You want to see where the trees are? Check out the Boulder County GIS map at
Boulder County. If you zoom in enough, you can see every single tree in Boulder County.
Boulder is a dry place. Seeing pictures from the late 1800s when CU was first founded gives you a taste of the parched landscape that was originally here. Back then we had cottonwoods in the draws and streams, but that was about it.
Now there is a fairly nice deciduous canopy that covers most of the town, which comes thanks to irrigation and is a big benefit of living in a city more than a century old. Sometimes humans can help a place become more beautiful. If you're looking for the natural trees, you'll need to look west of Boulder starting at about 5700 feet. Ponderosa pine is the dominant species in Boulder County, except for some of the shaded northern facing hillsides that attract Douglas Firs and other species that appreciate more shade.
Google Earth will also give you a good idea of tree cover. Flagstaff Mountain just west of town at almost 7000 ft (1600 ft higher than downtown) is covered with Ponderosas. At Chautauqua Park at 5700 feet you can hike the McClintock Trail which is completely forested as well. Here's a view from the McClintock Trail looking up at the 1st and 2nd Flatirons:
ProTrails: McClintock - Enchanted Mesa Loop Trail, Chautauqua Park Trailhead, Boulder - Denver - Golden and Lyons. It's a beautiful trail. Even better is the steep climbing access that ascends the thin ribbon of trees between the 1st and 2nd Flatirons.
I'm rambling now but I love it. It's a marvel to have such a beautiful trail a 5 minute drive from downtown.
Good luck.