The "Guść" reserve - with an area of 87.07 ha, it is the youngest reserve of the Kozienicka Primeval Forest. It was established by the Regulation No. 79 of the Mazowieckie Province Governor of September 11, 2002 (Journal of Laws of the Mazowieckie Province of 2002, item 6182). The current legal basis for the existence of this reserve is the Order of the Regional Director for Environmental Protection in Warsaw of April 10, 2017 (Journal of Laws of the Mazovia Province of 2017, item 3955). The aim of the reserve protection is to preserve the mosaic of forest habitats growing on the geologically and geomorphologically diversified terrace of the Vistula River. The ash alder covers the largest area here, which along with the abovementioned ash-alder riparian forest, which is the natural habitat of 91E0, is the most valuable part of the reserve. These habitats are located in a local depression filled with river sands with low peats in places. There are alder stands of various, sometimes advanced ages here. They are surrounded by dunes made of aeolian sands, overgrown with pine trees, mostly over 100 years old, with an admixture of oak and birch.