What he meant by "too many trees" isn't an aesthetic preference. Simply put, too many props kill your frame-rate.
Next you're going to say "but dood, I get 76 fps! No problemo!"
Wrong, that many trees is an epic problemo.
First, the most expensive thing to render in-game are player models. With 32 people running around ANY scene is going to take a frame hit.
Second, not everyone has your awesome machine. TF2 can run on many lower end machines, it is part of the appeal of the Source engine. The best way to judge framerate on ALL platforms is to load up dustbowl, set your settings as high as you can without dipping below 60 frames, than type +showbudget in console and run around. Notice what the bar graph does. Now run around on your map. If the frames ever drop below what you were getting on dustbowl than you have a problem.
Finially, that many of the same type of prop does look aesthetically wrong. It is a rookie mistake to rely on props for layout, and so far your map is a newbie cliche. I must warn you now, your "I will keep them" attitude is going to inhibit your proficiency as a mapper. Don't get married to your ideas, especially if you are going to ask for feedback. These forums are full of smart people who are really good at what they do, take their advice.