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Liriope's Muse: The Toxic Relationship Between Distressed Trees and Grass
A common concern amongst our customers is why grass does not grow beneath their trees. Often times they notice their grasses growing in patchy, weak, or just not at all around the trunk and under the canopy of their tree(s). In short, this is due to the tree’s allelopathy, a natural biochemical defense system that helps ward off pests, fight diseases, and suppress competing vegetation. It works by reducing nutrient availability in the soil and by affecting the germination, growth, survival, and reproduction of nearby plants.
Before we dive in, I highly recommend reading my previous post on allelopathy and its relationship to grasses and other plants. It will give you the background needed for the more in-depth discussion to come in this blog:
Lirope’s Muse: The Toxic Relationship Between Trees and Grass
My previous blog discussed the allelopathic relationship between healthy trees and grasses, in this blog we will discuss the relationship between distressed trees and their surrounding grasses and vegetation.
What causes a tree to become distressed?
There are many factors that may cause a tree to become distressed. The most common reasons that we come across in the field are: disease, over-pruning, improper application of mulch(volcano mulching), under-watering, improper planting, mechanical damage (like that caused by construction), root compaction, root damage, pest infestation, and chemical/herbicide damage. All of these stressors trigger the tree into a fight or flight like response, however the tree can’t move or leave the situation, all it can do is defend itself. So, it begins releasing stronger concentrations of allelopathic chemicals in attempt to ward off the pests, compartmentalize a wound, prevent pests from entering a wound, and source more water and nutrients it may be lacking. As a result, these potent biochemicals naturally are exuded into the surrounding soils, poisoning and choking out the existing vegetation, while also making it impossible for new grasses to sprout.
*The image above is from The Wild Nature Institue, it shows a giraffe browsing on a tree and if you look closely at the branches of the tree, you can even see the little ant colonies presents in the bublbs just beneath the thorns*

Giraffe and Acacia Tree Example
Trees aren’t entirely defenseless, and the Acacia tree is a great example of this. They not only can defend themselves chemically, but they have sharp thorns and spines of varying lengths oppositely arranged across the branches and limbs to protects the leaves and blooms from relentless herbivores… like the giraffe.
They also commonly host symbiotic ant colonies that have taken up residence in the swollen bases of the thorns. These ants help provide additional defense against menacing herbivores as they rush out to attack anything disturbing (eating) the leaves or branches of the tree.
Giraffes are known to eat more than 100 different kinds of plants, but the acacia tree is a favorite because its leaves and blooms are rich in protein, water, and calcium, all of which are especially valuable in the harsh savannah environment. It is essentially a superfood, and over generations, giraffes have adapted to be able to eat it in fascinating ways.
According to Sue, a Biology Professor, Giraffes have prehensile lips which can grasp and manipulate branches and leaves, their lips have developed a fine layer of hairs to help protect the lips from thorns. In addition, giraffes have exceptionally long tongues, up to 20 inches in some individuals, and the tongue is very muscular and flexible, which helps the animal pull leaves away from the branch. The inside of the giraffe’s mouth and its tongue are covered with hardened, bumpy skin, which protects them from being punctured by thorns and the attack of ants. Further, giraffe saliva is thick and sticky, which also helps coat the mouth and the digestive tract from accidental puncture by thorns.
So, it seems that giraffes have successfully countered the acacia tree’s spiny anti-herbivory defenses through their fascinating anatomical adaptations. What else is the tree left to do to defend itself? Use its allelochemicals, of course!
When an Acacia tree senses itself being eaten on, it launches into action and begins releasing biochemicals in the form of tannins into the leaves. The tannins are polyphenolic molecules that bind to proteins and make them unavailable for digestion, and their astringent, bitter taste makes the plant material unpalatable to the giraffes.
Some Acacia trees even release Hydrogen Cyanide compounds (Yes, like the poison!), not enough to kill the giraffe, but definitely enough to give them a stomachache and make them think twice about eating their leaves.
The longer the giraffes feed, the more allelopathic chemicals are released and after just 10 minutes the whole tree becomes unpalatable and the giraffe is forced to move on. But the Acacia’s have become intelligent, they know if they become toxic they’d just move to their neighboring tree and begin feeding there so they’ve learned to communicate. Yes, communicate you read that right! Plants that have been browsed, or eaten on, release a molecule called ethylene (the same molecule that is released by ripening fruit). Ethylene is carried through the air to neighboring plants and triggers the release of tannins in the plants it reaches. Chemically signaling to them that a group of giraffes is on the prowl and may come feed on them! In just a matter of minutes, all of the acacia trees for miles can become inedible to the giraffes… and they’ve picked up on this! They’ve found a way to outsmart the trees, giraffes will travel upwind once a tree has warded them off because they’ve “caught wind” on the trees’ strategy! It won’t be long, however, until the trees outsmart the giraffes yet again.
Something I found interesting upon my research is that the more giraffes feed on the tree, the more that the tree produces nectar for the ants that it hosts, so that they can continue to grow in numbers and protect the tree. But here’s the twist, if the giraffes go extinct or aren’t around to feed on the acacia trees, the ant numbers would begin to dwindle, more aggressive ants and beetles would be able to move in and take up residence in the tree and instead of being symbiotic they’d be predatorial and acacia trees all around would begin to suffer and possibly even die off! So essentially, if giraffes didn’t eat on acacia trees they’d probably go extinct or severely dwindle in numbers!
The Toxic Relationship Between Distressed Trees and Grass
The most common stressor that we come across affecting trees is over-pruning and regular or unnecessary pruning. Just like in the example of the Acacia trees above, their allelopathic response is a result of a pruning-like event. They lose their leaves and branches in large numbers due to the giraffes browsing, and in response, they release their tannins.
The response is very similar to when trees in urban landscapes are pruned on, the only difference is that they can’t defend themselves chemically against humans and ward them off with a bitter taste. Still, they release concentrated amounts of allelochemicals (the exact type varies by tree species)that travel throughout the tree and are released into the surrounding soils by their roots. This concentration of biochemecals in the soil make for a very inhospitable environment for other vegetation, especially grasses, to grow and germinate. Thus, leaving patchy, thin, or no grasses at all beneath the tree right around the trees CRZ (just under the canopy)
So, if you notice that you can't seem to keep grass under your trees, despite seeding and proper irrigation… Its’s time to rethink your pruning schedule because there is a chance you are stressing out your trees !
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