help change script GrassGenerator !!

hi

i need help script GrassGenerator There is a button named Generate When pressed (it clear tree)Is there a way to stop clear tree just add tree

 using Assets.Scripts.MapGenerator.Maps;
 using System.Collections.Generic;
 using UnityEngine;
 
 namespace Assets.Scripts.MapGenerator.Generators
 {
     public class GrassGenerator : MonoBehaviour, IGenerator
     {
         public int Octaves = 4;
         public float Scale = 40;
         public float Lacunarity = 2f;
         [Range(0, 1)]
         public float Persistance = 0.5f;
         public float Offset = 100f;
         public float MinLevel = 0;
         public float MaxLevel = 100;
         [Range(0, 90)]
         public float MaxSteepness = 70;
         [Range(-1, 1)]
         public float IslandsSize = 0;
         [Range(1, 100)]
         public int Density = 10;
         public bool Randomize;
         public bool AutoUpdate;
 
         public List<GameObject> GrassTextures;
         //DetailPrototype
         public void Generate()
         {
             if (Randomize)
             {
                 Offset = Random.Range(0f, 9999f);
             }
 
             List<TreePrototype> treePrototypes = new List<TreePrototype>();
             foreach (var t in GrassTextures)
             {
                 treePrototypes.Add(new TreePrototype() { prefab = t });
             }
 
             TerrainData terrainData = Terrain.activeTerrain.terrainData;
             terrainData.treePrototypes = treePrototypes.ToArray();
 
             terrainData.treeInstances = new TreeInstance[0];
 
             List<Vector3> treePos = new List<Vector3>();
 
             float maxLocalNoiseHeight;
             float minLocalNoiseHeight;
 
             float[,] noiseMap = new PerlinMap()
             {
                 Size = terrainData.alphamapWidth,
                 Octaves = Octaves,
                 Scale = Scale,
                 Offset = Offset,
                 Persistance = Persistance,
                 Lacunarity = Lacunarity
             }.Generate(out maxLocalNoiseHeight, out minLocalNoiseHeight);
 
             for (int x = 0; x < terrainData.alphamapWidth; x++)
             {
                 for (int y = 0; y < terrainData.alphamapHeight; y++)
                 {
                     float height = terrainData.GetHeight(x, y);
                     float heightScaled = height / terrainData.size.y;
                     float xScaled = (x + Random.Range(-1f, 1f)) / terrainData.alphamapWidth;
                     float yScaled = (y + Random.Range(-1f, 1f)) / terrainData.alphamapHeight;
                     float steepness = terrainData.GetSteepness(xScaled, yScaled);
 
                     float noiseStep = Random.Range(0f, 1f);
                     float noiseVal = noiseMap[x, y];
 
                     if
                     (
                         noiseStep < Density &&
                         noiseVal < IslandsSize &&
                         steepness < MaxSteepness &&
                         height > MinLevel &&
                         height < MaxLevel
                     )
                     {
                         treePos.Add(new Vector3(xScaled, heightScaled, yScaled));
                     }
                 }
             }
 
             TreeInstance[] treeInstances = new TreeInstance[treePos.Count];
 
             for (int ii = 0; ii < treeInstances.Length; ii++)
             {
                 treeInstances[ii].position = treePos[ii];
                 treeInstances[ii].prototypeIndex = Random.Range(0, treePrototypes.Count);
                 treeInstances[ii].color = new Color(Random.Range(100, 255), Random.Range(100, 255), Random.Range(100, 255));
                 treeInstances[ii].lightmapColor = Color.white;
                 treeInstances[ii].heightScale = 1.0f + Random.Range(-0.25f, 0.5f);
                 treeInstances[ii].widthScale = 1.0f + Random.Range(-0.5f, 0.25f);
             }
             terrainData.treeInstances = treeInstances;
 
             Debug.Log(treeInstances.Length + " trees were created");
         }
 
         public void Clear()
         {
             Terrain.activeTerrain.terrainData.treePrototypes = null;
         }
     }
 }

I appreciate any help