The ratio Chl a/b is a good indication of acclimation of the plant species to the environment such as nitrogen content and light. Fo further information, I advise you read the following article:
Increases of chlorophyll a/b ratios during acclimation of tropical woody seedlings to nitrogen limitation and high light
ChlB supports ChlA by supplying more light energy to Chla. Under shade when Chla fails to absorb more light, Chlb takes a leading part in supplying more light to Chla just to continue more photochemical reaction. ChlB is more adaptive to stress condition. Usually, the plant possesses more Chla than Chlb.