September 13, 2016
Two of the study co-authors, Carolin Seele and Stefan Meldau, collect buds for the subsequent analysis of plant hormones and tannins. (Bettina Ohse)

Picture the woods in early spring — the misty, pale green appearance of trees just starting to sprout, the young deer munching their way across the sun-dappled forest floor.

But behind that placid exterior, a battle of wits is raging. And despite not having a brain or any central nervous system to speak of, the trees are pretty wily.

In a study published in the most recent edition of the journal Functional Ecology, scientists in Leipzig, Germany describe the brilliant way that wild maple and beech trees figure out when roe deer are eating them — and enact a strategy to make sure the critters don’t return for another snack.

The stakes are high for young trees. If they can head off the threat from deer early, the saplings have a chance of catching up with their unbitten peers and making it to adulthood. But too much nibbling will make the trees stunted, or condemn them to death.

So the plants have evolved a complex set of responses specifically to deal with herbivore threats. Whenever a branch is snipped — by a deer, insect or human — trees released “wound hormones” called jasmonates. These chemicals help with the recovery process. They also play a role in interplant communication; when one plant releases jasmonates, their neighbors start to ramp up their defenses against disease and insect attacks as well. It’s like a forest-wide alarm system.

But the trees studied in Leipzig seemed able to recognize specific threats, and tailor their response accordingly. When a roe deer was eating their branches, the trees released a second set of chemicals: first the hormone salicylic acid, then bitter chemicals called tannins. The salicylic acid boosts protein production, allowing the trees to regrow what was lost, and the tannins make the trees distasteful to deer, which prevents further snacking.

“On the other hand, if a leaf or a bud snaps off without a roe deer being involved, the tree stimulates neither its production of the salicylic acid signal hormone nor the tannic substances. Instead, it predominantly produces wound hormones,” Bettina Ohse, lead author of the study, explained in a statement.

To test whether this was truly a tailored response to a roe deer threat, the scientists in Leipzig attempted do some outsmarting of their own. They simulated roe deer snacking by clipping the trees, then trickling deer saliva onto the broken branches with a tiny pipette. The fake deer attack sparked the same defensive response as an actual bite from a deer, suggesting to the scientists that trees are able to recognize deer saliva and respond to it. Now, Ohse and her team are performing the experiment on other tree species, to see whether some have better defensive mechanisms than others.

Keep that in mind the next time you consider matching wits with a tree.

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