Most parts of Ärjänsaari Island are covered by spacious heath forests, where walking is easy.

The forests of the island are even-aged, old pine forests in a sandy heath. The most common vegetation types are the Vaccinium-Myrtillus (lingonberry-bilberry) type and the Empetrum-Vaccinium (crowberry-lingonberry) type.
There have been felling activities numerous times, but time and peace increase the biodiversity of the forests and the extent to which they are in their natural state. The latest fellings on the island took place in 2016, when thinning and small-scale clearcutting were carried out in an area that covered about 65 hectares.
The most common tree species on the island is the pine. Due to the sandy soil, the forests will continue to be pine-dominant, and handsome old trees enrich the landscape.
There are not many spruces on the island. There are grey alder, silver birch and aspen trees, particularly in the more nutrient-rich areas, such as in the herb-rich forest that surrounds the old café building. In addition, there are small mire areas in the eastern and southern parts of the island.

As a counterbalance to the rugged esker forests in the inner part of the island, there is a herb-rich forest with humus soil in the surroundings of the old café building. The plants use groundwater that is generated in the area.

The herb-rich forest is home to the baneberry, the mezereon, the spring vetch, the fingered sedge and the wood vetch.In the herb-rich forest, there are polypore species that grow on deciduous trees and on the birch, in particular.
Amongst these are the artist’s conk, Phellinus punctatus syn. Fomitiporia punctata, the willow bracket, smoky bracket, gilled polypore, chaga mushroom, tinder fungus, birch polypore, mossy maze polypore and the ochre bracket. All these polypores indicate that there is decaying birch wood in the herb-rich forest.

Polypores have preliminarily been studied in the herb-rich forest that surrounds the café and along the trails. Amongst the indicator species of the old-growth forests on the island are the soft bracket and Phellinus pini, which is a common sight on old pines on the island.

Bird watching