How a Changing Climate Can Affect Tree Health – Climate, Forests and Woodlands

Written by Eli Sagor

A changing climate will affect different forest stands in different ways, depending on a wide variety of factors including stand vigor, species and age class diversity, soils, water availability, and the ways that climate changes. For example, reduced soil moisture as a result of changing rainfall patterns will affect some species differently than others.

Here is what we know:

The trees thriving in a given forest stand are well adapted to the current and recent historic conditions. Because most tree species are well adapted to a relatively narrow range of soil, light, and other factors, as conditions change their vigor may change. Even minor changes in summer or winter temperatures or soil moisture availability can seriously stress trees. Weather controls many aspects of tree health and vigor, which influences the ability of trees to ward off insects and diseases.
That said, weather also influences insects and diseases. Tree vigor may be a minor factor if the weather is favorable to the insect or disease. For example, wet springs and summers may favor tree growth and vigor but also provide ideal conditions for many foliage diseases, which may cause a loss of vigor. For example, recent heavy summer rainfall in northern British Columbia has allowed an epidemic of foliage disease to attack otherwise healthy lodgepole pine (Pinus contorta) trees. Even though they were vigorous at the outset, weather conditions favored the disease more than the trees.
As a tree’s vigor declines, its ability to deal with new or existing insect and disease threats will decrease. Most tree species can deal with most insect and disease issues on their own, if the trees are growing vigorously. For instance, bark beetles rarely cause widespread damage to healthy stands. However, if trees are stressed and not vigorous, even normally innocuous threats, like bark beetles, can cause serious damage. Pests that only seriously impact low-vigor trees are called secondary pests.
Reduced vigor and increased vulnerability will reduce forest health. Once some insect and disease threats become well established in a stand, they can overwhelm even healthy, vigorous trees. For example, if local bark beetle populations get very large, they can run rampant through a stand, causing widespread damage. This is not true of all insects and diseases, but it is true of some.

Because specific, local climate changes are difficult to predict, it is difficult to predict how a given forest stand will be affected. Different tree species on the same site will respond differently. Older and younger trees of the same species may respond differently. Stands of similar composition on different sites may respond differently.

Despite this variation and uncertainty, as the climate changes, trees will be under greater stress, which will make them more vulnerable to insect and disease outbreaks that might otherwise be negligible.

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