There are many situations where a raised screen of trees is useful. For instance, if you have an attractive wall which is, say, 6ft high and you wish to screen out a distant view above it without obscuring the wall you might like to use trees which have been grown as standards or high panels. What however do these different terms mean and how can these plants be used.
Standard trees are grown with a bare trunk around 6ft high (occasionally they are slightly higher at 2m and half standards are also available with shorter trunks). These bare trunks are usually easy to keep bare and you can allow the tops of the trees to grow to suit your needs. However you will need to buy trees to suit the situation. For instance a row of Birch trees may eventually grow to 45ft in good growing conditions and be 20ft across the crown but this could take many years. You would have to decide when you plant them just how far apart they should be. They will also be deciduous so the leaves will fall in winter and they will not form the same screen. There are alternatives however which will make it easier to provide a quicker and more controllable screen.
Both deciduous and evergreen standard trees, such as Holm Oaks (Quercus ilex), Laurels or the shorter Photinias, can be bought as standards and tightly clipped to create a screen limited to the size you want. Left to themselves Holm Oaks would grow as large as any Oak tree but clipped they can be kept to the size which suits you. This however requires a clip at least once a year. There are nurseries which sell a range of deciduous and evergreen trees clipped as high panels, that is bare trunks with a rectangular panel of branches carefully trained to shape on top of them. These can be planted next to each other to give an instant high screen. More decorative, but less effective as screens, are pleached trees where the high panel is trained along horizontal wires. Similarly espaliered fruit trees can be bought ready trained to give horizontal tiers of fruiting branches but these are usually on much shorter trunks for ease of picking and are less effective as screens. They are however a very effective way of dividing a garden into sections being both productive and extremely attractive.
To achieve the sort of high screens you want you will have to work out the overall width of the screen and the height which it needs to achieve. Then there is the qustion of tree variety. Do you want evergreen or will deciduous do (deciduous is often cheaper) and do you require anything else from the tree such as attractive foliage or flowers. Finally will the trees thrive in your garden conditions or will soil and drainage problems compromise their development. You will probably need to refer to a textbook or speak to an expert to answer these questions. A site visit may also be needed.
