When a branch is broken on a tree, the tree can sometimes identify the cause of the break and respond accordingly. Maple and beech seedlings, in particular, reacted differently depending on whether a branch was simply snapped off or browsed by a deer. A recent study found that when buds were snipped off, regardless of whether deer saliva was involved, the trees released hormones which helped the wound heal and triggered other buds to compensate for the missing buds.
If there was, however, deer saliva on the broken branch, the tree defended itself by releasing salicylic acid (which also happens to be the main ingredient in aspirin). Salicylic acid is part of a plant’s response to herbivore attack, whether it be from a deer or an aphid. The salicylic acid triggers the release of tannins, which make the plant tissue bitter and less palatable to the attacker.
Other scientists have studied the effect of salicylic acid sprays on crops ranging from potatoes to soybeans to tomatoes to peach trees. The spray seems to stimulate plant growth and help plants resist pests and diseases. As a result, the spray can sometimes lead to greater crop yields. A low-tech tactic sometimes used by organic growers is to make a solution of salicylic acid by soaking willow branches in water. (The willow water can also be used to stimulate rooting.)
Sources: Salivary cues: simulated roe deer browsing induces systemic changes in phytohormones and defence chemistry in wild-grown maple and beech saplings. Bettina Ohse, Almuth Hammerbacher, Carolin Seele, Stefan Meldau, Michael Reichelt, Sylvia Ortmann, Christian Wirth. Functional Ecology, August 2016;
Role of salicylic acid on yield improvement of ‘Elberta’ Peach (Prunus persica L. Batsch) tree. Hamideh Mohamadi and Zahra Pakkish. International Journal of Advanced Biological and Biomedical Research. Volume 2, Issue 4, Pages 970-973. 2014; How plants defend themselves. Jan Suszkiw. Agricultural Research, USDA. December, 2003.
- Janet Wallace