Tree topping is a far too common bad practice that that has a myriad of negative effects on tree structure and physiology. It results in permanent disfigurement and very often mortality. People often think topping somehow rejuvenates the tree due to the new flush of growth following topping. However, this is the tree's reaction to the severe stress of losing most or all of its ability to photosynthesize, or create food. Latent buds are activated as a survival mechanism resulting in rapid growth below the cuts in an attempt to regain the ability to photosynthesize again. If the tree does not have the reserves in its root system to activate this growth it will often lead to tree mortality. Additionally, topping results in weak branch attachment, wounds that will not properly heal, and could lead to decay, cankers, and splitting bark. Overall the tree becomes more unhealthy and hazardous. Moreover, this bad practice typically leads to removal of the tree later or a number of pruning measures to attempt to correct tree structure which inevitably costs the property owner more than having trees properly pruned from a young age. Topping is typically only accepted in special situations such as for fruit trees to increase production and accessibility. If you are looking for a contractor and they offer topping as one of their services it is my advice to look elsewhere for services. Below is a video that goes over the basics of topping and the consequences of this bad practice.