The French participe passé can - as also the English perfect participle - substitute a relative clause.

Example
Les arbres plantés le long de l' avenue présentent un risque pour le traffic.
  The trees planted along the alley were a risk for the traffic.
Les arbres qui sont plantés le long de l' avenue présentent un risque pour le traffic.
  The trees that were planted along the allwey were a risk for the traffic.






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