It might seem like a stupid question but ‘Why do trees need water?’
Trees draw water from the soil and transpire this moisture through the leaves, this moisture carries with it minerals from the soil that has been absorbed by the roots. The leaf uses these minerals to help build new leaves and twigs, buds and flowers.
Just a quick note here that we need to differentiate between evergreens and conifers because of deciduous conifers (larch, taxodium, etc) and evergreen broadleaves (holly, winter oak, etc)
Some water is retained within the tree and absorbs the sugary, energy rich products of photosynthesis and the oxygen from respiration, returning both to the roots to help them grow, depositing some to all the parts of the tree as it passes, and collecting some in the heart of the trunk as storage for future use. Basically water performs the same function in trees as it does for human blood.
Now when humans get hot they sweat, emitting water on the skin surface that gets evaporated off taking heat energy away from the core of the entity. This exact same process happens in trees, the ‘sweat pores’ are found in highest concentration on the underside of leaves, but also in lesser numbers on the woody areas such as trunk and twigs. If this process is prevented by lack of water the organism, human or plant, will overheat at a cellular level and die. An adequate supply of water is therefore critical. This dangerous overheating is worse in some species and you will see reference to scorch in some text books. Of all my trees it is the beech that are most prone to scorch. The easiest way to deal with this is to keep trees prone out of the full summer sun and drying winds so they only have to deal with ambient air temperatures.
So let’s focus for a moment on the actual practise of watering. Once the soil in a pot has dried out the ‘standard’ watering from a hose or can will simply run off – you must quickly learn to recognise this sign as you tree will be in peril. Rectify by dunking the whole pot in water until all the air bubbles are gone, leave for another 5 minutes just to be sure, and then return to its home.