@pending using photoshop, it's just tree assets retextured using edited tree textures. Then I constructed prefabs in game and saved them.

how did you make these? I want to make detailed birch trees, very white with dark, black markings etched all over

Cool!

I will be bringing an update in the next week to change the prebuilt trees to prefabs, and when loaded in they will be slightly smaller and spawn above ground. This should hopefully make it easier to use the trees.

I've been waitng ages for an environmental asset mod like this! I always had trouble with constructing custom trees.

The only other object that'd be really useful are dunes. Proper sand dunes for constructing deserts and making beaches. (scalable, with detail textures, I think they're called.)