It isn't the most beautiful thing in my yard, but it's still very special to me and performs a number of useful functions in the environment. Since its death, my snag continues to exert a big influence on the ecology of the yard. It offers nesting and roosting spots for birds and can serve as a den for raccoons and squirrels. Even the dead bark continues to contribute to the environment by housing insects that are food for woodpeckers. The fallen branches create concealment and protection for small animals.
A snag's existence depends primarily on its size and the hardness of the wood. Some snags such as cypress and redwood can remain intact for over a hundred years while others will rot away within a decade. In the spruce and western hemlock forests of Washington State's Olympic Peninsula, the majority of tree reproduction occurs in places where seeds have taken root in rotting trees.
Trees have the ability to withstand damaging factors in their environment. They can stave off many stressors that could damage their roots, leaves, trunks or limbs. A tree has the ability to compartmentalize itself and seal off dead wood and disease from the rest of the tree. It can defoliate itself to reduce effects of drought. It can even bleed to help extract insects that are damaging the bark.
Sometimes a tree dies very quickly due to severe insect infestation or disease. Most often, a tree's death is a slow process that results from numerous causes. These causes are defined as either abiotic (of or characterized by the absence of life or living organisms) or biotic (caused by a living organism). Abiotic causes of dead and dying trees include environmental stresses such as low temperatures, flooding, drought, excessive sun and heat, and ice storms. This type of tree death is often associated with the demise of seedlings. Biotic causes of tree death include overcrowded seedlings or trees losing the competition for light, nutrients or water. Any defoliation from insects, animals or disease can have the same effect. Decline in the vigor of a tree from periods of nutrient depletion, insect and disease infestations, and abiotic stresses can have effects that result in death.
All trees eventually die. For every mature tree, there are hundreds of seedlings and saplings that die. Only the most adaptable trees make it to old age. These five factors are the conditions to which a tree eventually succumbs: death because of the environment, death from insects and diseases, death from catastrophic events, death from age-related collapse due to starvation, and harvesting. In most cases, a tree's death is the result of a combination of causes.
The condition of the ground where a tree is growing will ultimately determine the environmental stressors that come to bear on the tree. A drought-sensitive tree situated on a dry site is very likely to succumb from the lack of water during a drought. That tree then becomes more susceptible to other life-threatening conditions.
Adverse environments include soils with poor drainage or too much salt, parched soil, pollution, extreme heat, and severe cold. It's especially important to be aware of the genetic tolerances of a tree species before planting. Not all trees adapt well. Trees that live to old age like my snag die gradually. That sometimes takes centuries. Growth declines after a tree reaches maturity and the ability of that tree to maintain itself starts to diminish. New branches may sprout in an attempt to help the tree maintain its vigor but are usually insufficient to sustain life for any length of time. An old tree will slowly collapse and crumble under its own weight. It then becomes nutrients for the soil and for the growth of new trees.
(http://fw.ky.gov/Wildlife/Documents/snagsandcavitytrees.pdf; http://www.huffingtonpost.com/mark-hostetler/snag-why-keep-that-dead-tree_b_4038549.html; http://edis.ifas.ufl.edu/uw277)