EnigmaToots has a look at creating trees in Blender using the Sapling Tree Generator Addon, showing how to build the trees and how to distribute them on a small plane.
In this video I go over how to use the awesome Sapling tree generator addon in BlenderEnigmaToots
The Sapling generator Addon creates trees for blender based on the method that was developed by Jason Weber and Joseph Penn in their paper “Creation and Rendering of Realistic Trees”. You can easily create a wide variety of tree models using a curve as the base element of construction. This also allows for dynamic geometry which can be used to change the level of detail for the tree models that are created.
To learn more about the Sapling Tree Generator Addon check the wiki page here: Blender Sapling Tree Generator Addon.
Going Further With Trees in Blender
Previously, Daniele De Luca had a look at how to build trees in Blender using the Sapling Tree Generator Addon, specifically how to style the leaves of the tree so that they looked more customized. Curving and Styling Leaves With the Blender Sapling Addon.
Michel Anders was creating his own Tree Addon for Blender. His uses the space colonization algorithm described and developed by Adam Runions. The space colonization algorithm can have the tree model parameters correspond visually to relevant tree characteristics identified in landscaping, offering convenient control of tree shape and structure. Essentially the tree will grow in the space provided for it, much like it would in real life. Check out the post Tree Addon for Blender Creates Trees Suited for Architectural Renders Based On The Space Colonization Algorithm here.
Not really tree based, but still within the same vein, we have Tamir Lousky who once shared an Addon for Blender that will allow you to animate the growth of Ivy or any branching object. Check out Ivy Growth Animator for Blender Creates a Growing Effect in Blender Easily.