If this natural area was deeded to the HOA, then the board of directors have the authority to maintain, or not. The owner can't force the HOA to maintain this. The HOA could, out of courtesy, budget for it next year. Likey was promised to the owner that it would be maintained by the sales person, they'll say anything to close the deal.
I manage on-site, an association with 3,300 homeowners, jurisdictional wetlands, and numerous amenities. Good luck. 4951