Applying damage to an object

My character needs to interact with items. In this case, he needs to be able to cut down trees. Most of the code for it i have worked out, the bit i am stuck on is how to apply damage to an object when clicked on.

So far i have this:

 function Update () {
 
 var hit : RaycastHit;
 
 if (Input.GetButtonDown("Fire1")) {
     if(Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), hit)) {
         Debug.Log(hit.collider.tag);
             if(hit.collider.tag == "Tree") {
                 Debug.Log ("tree");
                 SendMessage ("ApplyDamage", 5.0);
             }
         }
     }
 }

with this attached to the tree object

 function ApplyDamage (damage : float) {
     print (damage);
     }

However, although it can detect that the item is a tree on a click, it throws up the error:

SendMessage ApplyDamage has no receiver! UnityEngine.Component:SendMessage(String, Object) mousefire:Update() (at Assets/Scripts/Gameplay/mousefire.js:16)

I will be missing something here, still finding my way around code :) Any help will be welcome.