Cannot make static reference to non-static field (Java)

Question

Hey guys I'm having trouble figuring this out. Here is the code:

  package apps;

import java.util.Iterator;
import java.util.NoSuchElementException;

import structures.Vertex;


public class PartialTreeList implements Iterable<PartialTree> {

    /**
     * Inner class - to build the partial tree circular linked list 
     * 
     */
    public static class Node {
        /**
         * Partial tree
         */
        public PartialTree tree;

        /**
         * Next node in linked list
         */
        public Node next;

        /**
         * Initializes this node by setting the tree part to the given tree,
         * and setting next part to null
         * 
         * @param tree Partial tree
         */
        public Node(PartialTree tree) {
            this.tree = tree;
            next = null;
        }
    }

    /**
     * Pointer to last node of the circular linked list
     */
    private Node rear;

    /**
     * Number of nodes in the CLL
     */
    private int size;

    /**
     * Initializes this list to empty
     */
    public PartialTreeList() {
        rear = null;
        size = 0;
    }

    /**
     * Adds a new tree to the end of the list
     * 
     * @param tree Tree to be added to the end of the list
     */
    public void append(PartialTree tree) {
        Node ptr = new Node(tree);
        if (rear == null) {
            ptr.next = ptr;
        } else {
            ptr.next = rear.next;
            rear.next = ptr;
        }
        rear = ptr;
        size++;
    }

    /**
     * Removes the tree that is at the front of the list.
     * 
     * @return The tree that is removed from the front
     * @throws NoSuchElementException If the list is empty
     */
    public PartialTree remove() 
    throws NoSuchElementException {

            Node temp = new Node(Node.tree);
            return null;
    }

    /**
     * Removes the tree in this list that contains a given vertex.
     * 
     * @param vertex Vertex whose tree is to be removed
     * @return The tree that is removed
     * @throws NoSuchElementException If there is no matching tree
     */
    public PartialTree removeTreeContaining(Vertex vertex) 
    throws NoSuchElementException {
            /* COMPLETE THIS METHOD */

            return null;
     }

    /**
     * Gives the number of trees in this list
     * 
     * @return Number of trees
     */
    public int size() {
        return size;
    }

    /**
     * Returns an Iterator that can be used to step through the trees in this list.
     * The iterator does NOT support remove.
     * 
     * @return Iterator for this list
     */
    public Iterator<PartialTree> iterator() {
        return new PartialTreeListIterator(this);
    }

    private class PartialTreeListIterator implements Iterator<PartialTree> {

        private PartialTreeList.Node ptr;
        private int rest;

        public PartialTreeListIterator(PartialTreeList target) {
            rest = target.size;
            ptr = rest > 0 ? target.rear.next : null;
        }

        public PartialTree next() 
        throws NoSuchElementException {
            if (rest <= 0) {
                throw new NoSuchElementException();
            }
            PartialTree ret = ptr.tree;
            ptr = ptr.next;
            rest--;
            return ret;
        }

        public boolean hasNext() {
            return rest != 0;
        }

        public void remove() 
        throws UnsupportedOperationException {
            throw new UnsupportedOperationException();
        }

    }
}

I'm having trouble on writing the remove() method. I know all I need to do is set tree to tree.next and return what I removed from the linked list, but for the life of my I can't figure out how to access the tree when I don't have a parameter.

I get an error on the Node temp = new Node(Node.tree); line.