The tree mark is first seen in "One of Us". It seems to mark the place which Ben refers to as "Ethan's old drop-off point," near the caves. When Ben goes over his plan with Juliet, he indicates that the supplies necessary to treat Claire's condition will be planted at this location. Juliet falsely tells Jack that the medicine at that location had been there since Ethan's time among the survivors.
The symbol's most widely-accepted meaning is the alchemic symbol for ammonium salt. It can also be interpreted as a simplified fleur-de-lis, a symbol commonly associated with the French monarchy and its settlements including Canada.
The tree mark has certain visual similarities to Juliet's mark, but it is not the same symbol. The tree mark being almost identical to Juliet's mark is a set anomaly, and was intended to be the same symbol by the prop masters. [source needed]