"No longer able to fish, since they lacked the lumber to build canoes, the islanders established a diet based on rat meat. Their soil degraded by the rapid ecol

"No longer able to fish, since they lacked the lumber to build canoes, the islanders established a diet based on rat meat. Their soil degraded by the rapid ecological change, they burned trees and planted gardens of broken rock to enrich it;" I don't get it, did they burn some trees that could not be transformed into canoes?