Tree random creation / destruction around player using terrain object and treeInstances. I'm using it in a racing game.

URandomTreesAroundPlayer

// TREE Script - Generate random trees around player position, adjusting them to terrain height
// Script by: Manuel Fernández Yáñez - 9/5/2013 redgreenrefactor.blogsite.org @manufy

import System.Collections.Generic;

var treeList : List.< TreeInstance >;
var timer : float = 0;
var player  : Transform;
 
function Start () {
  
	// 1 - Get Tree Instances 
	
	var actualtreeInstances : TreeInstance[] = Terrain.activeTerrain.terrainData.treeInstances;		
    Debug.Log( "Removing Trees: " + actualtreeInstances.length );
	
	/* OPTIONAL FOR DEBUGGING // 2 - List Tree Coords
	
	for ( var t : int = 0; t < actualtreeInstances.length; t ++ ) {	
		Debug.Log("Existing Tree (XZ): " + actualtreeInstances[t].position.x + " " + actualtreeInstances[t].position.z + " Y: " + actualtreeInstances[t].position.y);	
	} */
	
	// 3 - Initialize tree list and remove ALL Trees
	
	treeList = new List.< TreeInstance >();
	Terrain.activeTerrain.terrainData.treeInstances = treeList.ToArray();
 	Debug.Log( "Trees removed: " + actualtreeInstances.length + " Trees actual: " + Terrain.activeTerrain.terrainData.treeInstances.length);
 	
 	// 4 - OPTIONAL: CREATE INITIAL TREES, OTHERWISE START WITH NO TREES
 	                                      
 	//Debug.Log( "Created Trees: " + " Trees actual: " + Terrain.activeTerrain.terrainData.treeInstances.length);
	
}

function Update () {

	var treeposition : Vector3 = player.position;
	var treeDisplacement: Vector3 = Vector3.zero;
    var terrain = Terrain.activeTerrain;
    
    // 1 - Create Tree from Prototype Tree 0
    
    var newtree : TreeInstance = new TreeInstance();;
    newtree.prototypeIndex = 0; // From terrain tree prototypes list index
    newtree.color = new Color (1, 1, 1);
	newtree.lightmapColor = new Color (1, 1, 1); 
    newtree.heightScale = 1;
    newtree.widthScale = 1;
	
	// 3 - Displace tree position randomly and height adjustment
	
	treeDisplacement = new Vector3(Random.Range(-100, 100),0,Random.Range(-100, 100)); // World Space Coords Random around player transform
    newtree.position = new Vector3(player.position.x + treeDisplacement.x,player.position.y + treeDisplacement.y,player.position.z + treeDisplacement.z );
    newtree.position.y = terrain.terrainData.GetInterpolatedHeight(newtree.position.x, newtree.position.z);  // Dont sure if I have to normalize coords 
    var newtreeterrainLocalPos = newtree.position - terrain.transform.position;
    var newtreenormalizedPos = Vector2(Mathf.InverseLerp(0.0, terrain.terrainData.size.x, newtreeterrainLocalPos.x), Mathf.InverseLerp(0.0, terrain.terrainData.size.z, newtreeterrainLocalPos.z));
    var newtreeterrainNormal = terrain.terrainData.GetInterpolatedNormal(newtreenormalizedPos.x, newtreenormalizedPos.y);
   
    // 3 - Calculate player coords
    
    var playerLocalPos = player.position - terrain.transform.position;
    var playernormalizedPos = Vector2(Mathf.InverseLerp(0.0, terrain.terrainData.size.x, playerLocalPos.x), Mathf.InverseLerp(0.0, terrain.terrainData.size.z, newtreeterrainLocalPos.z));
    var playerterrainNormal = terrain.terrainData.GetInterpolatedNormal(playernormalizedPos.x, playernormalizedPos.y);    
 	newtree.position =   new Vector3(newtreenormalizedPos.x , 0 ,newtreenormalizedPos.y );
     	
    // 4 - Add tree to the terrain
    
    treeList.Add(newtree);
    Terrain.activeTerrain.terrainData.treeInstances = treeList.ToArray();
   
    // 5 - Remove unwanted trees
    
    var numTrees = treeList.Count;
    var newtreeList : List.< TreeInstance > = new List.< TreeInstance >();
    for ( var t : int = 0; t < numTrees; t ++ ) {	
		var vecreal = Vector3.Scale(terrain.terrainData.treeInstances[t].position,terrain.terrainData.size) + terrain.transform.position;
		var distance = Vector3.Distance (player.transform.position, vecreal);
  		if(distance<80)
		{
				newtreeList.Add(terrain.terrainData.treeInstances[t]);
		}
		
	if (false)
		Debug.Log("Existing Tree (XZ): " + treeList[t].position.x + " " + treeList[t].position.z + " Y: " + treeList[t].position.y + " / " + vecreal.x + " " + vecreal.z);		
		
	}
	
	// Overwrite terrain trees with list trees
   
    Terrain.activeTerrain.terrainData.treeInstances = newtreeList.ToArray();
    treeList = newtreeList;
   
    // 6 - KLUDGE: Force colliders to work
    
    //terrain.terrainData.SetHeights(0, 0, new float[0, 0]);   
    //var heights: float[,]  = terrain.terrainData.GetHeights(0, 0, 0, 0);
	//terrain.terrainData.SetHeights(0, 0, heights);
	
	// 7 - Log
	
 	Debug.Log ("Player terrainPos: " + player.position + " normalizedPos: " + playernormalizedPos + " terrainNormal: " + playerterrainNormal + " numTrees: " + Terrain.activeTerrain.terrainData.treeInstances.length );
             
}