Tree resin (along with other gum and latex fluids) plays an extremely important function in trees by rapidly sealing over wounds used as introductory pathways by invading insects and fungal disease agents. Organisms that try to enter a tree via a wound can be flushed out, can become stuck and trapped in the seal and can be overcome by the resin's toxicity. It is also thought that resins have high antiseptic qualities that prevent decay and that they also lower the amount of water lost from the plant's tissues. In any event, consistent resin flow is essential to the continued health of most conifers.
If you have regularly handled or touched the bark or cones of pine, spruce or larch, you know about the fragrant "sticky" resin they copiously ooze. That resin is contained in ducts or blisters that run through the bark and wood and diminish in size and number as they enter roots and needles. Hemlocks, true cedars, and firs have resin mainly restricted to the bark.
Wound trauma to a tree can stimulate the production of "traumatic resin canals" that help in containing the injury and help in healing any resulting infection. Resin-laden blisters contained in the conifer secrete the light liquid, which immediately loses oils to evaporation and forms a heavy solid scab. It is interesting to note that this reaction to trauma by a tree is used in the manufacturing process of certain commercial resins and essential oils by stimulating resin flow by inflicting a purposeful injury or bark irritation (see tapping below).
The production of resin is very common in nature, but only a few plant families can be considered of commercial importance to resin collectors. These important resin producing plants include the Anacardiaceae (gum mastic), Burseraceae (incense tree), Hammamelidaceae (witch-hazel), Leguminosae, and Pinaceae (pine, spruce, fir, true cedar).