The trees, originally planted by the Carthaginians in 300 b.c. or thereabouts, thrive under irrigation.
The trees all die because there isn't enough irrigation to keep them hydrated. 
mnli hypothesis: The trees all die because there isn't enough irrigation to keep them hydrated.  premise: The trees, originally planted by the Carthaginians in 300 b.c. or thereabouts, thrive under irrigation.