Trees and bushes are different than other crops. You can place fruit trees and berries bushes in your farm and you can collect fruit or berries from them for three times in a row.
After that, you can ask your friends and fellow farmers to help you with an additional harvest. Just drop the "!" sign next to the dying tree / bush and any other farmer who will visit your farm will be able to revive them, giving you the possibility to collect fruits and berries one more time. If you are connected to Facebook, you can even use the "megaphone" to let your Facebook friends know that you need help in your farm!
When you've collected fruits or berries the fourth time, unfortunately the trees and bushes will die. You will have to cut them down to clear space in your farm and plant more!