Loki Tree

Loki Tree is a type of tree first mentioned in Episode 20-"We Are Family Part II" The tree was was never shown, but it was mentioned by Gobber. It is shown to have a horrible smell, and the ash of the Loki tree could be used to apply black colour. This type of tree grows only in one place: Outcast Island. Since Outcast Island is a dry and rocky area, this might suggest that the Loki tree doesn't need much moisture and soil minerals, and it might be a dry desert vegetation. The name of this tree is named after the Norse god Loki.

We Are Family Part II
When Stoick, Gobber and the gang come to the Isle of Night to look for Hiccup, the gang come across the fake Night Fury. Before anyone could stop them, Ruffnut and Tuffnut blow it up. When Stoick and Gobber arrive questioning if they found something, they reply that they found a fake Night Fury and was blown to pieces by Ruffnut and Tuffnut. Gobber examines a piece of the blown up Night Fury, and says that it was made of dragon skin, and then covered with the ash of the Loki tree. When Snotlout says that he has never heard of such a tree, Stoick and Gobber reply together that this tree grows only in one place- Outcast Island. Hence they conclude that Hiccup and Toothless are on Outcast Island.

Trivia

 * In Episode 16-"Defiant One", some trees are shown in Outcast Island. It is possible that these trees are Loki trees.