1) natural process during which a mature tree reduces its crown and increases its girth to consolidate resources and increase longevity. 2) the deliberate process of reducing tree height to mimic natural processes.
retrenchment (Wikipedia)
Retrenchment (French: retrenchment, an old form of retranchement, from retrancher, to cut down, cut short) is an act of cutting down or reduction, particularly of public expenditure.