Forest ecosystems cover 70% of the park's territory. The lowest zones (800-1500 m) are occupied with beech forests consisting primarily of European beech (Fagus sylvatica), riparian deciduous forests dominated by grey alder (Alnus incana) and mixed forests of beech, common hornbeam (Carpinus betulus) and hop hornbeam (Ostrya carpinifolia). Above these ecosystems are located xerothermic oak forests, dominated by sessile oak (Quercus petraea), whose upper zones are covered by European beech, European silver fir (Abies alba) and Bulgarian fir (Abies borisii-regis). Coniferous ecosystems occupy altitudes between 1300 and 2200 m and consist of Norway spruce (Picea abies), Macedonian pine (Pinus peuce), Scots pine (Pinus sylvestris) and firs. Secondary ecosystems that replace primeval forests cover 5% of the park consist mainly of common aspen (Populus tremula), silver birch (Betula pendula) and common hazel (Corylus avellana) replacing spruce and pine forests. The park is the only habitat of a local endemic tree species, the Rila oak (Quercus protoroburoides) that grows only in three localities along the Rilska River valley.