Forest health is affected by both natural and human factors. Defoliation is the most commonly used factor in describing forest health. It is expressed as the amount or proportion of trees whose crown is more than 25 per cent defoliated. Other symptoms of forest damage that reduce stand quality are the amount and percentage of dead, fallen and broken trees, decayed trees, stem defects, top damage, discoloration and multiple symptoms. It should be noted, however, that dead and decayed trees are also often seen as a forest characteristic that improves biodiversity.