By Carrie Black, Urban Forestry Community Service Aide II
Trees - like all other living things - need food, water, and nutrients to survive. Trees make their own food through photosynthesis, using energy from sunlight, water (from the roots), and carbon dioxide (from the air) to create sugar that is used to fuel the rest of the tree. Water is carried from the roots to the leaves through xylem cells. Sugar is transported back to the rest of the tree through phloem cells (see diagram below). Sugar is converted into starch, and can be stored in the trunk or the roots, or used for immediate growth.
Cross section diagram of a tree trunk.
The amount of photosynthesis that can occur in the leaves is directly related to the amount of water that the roots can absorb. A small root system can only support the photosynthesis of a small canopy, and roots can only grow larger if they have enough energy (sugar) provided by the leaves. In this way, the tree must balance its above ground and below ground growth. Topping a tree will stunt its growth by limiting the amount of photosynthesis that can occur in the leaves. Likewise, cutting roots or restricting the roots’ growing space will limit the amount of water that can be transported to the leaves for photosynthesis.
Topping a tree can limit its growth.
What happens after photosynthesis?
It is well known that trees act as carbon sinks, taking in carbon dioxide from the air during photosynthesis and releasing the oxygen that humans breathe. While trees use carbon dioxide to make their own food, they actually need oxygen (much like humans do) to process that food into energy.
In order to use stored starch for growth, trees must convert the sugars back into energy through a process called respiration. Respiration requires oxygen. During respiration, sugar and oxygen are combined to produce energy, with water and carbon dioxide created as byproducts. The energy that is released can then be used to make new tissues. Humans do the same thing when they process stored sugars. While trees take in oxygen from their surroundings, humans breathe it in with their lungs. Just as a person who is exercising needs to breathe deeply, a tree that is actively growing needs an immediate source of oxygen.
Roots need oxygen, too!
Most tree growth occurs at the tips of branches and the tips of roots. However, while the crown of a tree is usually surrounded by open air, roots need a source of oxygen in the soil in order to grow. In the ground, air and water are held in little pockets called soil pores. If the soil is dense and compacted (with no soil pores), there will not be enough oxygen available for respiration. Too much water in the soil will also limit the amount of oxygen the roots can take in.
Tree roots grow best when they have sufficient growing space and well-drained soil with enough oxygen and water (but not too much water). The depth that oxygen can reach depends on the type of soil and amount of compaction, and the most oxygen will be found near the surface of the soil. For this reason, roots tend to grow right under the surface.
What does this mean for the structure of a root system?
Many people imagine tree roots as a mirror image of the branches, but this is a common misconception. Tree roots actually grow outward horizontally from the base of the tree (picture a wine glass sitting on a dinner plate).
This wine glass sitting on a plate represents the basic shape of a tree and its roots.
Roots become smaller as they grow outward from the root collar (base of the trunk). Buttress roots right near the trunk help stabilize the tree. In the first 3-6 feet, woody lateral roots spread out and taper down to 2-4 inches in diameter. Small, ephemeral absorbing roots grow out of lateral roots, soaking up water and oxygen and other nutrients. These roots are opportunistic – they will grow wherever the conditions are suitable, and will die back if resources are not available.
Top, exposed roots of this tree growing on a hill side show their horizontal structure. Bottom, crowded roots are girdling this linden tree.
In the context of urban trees, infrastructure can severely limit the growing space available to the roots. Stay tuned for Part 2 of this blog post – Trees and Sidewalks. We will discuss how sidewalks impact tree health, and give alternatives to traditional sidewalks that could prevent tree/infrastructure conflicts.
UC Davis, Fruit & Nut Research & Information Center. No date. Photosynthesis and respiration. http://fruitandnuteducation.ucdavis.edu/generaltopics/Tree_Growth_Structure/Photosynthesis_Respiration/
Urban, James. 2008. Up by roots: Healthy soils and trees in the built environment. Champaign, IL: International Society of Arboriculture.