This post from the American Society of Landscape Architects' The Dirt blog explains.
"'The U.S. caused this real estate crisis with its housing policy. There were no down payment requirements, easy credit, and lots of capital moving into non-performing assets.' As a result, the federal government had to move in with $10 trillion investments and recovery programs ("real estate backstopping") to hold off further economic decline. To counter this trend, surplus land must be redeveloped as green space. Cities large or small can use green spaces as an "economic multiplier" that not only creates green infrastructure but also helps developers get developing again. "Parks can help unlock the real estate market." Also, tearing down underperforming, vacant housing can create wealth. "Land without buildings are still assets." If real estate entrepreneurs and parks managers collaborate on identifying opportunities, these types of program could not only lead to a "stock market explosion" but also make communities more livable."