uses for nickel began to develop. The town was reincorporated as a city in 1930. The city recovered from the Great Depression much more quickly than almost any other city in North America due to increased demand for nickel in the 1930s. Sudbury was the fastest-growing city and one of the wealthiest cities in Canada for most of the decade. Many of the city's social problems in the Great Depression era were not caused by unemployment or poverty, but due to the difficulty in keeping up with all of the new infrastructure demands created by rapid growth — for example, employed mineworkers sometimes ended up living in boarding houses or makeshift shanty towns, because demand for new housing was rising faster than supply. Between 1936 and 1941, the city was ordered into receivership by the Ontario Municipal Board. Another economic slowdown affected the city in 1937, but the city's fortunes rose again with wartime demands during the Second World War. The Frood Mine alone accounted for 40 percent of all the nickel used in Allied artillery production during the war. After the end of the war, Sudbury was in a good position to supply nickel to the United States government when it decided to stockpile non-Soviet supplies during the Cold War. The open coke beds used in the early to mid 20th century and logging for fuel resulted in a near-total loss of native vegetation in the area. Consequently, the terrain was made up of exposed rocky outcrops permanently stained charcoal black by the air pollution from the roasting yards. Acid rain added more staining, in a layer that penetrates up to into the once pink-grey granite. The construction of the Inco Superstack in 1972 dispersed sulphuric acid through the air over a much wider area, reducing the acidity of local precipitation. This enabled the municipality, province and Inco and academics from Laurentian University to begin an environmental recovery program in the late 1970s, labelled a "regreening" effort. Lime was spread over the charred soil by hand and by aircraft. Seeds of wild grasses and other vegetation were also spread. As of 2010, 9.2 million new trees have been planted in the city. Vale has begun to rehabilitate the slag heaps that surrounds their smelter in the Copper Cliff area with the planting of grass and trees, as well as the use of biosolids to stabilize and regreen tailings areas.