The interaction of complex climatic, hydrological, geological and topographical conditions make Bulgaria one of the most biologically diverse countries of Europe. Phytogeographically, Bulgaria straddles the Illyrian and Euxinian provinces of the Circumboreal Region within the Boreal Kingdom. The country falls within six terrestrial ecoregions of the Palearctic realm: Balkan mixed forests, Rodope montane mixed forests, Euxine-Colchic deciduous forests, Aegean and Western Turkey sclerophyllous and mixed forests, East European forest steppe and Pontic–Caspian steppe. Around 35% of Bulgaria's land area consists of forests, which include some of the oldest trees in the world, such as Baikushev's pine and the Granit oak. Bulgaria's flora contains between 3,800 and 4,200 vascular plant species of which 170 are endemic and 150 are considered endangered. There more than 6,500 species of non-vascular plants and fungi.