So will I be able to give her leaves from trees?The key to feeding rabbits is consistency, and doing things gradually. Rabbits have rather delicate digestive systems, and it doesn't take much to make an upset. It's good to have "usual" food, something they are used to that is nutritious. Then, slowly add in other foods a little at a time. Most people start with hay and pellets, and go from there.
I happen to feed whole grains, not pellets. There's nothing right or wrong about that, it's just a personal choice, and lets me use more locally available/ locally produced feeds. If I grow my own, great. If not, I can purchase them locally. When I get a new rabbit, I try to get a quart bag of the food the breeder is using as well. The first couple of days I just feed the pellets as is. The next day, I mix in 1/3 of my own feed mix. Slowly, I add more and more of my mix, until they are just getting a few spoonsful of their old feed, until it is all gone. By then, their digestive bacteria has had a chance to multiply strains that can digest well what I feed.
I do the same with seasonal feeds. In the spring, I start with just a few leaves of dandelion and plantain (the little Plantago sp. weeds that grow in many lawns (but never feed from lawns that have been chemically sprayed or are used as toilets by other pets). Gradually, I add more leaves, including some grasses, chickweed, chicory, rose and blackberry/raspberry leaves, burdock leaves; and some tree leaves like mulberry, apple, willow, pear, aspen, silver/sugar maple. Start slow. If you see signs that the bunny is getting diarrhea, bloat, or going off-feed, eliminate that new feed from the program (make a note of it, so you'll remember later not to use it.)
This is by no means an exhaustive list of potential feeds, there are many threads here on "safe to feed plants" for more information (and yes, there are a wide variety of disputed plants that some feed and others feel are not safe). Your choices will depend on what ecosystems are available to you, and where you live (warm or cold, dry or wet).
As fall approaches, there will be less greens available, and eventually only the tree twigs will be available. Of course, during the summer, you can dry those same edible fresh leaves and store them for the winter. Make sure they are completely dry, not moldy. They should smell fresh and clean. We do this when a tree needs pruned during the summer, or has broken in a storm. We take the branches into an airy barn, and let the leaves dry. When crispy and crumbly, we bag them in feed sacks. On a small scale, you can do this by putting a screen in your car between the front and back seats, putting the leaves on the screen, and leaving the windows cranked open a few inches for air circulation. They often dry in just one day. Or use a dehydrator, or hang them up in a bundle in the house until dry.