The oldest oak grove, which is over 300 years old, grows in the Širvintas forest in the regional park. Two trees of this forest, growing by the road to Seirija - Simnas, have been declared natural heritage sites. The height of the large Širvintas oak is 31 m, the circumference is 5 m. The height of the drilled Širvintas oak is 29 m, the circumference is 4.4 m.