I placed many trees with a terrain.

And now I want to destroy one tree if any object hits the tree.

But the tree dont executes the script.

Can you show us your script?

/// <summary>
/// AS_TreeTerrain written by Paul Tricklebank.
/// Add this script to a terrain positioned at 0,0,0 and do not add tree colliders.
/// This script will generate colliders and add the tree damage script to each tree instance.
/// Because changes to the terrain at runtime are saved well, not 100% accurate,
/// Changes made at runtime are not reversed this script records information about
///Tree instances and replaces them at the end of runtime.
/// use for your own purposes. no credit required.

/// </summary>

using UnityEngine;
using System.Collections;

public class AS_TreeTerrain : MonoBehaviour {
public Terrain terrain;
private TreeInstance[] _originalTrees;


void Start () {
terrain = GetComponent<Terrain>();
// backup original terrain trees
_originalTrees = terrain.terrainData.treeInstances;
// create capsule collider for every terrain tree
for (int i = 0; i < terrain.terrainData.treeInstances.Length; i++) {
TreeInstance treeInstance = terrain.terrainData.treeInstances[i];
GameObject capsule = GameObject.CreatePrimitive(PrimitiveType.Capsule);
CapsuleCollider capsuleCollider = capsule.collider as CapsuleCollider;
capsuleCollider.center = new Vector3(0, 5, 0);
capsuleCollider.height = 10;
DestroyableTree tree = capsule.AddComponent<AS_DestroyableTree>();
tree.terrainIndex = i;
capsule.transform.position = Vector3.Scale(treeInstance.position, terrain.terrainData.size);
capsule.tag = "Tree";
capsule.transform.parent = terrain.transform;
capsule.renderer.enabled = false;

}
}
void OnApplicationQuit() {
// restore original trees
terrain.terrainData.treeInstances = _originalTrees;
}
}

Add this script to the terrain. terrain must be at 0,0,0.

uncheck add tree colliders.

The above script will add the next script to each tree.

/// AS_DestroyableTree written by Paul Tricklebank.
///
/// This script gets added to each tree instance and handles tree damage and removal of tree instances.
/// use for your own purposes. no credit required.
/// </summary>

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class AS_DestroyableTree : MonoBehaviour {
public int terrainIndex;
//the prefab to instatiate when a tree is removed.
//this prefab is loaded from the Resources folder
private GameObject Log;

//Each trees vp_HealthPickup level.
public float health;



void Start()
{
//set the trees health
health = 20;
//set the log prefab
Log = Resources.Load<GameObject>("choppedTree"); //change this line and add your own prefab which will be instantiated when a tree is cut down
}


public void Damage(float damage)

{
//deplete health. if it falls below 0 destroy the tree
health = health - damage;


if(health <=0)

{
Delete ();
}
}


public void Delete() {
//remove the tree and instantiate the log prefab
Terrain terrain = Terrain.activeTerrain;
List<TreeInstance> trees = new List<TreeInstance>(terrain.terrainData.treeInstances);
trees[terrainIndex] = new TreeInstance();
terrain.terrainData.treeInstances = trees.ToArray();
Instantiate (Log, this.transform.position, this.transform.rotation);
Destroy(gameObject);
}


}

I wrote this a long time ago. It's based on a tutorial by someone who's name escapes me (Sorry.) Works great on Unity 4 or 5.

It may or may not still work on later versions.

Good luck.