Brix levels are a guideline to nutrient density and apparently this is built up in the leaves during each day and transported down to the roots each night. You can measure the brix levels with a refractometer which on a side note seems a great way to check that your trees are working like they should.
There has been a discovery that occasionally the brix level drops dramatically in the middle of the day. It is believed that the plants know when a bad storm is coming and secures all the sugar down in the base so that if can rebuild itself after the storm
It makes sense then that defoliation and pruning should be done first thing in the morning so the minimum amount of sugars are wasted. If you are religous with the brix testing you could even wait for a storm that induces the drop. Maybe the tree stores them in a different manner in this instance?
Anyway as we enter the warmer period remember that some trunks, branches or surface roots can be burnt by the sun after defoliation or major pruning. I found this happened with a hornbeam a couple of years ago
I like the idea of a refractometer to understand how healthy my trees are I might have to get one
No I'm serious 
taking the top half of trees of since 2005! 
Shari will it be OK will it 
