Yes, I know I promised the Official WTWWU Suggested Holiday Gift List today, but a matter of urgent importance came up. In a follow-on comment to yesterday’s post on Christmas Trees and Gondwanaland, my brother- let’s call him “Phil”- asked why Christmas trees are so flammable. Since my brother is the kind of absolutely-brilliant-but-vague guy who I could totally see listening to bootleg “Luna” MP3s on headphones while the tree caught on fire in the next room, I thought his concern worthy of a quick post.
About 500 Christmas trees per year catch on fire in the US, killing around 30 people annually*. The greatest danger of the tree catching on fire is not from something directly igniting the tree, but from stuff immediately adjacent to, or below the tree- such as gift wrapping- catching on fire. A strong bonfire at the base of even a well-hydrated tree can quickly ignite the crown (just like a strong, out-of-control forest fire can light the crown of a rooted tree in the forest.)
In fact, it’s almost impossible to ignite a fully-hydrated tree by holding a match or a live wire to it.
Tangent: And it’s been tried. Apparently back in the 90’s, some Department of Public Safety folks up in Minnesota ran whole bunch of experiments trying to light Christmas trees on fire using matches, cigarette lighters, an overheated wire and a blowtorch. (Man, how do I apply for a job at the Minnesota DPS?)
But when a tree gets dry, it’s another story.
Quick Plant-Plumbing Primer
OK, to review: plants get water from their roots to their leaves via a specialized tissue called xylem. (We originally talked about Standard Wood Architecture in this post.) In a tree truck, the xylem sits outside the sapwood, but inside the cambium layer. Xylem cells form long, vertical, straw-like tube to draw water up through the truck, out along the limbs, and eventually into the leaves.
There’s no “pump” or “muscle” in a tree; what pulls water up is transpirational pull, which works like this: water molecules evaporate from a leaf. But as they do so, the water molecules immediately behind and adjacent to them are pulled up and forward by surface tension of water molecules sticking together. Surface tension is what causes water to bead up into drops when you spill it on a desk, which you can see in this First Exceptionally Helpful Educational Video, which I personally filmed and produced- at great effort and expense- for this post. Surface tension works the way it does in water (and other liquids) because the individual water molecules are attracted to one another more than they are attracted to the gas molecules of the atmosphere immediately outside of the liquid/gas boundary. (Go here if you want to deep-end on details.) In this Second Exceptionally Helpful Educational Video, you can see the surface tension at work: As my finger extends the air/water boundary, water molecules are pulled along from behind my finger.
Cool Factoid About Surface Tension: Surface tension strength decreases with temperature, which a reason that washing clothes in hot water is more effective; the hotter water breaks into smaller droplets more easily and better penetrates the fabrics. Bonus Speculation: I wonder if this heat-weakened surface tension could be an additional cause of stress for plants in hot deserts?
OK, so when we cut a tree off from its roots, the thousands of transpiring water columns in the xylem tubes are in immediate danger of being broken. And when that happens, the tree starts drying rapidly. Dryness of a tree is measured by what is called foliar moisture content (FMC). When a tree’s leaves/needles are fully hydrated, it’s at 100% FMC, and almost impossible to ignite directly. But below 50% FMC, you can light a tree on fire directly from a match, and below 20% FMC a tree is pretty much a tinderbox.
But it gets worse. Once a tree is below around 80% FMC, it gets into a Dessication Death Spiral, and will continue to dry out, even if it’s standing in water. This is because embolisms have already formed in the water columns and the transpirational pull is failing in a critical portion of xylem tubes.
So the most important thing is to get your tree in water fast, and keep it in water.
Second, cut the bottom few inches off before putting the tree in water. By the time you get that tree home the xylem tubes are pretty well dried-out in that exposed stump, and if you stick in water without cutting you’ll introduce embolisms straightaway.
Third, use just water. Mixing corn syrup with your tree’s water lowers absorption by a factor of 7! Even mineral supplements lower absorption by a factor of 3. Remember, you’re not trying to feed this tree for a long life of growth; you’re just trying to keep the FMC as high as possible for a few weeks.
Phil also asked if some Christmas tree types are more or less flammable than others. In the Minnesota study it looked as though Blue Spruce took about twice as long to ignite as Balsam Fir (way to go Andrea!), but I can’t say whether this applies to all Spruces and all Firs. Also, Spruce Christmas trees lose their needles much more quickly than Fir Christmas trees, so pick your poison I guess…
And finally, Phil requested that I post more pics of the Dark Pope, to which my response is: Phil, that would be an awesome theme for your own blog. In fact, I just checked, and www.darkpope.blogspot.com is still available.
Next Up: Really, the Official WTWWU Suggested Holiday Gift List is coming!
*Figures from 1997