Treeseer,
It was simple deduction my dear watson!
words ending in "IST" typically mean one who does or studies.
I simply took the work Arbor to mean tree and voila!
Then Arboriculture and TaDa! definition number 2
And finally Arboricultural, meaning "of arboriculture," and BAM! definition number 3.
I really think that depending on where you are in your life journing with trees, at any given time you could be one, two or all three.
On any given day I'm an Arboriculturalist as I am looking at the whole scene in our region to anticipate and appreciate the forest as a whole.
When I examine a client's property I slip in to Arboriculturist mode for that is where I am.
And when I'm getting intimate with a single tree (purely monogamous, mind you
) in an aerial mode then I'm an arborist.
So, when we don all caps at one time or another, what does that make us?
I settle for simply arborist as my clients have a hard enough time understanding that that word even exists.