The NNR covers two glacial cirques with lakes on the slopes of Mt. Jezerní hora, 4 – 6 km north-west of the municipality of Železná Ruda on the Šumava PLA territory.
Parishes: Hojsova Stráž, Železná Ruda (Plzeňský kraj – Plzeň District)
Area: 174.86 ha
Elevations: 1008 – 1343.4 metres
The subject of the protection are the two lakes: Černé jezero (Black Lake) and Čertovo jezero (Devil’s Lake) which fill the floors of two distinctively modelled glacial cirques with well-developed talus cirque spruce stands on the slopes and fragments of sub-alpine, forest-free communities and the natural, climax spruce stands on the ridge of Mt. Jezerní hora. The only locality in the Czech Republic where the quillwort (Isoëtes lacustris) grows is in the Černé jezero glacial lake.
The character of the reserve is determined by the two glacial cirques containing natural lakes, which are held back by frontal moraines. The extensive and distinctively modelled cirque of Čertovo jezero lake – the largest and lowest-lying lake on the Czech side of the Šumava mountains at 18.43 ha, 1008 metres above sea level and a catchment of 1.286 km² belonging to the Labe (Elbe) catchment lies on the north-eastern slopes of Mt. Jezerní hora (1343.4 metres). The smaller and less distinctive cirque of the Čertovo jezero – 10.33 ha, 1030 metres above sea level and with a catchment of 0.857 km² lies on the south-eastern slopes of the same mountain and belongs to the Danube catchment. Rocky ledges and boulder flows have developed on the steep cirque walls and on the walls above the Černé jezero we can see rocky shelves which show evidence of being carved by glacial movements. Extensive talus fields lie on the upper south-western slopes of Mt. Jezerní hora.
The rocky basement under the whole reserve is built of biotite-muscovite mica schists with morphologically significant bodies of quartzites and quartzite schists up to several metres thick. Notable layers of Quaternary deposits can be found in places at the foot of the cirque cliffs. These deposits are of glaciogenic, periglacial and alluvial origins.
The soil cover is predominantly of podzols – typical podzols on the mountain ridge and cambic podzols on the slopes with areas of typical cryptopodzol. Cambic gleysols are developed in waterlogged areas with typical rankers found on the rocky slopes and open talus fields.
A notable natural factor which influences the distribution of plant communities (especially in the Černé jezero cirque) is the effects of the anemo-oreographic system which comes up the Weisser Regen stream valley and is reflected in the exceptional snow conditions in the locality. The regular snow movements and small avalanches on the steep slopes, together with the long-lasting snow beds under the rocky ledges significantly lower the upper border of the closed forest stands in the cirques. The stands above this border can be regarded as fragments of sub-alpine vegetation.
The short-stemmed communities of the rocky ledges and fissures of the Juncion trifidi alliance (Agrostis rupestris-Juncus trifidus association) in the Černé jezero cirque include the three-leaved rush (Juncus trifidus), rock bent-grass (Agrostis rupestris) and occasionally the parsley fern (Cryptogramma crispa). Contiguous growths of purple moor-grass (Molinia caerulea) occupy the rock fissures into which water flows.
Stands of dwarf pines (Pinus mugo) with small numbers of rowan (Sorbus aucuparia) and the willow Salix appendiculata can be found in places on the rocky ledges in both cirques and belong to the Pinion mugi alliance(Myrtillo-Pinetum mugi association).
The snow bed localities support the development of sub-alpine tall-herb vegetation of the Dryopterido-Athyrion alliance (Gentiano pannonicae-Athyrietum alpestris association). The dominant species here is Alpine lady fern (Athyrium distentifolium) and other species growing here are the gentian - Gentiana pannonica, baneberry (Actaea spicata), Alpine sow-thistle (Cicerbita alpina), large white buttercup (Ranunculus platanifolius), common monkshood (Aconitum callibotryon) and the goatsbeard (Aruncus vulgaris).
Fragments of two kinds of natural beech stands can be found on the slopes of both cirques. The beech stands with a tall-herb undergrowth and higher species diversity on the slopes above the Černé jezero lake can be classified as belonging to the beech-maple type (Aceri-Fagetum) whereas the poorer beech stands above the Čertovo jezero lake are of the acidiophilous beech with small-reed type (Calamagrostio villosae-Fagetum association).
Small fragments of montane athyrium spruce forest (Athyrio alpestris-Piceetum) can be found on the concave slopes with a damp substrate and with the dominant species – Alpine lady-fern – and also Alpine sow-thistle, the ragwort (Senecio hercynicus) and others. Climax spruce forest with hairy small-reed (Calamagrostio villosae-Piceetum) can be found in poorer localities on the cirque slopes and on the slopes and flat ridge of Mt. Jezerní hora and the dominant species here are hairy small-reed (Calamagrostis villosa) along with wavy hair-grass (Avenella flexuosa), wood-rush (Luzula sylvatica), purple coltsfoot (Homogyne alpina) and others.
Černé jezero lake has the more interesting aquatic flora with the unique submerged community of the Isoëtion lacustris alliance at depths of 2.5 – 6 metres with a strong population of quillwort (Isoëtes lacustris). The floating bur-reed (Sparganium angustifolium) which used to grow here has now disappeared from the lake. The second most common plant on the lake bed is now the moss - Sphagnum lescurii.
Both lakes, but especially the Černé jezero, have a relatively high species diversity of algae. So far, 205 species of algae and blue-green algae have been recorded in and around the glacial lakes. The most common planktons in both lakes are Peridinium umbonatum, Gymnodinium uberrimum and Dinobryon pediforme. A total of 144 kinds of diatoms were identified in profiles of undisturbed lake sediments which were collected from the Čertovo jezero lake, and this helps us to date the changes in chemical composition of the lake water over time.
The cirques of both glacial lakes also represent an exceptional territory for their wealth of mosses and lichens. The moss Rhabdoweisia crenulata does not grow anywhere else in the Czech Republic except in these two cirques. The moss Plagiothecium neckeroideum only grows here and in the Plešné jezero cirque in Šumava NP and the moss Andreaea crassinervia also grew here in the past. One of only two localities in the Czech Republic where the moss Nardia compressa grows is in the stream in the Čertovo jezero cirque.
The Černé jezero cirque (along with the Plešné jezero cirque in Šumava NP) has one of the highest lichen species diversities in the Czech Republic and species which are found nowhere else in the Czech Republic except for these two cirques include Caloplaca lucifuga, Porina hibernica and Rinodina interpolata. The NNR territory is the only known refuge in the Šumava mountain range (NP + PLA) for a large number of Alpine-Arctic lichens including Alectoria ochroleuca, Leptogium teretiusculum, Sphaerophorus fragilis, Thamnolia vermicularis and Umbilicaria proboscidea. Other rare and endangered lichens which have recently been recorded here include Lobaria pulmonaria, Nephroma parile and Sphaerophorus globosus, a number of saxicolous micro-lichens such as Lecidea commaculans, Rhizocarpon leptolepis and Umbilicaria torrefacta, several epiphytic species such Bryoria bicolor, Parmeliella triptophylla and Rinodina archaea as well as terricolous species such as Micarea turfosa and Polyblastia gothica.
Many notable fungi species can be found in the NNR and especially in the glacial cirques, including the protected wood-rotting fungus - Camarops tubulina and the rare bracket fungus Phellinus nigrolimitatus which is bound to the trunks of old fallen spruces. Several Ascomycete species which were new to science have been recorded in the reserve.
The fauna composition in the reserve is a reflection of the climax spruce forest zone of the Královský hvozd ridge which lies above the glacial cirques. The species diversity is relatively low but contains very significant relict elements. The rare Arctic-Alpine spider Acantholycosa norvegica lives in the periglacial talus fields on Mt. Jezerní hora. The Šumava endemic ground beetle form Oreonebria castanea sumavica lives on the summit areas of Mt. Jezerní hora and in the glacial cirques. Other significant beetle species found here include the ground beetles Calathus micropterus, Carabus linnei, Trechus alpicola and Amara erratica, the weevil Barypeithes araneiformis, the carrion beetles Aclypea souverbiei and Pteroloma forsstroemi, the rove beetles Mycetophorus monticola and Anthophagus omalinus and the longhorn beetles Evodinus clathratus and Judolia sexmaculata. The surroundings of the Černé jezero lake are a typical locality for the millipede Haasea pinivaga and the worm Dendrobaena vejdovskyi. The zooplankton in both lakes was studied in the late 19th century and again in the mid 20th century and this research is presently continuing and therefore we have data to evaluate the long-term changes in the chemical composition and richness of the lake water. The water flea Ceriodaphnia quadrangula still survives in the Černé jezero lake but other plankton-sized crustaceans which were previously recorded here have now died out. The larvae of the boreal caddis fly Molanna nigra, also develop in the lakes, which are on the southern border of its distribution here.
The vertebrate fauna, except for several eurytopic species, is formed of species which are typical for montane coniferous forests. Birds, which nest in the reserve include the capercaillie (Tetrao urogallus), three-toed woodpecker (Picoides tridactylus), ring ouzel (Turdus torquatus) and raven (Corvus corax). Lynx (Lynx lynx) regularly visit the whole Mt. Jezerní hora area.
Spruce stands cover the great majority of the area and are of varied origin. The old stands on the montane plateau of Mt. Jezerní hora are probably natural climax spruce forests. The oldest spruces are up to 300 years old and as they stand alone, they indicate that there were originally open spruce forests here. Large areas of these spruces were felled after being attacked by bark beetles in 1996 – 1998, but the wood was left lying in place. Otherwise the forests are left to natural development processes. The slopes of Mt. Jezerní hora and the cirque walls are covered with younger spruce stands which have grown in areas which were probably clear-cut in the early 19th century. None of the trees here are more than 250 years old. Beech trees grow as high as 1200 – 1250 metres above sea level, and in part of the Čertovo jezero cirque they are the dominant species. Firs grow in all parts of the reserve and especially below 1250 metres. The firs are naturally rejuvenating everywhere except for the summit ridge of Mt. Jezerní hora and in places they dominate the new growth. The forest stands were dramatically changed by the hurricane Kyrill in January 2007.
The reserve is a unique model territory for the study of relatively natural trends in the development of forested and forest-free ecosystems, without endangering or influencing the rich variety of communities in the glacial cirques and lakes. To maintain the undisturbed lake sediments and to protect the community of quillworts it is not permitted to enter the lakes or carry out destructive research of the lake beds. Public access to the territory is permitted along marked tourist trails.