Štirovača Virgin Forest Reserve, Croatia

Everyone in Croatia as well as a great many people in the world know the famous Plitvice Lakes National Park, the first park declared in Croatia in 1949. However, this is not the earliest national park founded in Croatia’s present day territory. Over 20 years earlier in 1928, the Štirovača Valley of Central Velebit was to be signed into law as a national park. It truly must have been an incredible sight, as other places on Velebit, Croatia’s largest and most legendary mountain, have more beautiful rugged landscapes and hiking opportunities. Štirovača Valley however, displayed a more peaceful side of this mountain which was in contrast to the windy and rugged slopes of the high peaks or coastal slopes above the Adriatic Sea, which is just some 10km away on the other side of the mountains. Here it is notably cooler and more sheltered, with far greater precipitation. This climate allows temperate rainforests to develop, with forests of large Norway spruce (Picea abies), European silver fir (Abies alba), and European beech (Fagus sylvatica) trees. Occasional sycamore maple (Acer pseudoplatanus) and European ash (Fraxinus excelcisor) trees can also be found. The elevations from the valley bottom (~1040m) to the high peaks above (1500–1600m) as well as the different slope exposures make this forest very diverse. Historically this was one of the largest unbroken areas of tall forest on Velebit, and perhaps the largest continuous spruce forest as well.

The only break in the valley’s forest cover are four idyllic meadows called “padezi”, some of which were settled and used for grazing in the past. The meadows may have been man made in relatively recent history, but some sources also state that they are naturally formed due to temperature inversion. There are many unique natural phenomena in the region. Spruce bogs may be found here, which are very rare for this part of Europe. Although, perhaps the greatest wonder of Štirovača is where water and karst topography meet and streams flow beside towering cliffs, cascading down into deep cave systems which are so vast that many are not fully explored. On gentle and steep slopes above the valley bottom there are mazes of giant sinkholes, terraces, ravines, and ridges, comprising an incredibly diverse topography with each area developing notably different forest characteristics. Still higher up, there are stunted subalpine forests which finally give way to meadows and dwarf mountain pines (Pinus mugo) on the highest peaks. But if this region in the heart of Velebit is truly so special, why have few outside of Croatia heard about it? Unfortunately, this is because Štirovača National Park no longer exists. In fact, it never really existed.

In a great failure of conservation, Štirovača lost its protected status one year after being declared a park, which may have been influenced by interests to exploit the forests. Regardless, this ended the dream of preserving what could have been the Croatia’s greatest and largest old growth forest. In the past, Štirovača could have been to Croatia what Perućica is to Bosnia and Hercegovina or Biogradska Gora is to Montenegro. Today, this once pristine sanctuary of fantastic forests and wilderness survives only in historical literature. Štirovača today is devastated by muddy forest roads and areas of barren stumps. Although still fully covered by forest, the old and large trees have mostly disappeared. There is no longer any indication of the profoundly beautiful old growth groves which once grew here, now only existing in a memory so distant it is like a dream, soon to be forgotten entirely.

However, there is still one place in Štirovača where its original forest can be observed in an untouched state. Covering some ~120 hectares, the reserve described here in this report protects the last true virgin forest of the valley, from the lowest depression to the highest peak. Štirovača is actually the first forest reserve to be protected in Croatia on the 26^th of April in 1965. The Štirovača Reserve, also called “Klepina Duliba”, encompasses incredible scenery, diversity, significance, and preservation. It is undoubtedly one of the most beautiful old growth forests of Croatia, and even Europe. Štirovača is not fully researched and although scientific study was done on the overall forest structure, much of it cannot be found in literature online. Because of this the information in this report is mainly compiled from personal exploration in the reserve, some local guides from the area, as well as a few documents about the forest written in Croatian. For access to the reserve, a guide should be found from the Velebit Nature Park headquarters in Krasno village. Due to the strict protection given to the forest, access is only permitted with a guided tour. Near the reserve there is a bumpy forest road, but vehicles are not heard often as people rarely drive this route. Occasionally logging sounds can be heard from the forests located outside the reserve. However, the old growth forest itself is a true wilderness and often the rustling trees and the flowing streams are all that can be heard.

The terrain ranges from elevations of 1090–1440m, comprising flat and vast valleys, gentle to steep slopes, sinkholes, terraces, ravines, ridges, and high peaks. The subalpine forests above the reserve are also not exploited due to inaccessibility, allowing for an unbroken virgin forest from lowest valley to the Šatorina peak above the tree line (1624m). In some places there are especially pronounced concentrations of stunning karst features with cliffs, caves, and outcrops of giant mossy stones characteristic of the valley. There are almost countless streams, mostly in the lower elevations of the reserve. Some of them have water year-round, and there is even a seasonal pond in one sinkhole. All the development stages can be observed and there are more beech, maple, and fir trees than in many other parts of Štirovača valley. This allows for the composition to vary between beech, fir, and spruce dominance, creating a fantastic contrast not seen in pure spruce or fir forests. There is no shortage of deadwood and snags, and there are far more animals when compared to surrounding exploited forests. Lower elevations of Štirovača contain exceedingly beautiful cathedral groves with large trees, while higher elevations encompass twisted and tortured forests battered by wind and snow. Almost no signs of human influence can be seen aside from some research marks on trees and a rare stump or two cut with an axe a long time ago. The preserved state of the forest is evident from species such as the lichen lungwort (Lobaria pulmonaria), and the rare ghost orchid (Epipogium aphyllum), which has only been found on 4 sites in Croatia over the last 80 years. Such a place reminds of a lost history when humans were at the mercy of nature, and such forests are often the settings for folklore tales of fairies, witches, and giants.

Average annual temperatures are 6^oC or less at the valley bottom, and soils in the reserve range from very deep to extremely shallow. Expositions of all directions may be found, but northeast and east are most prevalent. Precipitation is very high with a maximum reaching up to 3000mm a year, which makes Štirovača a true temperate rainforest. Although the reserve covers ~120 hectares, 90 ha of forest is beech, fir, and spruce, while 30 ha are subalpine beech forest with some maple. However, only about 40 ha of the Štirovača Valley bottom is protected in the reserve. The other ~50 ha of beech–fir forest grow on gentle to steep east and northeast facing slopes, or in hidden ravines and sinkholes above the main valley. Due to the interesting mix of tree compositions, the 90-hectare beech–fir forest association Omphalodo-Fagetum has been split into 3 sub-associations. They are aceretosum on ~40 ha, typicum on ~45 ha, and mercurialietosum perennis on ~5 ha. However, the forest composition is not homogenous and characteristics found in one area can sometimes be found in another.

The lowest regions of the forest cover ~40 ha and can be found at elevations of 1090–1180m, within the sheltered confines of the flat Štirovača Valley and its gentle slopes. This area is the most iconic to the Štirovača region, and represents an incredible example of cathedral forest. The forest floor may be relatively clean and open, but in many places the undergrowth of ferns and butterbur (Petasites) is lush due to the high humidity. In addition to the undergrowth, mosses cover all the great rocks and trees, which makes the forest look like a smaller version of a Pacific Northwest rainforest. Although most of the terrain is very gentle, some of the best examples of karst topography in Croatia’s forest reserves can also be found in Štirovača, with ravines, caves, cliffs, and slopes. Many streams cascade through these expanses, flowing down into caves and reemerging as springs elsewhere. One cave was explored by an expedition to a depth of over 230m, past a bend in the tunnel which held an underground pool of water. Further exploration was halted by another pool.

Photo right: Despite its mostly gentle terrain, a region of Štirovača Valley is characterized by stunning karst ravines, monoliths, sinkholes, and ice caves. The vast sinkhole in the photo is surrounded by over 40-meter-high cliffs. At the bottom is a seasonal torrent which carries trunks and rocks towards a large cave entrance (not visible). The cave continues into a long underground pit system typical of karst topography. There are also many smaller caves found, like the one visible to the right. Elevation is 1090m

The trees growing in this sheltered part of the valley seem to be on average the tallest, largest, and best developed examples of their species in Štirovača. This is to be expected, as the growth conditions for trees in Štirovača are extremely favourable, leading to the high productivity and biomass. The deep valley shelters trees from strong winds, provides them with water sources, maintains cool temperatures year-round, and contains soil rich with nutrients. However, finding the tallest trees is problematic due to the difficulty of laser measuring in this part of the forest. The tallest point of a tree is often nearly impossible to find through the dense broadleaf and conifer canopy. There are also no favourable vantage points on the treetops due to the flat or gentle terrain, meaning that the laser usually hits branches just below the top due to steep measurement angles. Finding the tree’s top from a greater distance is again made very difficult by the canopy, so perhaps only LiDAR scans could truly determine where the tallest trees are. Regardless, there is a great frequency of large trees rarely found elsewhere with such frequency and development. Exceptional spruce specimens may sometimes reach heights well over 55m, but trees of up to about 50m are common. Usually they dominate the lower elevations of Štirovača Valley. Some very large fir trees may be found at the lowest elevations, but the frequency of large firs in the composition increases higher up the valley on gentle slopes. Fir also seems to grow to its greatest heights in such regions, where exceptional individuals may reach well over 50m tall, though such heights are not as common compared to spruce. Both trees can reach a cbh (circumference at breast height) of well over 500cm, and estimated trunk volumes of over 40 m³. There are also incredible beech groves, with some individuals reaching well over 40m in height and 350cm in cbh.

The valley forests are heavily stocked, and even a beech dominated grove with only a few large conifers measured at ~1100 m³/ha of living wood. In plots dominated by many large conifers, this value would be far greater. Interestingly, some of the widest and most voluminous individual conifers found are not the tallest. A few are not even 45m tall and some of these specimens have sustained varying degrees of damage to their tops or trunks and would have been taller in the past. However, some massive trees simply did not seem to grow very tall, while other relatively skinny trees were far taller. This shows the importance of genetics and natural variation in determining how trees develop differently under the same conditions.

Photo right: This spruce was perhaps the largest tree of Štirovača until it fell down. Girth nearly 570 cm, height up to 57 m, volume estimate about 45 cubic metres

The tallest fir measured was a double-trunked tree with a cbh of ~440cm and maximum height of 53m, with a volume estimated at ~30 m³. Meanwhile, the tallest spruce measured was also one of the largest spruce trees recorded in Croatia, with a maximum height of 57m, cbh of ~570cm, and estimated volume of nearly 45 m³. Our guide from Velebit Nature Park also mentioned this tree as being of exceptional size. Both specimens are now wrecks and have fallen and partially decayed since the measurements were done. Both trees were quite a bit taller than any others of their species accurately measured in Štirovača, but it is not impossible that there are other trees of similar heights.

On gentle to steep east and northeast slopes higher above the valley, beech and fir forest with maple dominates elevations of 1180–1260m. Here the dense and dark valley forest gives way to more open groves from where the opposite side of the valley can sometimes be seen through the canopy. Beech is probably the most common tree of this region, and sometimes forms pure stands. In extensive parts of this area there are also beautiful cathedral fir groves often without almost any spruce trees. Sycamore maple obtains larger size in this better lit region, becoming a large tree which may reach well over 300cm in cbh and 30m in height. The largest tree measured here was a fir snag of nearly 500cm in cbh with a volume estimate of around 35 m³ based on the remaining trunk. The tallest measured fir was about 50m, but there may be taller trees at these sites. Beech reaches similar dimensions as it does in the valley.

High above the valley the topography generally becomes progressively steeper, but not everywhere. Formed between the steep slopes, there are mazes of deep sinkholes and ravines with beech, fir, spruce, and maple forests with characteristics unique to these sites. Such areas can be found at 1170–1260 meters in elevation, and have characteristics of both valley and slope forests. The topography is composed of varied elevations, and some of the sinkholes are so large that they act as small valleys in how they influence forest development. Here, one may stand on exposed ridges covered with subalpine forests and gaze down into depressions with large and tall trees. There do not seem to be any long flowing streams in this region. The largest trees found here were mostly fir and spruce exceeding 410cm in cbh, measuring up to about 50m tall. However, the largest fir found is nearly 520cm in cbh with an estimated volume of well over 35 m³, and height of over 46m with a large reiterated stem on the trunk. The largest spruce found is nearly 440cm in cbh. Beech reaches similar size as in the valley and on lower slopes. Little height measurements were done in these areas.

At elevations exceeding 1300m all the way to 1440m, there is a subalpine beech forest with sycamore maple covering the rockiest and windiest sites in Štirovača. This region has trees which are rarely over 25m tall, and sometimes even less. They are often bent at the bottom due to the pressure of snow, and usually grow on exposed stony slopes. Here, the elements shape a truly gnarled and twisted forest with strange trees. This is the least explored and hardest area of the forest to reach due to the terrain, and only a small section at lower elevations was explored for this report.

The Štirovača Reserve is a grand example of Velebit’s continental forests from lowest valley to the highest peak. It is also one of the last and perhaps even best remaining examples of productive mountain forest in the region.

Interestingly, another section of Štirovača Valley was protected in 1999 by the Northern Velebit National Park, covering a slightly larger area than the old growth reserve. This is about 100 ha of the valley, and 100 ha of slopes and terraces above. It is located in another region of Štirovača several kilometers from the reserve. Although the forest does not possess true old growth characteristics, it is still natural and valuable, with many trees of over 200 years old and ~400cm in cbh. Relatively young spruce trees have been measured at 52m tall, and the large meadow in the center has up to 134 species of vascular plants. The views on the large peaks, forests, streams, and water spring are beautiful. A notable feature is the log of a giant spruce tree cut nearby the border of the present day old growth reserve, with a dbh of ~180cm and estimated age of ~350 years. It was cut by the father of our nature park guide when the trunk broke. Such specimens may not have been very rare in the historical expanses of the region. Today the log has decayed notably, but in its original state it was probably wider than nearly all of the trees found in the old growth reserve today (https://www.kafotka.net/sites/default/files/styles/inline/public/foto/stirovaca_nekad.jpg?itok=qdOoJa1v).

DJ

References:

https://zir.nsk.hr/islandora/object/sumfak:1400/preview

https://np-sjeverni-velebit.hr/www/en/park-en/stirovaca

https://www.researchgate.net/figure/Epipogium-aphyllum-Swartz-Klepina-duliba-tirovaa-Northern-Velebit-photo-V-egota_fig2_298280703

https://indirektno.com/koja-ljepota-jensen-u-prasumi-klepina-duliba/

https://rewildingeurope.com/news/cooperation-started-with-zagreb-university-on-old-growth-forest-and-brown-bear/

https://www.sumari.hr/biblio/pdf/14056.pdf