Unity provides a tool called Tree Editor that lets you design trees directly within the editor. This is very useful when you want to create detailed forests and jungles with different tree types and variations.
This section of the manual explains how to use the Tree Editor.
From Unity 5, you can use SpeedTree Modeler from IDV, Inc. to create trees with advanced visual effects such as smooth LOD transition, fast billboarding and natural wind animation. See here for more information.
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