The Prašnik Oak Forest in Croatia

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Jeroen Philippona
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The Prašnik Oak Forest in Croatia

Post by Jeroen Philippona » Sat Nov 03, 2012 12:01 pm

Prašnik Oak Forest

At June 21 2012 we visited the Prašnik Special Reserve, an oak forest near Stara Gradiška, Croatia. This is a rather small (53.35 hectare) remnant and the only uncut part of the once huge virgin Slavonian oakforests along the Sava and Drava rivers.

This then nearly uninhabited area was settled by the Austro-Hungarian administration as a buffer-zone towards Turkey-occupied Bosnia. The history of the Slavonian oak forests is the history of vanishing of the last primeval forests in the Central European lowlands.
The Croatian-Slavonian Military Fronticr District registered in 1746 approximately 741 thousand hectares of virgin oakforests. Although the quality of the "Slavonian oak" was already known since the middle ages, exploitation on a larger, and ever increasing scale started only in the late 18th century.
The Slavonian oakforests were famous because of their high quality timber and the great dimensions the oaks could attain. In 1900 at the world exhibition in Paris a Slavonian oak stem section was displayed with a diameter at breast hight of 260 cm and a volume of 64 m3.

Originally the Slavonian floodplain forests were inundated every year and often for a long time in winter and spring. Research has revealed that (beside the pionier willows and poplars) from all hardwood treespecies Pedunculate oak (Quercus robur) can widstand flooding for the longest periods. This gave them a great advantage to other species like ash, elm, maples and hornbeam. The Slavonian oaks were from a special genetic type, very diverse but also late coming into leaf so even less susceptible to flooding in late spring.
The forest probably was rather open in the past and oak could regenerate here.
Because of measures against flooding this natural advantage for oak has vanished.

In the 1920's only 7500 hectares of the virgin oakforests had not been destroyed, but even these were cut. Only the tiny Prašnik forest of 53.35 hectares was left and declared a reserve. Oaks over 300 years old can be found here. The forest can not really be called virgin / pristine / primeval or old growth: the Sava river is behind dikes now and has not flooded this area for 60 years. The water tables are much lower than a century ago, because of the dikes, agricultural measures and also the climate change, resulting in warmer (and dryer?) summers (information Mrs Katica Nuspahić).
Some large oaks and lots of young hornbeams
Some large oaks and lots of young hornbeams
In Prašnik during recent decades Hornbeam (Carpinus betulus) has become much more prevalent, forming a dense second layer in the forest. Oaks can no longer regenerate in these circumstances. Many of the older oaks have died, probably because of the lower water table.
In the forest a few other tree species can be seen, among them Black Alder (Alnus glutinosa), Field Elm (Ulmus minor) and Field Maple (Acer campestre) and at relatively high locations a few large, old beeches (Fagus sylvatica).
One of the few old beeches in Prasnik Forest
One of the few old beeches in Prasnik Forest
We were able to visit this forest reserve thanks to Mrs. Katica Nuspahić, forest engineer of the Croatian Forestry Institute and Mr. Krunoslav Szabo, head-forester of Okučani Forest Office. They guided us in the forest and gave a lot of information.
Part of the forest is still suspected of land mines form the Croatian Serbian war in 1991-1992.
The reserve has suffered from a much lower water table in the reserve the last decades, because of dykes along the Sava river, agricultural measures and climate change. Many oaks have died or are dying and now hornbeam is increasing greatly wich limites the rejuvenation of oaks.

In the Slavonian floodplain now many more oak forests can be seen, but none of them are original old growth forests. Several of them have been planted from the mid nineteenth century. Especially in the eastern part of Croatia there are again large and beautiful oak forests with trees of around 150 years, which make a natural impression and are ecologically very valuable.
Katica and Krunoslav at the base of the largest oak
Katica and Krunoslav at the base of the largest oak
We measured the largest oak in the forest. It was 39.6 m (130 ft) tall with cbh of 730 cm (23.95 ft) @ 1.5 m (782 cm (25.39 ft) @ 1.3 m). Estimated age around 300 - 350 years, total wood volume at least 50 m3. Michael made about 200 photos to send to Michael Taylor to make a 3-dimensional model of the tree to estimate the volume.
The largest oak in Prašnik Forest
The largest oak in Prašnik Forest
The large buttresses give the impression the oaks grew up in a relative wet situation with high water table; the area in that time perhaps was rather often flooded by the Sava river.
Michael inspects the largest oak
Michael inspects the largest oak
There are more oaks of over 6 m girth in Prašnik forest. The height of most oaks we measured was between 35 and 38 m. They look as if they have grown up under rather open circumstances, not with a great density of the forest. Compared to the oaks in the Białowieża primeval forest in Poland they have shorter trunks, the crowns begin at a lesser height.

Jeroen, Kouta and Michael 2012.
Last edited by Jeroen Philippona on Mon Nov 05, 2012 6:48 am, edited 2 times in total.

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edfrank
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Re: The Prašnik Oak Forest in Croatia

Post by edfrank » Sat Nov 03, 2012 1:24 pm

Great post, already compiled into the November Magazine issue.

Ed
"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky

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jamesrobertsmith
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Re: The Prašnik Oak Forest in Croatia

Post by jamesrobertsmith » Sat Nov 03, 2012 3:55 pm

Wow! Looks a lot like the bottomland forests we toured at Congaree!

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Chris
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Re: The Prašnik Oak Forest in Croatia

Post by Chris » Sun Nov 04, 2012 9:46 pm

Excellent!

It really reminds me of Big Oak Tree State Park. Bottomland hardwood forest, now cut off from flooding (Mississippi River in this case). Changed hydrology is not allowing regeneration for the entire forest community.

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KoutaR
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Re: The Prašnik Oak Forest in Croatia

Post by KoutaR » Mon Nov 05, 2012 2:02 am

Chris,

From the few photos I found, those parks are not very dissimilar, indeed. Species diversity seems to be higher in Big Oak Tree SP. We identified 10 tree species in Prašnik, about half of them being potential upper storey species.

Kouta

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Rand
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Re: The Prašnik Oak Forest in Croatia

Post by Rand » Mon Nov 05, 2012 5:25 am

Chris wrote:Excellent!

It really reminds me of Big Oak Tree State Park. Bottomland hardwood forest, now cut off from flooding (Mississippi River in this case). Changed hydrology is not allowing regeneration for the entire forest community.
When I was there several years ago, the local naturalist bent my ear about this. I think she was overjoyed to see someone visiting besides the local fisherman contingent.

I've been curious whether last year's dunking was enough to have any affect at all or whether it takes multiple years worth to have any affect.

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