Response
Asking for citations is essential to good science. And I note that the person questioned didn't seem to consider it an improper or disrespectful question.

His word was never in doubt. He said something. Fine. He did not say that _he_ had discovered the trees and performed the tests. He had heard about this from elsewhere. I ask about the 'elsewhere' so as to be able to look in to the details. If you are a scientist, this is something you _want_ people to do. Nodding their heads and saying that's nice is about the worst possible response.

'This is true, take my word for it.' is a terrible travesty of science. You may be a fine, honest, careful scientist. But even the finest, most honest, most careful scientist makes mistakes. That's why the fine, honest, careful folks _want_ other fine honest, and careful folks to test their work, to challenge it, and to try to improve upon it.