In many places, Sanginjoki’s forests consist of former, slow-growing commercial forests whose most common tree species is pine (*Pinus sylvestris*). The value of areas reserved for conservation purposes lies in the future as the former commercial forests gradually age. In addition to these forests, there are old-growth forest areas that are magnificent and varying in terms of biodiversity, such as spruce mires.

The oldest and largest spruces (*Picea abies*) in the Sanginjoki Nature Reserve are located in Asmonkorpi and Iso Polvikangas, along the Korpilammen kierros trail. These spruce mires are one of the oldest parts of the nature reserve, where nature has been left alone to grow on its own terms for decades.
The wildnerness-like shores of Lake Isokankaanjärvi are surrounded by an old coniferous forest. Behind the north shore of the lake, forests turn into dry lichen heaths. Takaharju esker, which rises to a height of approximately 90 metres at Isokangas, is one of the highest points in Oulu.
Aspen (*Populus tremula*) is a keystone species in forests. There are over one hundred species that depend on the aspen, with approximately one thousand species able to live in them. Aspen is an important growth substrate for fungus, moss and lichen species. It also serves as a nesting tree for woodpeckers, such as the black woodpecker (*Dryocopus martius*), which thrive in Sanginjoki. Other species, such as the European pine marten (*Martes martes*) and Boreal owl (*Aegolius funereus*), use old black woodpecker nesting holes.

Former commercial forests are largely heath forests with a blueberry understory. Many species thrive in these forests, which are also enjoyed by berry pickers and hikers. Blueberries are an important food for not only humans, but also many bird species, which means that a large number of pollinators are needed to maintain a high berry yield. Indeed, it has been calculated that six bumblebee queens are needed to pollinate enough berries to make a single blueberry pie.

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