The truth is simply that when planting or when land with trees is inherited, it is essential to allow for an annual budget of maintenance. The person you asked at the garden centre for a small tree, which will be 'maintenance free', is usually wrong. And together with other massive mistakes, (including the classic - bareroot christmas tree planted at the bottom of the garden - a norway spruce gains a huge height, is used to a forest habitat and will sap most nutrients from your land as well as water), a walk down a suburban road in England will highlight tree management disaster after tree management disaster. The professionals are rarely called until it is too late, when only a fell can alleviate the problem to great cost.
Organisations such as the ITF amongst others have attempted to redress the balance, education being the primary tool. However against a continual decline in curriculum based activities to promote trees, little headway can be made. We are very lucky to have one of the best quango's in the Forestry Commission out there assisting and together with many other NGO's this has helped to prevent tree health from disappearing from the agenda altogether.
A huge factor recently has been a monumental change in suggested management techniques, notably Dr Shigo's work on pruning and chemical applications to treat fungal attacks. Many 'old boys' including some still hired by the BBC, continue to ignore and criticise this advice, much to their discredit. After 'real' and intense research these new ideals are the way forward.
But it is paramount to avoid problems occurring in the first place, and to this end some simple techniques to allow for a diagnosis. These simple tricks may come in handy:
An ailing tree, will normally always respond to some root stimulus, this doesn't cost much and if it does not work after a season - it is time to revise the situation: A healthy dose of mulch - very well rotted horse manure compost or similar is always good. If you can, very gently spike around the root plate area, to allow penetration of the compost deeper into the soil. Most modern gardening techniques go against the necessary tree management - collecting and tidying up all the fallen leaves, collecting grass clippings etc., is basically removing the future essential nutrients the tree requires.
Brashing, (the removal of smaller limbs towards the bottom of trunk): Many trees are self brashing, and will happily drop their branches onto the nearest car roof. To avoid this and to increase the health of the tree simply prune when young.
A very simple method of checking the health of a tree 'there and then' is the ''O'Neill factor': Simply measure last years growing season and divide against the previous years. If the tree is still growing the factor will be >1. If below 1 the tree is slowing down and needs some serious management devoted to it. This was developed in the 1980's as a quick method of determining tree healt for Spruce, but can be used for almost all species.
A normal household hammer, when lightly tapped around the whole circumference of a trunk or limb, will easily highlight internal problems when a hollow thud occurs. It is possible with practice to discover the whole area in question and as such make a simple decision with regards any necessary work.
I once heard a retired tree man advising a client that "it is essential to spend more time looking at the tree, than working on it". Whilst simplistic it has been absolutely true for me and many others. Get to know your tree, treat it with the respect it deserves.