How to randomly spawn GameObject on a Terrain

Hello, I have an issue with my script - I want to spawn random amount of trees on a terrain. The problem is - trees just ignore the terrain and spawn inside of it, but not on a terrain. Please, help! Here's my script:

using System.Collections;

using System.Collections.Generic;

using UnityEngine;

public class TreeSpawner : MonoBehaviour {

 public GameObject Tree;
 public Terrain terrain;
 private int terrainWidth;
 private int terrainLength;
 private int terrainPosX;
 private int terrainPosZ;


 void Start () {
     terrainWidth = (int)terrain.terrainData.size.x;
     terrainLength = (int)terrain.terrainData.size.z;
     terrainPosX = (int)terrain.transform.position.x;
     terrainPosZ = (int)terrain.transform.position.z;
 }


 void Update () {

     int randNumber = Random.Range (50, 100);

     if (Input.GetKeyDown (KeyCode.Space)) {
         foreach (Transform child in gameObject.transform) {
             GameObject.Destroy (child.gameObject);
         }


         for (int i = 0; i < randNumber; i++) {
             int posX = Random.Range (terrainPosX, terrainPosX + terrainWidth);
             int posZ = Random.Range (terrainPosZ, terrainPosZ + terrainLength);
             float posY = Terrain.activeTerrain.SampleHeight (new Vector3 (posX, 0, posZ));


             GameObject newTree = (GameObject)Instantiate (Tree, new Vector3 (posX,posY,posZ), Quaternion.identity);
             newTree.transform.parent = transform;
         }
     }
 }

}