Many governments mandate healthy forests, although there currently is no universal agreement among forest health professionals or world governments with regard to the definition of a healthy forest. In 1995, the Santiago Declaration of the Montreal Process Working Group adopted seven criteria for conservation and sustainable management of temperate and boreal forests, one of which was the maintenance of forest ecosystem health and vitality. To assess trends in forest health 67 indicators were devised, but no definition of a healthy forest was devised.
I define forest health, and then propose an objective and quantitative approach by which to assess and to compare the health of the world’s forests in both time and space. The approach is based on the concept that sustainability is the key to health, whereby sustainability is pragmatically defined as the maintenance of the current structure. The current structure provides the framework by which changes can be evaluated by comparing current observed mortality to a theoretical value known as baseline mortality. A healthy forest is thus defined as one that is sustainable and meets management objectives. A sustainability index and threshold score were developed that can be used to compare the sustainability of the world’s forests in time and space, and to evaluate the impacts of various disturbances on sustainability. Several examples are discussed.