by Holly Schubert
While walking in the forests at Lutherlyn and other places, we have occasionally noticed a sticky substance oozing out of trees, usually evergreen trees. Maybe you've seen it too, and if you've played in pine or spruce trees as a kid, maybe you've gotten this sticky goop on your hands, clothes, or even in your hair (yuck). I've wondered, and maybe you have too: what exactly is that stuff? (And for me, when I find something new in nature, another question that usually follows closely behind that is "can I eat it?" Or in general "how can I use it?")
While walking in the forests at Lutherlyn and other places, we have occasionally noticed a sticky substance oozing out of trees, usually evergreen trees. Maybe you've seen it too, and if you've played in pine or spruce trees as a kid, maybe you've gotten this sticky goop on your hands, clothes, or even in your hair (yuck). I've wondered, and maybe you have too: what exactly is that stuff? (And for me, when I find something new in nature, another question that usually follows closely behind that is "can I eat it?" Or in general "how can I use it?")
Sap
Many people assume that this sticky substance is sap, because it oozes out of trees. It is not sap. Sap is made up mostly of water, and carries water and minerals from the roots to the leaves through the sapwood (xylem) of a tree and carries water, sugar, and other nutrients from the leaves to the rest of the tree through the inner bark (phloem). Sap "runs" in sapwood when temperatures vary between above freezing during the day and below freezing at night. The temperature variation causes a difference in pressure inside the plant tissues, which pushes the sap up the sapwood during the day, and allows it to drop back to the roots at night. (This is when we can gather maple sap to produce syrup.)
Resin
The sticky stuff we see on evergreen trees (and some other plants) is not sap, or even hardened or thickened sap. It is a different substance altogether, called resin. The US Forest Service tells us that "resins are plant products that,
- are not soluble in water,
- harden when exposed to air,
- do not play a role in the fundamental processes of the plant, and
- are generally produced by woody plants.
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| buck rub and resin flow on young norway spruce tree, Venango Trail at Camp Lutherlyn |
While resins do not "play a role in the fundamental processes of the plant" and are usually considered a waste product of cellular processes in the plant, they do provide a number of benefits to the plants that produce them. Resins seal over wounds, decrease water loss, protect from pests, and have antimicrobial properties that help prevent decay and infections.
Resins have also proven to be useful to humans. People have gathered and used resins from plants for thousands of years. Resins have been used for waterproofing, varnishes, adhesives, art, incense, medicines, food, and many other purposes. Some famous resins include frankincense, myrrh, balm of Gilead, amber, and balsam. Some products that have been made from resins include pine tar, pitch, rosin, and turpentine, though some of these are now made synthetically or from petroleum products instead of plant resins.
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All of the conifers common in Pennsylvania produce resins. Of the trees growing in our area, spruces, Scots pine, and larch have historically been used to produce products like turpentine from resin. At Lutherly, Norway spruce seems to produce the most resin, followed by white pine, and then other evergreens. In the northern woodlands that cover the upper reaches of the northeastern United States, balsam fir produces the most renowned resin. The resin is present in blister-like pockets that cover the bark, and its high oil content makes it simultaneously flammable, edible, and medicinal (though I imagine you might have to develop a taste for the flavor).
Some animals even use resin. One of the substances honey bees create in their hives is propolis. This sticky "bee-glue" is used to fill small cracks in the hive, smooth surfaces, and even seal off sources of contamination in the hive and keep the hive healthy. It is made from resins gathered from plants such as poplars and conifers, and mixed with saliva and wax. Propolis has also been used by humans for everything from embalming to antibiotics.
Rosin is another product made from resin, used in fields from music to sports to industry. Musicians add it to the bow hairs of stringed musical instruments like violins; dancers, gymnasts, rock climbers and baseball players add it to shoes and hands to improve grip; and many industrial processes use it to create the right consistency in products ranging from chewing gum to lead soldering.
Pitch
People sometimes seem to use the word pitch to refer to the same substance as resin - that sticky stuff coming out of the trees. (I grew up calling it "pine pitch.") Or, in common usage it sometimes refers to the more fluid state of this substance, while resin is assumed to have hardened. But more properly, pitch refers to a product made from resin. Heating resin and mixing in a fiber and possibly a wax creates pitch, which is useful as an adhesive, waterproofing layer, or flammable material for firestarter, torches and crude candles. This is the type of pitch that native Americans used to waterproof the seams of birchbark canoes. 
At the same time, pitch is also a more general term for a number of "viscoelastic solid polymers." This means that even though it seems to be solid at room temperature and can be shattered with a hard impact, it is actually fluid and will flow over time, but extremely slowly. In this general sense, pitch can be natural or manufactured, from petroleum products or plants.
Naturally occurring asphalt or bitumen, a viscous semi-solid form of petroleum, is a type of pitch that has been used in "pitch drop experiments" to study and demonstrate the flow of a seemingly solid material over a period of many years. The University of Queensland in Australia and Trinity College in Ireland have both observed pitch over many decades, recording only nine "drips" of the pitch in the course of their observations. In 2013 scientists at Trinity College were able to record pitch dripping from a funnel on camera for the first time.
We still
say something is "pitch-black" if it is very dark, because whether it
is made from trees or petroleum, pitch is very dark in color.
We have
also noticed blobs of a gummy substance on wild black cherry trees, as well as
cultivated cherry and peach trees. It is often mostly clear, or a little bit
amber in color, and feels rubbery but not as sticky as the resin found on
evergreens. I was surprised to discover that unlike the resin on evergreens,
this IS a result of sap flowing out of trees. It is fairly common in
"stone fruit" trees like cherries and peaches, and is referred to as
gummosis. Sap from the tree oozes out of wounds in the bark caused by insects,
fungus, or broken branches, and as it dries creates the gummosis. It is
sometimes an indication that the tree is unhealthy because it is being attacked
by insects or disease, but is sometimes simply the result of cracks in the bark
or branches caused by wind or other damage.
It is amazing that the simple sticky blobs we see on trees are connected to so much history and fascinating information! For more reading about some of the topics mentioned, check out the
links below.
US Forest Service on resin
Temperate Climate Permaculture on resin (including biology,
history and human use)
Balsam fir,
and resins in general
Creation of
amber from resin:
Propolis
Pitch
Historical
production of pitch
Gummosis
