Trees and Water: A Critical Relationship

I pride myself on being a knowledgeable Arborist, always a student, staying current and being as ethical and honest as I can be. So I told myself, let’s get your dry-erase board out and start jotting down everything you know about water and trees (Figure 2). Wow, there is a lot! Then it came down to what to focus on for the article for TCI Magazine readers. I decided I truly wanted to emphasize the incredible role water plays for our planet and our lives, yes, but also for our trees.

Water is an inorganic, transparent, tasteless, odorless and nearly colourless chemical substance. It is vital for all forms of life, even though it provides no calories or organic nutrients. Its chemical formulation is H2O, meaning that each of its molecules contains two hydrogen atoms and one oxygen atom held together with a covalent bond. And just in case you were wondering, the scientific name for water is dihydrogen oxide or dihydrogen monoxide.

Figure 1: Water’s chemical formula is H2O, meaning that each of its molecules contains two hydrogen atoms and one oxygen atom held together with a covalent bond. Graphic courtesy of Pearson Education, Inc.

Next, before we get into all the incredible things water is responsible for, we need to better understand exactly what a tree is. By just about all definitions, a tree is a long-lived perennial plant with a woody stem. For this article, let’s focus on the living part of that definition. Again, a tree is a living organism. And, as previously stated, water is vital for all living forms of life, including trees.So, what does water do for trees? For starters, trees are about 50% water, and they need a consistent supply of it to be healthy and to grow. Water plays a key role in the movement of nutrients and is critical for many internal processes, including photosynthesis. Trees also use water to stay cool through the process of transpiration.

As we move forward with trees and water, we have to cover one of the most important topics on the availability of water in urban forestry, and that is soil. I will be brief, but the water doesn’t just magically jump into the trees. While it’s kind of like magic, trees rely on osmosis. This is the transfer of water molecules through a semi-permeable membrane from a low-concentration solute to a higher-
concentration solute, equalizing the concentrations. This process takes place in the root zone by the root hairs, and many factors affect the relationship between the root hairs and the soil. While there is enough material on that subject for a different article, let’s consider a few important ones just to refresh our memory – soil type, amount of organic matter, compaction, salts and hydrophobic conditions are a few factors that affect soil-water availability.

After we’ve examined and treated any existing soil conditions and validated that the soil is picture perfect and the root hairs are doing what root hairs do, what’s next?

Osmosis is the transfer of water molecules through a semi-permeable membrane from a low-concentration solute to a higher-concentration solute, equalizing the concentrations. This process takes place in the root zone by the root hairs. Graphic courtesy of www.eschooltoday.com.Walking a property and identifying the tree and plant species and determining overall plant health would be the next step in working on a water-management plan. Most plants, trees and turf can coexist, but each will need to have specific requirements met. Some of those basic requirements will be healthy soil, proper nutrition, being in the right spot – as in sun or shade – and the correct amount of water. This is where it may start to get difficult. So, what do we do?

One plan, and an option frequently used, is to do nothing. Let Mother Nature have it. The fact is, trees have been around for more than 370 million years and, for the most part, they’ve survived without our help. Or have they? Seeing trees thrive in forests and other natural areas gives the impression that trees don’t need help. But there are critical differences between the environments of forest trees versus those planted in urban or suburban landscapes.Figure 2: The author’s meticulous and concise notes. Courtesy of Joe Aiken.

In wooded areas, decomposing leaves and other organic matter provide trees with a constant source of nutrients. That same organic matter also improves water retention and availability. This natural process is disrupted in urban environments, where we rake leaves and remove other decaying plant material. Furthermore, soil’s organic matter is often removed or compacted during the construction process, and vast areas of concrete or grass surrounding trees can limit root development and hinder the uptake of water, oxygen and nutrients.

The other plan is to get involved. Figure out a program, because you were called by a client who has a problem, and they love their trees. The practice of arboriculture goes way back, but I feel the beginning of modern arboriculture started with John Davey and his book, “The Tree Doctor.” Even in 1902, Mr Davey covered the value of water and trees with the invention of a tool called the rain machine. This was designed to provide the tree with the proper amount of water.

Water by any other name … Graphic courtesy of waterandsociety/wordpress.com.

Times have changed since 1902, but the fact is, trees still need sun, quality air, quality soil and, yes again, water to survive. Spring forward to today, with more people, more landscapes and more and more water restrictions. So, depending where geographically you serve trees, water may be more important, and you will need to be more strategic in your approach to urban hydrology.

In the last decade, from my understanding, the methodology used more frequently with urban trees is irrigation. In some instances, we are finding the installation of retention ponds, both above and below ground.

Finally, we need to discuss the value of soil amendments. Proper tree watering starts with the soil, and applying compounds to the soil that help retain water is proving to be extremely valuable. These include products such as biochar, humate, organic matter and, more recently, hygroscopic humectants. Hygroscopic humectants are materials that attract water vapor from the atmosphere within the soil, condense it back into a liquid form and retain the liquid for the plant to absorb (sportsfieldmanagementonline.com). The science behind these humectants is really amazing. This technology has been used for years in turf and agriculture, and its time for arboriculture has arrived. There are a lot of data and articles to be discovered with just a few clicks on the computer, or give me a call and I can point you in the right direction.

I want to reiterate the important role water plays in life – not only in our lives but also in the lives of the trees we are hired to protect.

Joe Aiken is senior regional technical manager with Arborjet, a 19-year TCIA Corporate Member company based in Woburn, Massachusetts. He lives and works in Michigan.