Thanks, Caenwyr! You are definitely right about the line thickness. I made two brushes from my hand-drawn trees (one brush is a single tree, the other is a small cluster of trees). The line thickness increases or decreases based on the size of the brush. Unfortunately I'm not sure how to remedy that in Photoshop.

Based on your feedback, I may just erase the large trees outside of the town and replace them with trees in a smaller brush size. The line thickness would be still inconsistent, but it would not be as noticeable.