unity - How do I reference one game object in a script attached to another?

This is all in Unity, with C#.

I have attached a script to my player that should destroy a TreeGroup (as in a forest) whenever I press 'Q' and instantiate one when I press 'E'.

Here is the code:

using UnityEngine;
using System.Collections;

public class DisableTrees : MonoBehaviour {

    // Use this for initialization
    void Start () {

    }


    // Update is called once per frame
    void Update () {

        if (Input.GetKeyDown (KeyCode.A)) {
            //Disable Trees on A Keypress

            Debug.Log("Trees Disabled!");
        }

        if (Input.GetKeyDown (KeyCode.E)) {

            //Enable Trees on E Keypress

            Debug.Log("Trees Enabled!");
        }

    }
}

How do I access and find my 'TreeGroup' object group while script not being attached to 'TreeGroup' but my character instead and then Destroy on Q Keypress? And how do I Instantiate it?