Slovenia's geography: the country that named karst

Why a plateau in Slovenia named a landform

Between Ljubljana and the Gulf of Trieste the bedrock is limestone, and water has been working on it long enough that most of the drainage has moved underground. Slovenes call the region the Kras. Geologists everywhere call the process karst, and the word came off that plateau. UNESCO's citation for the Škocjan Caves records that the terms karst and doline both originate in this region, and that specialists refer to the surrounding country as Classical Karst.

The Kras is a spur of the Dinaric Alps in the south-west, and it takes up about one quarter of Slovenia's area while holding a small share of its people. Surface water is scarce. Settlement sits between wooded limestone ridges, in dry and blind valleys, in hollows and in poljes, the flat-floored basins that karst country produces instead of ordinary valleys. Sinkholes and poljes hold terra rossa, the red soil left behind as the limestone underneath it breaks down.

The Reka River shows the mechanism at full size. It runs on the surface, drops into the ground, and cuts a canyon roughly two kilometres long inside the rock, in places 150 metres high and more than 120 metres across. The Martel Chamber on that course exceeds two million cubic metres in volume. The protected park above it covers 413 hectares and takes in some six kilometres of passages reaching more than 200 metres below the surface.

The study of this ground is older than the English word for it. Karst research began in the eighteenth century in Habsburg Carniola; Valvasor drew the collapsed dolines at Škocjan in 1689; the cave system was recorded as discovered in 1815; and the rimstone pools that made it famous were formally described in 1888. More than 28 million people have since walked through Postojna Cave, a quarter of an hour away by road, and Vilenica is one of several others open to visitors. Predjama Castle, near Postojna, is built half inside a cave mouth.

The dark has a fauna of its own. Slovenia holds a few tens of endemic cave species, and the only cave vertebrate known there is the olm, which lives in the Karst, in Lower Carniola and in White Carniola.

How four of Europe's physiographic regions meet

Slovenia carries pieces of four of the continent's major physiographic units at once: the Alps, the karstic Dinaric Alps, the Pannonian and Danubian lowlands, and the Mediterranean coast. Britannica divides the country into four regions on that basis, and the shares are markedly uneven.

The country that results is mostly elevated. Average height above sea level is 557 metres, and around 90% of the land surface stands at 200 metres or more. The only substantial flat area is in the north-east, where the Pannonian plain runs out through Prekmurje toward the Mura. Ljubljana sits in a basin on the southern edge of the Alpine region, close to the geometric middle of the country, which is fixed at Slivna in the municipality of Litija.

All of it fits into a small box. The maximum distance north to south is 163 km and east to west 248 km. Total area is 20,271 km2, of which 122 km2 is water, and the fifteenth meridian east runs almost exactly down the middle from north to south. The land boundary totals 1,086 km: 670 km with Croatia, 330 km with Austria, 280 km with Italy and 102 km with Hungary.

The regions Slovenes actually name are older than the physiography and come from the Habsburg crown lands rather than from the relief. Upper, Lower and Inner Carniola, Styria, Carinthia, Gorizia, Slovenian Istria and Prekmurje are the traditional set, with White Carniola and the Central Sava Valley usually counted apart. Each of them describes a basin, a valley or a plateau, which is what happens when administrative lines are drawn in country this broken.

What the Julian Alps hold above the treeline

The Alpine region takes about two fifths of the surface. Along the borders with Italy and Austria stand the High Alps: the Kamnik and Savinja group, the Karavanke, and the Julian Alps, which carry Triglav at 2,864 metres, the highest point in the country. The peak appears on the coat of arms and on the flag. In a vale beneath it lie Lake Bohinj and Lake Bled, and Bled Island in the second of them is the only natural island Slovenia has.

Slightly lower than the High Alps is subalpine ridge-and-valley terrain, whose main range is the Pohorje, south of the Drava. The historical name for the central Alpine lands is Gorenjska, Upper Carniola, and Slovenes still use it.

The treeline runs at 1,700 to 1,800 metres. Above it the climate turns subpolar and then, on the tallest peaks, tundra, and the plants are alpine: Daphne blagayana, gentians, Primula auricula, the lady's slipper orchid, and edelweiss, which is the emblem of Slovene mountaineering. Marmots, ibex and chamois live at that height.

Triglav National Park is what protects most of it, and it took seventy-three years to arrive at its present size. Conservation was proposed in 1908 and first realised in 1924, when the Nature Protection Section of the Slovene Museum Society and the Slovene Mountaineering Society took a twenty-year lease over some 14 km2 of the Triglav Lakes Valley. Permanent protection came in 1961 over around 20 km2, on terms that satisfied nobody, and two decades of further proposals ended in the 1981 rearrangement that gave the park the 838 km2 it still covers.

From Triglav the valleys spread outward radially, and they feed two river systems that end in different seas. The Soča runs to the Adriatic. The Sava runs to the Black Sea.

Where the water goes after it leaves the mountains

Slovenia has a coast and drains away from it. 81% of the territory, 16,423 km2, lies in the Black Sea basin, and the remaining 19%, 3,850 km2, in the Adriatic basin. The sub-basins are those of the Mura, the Drava, the Sava together with the Kolpa, and the short rivers that reach the Adriatic directly.

The Sava rises in the Julian Alps, passes Ljubljana and crosses into Croatia, and its narrow valley carries the rail line toward Zagreb and on to Belgrade. The Drava enters from Kärnten and the Mura from Steiermark; the two meet in Croatia and reach the Danube, as the Sava does. Only in the west does the pattern reverse. The Soča rises beneath Triglav and, after a steep course, reaches the Gulf of Venice through Italian territory.

Steep gradients make the runoff fast. That gives the country a large hydroelectric resource and strips nutrients out of its soil at the same time. Hydropower supplies around a third of Slovenian electricity; in 2018, hydroelectric plants produced 4,421 GWh out of net national production of 12,262 GWh, with thermal plants at 4,049 GWh and the Krško nuclear plant contributing the Slovenian half of its 2,742 GWh.

The exception to fast water is the Ljubljana Marsh, a wetland on the floor of the Ljubljana basin. It was largely drained under the Habsburg monarchy and it still floods; the prehistoric pile dwellings preserved in it are on the UNESCO World Heritage list. Minor flooding and earthquakes are the natural hazards the country records.

Water quality is a working problem. The Sava carries domestic and industrial waste. Coastal waters near Koper carry heavy metals and toxic chemicals, and air pollution from chemical plants there has damaged forest through acid rain. The Reka, the river that cut Škocjan, has had water quality that varies with industrial pollution, sewage and agricultural waste, and what it carries goes straight into the cave system underneath.

How the sea reaches only one corner of Slovenia

The coastline is 46.6 km long, it runs between Italy and Croatia, and all of it lies on the Gulf of Trieste. Koper, Izola, Piran and Portorož are the towns along it. The Littoral has no natural island at all, though a plan to build an artificial one exists, and the country's single natural island sits in an Alpine lake sixty kilometres inland.

Primorska occupies barely one twelfth of the country and takes in the Slovene portion of Istria, the Adriatic hinterland, and the Soča and Vipava valleys. Koper is the main port, handling container traffic of almost 590,000 TEU a year with lines to major world ports, and it stands closer to destinations east of Suez than the northern European ports do. The rail link inland from Koper to Divača runs on a single track, and a second one has been at an early construction stage, driven by the freight increase the port projects.

That short strip also fixes the country's vertical datum. Since 2016 the Slovenian geodetic system has taken its zero-metre elevation benchmark from the tide gauge station at Koper. Until then it referred to the Sartorio mole at Trieste, in Italy.

Where the coast ends was arbitrated rather than agreed. Croatia and Slovenia signed an arbitration agreement on 4 November 2009 submitting the course of their land and maritime boundary, Slovenia's junction to the high sea, and the regime for the use of the relevant maritime areas to a tribunal. Three rounds of written pleadings followed, with more than two thousand documentary exhibits and maps, a hearing ran at the Peace Palace from 2 to 13 June 2014, and the tribunal rendered its final award on 29 June 2017. The award is part of the settlement of the country's boundaries, and this catalogue records it without taking a position on the line.

Why the weather changes over short distances

Four climate systems press on a country 248 km wide, and they interact across most of it. The north-east is continental and has the widest gap between winter and summer temperatures. The coast is sub-Mediterranean, classified as humid subtropical, and the sea's moderating effect reaches inland up the Soča valley. Most of the rest of the country is oceanic. The northern plateaus and mountains are continental with mild to hot summers and cold winters, and above the treeline the classification runs through subpolar to tundra.

Precipitation makes the contrast plainer than temperature does. Rain often arrives from the Gulf of Genoa, and some western regions take over 3,500 mm of it a year while Prekmurje in the north-east takes about 800 mm. Spring is particularly prone to rainfall. Snow is frequent in winter, and the record cover measured in Ljubljana was 146 cm in 1952.

Wind is the quiet exception to all this variety. Slovenia lies in the slipstream of the Alps, so average wind speeds are lower than on the plains of the neighbouring countries, and a good deal of what does blow is local and vertical, on a daily cycle set by the terrain. Three named winds carry regional weight: the bora, cold and gusty, and the jugo, humid and warm, both characteristic of the Littoral, and the foehn, typical of the Alpine north.

What grows on thin limestone soils

Slovenia's geology produced a soil mosaic instead of a soil. Carbonate bedrock underlies much of the country and weathers to thin lithosols, which suit forest growth and little else. The permeable brown podzols, cambisols and fluvisols that repay fertiliser cover only about a tenth of the surface, chiefly in the north-east, and the good alluvial soils sit in Subpannonia.

That is a large part of why the country is wooded. As of 2020, 60.4% of Slovenia, 12,380 km2, was under forest, mostly beech, fir-beech and beech-oak stands with a relatively high production capacity. The interior carries Central European oak and beech; the mountains carry spruce, fir and pine; pine grows on the Karst Plateau, although only about a third of that region is under pine forest. The lime tree, common throughout, is a national symbol. Remnants of primeval forest survive, the widest of them in the Kočevje area, and the old stands at Goteniški Snežnik and Kočevski Rog are on the UNESCO World Heritage list.

The remaining ground divides into grassland at 5,593 km2, fields and gardens at 954 km2, orchards at 363 km2 and vineyards at 216 km2. Arable land stood at 8.53% of the territory and permanent crops at 1.43% in 2005, and 100 km2 was irrigated in 2003.

Protection is extensive on paper. There are 286 designated Natura 2000 areas covering 36% of the land surface, and about 12.5% of the territory is protected under national designations, the largest being Triglav National Park. Diversity has been declining under pollution and environmental degradation even so, and the country's mean score on the 2019 Forest Landscape Integrity Index was 3.78 out of 10, a measure of how far a forest has been modified rather than of how much of it there is.

The species count is high for the area: around 1% of the world's organisms live on 0.004% of the Earth's surface. There are 75 mammal species. Recent estimates put wolves at 40 to 60 animals and brown bears at about 450, and the edible dormouse of the beech forests has been trapped long enough for the practice to count as tradition. Native domestic breeds include the Carniolan honeybee, the Karst Shepherd and the Lipizzan horse, and the marble trout is the indigenous fish, restocked by breeding programmes into streams that introduced trout had taken over.

What was taken out of the rock

Mining was never done here on a large scale, and the deposits are unusual rather than big. Idrija in the west sits on one of the few places on earth where mercury occurs both in its elemental form and as cinnabar ore, and the mercury mining site there is on the UNESCO World Heritage list. Lead and zinc came out of the Mežica mine. Both have closed, and both now take visitors underground.

Uranium was found near Žirovski Vrh in the 1960s. The mine ran for eight years and closed in 1990 when uranium prices fell. Lignite is the one fuel still worked, mostly in the north-east, and it feeds the Šoštanj power plant; output was 2.4 million tonnes in 2022, short of what the country burns, so imports continue.

Building stone has been quarried longer than any of it. Marble and limestone have come out of Slovenian ground for centuries, and the Podpeč quarry has produced marble since Roman times.

The rock also moves. Slovenia sits on the small Adriatic Plate, squeezed between the Eurasian Plate to the north and the African Plate to the south and rotating counter-clockwise, at the junction of the Alps, the Dinaric Alps and the Pannonian Basin. Tectonic fault lines cross the country, sixty destructive earthquakes have been identified in its past, and Ljubljana was wrecked by one in 1895. A network of seismic stations is active throughout the country.