The behavior of the D1 center in semi-insulating 4H-SiC substrates revealed by low-temperature photoluminescence was investigated after post-growth high temperature anneals between 1400 and 2400oC. The influence of different post-anneal cooling rates was also studied. The optical signature of D1 was observed up to 2400oC with intensity maxima at 1700 and 2200oC. We propose that the peak at 1700°C can be related to the formation and subsequent dissociation of SiC native defects. It was found that changes in the post-annealing cooling rate drastically influence the behavior of the D1 center and the concentrations of the VC, VSi, VC-VSi and VC-CSi lattice defects.