Tree Decline is one of the most common causes of why your tree doesn't look well or is dying.
Unlike animals - trees do not age. There is no genetic code that tells the cells to age and eventually fail. In theory, a tree could last for hundreds if not thousands of years. There are cedars on the Niagara Escarpment that are hundreds of years of age and there are oaks in England, and Redwoods in British Columbia that are approaching 1,000 years of age.
Most trees do not die suddenly - Its usually a slow process called Decline. In hardwoods (trees that drop their leaves each fall), decline involves the slow loss of the size of the canopy. it usually starts at the top with the thinning of the tree or gradual thinning of the interior - it also can manifest by the tree turning fall colours and dropping its leaves earlier than most. Conifers (evergreens) can show Decline though thinning of the interior or complete branches suddenly dying off ( as certain diseases move in on a weakened tree).
Depending on the tree and what's causing the decline, many trees can be saved through good management practices and target pruning.
So why do trees die. There are two reasons that can work together or separately; 1) Trees don't stop growing and as a result outgrow themselves or where they live and/or 2) Severe environmental stress or disease gets to them.
1) Trees don't stop growing . A tree has to keep making new growth to survive. It's leaves have to keep competing for sunlight from other trees and its roots keep having to move through the soil to ensure enough nutrients. As a tree keeps getting bigger, eventually the tree gets so big that physics take over. Either the tree trunk or the branches breaks under its own weight. Once these breaks occur, there are now large wounds in a tree near or on the trunk - Heart rot will usually follow. Heart rot is a fungus that attacks the dead wood in the middle of the tree - its that dead wood that holds the tree up and if it rots, its only a matter of time before the whole tree comes crashing down.
A sugar Maple in decline - Starting to turn Colour in early August. Also note the thin canopy. ( Dead Ash tree from Emerald ash borer to the right )
Also, as trees get bigger and bigger, its often the case where the canopy or the roots can no longer keep up with the needs of the entire tree. Once the Canopy is damaged and gets smaller (e.g the canopy may shrink due to competition for sunlight. defoliated by bugs, or limbs broken in a storm), there will not be enough leaves to produce enough food, or if the roots get damaged (for example from severe drought or extensive construction, or they run out of space to grow) there will not be enough water to supply the tree. All this leads to a vicious cycle where one end of the tree cant provide what the other needs and this in turn limits resources going the other way and so on until the tree dies Trees also don't know how to cut their losses, they tend to try to keep the whole tree alive ( as opposed to trying to save just a portion of itself) and this adds to its stress or demise - this is especially true with the conifers.
All trees make special chemicals that make them taste bad to bugs (and some of these chemicals even fight some fungi) . These chemicals are called secondary metabolites and for those of you who care they are Alkaloids, Phenolics and Terpenoids. However, if a tree suffers from an environmental stress (drought, competition, late frost... ) and is not at optimum health, the tree will stop making these chemicals and just focus its resources on keeping itself alive. This means the tree can fall further behind the 8 ball because it will now be susceptible to more pests and more pests which means even less resources to try and stay alive.
Finally, extreme climate events or diseases are the other way trees die. Storms knocking a tree over or extreme drought are the most common climate events. Some trees don't show signs of drought until its too late; For example a common symptom that a spruce tree is experiencing drought is that the whole tree turns brown (and then dies). Certain diseases are guaranteed fatal such as Dutch Elm Disease , Emerald Ash Borer and Butternut Canker . Others can kill a tree if not caught early such as Nectria Canker in Walnut.
Occasionally, tree decline can be confused with a root pathogen (most often a fungus) and often that pathogen is moving in a result of decline.
Maintaining good tree health and controlling the size of the canopy in limited space situations are the best ways to ensure your trees last for a long, long time.
Note: Urban tree decline and decline because of vascular wilts ( verticillium, nectria, botryosphaeria, etc.) can often look very similar and the differences can be subtle. The main differences are that vascular wilts tend to be very sudden (within one growing season) and the leaves tend to brown and stay attached while decline usually is slow over several years and results in leaf chlorosis (rather than browning) and early leaf drop.
A spruce tree in decline. Note the thinning branches