Proper Pruning = Proper healing

Making cuts at the correct angles avoids damage to the vascular tissue of the tree, enabling the tree to form 'reaction wood' (callus wood) - proper sealing of pruning wounds is critical for the tree to compartmentalize and seal itself from insects and disease. 

Progression of reaction wood growth from pruning cut to full sealing of pruning wound.

juvenile tree pruning

Illustrations courtesy of Brian Kempf - Urban Tree Foundation

imProper Pruning = irreparable damage

Although trees possess impressive healing processes, poorly made cuts can damage the cambium (vascular tissue) of the tree.  Unfortunately, most of these cuts are irreparable and should be avoided at all costs, as trees cannot fully recover and sustain permanent damage.