Replacing Damaged Power Lines
Power lines that are low enough where trees can fall on them have a steel cable for mechanical support.  When a tree falls, the steel cable takes the load.  It will elongate slightly and put a little more tension on the conducting wires, but not a lot.

The steel cables are often strong enough that the wooden pole holding the span at either end breaks before the cable when something sufficiently heavy falls on the cable.

Around here (north central Massachusetts), local feeders are often 13.8 kV three phase.  These are in a bundle of 4 wires, with the steel support cable being the top, and the three conducting cables on spacers hung from the steel cable.  The top steel cable also acts as lightning protection.  It is grounded regularly.