It covers a section of a peaty lowering that is adjacent to a dune hill. The protection here focuses on the habitat of the naturally-renewing European yew in a multi-species mixed forest.
In Poland, European yew can be found in the southern, western, and northern part of the country, where it grows individually and in small insular habitats. It thrives within a mild maritime climate and on fresh fertile soil. However, large populations of European yew constitute a rarity and therefore they are protected in reserves.
European yew belongs to the slowest-growing, shade-tolerant and long-living coniferous trees. It grows in the form of low trees and shrubs. It does not produce resin. Its wood is strong and flexible, of a reddish colour, and its needles are dark-green, soft, and lighter at the bottom. Due to its edible seeds, it is spread by birds.
60% of the Choczewskie Cisy Reserve population consist of one- and two-year seedlings. There are several dozen of the trees with a circumference of over 30 cm, and some of oldest reach 140-150 cm in circumference and 10-13 m in height. Probably the European yew population in the reserve grew from seeds brought here from the palace park in Sasino, created in 1868. Due to the wetness of the area, extensive drainage work was conducted here, resulting in a network of ditches. Despite this, the level of ground water is high, which, accompanied with high air humidity, enhances the development of an extensive stand of European yew.
As a result of anthropogenic transformations of the natural plant community, new combinations of species and community-associations have come into being. The ash-alder carr has preserved the most natural features. The presence of European yew in the layer of shrubs, undergrowth and underbrush is an evidence of the diversity of the age structure of its population.
Mixed forests with beech, oak and birch are dominant in the area, but there are locations where pine prevails, constituting the oldest tree stand. Hornbeam and hazel can also be found here. Among the rare plants, and the ones covered by species protection, there are heath spotted orchid, cross-leaved heath, bog myrtle, Northern firmoss, Glyceria nemoralis, and yellow pimpernel.
Because the under-bush and the seedlings of European yew are sensitive to treading, it is forbidden to enter the reserve. The inside of the reserve can be seen by moving along the bordering road.