Random Forest Scene Node
This is a simple plugin where you define an area and some other parameters, hit a button and have a whole lot of trees planted there by random.

This is how it works:

You can specify a terrain scenenode to be used, and all trees will get an appropriate height position. If no terrain scenenode is specified all trees are in the same height. There is also a "resize" attribute to variy the size of the trees. You specify how many percent the size may vary (i.e. if you enter "0.25" the tree size may range from 25% smaller to 25% bigger (scale 0.75 to 1.25))

There are some parameters you can specify:

If you store your scene the trees won't be stored, only the parameters used to generate the forest. I added an own random number generator to the plugin from a random tutorial on the web so that I (hopefully) get the same random number sequence on all computers and compilers (I don't want to rely on the rand() function, implementations may vary between compilers I guess). This way of saving the forest reduced the filesize of the scene file from 1.5 MB to 50 KB in a test I made. You can still have the trees serialized though, there is an option for that.

I hope this plugin will be useful to some people here. screenshot
23.05.2011 |bulletbyte.de: "RandomForestPlugin" update
 I have now integrated the "MeshCombiner" provided by Lonesome Ducky on the (official Irrlicht Forum which brings a huge performance boost (before: 13 FPS, after: 200 FPS).
22.02.2011 |bulletbyte.de: "RandomForestPlugin" update
 Added some functionality to the RandomForestPlugin: now it shows the border of the forest in IrrEdit. Also uploaded a new version of IrrOde where the IrrOdeCar demo reflects this change.
19.02.2011 |bulletbyte.de: new Plugin "RandomForest"
 I have created a new plugin for the IrrEdit scene editor: the Random Forest Plugin. You can specify an area in your scene and have it populated with trees. It's also possible to add more than one forest to a scene. I have also added two forests to the IrrOdeCar demo of my IrrOde wrapper as a demo.