Abiotic and Site Factors-Interactions with Insects

Many if not most “sick tree” problems can be traced back to underlying stresses that have reduced the tree's vigor, making it more vulnerable to diseases or insect pests.  Just as people become more vulnerable to illness when tired, stressed, and run down, trees are more likely to acquire problems when they are low in vitality.  Insect and disease problems are often the result of poor tree health, not the cause.

What can reduce tree health and vigor?   There are many possible causes.  Some are related to environmental factors we can’t do anything about, such as drought.  Others result from human action, or can be improved by human intervention (for example, thinning overly dense stands of trees).   

When it comes to trees, it’s usually much easier to cause a problem than to solve it.  As a result, the best approach is to anticipate and prevent problems before they occur. 

Factors contributing to tree stress
·        Drought
·        Shallow, rocky soils
·        Solar exposure (e.g., hot, dry southwest slope)
·        Poor soil drainage
·        Heavy clay soils
·        Competition with surrounding trees and brush for limited resources, usually soil moisture
·        Mechanical damage to tree trunks
·        Damage from fire
·        Sudden exposure of shade-grown trees to intense sunlight
·        Shade (for trees that are intolerant of shade)
·        Soil compaction
·        Trenching
·        Backfilling over the existing soil surface
·        Over-watering
·        Flooding
·        Exposure to high temperatures (pavement, house siding)
·        Frost, unseasonal cold snaps
·        Species not well suited to site conditions (e.g., site is too droughty)

Many of these factors, such as drought, thin soils, too much competition, and soil compaction, are directly related to moisture availability.  Soil moisture is typically the limiting factor in tree growth in natural areas in SW Oregon, not lack of nutrients or sun. 

View a short presentation with pictures of site factors contributing to tree stress and health problems.

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