Forest health encompasses many factors and a number of components. For example, a healthy forest includes healthy soils, water, plants and a functioning ecosystem. A dying or dead tree or a rotting log on the forest floor are parts of a healthy forest. Tree health focusses on what is traditionally referred to as forest health. Tree health can be affected by many abiotic factors – mechanical wounds, soil compaction, impacts of acid rain, the effects of heavy wind or ice damage, sunscald, drought, late or early frosts and forest fire and, by extension, climate change.
Most discussions about tree health deal with insects and disease are a known together as forest pests. Some forest pests can affect a wide variety of species – e.g. forest tent caterpillar will defoliate aspen, maple and other tree species – while other pests are very host specific – i.e. Beech Bark Disease only affects beech. Some pests may attack a variety of conifer species, while others a variety of hardwood species and a few may attack conifer or hardwood trees.