Not being able to escape form your predators makes for an interesting challenge. Plants, being the static organisms that they are, have risen to this challenge in some interesting ways. From thorns, to hairs, and even chemical warfare, there is no end to the strategies plants have evolved to discourage herbivores. However, these adaptations come at a cost. Whether its physical or chemical, defenses are costly to produce. That is why so many plant species have evolved induced defense mechanisms. They don't bother producing their chemical cocktails until they sense that herbivores are munching on their tissues. Whereas the literature is rife with examples of insect induced defenses in plants, few studies have ever investigated whether plants can sense mammalian herbivory.
A 2016 paper published in Functional Ecology suggests that some trees certainly can. By looking at the responses of two different tree species, sycamore maple (Acer pseudoplantanus) and European beech (Fagus sylvatica), the research team found that trees seem to be able to distinguish between being clipped and being browsed. This experiment focused on deer browse. Deer populations are at historically high densities. This has had severe ramifications on forest composition. Tree saplings are especially vulnerable, so much so that we are witnessing an alarming decrease in tree replacement over time.
The research team tested if trees could sense herbivory in a pretty ingenious way. They set out into the forest with pruners. Saplings were subjected to two different treatments - simple pruning and pruning followed by the addition of deer saliva. The team then took a molecular look at how each tree responded. What they found was quite startling. Trees that were subjected to pruning alone began producing a class of hormones called jasmonates. This was not surprising as jasmonates are involved in some generic plant defenses. The most interesting results came from the treatments in which deer saliva was added.
When saliva was added to pruned beech twigs, the researchers found that the trees increased their production of metabolites related to growth of buds and leaves. They also found that the addition of deer saliva caused an increase in the production of defense compounds, specifically tannins. Tannins bind to proteins in animal guts, making them harder to digest. In maples specifically, they also found an increase in certain types of flavanols, which have shown to have anti-herbivory properties in insects and humans, however, more work is needed to see if they do in fact deter other mammalian herbivores.
Although we still don't know what exactly the trees are responding to in deer saliva, these results nonetheless offer the first evidence of trees not only being able to perceive mammalian herbivores but also responding with an increase in defense compounds. Although they only looked at two tree species, it stands to reason that such responses are wide spread throughout many plant lineages. It also calls into question previous research that used simple pruning as a proxy for herbivory. Taken together, the picture of plants being unresponsive backdrops to more charismatic fauna is entirely erroneous. Plants are proving to be quite "aware" of their environment.
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