Some thoughts on Mr Red's hints.
At guess #11, SOAR's trend improved after a dip in the middle. Now I don't know whether this applies to the total score or to a particular entry, but it may be significant that for No 10, he guessed Pine (Scots), switched to Hemlock, and then returned to Pine.
At guess #10, two trees have not been named correctly yet. I still don't know if this means that the names of these two trees have not appeared at all in peoples' guesses, or whether it means that they have appeared somewhere but against the incorrect number.
At guess #7, tree number 5 has been named, but not against number 5, which means that no 5 must be one of: Alder, Aspen, Cypress, Elder, False Acacia, Field maple, Goat Willow, Hemlock, Holly, Hornbeam, Horse Chestnut, London Plane, Mountain Ash, Oak, Pine ( Scots I think), Salix I think, I'm going with poplar, Sycamore, umberella pine, Walnut, White Pine, Whitebeam, Willow, Yew.
If we then take Mirius' comments (good correction on my second posting
), this list comes down to: Alder, Aspen, Cypress, Field maple, Goat Willow, Hemlock, Hornbeam, Oak, Pine ( Scots I think), Salix I think, I'm going with poplar, Sycamore, umberella pine, Walnut, White Pine, Whitebeam, Willow. And if we then take andyn's comments, it can't be alder.
At guess #15, one tree has been correctly identified, but an incorrect one has been given more times than the correct one. No tree has been named consistently by everyone. If I look at the most popular tries: London Plane for 8 (13 guesses); Oak for 8 (10 guesses); Scots Pine for 10 (9 guesses); Ash for 4 (9 tries). I still can't conclude anything from this, unless there's something similar to London Plane that isn't London Plane.
At guess #20 there is an interesting difference of opinion on no 1. Commonest is Sycamore and Horse Chestnut.
I'm not sure yet where this gets us. but I bet that when we see the answers eventually it's going to be "Oh, of course!".
Geoff