If you've been following along with the blog lately, you'll remember that we've been running a series on extreme soils. Here is what we've established so far:
1.) Water-logged soils inhibit root respiration in low oxygenated/anaerobic soils, increase nutrient dilution and can kill micro-flora and fauna.
2.) Soil temperatures over 90°F will kill roots! Dead roots kill trees.
3.) Desiccated (extremely dry) soils will kill roots. Again, dead roots kill trees.
Today, we want to discuss the idea of tree engineering. Even though we don't tend to think of trees as engineered structures, they are! Just like any other structure, healthy trees use good materials arranged in a way that utilizes strength and flexibility to create the best advantage. In fact, wood probably has some of the best engineering properties available. It's both strong and flexible, it's light and yet tough. Trees have to be able to able to stand up all day long (and all night, too!); they have to bend without breaking in severe wind and storms. A tree must be strong, but not brittle; it can't shatter if it's damaged. A tree has to do all of those things and not buckle under its own weight. What human-engineered structure can do all of those things as successfully as a tree? Buildings that come close attempt to follow a tree's example.
The interior of a tree is composed of tiny cells and tubes which form the tree's transport system. Tubes called "xylem" bring water and nutrients up from the roots. Tubes called "phloem" bring sugar (energy) and oxygen from the leaves down through the tree. In fact, about 90% of the tree's structure is composed of these tubes. The xylem and phloem also help contribute to the strength of the tree because they run in the same direction as the tree's fibers.
So: Trees are wood structures which are built of tubes. In this process there is flux and responses depending on conditions. Drought is most often seen as an agent of wilting, or “burnt leaves”. Concerning the structural risks caused by drought, Wilt shows that the plant is not manufacturing sugars. There is no transport up the xylem, and therefore there is no transport down the phloem. Leaves don't get the water they need to produce sugar; roots do not get the sugar and oxygen they need to live. So in dry periods, roots die due both to direct desiccation and starvation.
While there are natural annual ebbs and flows in root growth, a summer like ours is catastrophic for the root mass of trees and shrubs. This lost mass will impact the next year’s growth, but it can create near-term hazards if the tree is already weak, or had roots cut by construction. Often, when evergreens “flip” out of the ground, it is due to loss of root mass. The tops are the same size, but the “anchor” has become much smaller, and the ‘release”.
Diminished water stops the formation of wood in the tops of trees, too. Branches may have enough strength to grow leaves and twigs, but not the additional wood in the limbs. It there are wounds, rots or hollows, the tree can not add enough wood to balance the additional growth at the ends and the branches will break. Likewise, leader wood and trunk can be compromised in the short- and long-term by lack of water.
As with most situations in horticulture, the axiom, “an ounce of prevention is worth a pound of cure" is descriptive and directive of the drought scenario in the landscape. Proper care will produce vigorous plant can fight off environmental effects and not just survive, but thrive. Because a tree can look healthy but be structurally unstable, it is necessary to contact an arborist to have a structural assessment done. On healthy trees, this could be done as infrequently as once every 5-7 years. Trees that are currently undergoing care should have regular assessments included in their care plans.
Do you have questions? You can leave it in the comments or contact us!
Jeff Ling is a Registered Consulting Arborist and Co-Founder of TreeMasters, Inc., a full service arbor-care company, located in Fort Wayne, Indiana. It services tree owners with scientific tree management services throughout Indiana, southern Michigan and western Ohio.