Photoshop CC has the ability to create trees since the early release of the CC version. Version 2014 however has improved even more on this nice feature. You can choose from a rich collection of circa 34 types of trees and adjust needed parameters to change the size and type of leaves, height of branches,etc. Below is a sample of the tree types you can find in Photoshop.
You can change leaves types and density, change the direction of light, your camera perspective angle, and you can create your own tree/leaf combination as well as change the leaf color to be aligned with different seasons like summer, autumn, spring, and winter.
To be more flexible, Photoshop also lets you “Randomize Shapes”. Using this option, the branches and leaves change every time - random. This is useful if you are adding trees and you don't want them to appear the same.
To access this feature, create a blank layer for every tree you want to insert. Then go to menu - Filter - Render - Tree. You will be prompted with the above dialog box where you can tweak leaves, select tree type, change camera perspective, etc. As an example, I have added 3 Redwood trees with random shapes of leaves in order to make the leading lines more obvious.
Now it's your turn to try. Go ahead and enjoy building your forest!