The Magic of Mycorrhizae

Trees are amazing organisms that provide food, shelter and other resources for a variety of living things, including everything from mistletoe to humans. But while it is important to recognize the contributions trees provide to the rest of the planet, it is equally important to recognize the ways other species actually help trees to survive.

Many times, these relationships are symbiotic, or mutually beneficial. Some such relationships are obvious and visible — squirrels and jays, for example, help spread acorns throughout the landscape as part of their feeding and caching behaviors, which helps the trees colonize new areas and perpetuate the species. However, other symbiotic relationships occur out of plain sight.

Microorganisms in the Soil

Although soil appears to be little more than a mixture of dirt, rocks and bits of dead leaves, it is actually a thriving ecosystem, containing an unthinkable population of microorganisms. Some of these microorganisms are beneficial to trees, while others are harmful. Still others fail to interact with trees in any meaningful way.

One of the most important groups of microorganisms that interact with trees are beneficial fungi that live on and inside their roots. These fungi – called mycorrhizal fungi – play important roles in the biology of the trees. Mycorrhizae are fairly ubiquitous in healthy, undisturbed soils and they naturally colonize the roots growing in the area. However, urban soils, which often suffer from compaction are often deficient in these beneficial fungi, which can cause local trees to struggle.

The Mycorrhizal Connection

Mycorrhizal fungi and tree roots form a very tight-knit partnership. In fact, they are often referred to as a single entity: mycorrhizal roots, thanks to their strong bond and blurry boundaries. There are two basic ways in which the fungi interact with the roots: Many mycorrhizae coat the find absorbing roots of plants and trees, forming a type of living shield around the roots. However, some are actually able to penetrate the roots of trees, colonizing the spaces between the individual cells.

The Effects of Mycorrhizae

Mycorrhizal fungi help support tree health in a number of important ways. Some of the most important include:

  • Mycorrhizae help tree roots to absorb more water from the surrounding soil, via the increased surface area they provide to the roots. This not only makes the trees more efficient, it makes them better prepared to survive droughts and reduces their supplemental irrigation needs.
  • Mycorrhizae also helps to absorb more of the life-sustaining nutrients necessary from the soil. This means that trees with mycorrhizal roots require fewer fertilizers and soil amendments to remain healthy.
  • Mycorrhizae help protect tree roots from pathogens and harmful fungi. This is accomplished by helping to support the tree’s health (which naturally increases its ability to fight off pathogens), partially shielding the roots from contact with the pathogens, and by out-competing many of these harmful microorganisms for the resources they need.
  • Mycorrhizae help plants survive transplanting efforts, as they improve the tree’s ability to absorb water and nutrients from the new soil, which helps them to acclimate to their new location more quickly.


If you believe that your trees may be struggling in the often-sterile soils that characterize urban areas, give your favorite local arborists a call! We’d love to help analyze the soil situation of your property and make suggestions to improve it, so that your trees can reach their full potential.