It’s a Sticky Situation
The Sticky Stuff in the Forest
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
To the Native Americans who lived in the Tryon Creek area, it was ”the super glue of the forest.” Among other uses, they used it to help fasten together parts of tools. For most people today, it’s just something you try not to get on your clothes. What is it? It’s resin from one of Tryon Creek’s native plants.
Just how sticky is resin?
For the photo below, I smeared some resin from a Douglas-fir (Pseudotsuga menziesii) on three quarters. I pressed my finger onto the first quarter, then pressed that quarter onto the second one, and then pressed the second quarter onto the third one. I was able to lift all three quarters. [Note: I only had enough resin to smear on three quarters. I don’t know how many I could have suspended with more resin!]
Isn’t resin just a fancy name for sap?
This is one of the most frequent questions on my nature hikes. Sorry, but resin is not sap. Sap is the watery fluid that moves dissolved minerals (like iron and phosphorus), organic nutrients (like sugar) and even water itself between different parts of the tree. The resin doesn’t do that at all. While all conifers and flowering plants have sap, a relatively small number of them make resin. At Tryon Creek the two prominent producers of resin are Douglas-fir and black cottonwood (Populus balsamifera spp. trichocarpa).
Okay, so what is resin?
Resin is a mixture of different chemicals, depending upon the tree species. In conifers, resin is primarily a special group of hydrocarbons called “terpenes.” These terpenes are chemicals that are quite a bit like some of the chemicals in gasoline. Below are chemical structures of two different hydrocarbons; one found in Douglas-fir, one in gasoline.
Look pretty similar, don’t they? Ever wonder why Douglas-fir burns so fast in a forest fire? This could be your clue!
Where does the resin come from?
That depends! In Douglas-fir the resin is made by cells lining special tubes in the tree called resin ducts. These resin ducts are located in the twigs, the trunk, cones or needles of the tree. Resin sometimes leaks out of the tree when it is injured by insects, storms, disease or people. When it first leaks out of a conifer, it is typically crystal clear, like these resin drops hanging from a Douglas-fir cone.
Often it will later turn a yellowish color, like the drop of Douglas-fir resin in the spider web below.
Finally the resin will harden into a whitish mass like this drip on a Douglas-fir trunk below.
In black cottonwood, the reddish-brown resin is primarily found in the tree’s buds. In the picture below, you can see the resin primarily in the seams between the bud scales. In addition a drop of resin is sitting on the bud.
When the black cottonwood buds open in the spring, the resin perfumes the air with one of the forest’s sweetest smells. You can experience this on the Old Main Trail within a couple hundred feet of the Nature Center, but timing is key. It is so enticing that one third-grade girl on my hike asked if anyone made perfume out of cottonwood buds.
So why do trees produce resin?
Resin helps the tree in several different ways.
- First of all, many of the chemicals in resin repel insects because of their smell or bad taste (I’ll take the insects’ word on that; I won’t be doing a personal taste test!) In addition, when insects bore into the tree, the tree produces extra resin near the wound, and tried to encase the insect in resin to suffocate it. This strategy works if there are a small number of insect attacks. However if a large number of insects attack, the tree’s resin defense is overwhelmed.
- Second, the resin has strong anti-microbial properties that help prevent the tree from getting any number of nasty tree diseases.
- Finally, resin is a great sealant. In the case of the black cottonwood bud, the resin seals in moisture, so the tree doesn’t dry out over winter. In addition, sealing up wounds prevents diseases from entering the tree.
One kind of wound is shown in the picture below. [Note: this picture is not from Tryon Creek SNA]. The crack in the butt log of this large Douglas-fir (arrows mark both ends and the middle) filled with resin (now turned white) in an effort to seal the wound from diseases. When the tree was cut down, some of the white resin was smeared across the log by the saw. [The gray is dried mud.]
So other than helping the tree, is resin useful for anything else?
Oh yes! Plant resins have a long history of use by animals. Two of the animals that use plant resins are bees and humans. Bees collect plants resins, perhaps add a bit of beeswax and make a substance that beekeepers call “propolis”. They incorporate this propolis into the hive. This propolis is highly effective in inhibiting the growth of some microorganisms that cause important bee diseases. In north temperate areas, including Tryon Creek, cottonwood trees are the bee’s preferred source of resin.
People have been using plant resins for a long time too. In addition to the gluing together of tools mentioned earlier, Northwest Native Americans smeared resin from both Douglas-fir and black cottonwood on wounds because of resin’s antibiotic properties, and some even have anti-inflammatory effects. Finally, Douglas-fir resin was used to seal canoes tight, and to waterproof some baskets.
Resins are produced by many plants to protect and defend themselves. In terms of the energy required to produce them, they are expensive to make. Nonetheless, they are such useful materials that many plants produce them.
Look for resin the next time you are at Tryon Creek, think about what a useful material it is, and yeah, try not to get any on your clothes!