CAPE TOWN (Reuters) - South Africa cut its 2016 GDP growth forecast by almost half on Wednesday and warned Africa's most developed economy risked slipping into a “low growth trap”. South Africa's economy is expected to grow by 0.5 percent this year, down from an estimate of 0.9 percent in February, Treasury said in its medium term outlook. Treasury's growth forecast for 2017 was cut to 1.3 percent from 1.7 percent previously, and its 2018 estimate was trimmed to 2 percent from 2.4 percent. Lower growth estimates were partly due to weak investor confidence linked to perceptions of elevated political risk. (Reporting by Mfuneko Toyana; Writing by Joe Brock)