can i send it to you via twitter? I'm @triadne on twitter

no worries, I'm just copying my files to a new folder to try on 2018. 10GB, yikes. in any case, it's still useful for me to help you debugging your 12017 version though isn't it?

interesting. even if i set it to billboards only, it does not show a billboard. on any trees. the behaviour is the same regardless of which tree it is, mine or yours.

yes I'm thinking of trying upgrading my unity, but I'm worried that I won't catch all possible errors. I've been playtesting my game for 4 years now and I often forget which parts to test to see if there are any errors. 

okay, new scene, no change, with your tree, no change. it doesn't change to billboard at all

good point, I'll try that now

Thank you, this seems to have resolved that issue completely! 

Still an issue with no ambient occlusion though... but did you say I had to upgrade my unity version to get that? 

now that the trees are being created correctly, I have noticed another issue. the tree is not transitioning to billboard. I set it to 10, the minimum distance to turn into a billboard, and yet it does not change at all. I've tried moving the camera really really far away and also, the editor window really far away but it's definitely not showing the billboard version. I selected the Lod_1 and moved it about just to make sure.

As I try to resize my trees manually, the billboards refuse to resize as well. they end up becoming bunched up and will result  in the screen being filled with leaves if you're photographing them, at a reduced size.  There is a leaf size control in my tree prefab but I don't really want to mess with that as it could ruin my tree irreparably. 

I have tested your unchanged scene, all perfect.  I pulled your trees into my scene. I put one of your trees in the 'put trees here' without any others. it performed as intended. I put my tree in as well as one of yours, yours worked as intended, mine did not.  I looked to identify differences in the leaves, because the bark appears on my screen at the correct size. my leaves are billboards while yours are mesh. I temporarily changed my leaves to tricross... it rendered as intended. 
So, the billboards are failing to resize correctly.  thinking about this now, it does not surprise me that it is unity billboards causing a problem. they have been the source of many problems for me and actually the reason I bought your asset to replace all my trees and have better billboards.

Actually yes that is what happens, like I said earlier, the billboard tree just appears as a close up of the leaves in a crisscross pattern. Like three big rectangles of leaves stuck together. So it seems like that must be what happened, its taking the screenshot really close, or the tree is too large for it. I don't know what to do about that. I don't think my trees are abnormal sizes.