Health + Risk Assessment

Health Assessment

Trees are living beings, and like people, they can have health problems. When trees become stressed they can become more susceptible to infestation by insects and/or infection by fungal pathogens. These “diseases” are often treatable, but assessing trees for causes of stress is key in order to correct health problems in the long term.

For example, when pine trees are under drought stress due to a lack of rainfall and/or adequate irrigation, they can be infested by bark beetles. Between 2010 and 2019, the U.S. Forest Service estimated that over 147 million trees died in California forests due to the combined effects of record drought and bark beetle infestation. In landscape situations, pines can be treated with insecticides to protect them from bark beetles, but soil conditions and irrigation also need to be assessed in order to provide recommendations to correct any drought stress that may exist.

Due to the ever increasing amount of global trade, new tree pests are accidentally introduced into California. Invasive tree pests often attack regardless of whether trees are under stress or not. In these cases, trees often need to be protected through the use of chemicals since our natural environment offers trees no or not enough protection from pests that come from other ecosystems. The Center for Invasive Species Research (UC Riverside) is a good source of information on this subject.

A few examples of invasive tree pests in Southern California include:

Shot hole borers on California sycamores

HLB (citrus greening disease) on citrus trees

Gold spotted oak borer on some native oak species

Risk Assessment

Trees can be viewed as living tissue supported by a dead wooden structure, almost like a living skin covering a wooden statue shaped like a tree. The living tissue of a tree adds to this wooden structure every year by using the energy of sunlight to turn atmospheric carbon and other natural elements into wood mass. The older trees get, the more massive they become.

Trees, like any wooden structure, can fail if their load bearing capacity is exceeded by natural forces, such as gravity and wind load. And since wood is a carbon-based material, fungal organisms have evolved to eat it, which can also lower the load bearing capacity of a wooden structure.

The International Society of Arboriculture has developed a methodology for assessing tree risk and communicating it to tree owners and managers. While there is no way to guarantee that a tree will not fail, having a tree assessed for risk can help make management decisions. For example, a person who is interested in buying a home, or who has just bought a home, may want to have large trees on the property assessed to see if there is a risk present beyond their level of comfort, or tolerance.