to be able to mine some bracket you actually need to be in that bracket
that means, to be able to log 31-35 you have to have atleast 31 lvl, for 21-25 you have to have atleast 21 lvl etc etc etc
Tree whisper have nothing to do with it, its the trait allowing you to find "+" nodes where if you meet the condition of said node you will get for example +5% to HQ bonus