Located in the forest area of the municipality of Stara Gradiška, this special reserve of forest vegetation covers an area of 52 ha of the last remnants of the Slavonian oak rainforest, today it is used for scientific research and for the needs of foresters. In ancient times, such forests covered the entire area between the Sava and Drava rivers. Prašnik is a unique secondary rainforest of old Slavonian oaks in the area of Stara Gradiška municipality. It is considered a rainforest because it has never been managed. It was excluded from regular management as early as 1928, with an area of about 53 ha, and is located at approximately 96 m above sea level. Today it is a special reserve of forest vegetation with about 1,000 old oak trees and about 150 hornbeam and beech trees. Between the old trees, which are quite rare, a young mixed forest appears. Oak trees are between 250 and 300 and even more years old, have a chest diameter of 70 to 220 cm, and the height of individual giants is even greater than 40 m. The volume of such trees reaches an astonishing 50 m3.