Deciduous trees should be pruned in their dormant season, which in Mediterranean and similar mild winter climates, means towards the end of winter. They should not be pruned in the spring as pruning causes the sap that is rising in the plant to "bleed" thereby depleting the tree of valuable energy and nutrients. If you have missed the boat, either wait till next year or carryout a very light pruning in the summer.
On the other hand, broadleaved evergreens and trees that come from tropical or sub-tropical climates are likely to be sensitive to cold and late frosts, and can incur severe damage if pruned too early in the year. They should not be touched until all possibility of frosts has passed.
In Mediterranean climates, spring is the best time for pruning because it anticipates the plants' principle growing season. There is little point in allowing the tree to expend energy on new growth only to remove that growth a month or so later.
As opposed to shrubs and bushes, the aim of pruning landscape trees is to develop and maintain the tree's natural shape. For this reason, it is a mistake to shorten branches as this detracts from the natural "flow" of the tree.
Instead, whole branches and limbs should be removed at their base, which is either where the branch is attached to the main trunk or a thicker branch. An exception is when a stem shoots forward into a long, thin branch that is out of proportion to the rest of the branches on the tree. It is best to clip such stems.
Pruning Fruit Trees
There is considerable confusion amongst home gardeners as to how best to prune their fruit trees. It is common to see the branches cut back as though the tree is a rose bush or some other flowering shrub. This is a technique borrowed from agriculture where the farmer is more interested in maximizing yields than in maintaining the natural look and beauty of the tree.
In my view, garden trees are ornamental, landscaping elements first, and providers of fruit second. In fact, by pruning fruit trees as one would landscape trees, (i.e. by removing a few limbs entirely) one gets the best of both worlds - a worthy, natural-looking specimen that produces enough fruit to supply the needs of the average family.
The Health of the Tree
Another reason for not pruning fruit trees according to the agricultural method concerns the long-term health of the tree. Persistent pruning that involves the removal of large volumes of material, depletes the energy potential of the tree, and renders it more susceptible to pests and disease. Remember that the tree, whether fruiting or otherwise, is the most precious element in the garden, and that pruning should be carried out as carefully, conservatively, and judiciously as possible.