Trees know when deer have eaten their leaves. It's a type of automatic chemical response.
Plants can't run to escape being eaten, so they need other ways to defend themselves from animals like deer that can eat enough leaves to kill young saplings.
Plantcon 1
Thorns are one type of defense. Plants can also produce different types of chemicals that make their leaves taste bad and be difficult to digest.
German scientists studying young beeches and maples knew plants responded to predation, but they wanted to know if plants could tell the difference between a deer chewing their leaves versus being damaged by other factors like storms, or being trampled upon.
The scientists discovered that deer saliva was the key. When a deer feeds on leaves, saliva is left behind. The saliva sends a chemical signal to the trees' cells and they increase their production of salicylic acid.
The acid is a hormone that signals the tree to deposit more bad-tasting chemicals called tannins in its new leaves. It also increases the amount of growth hormone made to grow more leaves.
In their experiment, the scientists broke leaves on the trees and either added saliva, or left them alone. The tannin and growth changes only occurred when the saliva was present. Only wound-healing hormones were produced when there was no saliva.
Sources And Further Reading:
Bettina Ohse, et al. Salivary cues: simulated roe deer browsing induces systemic changes in phytohormones and defence chemistry in wild‑grown maple and beech saplings. Functional Ecology, 2016; DOI:10.1111/1365‑2435.12717. Accessed December 20, 2016.
