Ahhh you guys... PTS, you're getting what you didn't want for an answer, and it's bothering you. It's a public forum, and people were staying on topic. Clearly, the advice is sound - to hire for attitude and train for aptitude as Dakota summed up.
Maybe you do get get a lot of applicants, we've gotten about 400 applications in the last couple of months - is that a lot to you? We haven't used a standard test for any of them. A couple years ago, we were meeting with people the temp agency was sending over, and it was up to two of us to select the person with whom we wanted to work. In that case, we had a single well-defined position and were interviewing on a near-daily basis. We used a series of a half-dozen questions to guide our interview and make the comparisons easy.
I've talked about this before, but in hiring an executive six-figure position when I worked somewhere else, we crafted a more extensive list of questions under the supervision of the state. Again, there were no right answers. The point was to direct the discussion during the interviews and share the role of asking questions amongst the people in the room.
Contrasted with your experience in municipal governments, I say cherish your freedom! Developing a sense of getting to know a person in a brief interview is an important skill. Hiring should be done by the person at your company best able to do that, and I would avoid handicapping them with a test. I've said this before, but buy the book
Blink by Malcom Gladwell. He has an excellent discussion in the beginning of the book about how we in Western societies have successfully disabled our own intuition by overvaluing standardized metrics.
This is not what you wanted to hear, and you will resent my saying this. But listen to it nonetheless. You will believe it more over time. If you want, make a test and give it to all your applicants. Try giving it to some and not others and see if it helps. Let us know how it works! You don't know for sure until you try it out, and I would be interested in hearing about it. Your best source would be your own background experience, so I would try inquiring with the person responsible for hiring arborists in your own community to get a copy of their test.