Plant resins: Sticky stuff exuded for security

Did you ever lean back against a nice big spruce tree to enjoy a view as you relaxed with a sandwich and a cup of something tasty and warm? If you did, you may have found later that your jacket had acquired some sticky spots of resin from the tree. (I've noticed that this happens more frequently with new jackets than with old scruffy ones ...)

That resin is part of the tree's defense against natural enemies (You were just an accident). There are many kinds of sticky materials exuded by plants, but not all sticky exudates are resins. In the strict sense, resins do not include gums, latex, waxes or mucilages. Resins differ from the other exudates in their chemical composition, detailed in Jean Langenheim's tome titled "Plant Resins: Chemistry Evolution Ecology Ethnobotany." The following essay is based on information from this work, and once I started the book, I got hooked.

Virtually all of our local trees produce resin of some sort, not only the conifers, but also the deciduous species. The sticky resins of conifers generally deter attacks of insects and fungi, and this interaction goes back millions of years. Some conifers store quantities of resin in case of attack, but others produce most of their resin only in response to attack and wounding. Sticky resins of some conifers also deter birds or rodents that try to extract seeds from the cones.

Resin production and mobilization requires water. So, in drought years, less resin is produced and natural enemies have a heyday. That's why spruce bark beetles can devastate spruce forests in dry summers.

Among the deciduous trees, willows, cottonwoods, alders and birches produce resin on young twigs, buds and leaves. In these species, the resins on buds and leaves may protect not only against insects and pathogens, but also against desiccation. Resins stored in the bark of young birch trees deter the winter foraging of snowshoe hares. The buds and young twigs of balsam poplar and alders, found in interior Alaska, are also loaded with resins that hares don't like.

Sometimes animals turn the interactions around and use resins as a resource. Some bark beetles, for example, use the aroma of plant resins to locate their host and to avoid trees already invaded by beetles. Thus, the tree's defense has been turned against the them - an example of a co-evolutionary arms race in which a defense is overcome by an attacker, leading eventually to other defenses and subsequent counter-defenses.

Tropical bees of various kinds collect plant resins to construct their nests. In addition to the structural function, resins provide some defense against fungi and bacteria. Some of these resins are typical ones produced from wounds on a tree, and the bees have to be careful to get it while it's fresh so they don't get caught as it hardens. Surprisingly, the flowers of some tropical plants produce resins; in these species resin instead of nectar is the reward for the pollinators (the bees).

Honeybees collect resin from buds of some cottonwood species (but apparently not from our black cottonwood), as well as from willows. They use it to seal their hives and reinforce the wax combs (That mix of resin and wax - called propolis - has been used in human medicine, for a variety of ailments, for thousands of years).

Vertebrate animals are known to use plant resins too. Tropical forest creatures called coatis, which are relatives of raccoons, deliberately seek out certain trees and groom the resin into their fur. What the resin does for them is apparently still to be discovered. Here in Southeast Alaska, two observers have told me that wounded beavers were later seen with conifer resin plastered over the wound. Is it possible that beavers have discovered the antiseptic properties of resins?

Documented human use of plant resins dates back more than 5,000 years. Humans long ago discovered the value of resins in treating wounds and various diseases, as well as for incense, fragrances, preservatives, glues and waterproofing. But that's the subject for another essay.

• Mary F. Willson is a retired professor of ecology.