We’re fast approaching the time of year for tapping birch trees.
If you’re not familiar with what’s meant by the phrase ‘tapping a birch’, it simply means putting a hole in a birch tree to collect its sap, a bit like in the photo below where I used an old fashioned brace and bit to drill a hole in a birch and then inserted a hollowed out length of elder into the hole to direct the sap into a billy can.
Before rushing off to copy me, I’d advise that you either read the whole post or skip to the end where I suggest pruning a branch instead of tapping.
What’s sap and where does it come from?
But first I want to go over a little tree physiology. Bear in mind I’m but a keen amateur when it comes to trees so feel free to comment below if anything here is wrong and I’ll correct it.
Birch is a deciduous tree, essentially a type of large flowering plant. Deciduous literally means “falling off at maturity” and so when we refer to a deciduous tree we usually mean a tree that sheds its leaves. In the northern hemisphere deciduous trees typically go dormant over the winter and as part of that process they shed their leaves in the autumn (or fall) as part of an evolved survival strategy.
There are at least two strands to this strategy. One is that the water in the cells of the leaves would freeze over the winter, rupturing the cells and killing the leaves. Another strand is that it protects the tree from being blown over in the windier winter months.
The process whereby a trees sheds its leaves is called abscission. As the levels of light and the temperature drop, the flow of a hormone, called auxin, to the leaves slows down and the levels of another hormone, ethene, rise. This tells the cells at the base of the leaf to weaken their walls and at the same time other cells expand to break the connections between those weakened cells, and so the leaves first change colour and then fall off the tree. (Note that this process can also be caused by other factors such as the dry season for trees in the tropics.)
Tree buds are tiny leaves, stems and flowers all enclosed in protective scales. It’s surprising how many people think that buds appear in the spring but this isn’t the case. A deciduous tree creates buds in the summer when there’s more sunshine to convert into energy through photosynthesis. Some of this energy is stored in the buds, which along with the rest of the tree lay dormant until the spring.
The photo below is of birch buds.
An interesting point here is what triggers the trees to come out of dormancy, and it’s a little more complicated than just higher temperature and more sunlight. It turns out that deciduous trees need to undergo a period of cold before being exposed to warmer temeratures. This combination of cold and then warmer temperatures triggers a physiological responses in trees. (And has worrying implications with climate change)
One of the first things that happens in the spring is that tree roots explode into action, moving water and nutrients from the soil into the rest of the tree. Under the layer of bark, water begins to move. This water mixes with simple sugars, the result of photosynthesis. The result is called sap.
If you’re wondering how the tree gets water from the soil up to its branches, a couple of different mechanisms are in place. Initially water gets into the roots through osmosis. The roots of the tree contain higher concentrations of minerals (mostly sugars and salts) than the soil that surrounds them, which causes root pressure. This is where the roots draw water in from the adjacent ground via osmosis. Osmosis is where a solvent (in this case water) passes through a semi-permeable mebrane (the root hairs) to equalise the pressure difference between the minerals in the soil and the minerals in the tree roots. As new water is drawn in through the roots via osmosis, it forces the existing water upwards through xylem. A tree has countless xylem running uninterrupted from the roots to the very tips of the branches.
Once the leaves are out they release water into the atmosphere via transpiration. This loss of water causes a reduction in hydrostatic pressure which then draws more water upwards.
Tapping birch trees
Now that we know how and why the sap is moving, let’s look at how to extract it.
For many of us tapping birch trees was brought to our attention by Ray Mears on his TV show ‘Ray Mears Bushcraft’ in 2004 and it quickly entered bushcraft folklore as a must do activity, me included as you can see from the photo at the start of this post.
The best time to collect sap from a birch varies from year to year and there is also some variation based on where you are. In the British Isles it’s generally in March, although spring arrives in the south before the north and the west before the east. There can be several weeks difference between Land’s End and John O’Groats! Typically here in Kent I do it in the last couple of weeks of March, although this year things are happening a little earlier.
Is it harmful to the tree?
Tapping birch trees has become a hotly debated subject in the last few years with a genuine question around whether, as bushcrafters, this is something we should be doing at all. Clearly putting a hole into a tree causes it harm, but how much harm and can the tree recover from it?
I’ve seen discussions on various online forums with claims that :
- No harm is done to the tree
- Putting a hole in a tree will definitely kill it.
Neither of these claims seem particularly likely; if you put a hole in a tree there will inevitably be some degree of harm.
But at the other end of the scale, that putting a hole in a tree will kill it, seems equally unlikely. One of the many things I love about trees is their resilience. They have evolved to survive high winds, lightning strikes and grazing by animals. We coppice our woodland regularly and the trees grow back. So will putting a single hole in a tree kill it?
How harmful is it?
I was interested to find any evidence that might exist as to exactly how harmful tapping birch trees is. Here’s what I found out.
We know that people have been tapping birch for a very long time in Europe, this article provides a fascinating glimpse into the different ways that birch have been tapped in different places at different times. But that doesn’t mean the practice is without harm and the article doesn’t purport to assess that.
We could look to the maple syrup industry which has been tapping maples for a few centuries. Presumambly if tapping a maple caused irrecoverable harm the industry would have died out by now. However it turns out that birch has a much lower tolerance to wounding and compartmentalisation of wounded tissues than maple (Shigo and Larson 1969).
The US has an expanding industry of birch syrup so we could look to the Alaska Birch Syrupmakers’ Association for guidance. I’ve copied word for word the advice that ABSA has produced:
- Time to tap varies by location; usually first part of April.
- Tap holes: 1 ½ – 1 ¾” deep, slight upward angle, using a 5/16-7/16” bit, depending on spout used.
- Location of hole: 2-4 feet high, to the side of previous holes
- Tap healthy trees; 8” dbh or larger
- Do not tap trees that have ever had pesticides sprayed on or around.
- One tap per tree.
- Use plastic, nylon, or stainless steel spouts, or tubing supplies commercially available through local and maple syrup equipment suppliers.
- Do not drive taps too deep – wood can split causing leakage.
- Sterilize taps before use.
- Tap trees when the sap flow is continuous.
- Tap trees only where access is good and equipment will not compromise ground cover. Minimize damage to trails during break-up.
- Remove spouts at end of season; may spray hole with pure water. Cork tap holes upon removal with appropriate sized cork (available through local suppliers).
The State of Alaska Plant Materials Center and Division of Mining, Land and Water recomends that
“Each tree may be tapped once a year for five years. After a two year recovery period, tapping for another 5-year period may resume. This cycle may be repeated three times for a total of 15 tapping seasons per tree in a 19-year period, after which the tree must be retired.”
A study in 2007 reported in a literature review by Lori Trummer and Tom Malone found that a sample of 21 trees that had been tapped all showed sapwood red heart staining (regardless of how many times they’d been tapped) and 9 of the trees showed signs of decay at the tap holes. Of those 9 some had been plugged, others not. Which suggests that putting a plug in the hole, even a sterilised plug, isn’t helpful. In fact “plugging increment borer wounds in birch with sterile dowls has not been shown to be effective in reducing the development of stain or canker fungi”. Lorenz (1944)
My conclusion? Tapping doesn’t kill the tree. But not killing the tree isn’t a very high bar to set if you’re concerned with conservation. Furthermore the evidence suggests that putting a hole in a tree is likely to allow infection to enter it and that’s something we should avoid.
The alternative to tapping birch trees
So instead of putting a hole in a tree to collect the sap, simply prune off a branch and collect the sap as it drips out. This is easier to do, much easier for the tree to deal with, and in my experience doesn’t seem to have much effect at all on the volume of sap collected.
You can see here that the birch I used wasn’t very big at all, probably only 10 – 12 cm in diameter but it still gave a steady flow of sap.
What to do with the sap
Once you’ve collected your birch sap you have plenty of options. Generally I just drink it! But you could reduce it down to make birch syrup, use it for birch sap wine or beer or birch sap vinegar. As far as I know there aren’t any recorded cases of anyone becoming ill from drinking birch sap.
References for tapping birch trees
Ingvar Svanberg, Renata Sõukand, Łukasz Łuczaj, Raivo Kalle, Olga Zyryanova, Andrea Dénes, Nóra Papp>, Aneli Nedelcheva, Daiva Šeškauskaitė, Iwona Kołodziejska-Degórska, Valeria Kolosova. 2012. Uses of tree saps in northern and eastern parts of Europe. Acta Societatis Botanicorum Poloniae.
Larsen et al. 2008. Alaska Non-Timber Forest Products Harvest Manual for Commercial Harvest on State Owned Lands. The State of Alaska Plant Materials Center and Division of Mining, Land and Water
Lorenz, R.C. 1944. Discoloration and decay resulting from increment borings in hardwoods. J. Foresty 42: 37-43.
Shigo, A.L.; Larson, E.vH. 1969. A photoguide to the patterns of discoloration and decay in living northern hardwood trees. USDA Forest Service, Northeast Forest Experiment Station, Research Paper NE-127, Upper Darby, PA. 100p.
Trummer, Lori; Malone, Tom. 2009. Some impacts to paper birch trees tapped for sap harvesting in Alaska. United States Department of Agriculture, US Forest Service Alaska Region, State and Private Forestry Forest Health Protection, Anchorage Office
Yoon, Seung-Lak & Jo, Jong-Soo & Kim, Tae-Ok. (1992). Utilization and Tapping of the Sap from Birches and Maples. Journal of the Korean Wood Science and Technology. 20.