The assessed value of property in Harvard Square nearly doubled between fiscal years 2012 and 2017, an explosive growth that has contributed to rising rising rents and high turnover among Square businesses. According to data collected by the City of Cambridge, assessed property values in Harvard Square rose from $1.8 billion in fiscal year 2012 to $3.2 billion in fiscal year 2017. And as property values rise, so do rent rates, according to Michael H. Shuman, an expert on community economics, a trend he said poses problems for local retailers.