Above- & UnderGround Partners: That is what urban trees are missing

by Wei Zhang, Tree Diaper & Dean Alberty, Deep Roots Microbiome

A third-grader can tell you that trees need water, sunlight, CO2, nutrients, and minerals. That probably is enough for a tree in the forest, but not enough for urban trees. What is missing from urban trees? Partners, above- and underground.

Almost all species we use in urban environments evolved in forests with their friends and family over millions of years. None of them, including those cultivated, evolved in forests of concrete and asphalt. The difference is obvious.

Young trees get shade from mature trees in the forests to protect them from the scorching sun. The leaf shade from mature trees protects the young trees over the winter and leaf litter also makes great compost. These are the above-ground partners. Equally important are the underground partners in the root and soil microbiome. It is critical for the survival of saplings and for them to thrive. In the natural forest, it is easily passed on from the mother trees to the offspring.

How do we build the modern urban environment? We clear all the trees and sell them for lumber, strip off the topsoil, compact this soil with heavy machinery, and pour concrete and asphalt on top of it before planting some saplings in the tree pits dug into the compacted soil. After that, we spray tons of herbicides and pesticides on the soil. Many trees have their rootballs dipped into insecticide baths when crossing state borders. We apply truckloads of deicing salt on roads during wintry weather conditions. After all the above processes, how many of the beneficial microorganisms are still there? Even if we have a few survivors, is there any food (organic matter), oxygen, or moisture for them to survive and thrive?

Below we use three cases to illustrate why these aboveground and under-ground partners are so important.

Case #1, the Big Willow Oak Brother: This case was discovered in Ashland, VA at the intersection of England St. and N. Washington Hwy. 4. Willow Oak trees with about 8” caliper were planted beside a bank building in 2016. Every one of these trees has diebacks. Two were removed by April 2023. It won’t be very long before someone removes the third tree. We did notice the 4th tree is doing fine. For 7 years, it has had minimal dieback and we believe it will survive. What is the difference? The 4th tree has a big brother next to it.

Case #2 Proper Soil Moisture and Soil Microbiome Makes the Difference: We all know that soil moisture is crucial for the survival of urban trees. Promoting a healthy soil microbiome is also very important to the survival of urban trees. Let’s delve into the importance of these factors and their relationship to tree well-being.

Adequate moisture in the soil is necessary for the uptake of water through the tree’s root system. It serves as a transport medium for essential nutrients, ensuring their availability to the tree. Insufficient moisture can lead to water stress, which weakens the tree, makes it more susceptible to diseases and pests, and hampers its ability to photosynthesize effectively. Conversely, excessive soil moisture can lead to root rot and oxygen deprivation, causing the tree to decline or even die. Maintaining a proper balance of soil moisture is vital for optimal tree health.

The soil microbiome plays a significant role in supporting tree health. Beneficial microorganisms, such as bacteria and fungi, contribute to soil structure by creating aggregates that improve waterholding capacity. They also assist in the decomposition of organic matter, enhancing soil aeration and preventing waterlogging. At the same time, proper soil moisture boosts the population of these microorganisms and keeps the bad ones in check. Additionally, certain microorganisms form symbiotic relationships with tree roots, facilitating the absorption of water and nutrients. For example, the mycorrhizal associations extend the root system’s reach, enabling trees to access moisture in a larger soil volume. Below is a picture taken from our backyard where soil moisture was kept at an optimum level and the microbial community was maintained at a high level.

Case #3 Earthworms are valuable allies for urban trees: Their activities improve soil structure and soil aeration, enhance nutrient cycling and soil fertility, regulate soil moisture, suppress diseases, and promote biodiversity, especially soil microbiome. Healthy earthworms require organic matter as food and proper soil moisture. Their soft and permeable skin needs to stay moist for efficient gas exchange. Soil moisture influences the microbial com- munities present in the soil, and these microorganisms play a significant role in earthworm ecology. Moist soil conditions promote the activity and growth of beneficial soil microbes, such as bacteria and fungi, which contribute to nutrient cycling and provide a food source for earthworms. During the 2021 Central Virginia drought, we visited a few sites where we have TreeDiaper treemats installed. We were astonished by the number of earthworms under these treemats. See the whole video on youtube: https://youtu.be/cN-vH0zs0Uo.

At the same time, the soil microbiome with healthy and active earthworms is significantly improved. See the picture below for a site where bacillus bacteria mycorrhizae were carried around by earthworms to areas that are far from the area we applied the soil microbials.

Wei Zhang holds a Ph.D. degree in Engineering from Virginia Tech. He has been doing research & development for the green industry for 15 years. His research areas include watering solutions for landscape plants, relationship of soil/ water/air, plant protection against stresses like drought and road salt. His work also provides novel solutions to green stormwater infrastructures, for which plants are used for the treatment of pollutants in stormwater.

Dean is the Senior Chair of the Missouri Community Forestry Council. The Chair position helps set the course of our council and promotes growth and development for our organization. The Council covers the State of Missouri Urban Forestry professionals and stretches to neighboring states that benefit from the extensive organization and growth of the Missouri Council. In addition, Dean is involved in growing better trees and helping people grow better trees as CEO of Deep Roots Microbiome Company from the years of experience improving tree growth and production.