HOUSTON - The rock-bottom mortgage rates that propped up the housing market during the downturn will hold it back in the years ahead, real estate economists say. Mortgage rates will inevitably increase as the economy continues to recover and the Federal Reserve winds down a stimulus program that has kept rates artificially low, according to several economists who spoke on a panel on the future of the housing market at the National Association of Real Estate Editors' (NAREE) spring conference. As that shift occurs, many homeowners who might consider selling their homes to buy new homes will choose to hold onto them in order to avoid trading in mortgages with historically low rates for loans with steeper ones, they said. Almost half of all U.S. mortgages have rates of 4.5 percent o...