Create objects above the terrain

I did a terrain generator code, and a trees generator code too, the trees "spawn" sometimes fying and sometimes half under the terrain and half above, what can I do to make the trees spawn exatcly with the bottom in the terrain? Here is the code:

 using UnityEngine;
 
 public class TreeGenerator : MonoBehaviour
 {
     [SerializeField] GameObject treeType1;
     [SerializeField] GameObject treeType2;
     [SerializeField] int treeAmount;
     [SerializeField] int treeAmount2;
 
     void Start(){
         GenerateTrees();
     }
 
     void GenerateTrees(){
         for(int i = 0; i < treeAmount; i++){
             GameObject trees = Instantiate(
               treeType1, new Vector3(
               Random.Range(1, 2000), 
               Random.Range(1, 20), 
               Random.Range(1, 2000)), 
               Quaternion.identity
                     
             );}
         
 
         for(int i = 0; i < treeAmount2; i++){
             GameObject trees = Instantiate(
                 treeType2, new Vector3(
                 Random.Range(1, 2000),
                 Random.Range(1, 20),
                 Random.Range(1, 2000)),
                 Quaternion.identity);
         }
 
     }
 }