How the bottom half of your tree is affecting the top

I spend most days talking to people about tree issues. Easy ones are when I can point to a big, hairy bug or mushroom and say “there ya go – that’s the reason your tree is declining”. Unfortunately this is rarely the case and if there is something present it’s only been able to cause harm because the tree is suffering from another issue. As homeowners, we spend a lot of time looking at about 60% of our trees’ mass and zero time looking at what’s going on below ground. There’s an old adage that says “pay attention to what comes between you and the ground – don’t skimp on shoes, tires, or mattresses,” and the same should be said for the roots of your trees. In order to thrive, a tree needs to have the following items available to it: oxygen, water, nutrients, and light. With the exception of light, all of these have to be available below the soil surface.

Let’s think for a moment about a tree in the forest. It’s cool and quiet below the canopy and the sun barely reaches the forest floor. The soil below your feet is soft and covered with what remains of last year’s leaves. Contrast this image with a tree in the middle of a typical suburban yard. It’s warmer under that tree and the grass below your feet is pinned between you and a hard soil surface. Last year’s leaves are no longer here; they have been collected or carried away by the wind. The surface of the ground here is covered with well-manicured grass rather than with deep leaf litter.

Why are the leaves on the forest floor important? Those leaves (along with fallen trees and broken limbs) are in the process of breaking down and returning many of the nutrients collected by the tree back to the soil. The give-and-take between the soil and the tree creates a balance of nutrients in and out of the soil with trees taking nitrogen up from the roots and returning it to the soil each fall. Without this process trees in an urban environment can quickly exhaust the available nutrients and suffer from slow growth, poor fall color, discoloration, early leaf drop, and even death. In addition to this drop in available nutrients being returned to the soil, add the blanket of grass now covering the area where only shrubs and other small trees were in the woods. The competition for resources in the lawn is extreme with grass winning the lion’s share of both water and nutrients. If we want to help the trees in our yard we have to do two things, mulch (to reduce the competition in as much of the space as we can) and fertilize to replenish the nutrients we are raking up each fall.

So what about the texture of the soil? The ground in the yard is firm because it’s been compacted during construction of the home followed by countless soccer games and backyard barbeques. Compaction is often overlooked when discussing the well-being of trees, but it is often the cause of slow growth and early death. At some point your yard was forest, a prairie, or a farm, but when construction began the topsoil was pushed aside or carried away to make nice level lots to build on. Along with topsoil we lost many of the nutrients required for healthy tree growth and exposed a soil surface that had long been buried and nearly void of life. Often we are left with heavy clay that is easily compacted. These compacted soils make growth of new roots difficult for trees and in time can stop root growth completely. If you are concerned about the structure of your soil, a reputable arborist can perform a bulk density analysis to see if this is an issue. There are very effective treatments for this condition developed by the team of researchers at the Bartlett Tree Research Laboratories. I know that was a shameless pug, but I have seen the benefits of changing soil structure and it’s one of the best things available to reinvigorate a tree that’s been declining. Softer soils allow more root growth, and those roots can take up more water and nutrients, giving the tree what it needs to grow and be healthy.

As you might imagine, I explain these issues on a regular basis to people concerned about their trees. The number one response I get is “this tree has been here for 20 years, why are we only seeing a problem now”? This is actually a great question with a fairly simple answer: we’re not. Usually the tree has been struggling for a long time before we see signs severe enough to get our attention. As trees age they become less resilient and more susceptible to damage from these issues. Decline from poor soil conditions can start long before the natural life span of a tree is over; this is why it’s important to talk to an arborist as soon as you see a change in the tree.

Mature trees are a treasure, and by paying attention to what’s happening below ground we can help them continue to be part of our lives and those who come after us. Thank you taking the time to read this blog and please always consult an ISA-certified arborist for the health of your trees.

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