clearly no one there is very up to date on maple.

Most journalists are hacks. Well spoken in many cases, but still hacks. You truly can't trust a news article to provide you with good, accurate information and accounts anymore. Real, proper research takes time and journalists just don't do it these days.

Most also have an agenda and cherrypick (and distort) facts and quotes that support their beliefs and not necessarily reality. I rarely see an article without at least some of this. The more you personally know about a subject or event, the more evident this becomes in the articles.

Can you tell that I am disappointed with the news?

I know that in this article, it is just ignorance. So I am not saying that there is some conspiracy against the trees or something, but I just get frustrated when I see gross inaccuracies abound and it makes me question everything else that I have read. What can you really believe? end of unnecessary rant... wish the sap would get going already.

[DrTimPerkins]

The article is misleading as is several other articles I've seen taken from the same "Modern Farmer" article. It seems going to the real source (UVM) is too much for modern journalists so bad info keeps getting regurgitated. It's like a bad case of the "telephone" game imho.

You said it....not me. Although I do agree with you. It was a very poor piece, and contains a lot of inaccuracies.

[DrTimPerkins]

http://www.npr.org/blogs/thesalt/2014/02/02/270204651/sap-discovery-could-turn-syrup-making-upside-down
Maybe I'm missing something here but after reading and listening to this article I'm confused. The article suggests that researchers have just now discovered that sap comes from the roots and not the top. Take a look and let me know what you think.

In the spring of the year, in maple trees, freezing causes water uptake (soil water pulled up through the roots, through the stem, into branches). During that process, the bulk flow of liquid is UPWARD. However we do not collect sap during this time, since the stem is under a slight negative pressure (vacuum) during the uptake phase.

During a subsequent thaw, the bulk flow of the sap (the mixture of water, sugars and some other stuff) that we collect is DOWNWARD by simple gravitational forces. This phase is termed "exudation", or more commonly by maple producers, a "sap run".

Under vacuum, we start off a flow with most sap moving DOWN, but there is also more lateral (side-to-side) movement of sap, and some UPWARD movement of sap as well, depending upon how long the thaw lasts and what the vacuum level is. We call this "vacuum-induced flow" or "vacuum-induced exudation".

What we found in this research (which was very poorly explained in the NPR piece) is that during an extended thaw (beyond a few days) and with good vacuum, the bulk of the liquid movement is UPWARD, and that during this type of sap flow dynamics, we are pulling (with vacuum) the soil water up through the roots and into the stem and out of the taphole. Under this flow regime, the top of the tree is unnecessary. Despite what some of the reports say, this wasn't something we "stumbled" across, or observed naturally in a broken-off stem. It was a research projected aimed specifically at understanding in more detail the phases of sap flow dynamics at different times and especially under vacuum conditions. The application of this new information to the process of sap collection is what led to the development of the method of collecting sap from cut-off saplings.

[DrTimPerkins]

The sugars of a plant are stored in the roots. The moisture of the ground is absorbed into the roots and used to carry the sugar to the buds to start the green up of spring. This is a simple version of a very complex proceeder.

Yes, it is complex....and in maple it is even more complex than normal. In maple trees, carbohydrates (starches and sugar) are stored in all wood tissue, so roots, stems and branches. In most plants we think of sap (mostly water) moving up from the roots to the leaves via transpiration. Water (as vapor) is literally being "pulled" out of the leaf by the atmosphere. The cohesiveness (stickiness to itself) and capillarity of water cause this pull to pull on the liquid in the leaf, then the branch, stem, and root to pull water from the soil. Everyone pretty much understands that fairly well.

However keep in mind....when the sap runs in maple trees in the spring....there are no leaves on the tree. The atmosphere has relatively little to pull on, since there are no leaves with their stomates open for water to evaporate from.

Instead, it is forces within the wood cells themselves, caused by contraction of gas bubbles (gases shrink a lot when cooled) in the lumen (wood cells in maple are filled with gas, most other plants have liquid filled lumen spaces), combined with the massive drop in vapor pressure (like humidity) when the liquid begins to transition into solid (ice formation). These forces provide the driving force to "pull" water up into the stem during the freeze phase.

Sap exudation, or flow, occurs when the frozen sap subsequently thaws out. Any break in the xylem (wood) of the tree will ooze sap, which generally moves down the tree due to head pressure (it is essentially like a very long straw filled with water that you put a pinhole in the bottom of). Once the head pressure is dissipated due to sap flow out, the sap will stop running out. There may be some additional slow weeping if the temperature goes up (again due to lumen gas expansion).

So you might wonder....why do maples do this? Other trees don't. It is thought that this is an evolutionary adaptation to xylem embolism. The vessels in the tree embolize (have bubbles form in them) when the temperatures drop below freezing. Because they are normally under tension, bubbles will form spontaneously from dissolved gases as the temperature drops and less gas can be held in solution. This is problematic because (as we all know), it is difficult to move liquid effectively through a small pipe when there are bubbles that get in the way. So how do you get the bubbles out? Different trees do it different ways. The way maple does it is by generating stem pressure within the vessels via this freeze/thaw process. The pressure will allow the bubbles to redissolve in the liquid, thus restoring the capacity of the vessel to move liquid. So by the end of the spring season, all the vessels are again completely liquid filled....then the buds break and the leaves emerge, and the atmosphere can once again start "pulling" water out of the leaves.

In any case....sorry for the long explanation (which is actually just the simplified version)....figured that some of you might be interested.

[maple flats]

And I thought it was simple. I'm glad my trees knew how to do it, I'd never manage training them. Too scientific!

[bchncamaro]

Thanks Dr. Tim, and everyone else who contributed to this thread. This has been very informative to me :)

[wiam]

Unfortunately I have seen articles in maple news that could have been written by the same reporter.

[maple marc]

Dr. Tim,

Thank you! Regarding gravity and head pressure: how important to sap flow is the height of the tap hole? I have been tapping my trees for 10 years, in a spiral-down pattern. I am now near the ground, and it is time to start up high again (5'?). Will I notice any difference in flow rates (buckets)?

Thanks!

[DrTimPerkins]

Will I notice any difference in flow rates (buckets)?

It is relatively minor. Better to ensure that you spread the wounds all around the tapping zone than to cluster tap down low.

[SevenCreeksSap]

Thanks Dr. Tim for the explanation of the process. I've see several comments on other forums ( fora ?) that have been negative such as - wow, how much did they spend to find water comes from the roots? - And these commentators don't get that it is the downward flow we are tapping in to. The new process of capturing the sap from a sapling is getting that upward flow, correct?

The next logical question(s) if the new process is capturing the upward flow, is where is the sugar coming from, and what is the average sugar content from the saplings? is it gathered or processed in the root system, or as it works through the stem?

Many of those same commenters indicate the sugars are coming from the leaf buds, leaves, twigs, etc, but as you pointed out, No Leaves!

[DrTimPerkins]

... it is the downward flow we are tapping in to. The new process of capturing the sap from a sapling is getting that upward flow, correct?

Yes, most people still remember their grade school botany with sap flowing upward, but neglect to remember that this is during the summer with leaves on the trees.


The next logical question(s) if the new process is capturing the upward flow, is where is the sugar coming from, and what is the average sugar content from the saplings? is it gathered or processed in the root system, or as it works through the stem?

Carbohydrates are stored in the sapwood (xylem) as both sugar and starch. The average sugar content is the same as most typical trees because the saplings are growing in the open. If they were understory trees, the sugar content would be lower. The water comes in through the roots, then mixes with sugars in the wood as it moves through the system. Over time, the sap sugar content would drop off slowly, until there was another freeze which would stimulate the conversion of some of the starch into sugar.


Many of those same commenters indicate the sugars are coming from the leaf buds, leaves, twigs, etc, but as you pointed out, No Leaves!

Sugars are formed through photosynthesis in leaves during the summer, and are moved into the wood where they are converted to starches for storage.

[bees1st]

What scares me is that when a journalist does a piece with wrong information , which they often do . How much bad information about the world and current events is out there ? But that's a whole nother thread!!!!

[Ausable]

Thank you Gentlemen and especially Dr. Tim. I'm always amazed at how little I really know. It all does make sense - I never realized what an involved process maple sap flow is - especially the sap we are after that contains sugar. Now - I also better understand why the condition of and spile size is important. ---Mike---