Shade Tree Decline

Shade tree decline, or shade tree disorder, is a term used to describe branch and twig dieback in the crown of a tree. These trees are not being affected by pests or disease, per se, but they are very unhappy trees.


Before we begin, go outside and take a quick look at the trees. Go ahead. I’ll wait.

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Canopy of healthy sweetgum tree (Kate Russell)

Symptom of shade tree decline (Kate Russell)

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Okay, so, what did you see? Of course, it depends on the season. In winter, most deciduous trees will be bare naked. By late spring and early summer, healthy trees should be covered with leaves, a thick blanket of sugar producing machinery! Every leaf surface should be cranking out energy as fast as it can. What you probably saw, especially if you are in California, is sort of healthy-looking trees that were a bit thin on top. This is not a hormonal problem that can be treated with hair tonic. This is serious business and those trees are dying.


Drought (or lack of proper irrigation) is the number one cause of shade tree decline, but there are other causes, and it is not only seen in ornamentals. Canada’s sugar maple trees have been exhibiting shade tree decline for decades. In that case, the decline is due to air pollution blowing northward along our Eastern seaboard. [My sincere apologies, Canada.] There are many other causes (and few solutions) to the problem of shade tree decline.


Symptoms of shade tree decline

Symptoms of shade tree decline appear gradually and, like the frog in the pot, we often don’t even notice until severe damage is done. Initial signs of shade tree decline include:

• reduced new growth
• die off of new twig and leaf buds
• leaves are smaller and fewer
• premature fall color change 
• early leaf drop

     
     Later symptoms include:

• thinning of the crown
• excessive seed production
• dieback of larger limbs and branches
• increased sucker growth

These are all signs that the tree knows it is dying and is trying to put what little energy it has remaining into the next generation. Most trees affected by shade tree decline die within a few years.


What causes shade tree decline?

Imagine getting pummeled all day, every day. Professional boxers do it, but they have strict nutritional support, medical teams, and proper hydration. Our trees have been getting pummeled by drought, pollution, and injury for many years. In addition to drought and pollution, the following injuries can contribute to shade tree decline:

• extensive bark damage from lawn mowers and weed wackers
• herbicides and other chemical damage
• soil infertility
• soil compaction
• extreme temperature changes
• flooding or changes in the water table
• root damage due to rototilling and digging
• girdling by tree supports left in place too long
• sprinkler misplacement (never let the spray hit tree trunks)

The down side of shade tree decline

Trees are a big investment, financially, and they take time to grow. They also prevent erosion, provide shade (and food), store carbon, create the oxygen we breath and filter pollutants out of the air. They provide habitat and food for many other living things, promoting biodiversity. As a tree begins to suffer shade tree decline, it becomes less able to defend itself against pests and diseases, and it is more likely to suffer sunburn damage, providing easy entry to borers and other insect pests.


The bigger problem

The Carnegie Institute for Science reports that California has lost an estimated 66 million trees in the last five years. Even if heavy rains appear, they say that won’t be enough to prevent the future loss of “tens of millions of trees” in California alone. They are using a type of spectrometer that analyzes the molecular composition of trees as they fly overhead. This amazing equipment can tell when a tree has lost too much water to recover. These dead and dying trees now harbor pests and diseases in epidemic proportions. This large-scale die off also makes it easier for invasives to get established, and they pose a major fire hazard. 


Preventing shade tree decline

Prevention is much easier, and more likely to succeed, than treatment. If you have trees, such as birch, willow, or Japanese maple, that evolved to live in the shaded understory, or near a creek or stream, you will have to recreate those conditions to prevent shade tree decline. You can do this by:

• pruning to keep the tree in the shade of a more heat tolerant tree
• coiling a soaker hose in a spiral around the tree trunk, starting a few inches away from the base and ending up out by the drip line, and watering deeply at regular intervals
• fertilize regularly and not excessively
• mulch under trees, keeping the mulch 6-8 inches away from the trunk
• avoid overspray of herbicides and other chemicals
• install trees a safe distance from concrete surfaces that heat the soil
• plant trees in soil that suits them (pH)
• remove tree supports when they are no longer needed
• install footpaths around trees to reduce soil compaction

Treating shade tree decline

Prospects for an affected tree are not good. The damage has already been done and even the most diligent care may not be enough. If you are determined to try, these actions may save your tree, or help it to survive a little longer:

1. fertilize the tree in spring
2. prune out all dead branches and twigs
3. mulch 
4. provide extra water throughout the growing season, irrigating properly

How to irrigate trees

Most trees have the majority of their roots in the top 12 to 18 inches of soil. When these roots get too dried out for too long, they die. This reduces the amount of water available to the tree for life processes and cooling (and growing new roots). While it ‘feels’ right to water a little bit every day, this actually encourages shallow root development. Deeper roots are better protected from the elements. Properly irrigating trees means getting the water to a depth of at least 5 to 7 inches every 3 or 4 days, during drought. Before we knew better, we used to plant trees in bowl-shaped depressions that would then be filled with water. Sounds good, but it’s a bad idea. This practice leads to crown rot and several other fungal diseases. Better methods include soaker hoses and irrigation rings. Irrigation rings can be mechanical devices, or they can simply be trenches dug around trees at the drip line. Fill the trench every few days during the peak of summer and your trees will thank you.


Selecting the right trees

If it is too late to save your tree, invest in a reputable arborist for professional removal. Trees can be deadly. Seriously. Then, use the following information to select trees best suited to your microclimate (and be sure to irrigate them properly): 

• plant trees where they can reach full size (big trees in small spaces are rarely healthy)
• avoid planting acid-loving trees, such as beech, dogwood, magnolia, pin oak, and willow oak in California’s alkaline soil
• avoid planting trees that are sensitive to air pollution, such as larch, sugar maple, white ask, white, pine, yellow poplar), installing more tolerant trees (red cedar, sycamore, sweetgum, bald cypress, or pin oak) instead 
• plant trees where they will get the amount of sunlight they have evolved to use

Fighting Mother Nature is a full-time job and, honestly, it’s mostly a waste of time. Rather than setting ourselves up for tons of maintenance in a never-ending battle, put the very things that occur naturally to work for you. There are plenty of trees and shrubs that have millions of years of evolution behind them, making them a perfect choice for a specific area. 


Plant those instead.