Forest health is measured in many ways

At the moment, forest health is most generally taken to mean the visible impact of airborne pollutants on the trees. However, health problems also include natural disasters, such as destruction by fungi and insects. In this sense forestry does not pose a health risk to forests, but nevertheless, forest health is an environmental factor which has an impact on forestry.

Forest health can be measured in many ways. Damage to trees, such as cellular damage, deficient nutrition of the needles, growth changes and visible symptoms in needles and foliage are a direct indication of the state of health. The effect of pollution can also be determined by using what are called bioindicators – in other words, plant species which are especially vulnerable to pollution.

A good bioindicator is the beard lichen living on the branches of trees. It has no roots, but it takes all its nutrients directly from the air and from the surface of the tree, through its epithelis. Thus, pollutants enter the beard lichen's metabolism directly. It is a positive indicator of the state of health of Finnish forests that during the past ten years, beard lichens have clearly proliferated in southern Finland.

Mosses, too, take most of their nutrients directly from rainwater and the particles attached to their surface, so that measurements of the content of various elements in mosses provide information on airborne pollutants. The puffed shield lichen (Hypogymnia physodes), growing on tree branches, is a lichen which withstands airborne pollution very well. It accumulates pollutants, which is why an analysis of the puffed shield lichen allows researchers to estimate the accumulation of airborne pollutants in the forests.

Defoliation means the loss of needles or leaves of trees. One cause of this is assumed to be airborne pollution. However, defoliation is also affected by changing weather conditions, the tree's age and growth site, the prevalence of pests and the operations carried out in the forests. Defoliation is studied by comparing visually the selected tree with the crown of a standard tree regarded as normal. The assessment is subjective, and the result depends on the training and experience of the assessor. Compared to a healthy tree, a defoliated tree is likely to be more vulnerable against various stresses, such as exceptional weather conditions or damage by fungi and insects. Moreover, the tree's growth will slow down as the mass of its foliage decreases.

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