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Home & Garden: Causes of tree limb failure

Curt Swift
CURT’S CORNER
Free Press Gardening Columnist

Quite often when I watch The Weather Channel, it shows trees downed by winds wreaking havoc in different parts of the country. I find the same downed and damaged trees when I drive around western Colorado after a storm.

In many cases you can tell the cause for the failure from the internal rot and discoloration in the broken limb. In other cases, however, one needs to look at other parts of the tree to identify the reason for the damage. Internal rot is not always the reason branches break.

Trees are often ignored the first few years after planting, yet these are the years when trees need to receive serious training — to properly space branches, correct codominant stems, remove bad branch attachments, and control disease, insect, and mite problems. How many times have you seen trees break at a weak branch attachment that should have been corrected in its earlier years? I realize there are wind speeds that will destroy pretty much any tree, but we don’t typically experience hurricane strength winds in this area.

Some forest trees shed branches naturally as they age, especially when the branch is shaded by other trees. Trees in our urban forest are seldom prone to this shading effect unless they have not been properly pruned or have been planted too close to each other. Urban trees that experience limb breakage typically are reacting to the lack of inadequate training in the first 10 years of their lives.

Pruning to thin out excessive branches so the remaining branches receive adequate light is critical for the development of strong branches. Adequate light is necessary for the formation of buds and the development of leaves. Without an adequate supply of functional leaves on the branch, the formation of wood fibers (xylem) necessary to strengthen the branch is limited. The carbohydrates produced by leaves through photosynthesis are also necessary for the production of disease and insect protective compounds (chemicals) the tree needs to protect itself from these invaders.

Most arborists understand they cannot remove more than one-third of the leaf-bearing tissue on a young tree without causing serious damage to the tree’s health. With mature trees, removing more than 25 percent of these twigs, limbs, and branches can kill the tree. Excessive pruning results in the death of roots due to a lack of the carbs they require for survival and root development. Tree death due to insufficient root growth is not instant, but takes years for most trees. During these years of decline the tree dips into its store of carbohydrates. As carb levels drop the tree is no longer able to produce the protective chemical substances it needs to prevent disease infections and attack by insects and mites. The lack of these protectants leads to further branch and tree failure. During these years of decline, limb breakage is more likely to occur.

The death of nutrient and water absorbing roots is a natural cycle of life. Depending on the species of tree, these fine roots may survive for mere minutes or last for weeks even under the best of conditions. In order for the tree to continue to absorb water and nutrients, new roots need to be created to replace dead and dying roots. This root initiation and development is dependent upon a specific level of stored carbohydrates. Excessive pruning reduces carbohydrate production to the point root initiation and development is severely limited or completely stops. This lack of root growth directly impacts photosynthesis and carbohydrate production. After all, the fertilizer elements and water necessary for photosynthesis move from young roots to the leaves. The lack of young roots due to excessive pruning consequently results in a decline in photosynthesis, causing a “Catch 22” problem from which the tree cannot escape. Root damage leads to reduced photosynthesis, which results in additional root death … and on and on until the tree dies.

New roots do not form from the epithelial cells covering the root surface, but originate from inside the root pushing their way through the root tissue until they break through into the soil. This process requires energy, which comes from carbohydrates. The epithelial tissue previously serving as a protective layer against invasion by root-rot organisms is now broken due to new roots. Even if adequate levels of carbs are available for root initiation, there may be inadequate carbs available to form the chemical compounds the root needs to protect the wound from attack by root pathogens. In addition to these chemical protectants, mycorrhizal-forming fungi provide additional protection. In order for these fungi to be effective, they obtain carbohydrates from the root. Without carbohydrates, mycorrhizal-forming fungi are not effective. Due to the lack of the tree’s chemical protectants and mycorrhizal-forming fungi, fungal and bacterial root-rot organisms can easily gain entry through the wounds from which the new roots emerged causing additional root death.

GJ Free Press columnist Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at Curtis.Swift@alumni.colostate.edu, 970-778-7866 or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.


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