public class Behaviour
{
    Item owner;

    public Behaviour()
    {
        // base class, does nothing.
    }

    public void Attach(Item item)
    {
        owner = item;
    }
}

public class Harvestable : Behaviour()
{
    public Harvestable()
    {
        // This is just a base class so it doesn't do anything?
    }

    public virtual void Harvest()
    {
       // maybe log that the class was not set correctly
    }
}

public class HarvestReplace : Harvestable
{
    ItemType replaceType;

    public HarvestReplace(ItemType replaceWith)
    {
         this.replaceType = replaceWith;
    }

    public override void Harvest()
    {
        owner.Remove();
        owner.World.AddItem(replaceType);
    }
}

public class ItemType
{
    someVar someProperty;
    List<Behaviour> behaviours;

    public ItemType(ushort id)
    {
        // EXAMPLE if it was a tree, in reality there's a load of different types here.
        ItemType tree = new ItemType;
        tree.someProperty = someValue;
        tree.behaviours = new List<Behaviour>();
        tree.behaviours.Add(new HarvestReplace(ItemTypes.Log);
    }
}

public class Item
{
    World world;
    someVar someProperty;
    Harvestable harvestable;

    public Item(World parent, ItemType type, Vector3 position
    {
        // Get all the basics
        world = parent;
        this.someProperty = type.property;

        // now assign behaviours from type
        // Uh this won't actually work with multiple objects as you're assigning an object reference
        // I.e: Create TreeA then TreeB and Harvest TreeA, TreeB will get deleted.
        foreach(Behaviour behaviour in type.behaviours)
        {
            behaviour.Attach(this);
            if(behaviour is Harvestable)
            {
                this.harvestable = behaviour;
            }
        }

    }
}