public int NumberOfTrees = 100;
  List<GameObject> trees;

  void Start()
  {
      trees = new List<GameObject>();

      for (int i = 0; i < Mathf.RoundToInt(NumberOfTrees / 2); i++)
      {
          for (int y = 0; y < Mathf.RoundToInt(NumberOfTrees / 2); y++)
          {
              TreeData data = ScriptableObject.CreateInstance<TreeData>();
              data.RandomiseParameters();
              TreeGeneratorManager.instance.RequestTree(data, new Vector3(i * 10, 0, y * 10), callback);
          }
      }
  }

  public void callback(GameObject g)
  {
      trees.Add(g);
  }