We know that all trees do eventually die. There are many hundreds of seedlings and saplings that succumb for every mature tree left in the forest. All ages of trees eventually die to basically the same agents and only the most adaptive (and often lucky) individuals make it to old age. There are five factors to which a tree eventually succumbs: death from its environment, death from harmful insects and diseases, death from a catastrophic event, death from age related collapse (starvation) and of course, death from harvest. In most cases, death is the result of several, if not all of these conditions taking place simultaneously. Lets take a look at each of these.
Ground and site conditions on which a tree lives ultimately determines the environmental stressors placed on that tree. If a drought-sensitive tree lives on a dry site during droughty conditions it may indeed die from lack of water. But that same tree can also be more susceptible to each and every other life-threatening factor placed upon it. For example, a disease that appears to be killing the tree may in effect be only a secondary issue to the initial environmental problem.
Examples of adverse environments to trees are poorly draining soils, salty soils, droughty soils, air and ground pollution, extreme sun heating or cold spots and many, many others. It is particularly important to understand a tree species' genetic tolerance to environmental conditions when planting. Many trees adapt very well to poor sites but you need to understand which species fits where.
Harmful Insects and Disease
Virulent diseases like Dutch elm disease and the chestnut blight have caused sudden death to entire forests in North America. However most common diseased are more subtle in their work, kill many more trees in total than virulent types and cost forest and yard tree owners billions of dollars in forest product and specimen tree value.
These "common" diseases include three bad ones - armillaria root rot, oak wilt and anthracnose. These pathogens invade the tree through leaves, roots and bark wounds and damage a trees vascular system if not prevented or treated. In natural forests prevention is the only economic option available and is a major part of a forester's silvicutural management plan.
Harmful insects are opportunistic and often invade trees under stress from environmental problems and/or disease. They not only can directly cause tree death but will actually spread harmful disease fungi from a host tree to surrounding trees. Insects can attack a tree's cambial layer by boring for food and for nesting cavities or they can defoliate a tree to the point of death. Bad insects include pine beetles, the gypsy moth and emerald ash borers.
A catastrophic event is always possible in a large forest as well as in an urban setting. All property, including trees, are subject to being damaged or completely destroyed. In many cases, trees are not actually killed but are damaged to the point where their vigor is lost and insects and disease take advantage of a tree's loss of resistance.
Major tree losses can occur during a forest fire or when exposed to tornado-strength winds. Trees take a terrible hit when heavy ice is deposited on species sensitive to limb weight which results in breakage. Floods that do not recede quickly can cause root oxygen levels to diminish to the point where tree damage can occur. Extraordinary drought makes quick work of moisture-loving tree species and can harm all trees when extended over a long period.
For trees who beat the odds and live through maturity to old age, there is a slow dying process that may take centuries to complete (in long-lived species). The modular tree compartmentalizes around damage and diseased areas and continues to grow. Still, growth starts slowing after a tree matures, the ability of the plant to support itself diminishes and incurs the loss of adequate foliage for hydration and food.
New immature branches, called epicormic sprouts, try to assist in maintaining an old tree's vigor but are weak and are insufficient to sustain life for very long. An old tree slowly collapses under its own weight and crumbles to become the nutrients and top soil for future trees.
Timber HarvestsI want to include this only to remind you that trees do die to the axe. Trees via their wood have supported mankind and civilization for millennia and continue to be a necessary part of the human condition. The practice of forestry through professional foresters work continually with much success to provide a sustained flow of usable wood volume and at the same time, insure a surplus of trees.