Christmas tree cultivation major Christmas tree growing region, Douglas-fir has always been the primary species grown. A full one-half of all trees produced in the Pacific Northwest are Douglas-fir. Douglas-firs typically take five to seven years before they are mature enough to sell as Christmas trees. Also common in the region are noble fir, a tree which commands a higher price than Douglas-fir, and grand fir, which accounts for about 10 percent of the annual harvest in the Northwest. Other species collectively account for only 3–5 percent of the total Northwest harvest. In North America, Fraser fir, grown in the Appalachian Mountains of
Tree walking automaton Tree walking automaton A tree walking automaton (TWA) is a type of finite automaton that deals with tree structures rather than strings. The concept was originally proposed by Aho and Ullman. The following article deals with tree walking automata. For a different notion of tree automaton, closely related to regular tree languages, see branching automaton. All trees are assumed to be binary, with labels from a fixed alphabet Σ. Informally, a tree walking automaton "A" (TWA) is a finite state device which walks over the tree in a sequential manner. At each moment "A" visits a node "v" in state