In my world the term “Whisperer” was first used about 15 years ago by a customer who liked my approach. The word whisperer comes from the horse whisperer, Monty Roberts and his book The Man Who listens to Horses. The short answer is someone who can listen and see and feel and do what is right. Being able to read and work with an existing natural system. Whispering in this sense is beautiful and cuts through all the bluster, bluff and and standard procedures to address what is really going on. You are already whispering with plants if you realize that they are designed with no extra leaves, and that the removal of their leaves is always injurious to some degree.
Qualifications for whisperers are varied - lots of time on the ground, in the field, and doing the work is always good. Extensive reading is also good. As important is an open mind and a clear eye, asking what is going on here anyway? The whisperer's greatest tool is his/her heart, yup, lots of love, a strong desire to help and do what is needed. Attitude is what makes a whisperer, loving, caring, curious with empathy and respect. If those horses would have let him, Monty would have walked right up and given them the hug of their lives. Try it with your tree, see what comes back.
Being a tree whisperer is what I aspire to everyday.