The Fascinating Defense Mechanism of Holly Trees Against Deer Browsing

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Image credit: Steve Slater/Leif Bersweden

The interesting way that Holly Trees protect themselves from deer browsing

People love Holly trees (Ilex aquifolium) because they have beautiful evergreen leaves and bright red berries. This makes them a popular choice for gardening and landscaping. These trees are also known for being deer food; the deer love to nibble on the soft leaves of the lower branches.

Holly trees have an interesting way of keeping hungry deer away that has evolved over time. Deer like to nibble on holly trees’ leaves. To protect itself, the tree grows spiky, prickly leaves on the lower branches that the deer don’t like as much. This lets the tree keep growing and doing well even though there are always herbivores around.

The sharp leaves that holly trees make when deer eat them are called “bristles,” and they are much tougher and more rigid than the smooth, glossy leaves that holly trees normally make. Additionally, the hairs are not as healthy as the leaves, so deer are not as likely to eat them.

Image credit: Steve Slater/Leif Bersweden

That being said, holly trees don’t just grow bristles when deer eat them. This is also a way for them to protect themselves from other threats, like bugs and diseases. When pests or diseases attack a holly tree, it may grow bristles to stop the damage from getting worse.

Holly trees make spines, which is an interesting example of how plants have changed over time to fit their surroundings. Holly trees protect themselves from deer eating by making spiky leaves. They can keep growing and reproducing even though they are constantly being eaten by herbivores.

Pairs of prickly and nonprickly leaves borne on contiguous nodal positions of the same branchlet for four of the heterophyllous Ilex aquifolium trees sampled for comparative DNA methylation analyses. Image credit: Carlos M. Herrera

Holly (Ilex aquifolium) knows that deer are nibbling on its leaves if it turns on genes that make the new leaves spikey. The upper leaves of Holly trees, which are out of reach, have smooth edges, while the lower leaves are sharp.

If Holly (Ilex aquifolium) finds its leaves are being nibbled by deer, it switches genes on to make them spiky when they regrow. So on taller Holly trees, the upper leaves (which are out of reach) have smooth edges, while the lower leaves are prickly

This is a unique and effective way for deer to protect themselves. If you have a holly tree in your yard and notice that the lower branches are covered in spiky leaves, you can thank them. What about deer? If you’re hungry, you might want to think twice about nibbling on a holly tree!

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