替代木材的替代燃料来源

In my world, humans live on an island. The island is not real big, there is a volcano, a river around which is a marsh.The forest on the island is sacred for them so they cannot enter it and cannot harvest any trees. They use mud for construction but I can't figure out what would they use as fuel I was thinking of palm wood but there would have to be quite a lot of these I guess. And I forgot to mention that they are bronze age. Any tips or suggestions?In my world, humans live on an island. The isla