Trees greatly reduces FPS

Recently I reduced my terrain from 10241024 in size to 256256, since my level was way too big. The level is a forest, and I used to have around 10000 trees on the old map.
After I’ve reduced it in size, the fps is severely reduced (by 30-50fps) if I have more than 1500 trees on my map. Why is this?
I am using Unity Free, so there’s no occlusion culling (as far as I know) that should do anything different.
I’ve posted a pic below, I hope there’s enough info there.

Thanks

25011-fpsissues.jpg

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