The Sobień Mountain Forest Nature Reserve was established in 1971 and covers an area of 5.34 ha. As the name indicates, the reserve covers with its range the Sobień Mountain with a height of 360 m a. s. l., situated in the Słonne Mountains Range, in Manasterzec village. This peak is primarily known for the ruins of the Kmita castle from the 14th century which are on the top of the peak. A special viewing platform was built on it, from which a panorama of the San River and the Bieszczady River stretches. Sobień is also especially valuable in terms of nature. The reserve contains fragments of oak-hornbeam, lime-hornbeam forests, Carpathian beech forest and the submontane forest. The reserve looks particularly good in early spring before beeches flower leaves and the biggest amount of light gets to the undergrowth. Flowering, among others Common snowdrops, numerous hepatica, alpine squill, dentariaglandulosa, unspotted lungwort. Somewhat later, one can observe lilies of the martagon lily and 3 types of poisonous aconites: Moldavian, paniculate and platypus.
At the castle ruins area and on the southern slopes of the hill, thermophilic plants grow, such as common agrimony, willow bell, flat pea and white swallow-wort. A marked path leads to the top.