Question: A Christmas tree was a type of alert area created during the tension that began in which year?

Context paragraphs:
White House Christmas tree: The White House Christmas Tree, also known as the Blue Room Christmas Tree, is the official indoor Christmas tree at the residence of the President of the United States, the White House.  The first indoor Christmas tree was installed in the White House sometime in the 19th century (there are varying claims as to the exact year) and since 1961 the tree has had a themed motif at the discretion of the First Lady of the United States.

Cold War: The Cold War was a state of geopolitical tension after World War II between powers in the Eastern Bloc (the Soviet Union and its satellite states) and powers in the Western Bloc (the United States, its NATO allies and others).  Historians do not fully agree on the dates, but a common timeframe is the period between 1947, the year the Truman Doctrine (a U.S. foreign policy pledging to aid nations threatened by Soviet expansionism) was announced, and 1991, the year the Soviet Union collapsed.

Loring Air Force Base Alert Area: The Loring Air Force Base Alert Area is a former alert area for B-52 Stratofortress aircraft of the 42d Bombardment Wing situated at the former Loring Air Force Base at Limestone, Maine.  It was constructed in 1960 due to a demand by the Strategic Air Command that its bomber bases have a staging area to launch alerts from.

Christmas tree (aviation): A "Christmas tree" was a type of alert area constructed by the United States Air Force for the Strategic Air Command (SAC) during the Cold War.  Oftentimes, bombers or tanker aircraft were stationed next to a readiness crew building (RCB), also known as "mole hole" facilities.  The alert apron, also known as an alert ramp, received the name "Christmas tree", because in planform it resembled a tree of the same name.

Christmas tree production in the United States: While the first Christmas tree farm may have appeared as early as 1901, Christmas tree production in the United States was largely limited to what could be harvested from natural forests until the 1950s.  Among the important Christmas tree producing areas in the U.S. are Wisconsin, North Carolina, Pennsylvania and the Pacific Northwest.  In 2002 Christmas tree production in the United States totaled 20.8 million trees and the U.S. was one of the world's leading producers of natural Christmas trees.  That same year, Pennsylvania was the top producer in the United States.

Christmas tree stand: A Christmas tree stand is an object designed to support a cut, natural or an artificial Christmas tree.  Christmas tree stands appeared as early as 1876 and have had various designs over the years.  Those stands designed for natural trees have a water-well, which, in many cases may not hold enough water to adequately supply the cut tree.  Some specialty Christmas tree stands have value on the secondary antiques market.

Aluminum Christmas tree: An aluminum Christmas tree is a type of artificial Christmas tree that was popular in the United States from 1958 until about the mid-1960s.  As its name suggests, the tree is made of aluminum, featuring foil needles and illumination from below via a rotating color wheel.  The aluminum Christmas tree was used as symbol of the commercialization of Christmas in the highly acclaimed and successful 1965 television special, "A Charlie Brown Christmas", which discredited its suitability as holiday decoration.  By the mid-2000s aluminum trees found a secondary market online, often selling for high premiums.  The trees have also appeared in museum collections.

Alert area: An Alert Area is a place of alert for military aircraft at United States Air Force bases.  During the Cold War, bomber aircraft would be stationed on the Christmas trees within the area, ready to take off at a moment's notice.  Oftentimes, tanker aircraft would be stationed in a separate area of the base, and would take off along with their bomber counterparts.  At the Alert Area at Loring Air Force Base, the area consisted of a forty acre high-security area located to the southeast corner of the runway, and bordered to the east by forest.  It also contains a Christmas tree and mole hole, among other buildings.

Mount Ingino Christmas Tree: The Mount Ingino Christmas Tree is a lighting illumination in the shape of a Christmas tree that is installed annually on the slopes of Mount Ingino (Monte Ingino in Italian) outside the city of Gubbio, in Umbria region in Italy.  The tree is also called the Gubbio Christmas Tree or "the biggest Christmas tree in the world".  In 1991 the Guinness Book of Records named it "The World's Largest Christmas Tree".

Singing Christmas Tree: A Singing Christmas Tree, sometimes called a Living Christmas Tree, is an artificial Christmas tree filled with singers used as part of nativity plays. First appearing in 1933 at Belhaven University, they later made their debut indoors at West High School in Denver, Colorado, and then in 1957 in Sacramento, California.  Constructed of steel, the tree is actually a conical circular sector where between one-third and one-half of an actual Christmas tree is shown.  Depending upon tree size, they can accompany between 30 and 450 singers.  These trees can be put up by churches or communities depending upon the location.  Since then, the singing Christmas tree concept has spread to Canada, the Philippines, South Korea, Switzerland and Sri Lanka.  The most attended tree takes place in Knoxville, Tennessee, with 60,000 attendees in 2007.

Supporting paragraphs: Christmas tree (aviation) Cold War
Answer: 1947