The trees, originally planted by the Carthaginians in 300 b.c. or thereabouts, thrive under irrigation.
The apple trees were planted around 300 b.c. and are kept alive by irrigation. 
mnli hypothesis: The apple trees were planted around 300 b.c. and are kept alive by irrigation.  premise: The trees, originally planted by the Carthaginians in 300 b.c. or thereabouts, thrive under irrigation.