Thinning out the crown of a tree is a process that reduces the overall density of the crown.
This method of pruning a tree is also a viable alternative technique when considering crown reduction. Although the overall height and spread of the tree will remain the same, the resultant effect of crown thinning is one that gives the impression of a less oppressive ‘smaller’ tree. This option would be a suitable service to provide when the overall shape of the tree is something that is prized by the owner and therefore reticent to lose the natural aesthetic of the tree through the more extensive measure of crown reduction.
Thinning out the crown will also reduce the overall sail effect on a tree so if you have a specimen that is exposed to high winds this may be an option to lessen the risk of major branch or trunk failure. Another benefit would be an increase in light and air circulation which will allow more light through the tree canopy which may appease a lawn struggling in heavily shaded areas.
All species of tree even in the same genus will respond differently to such a procedure and it would be worth discussing the ultimate result of different methods of pruning in order to make the most informed decision.
There is some science to suggest that crown thinning can be more effective in stalling or slowing a tree’s growth compared with crown reductions, but again this will depend on which species of tree one is dealing with. For instance, the very common wild plum or purple plum, which its delicate abundant white through pink blooms in early spring, are often seen to have vast amounts of vertical epicormic shoots arising from the primary branch structure throughout the crown. This often gives the appearance of an exceptionally dense and crowded habit and the temptation will be to thin out the epicormic growth. Doing this will absolutely result in an increased regenerative response from the tree and the problem will rapidly reoccur.
Unfortunately, a wild plum with this habit is probably the result of a previous crown reduction or some other natural wounding and so continual maintenance using a crown thinning system will become routine. The frequency of the routine will depend upon individual specimens.
Crown thinning would also include the removal or damaged and crossing branches. Over time branches that are grouped closely will sway in the wind and damage one another or in some instances, such as in many Beech trees, will result in two or more branches grafting together at the point where they cross. In this circumstance thinning out either of the two grafted branches would be ill advised except under certain conditions. There are many reasons for this, but most important among these is a tree’s ability to engineer its own growth by responding to the stresses and strains of its exposure to immediate climatic conditions, how vigorously it grows and therefore how much structure needs to be supported by the primary scaffold as a whole. In other words, if conditions in any given year result in a vigorous growth response, following this the tree may slow its lateral and vertical growth in order to provide more support at key stress points within the canopy by thickening those points with harder, denser wood.