Right, extra wire at the base of the tree is a good idea for all the reasons mentioned, and one more.... Last week I serviced one of my jobs and found a downlight's wire stretched much too tightly. The cause? I had run the wire run up the tree on the OUTSIDE of a bent branch and over a root whose growth is limited by rock ledge. The wire had grown tight because there was no where for it to go as the branch and root grew in diameter. The absolute height of the fixture remained the same but the distance the wire had to travel increased considerably, particularly at the root that could not swell downward because of the ledge. It's always in the details!!! I should spend more time thinking like a tree. In this case, I was so pleased of the clever way I'd hidden the wire from view that I completely missed considering how the tree will grow.