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240 WISSEN Schweiz Z Forstwes 163 (2012) 6: 240–246
Old-growth forest reserves in Slovenia:
the past, present, and future
Thomas A. Nagel Department of Forestry and Renewable Forest Resources, University of Ljubljana (SI)*
Jurij Diaci Department of Forestry and Renewable Forest Resources, University of Ljubljana (SI)
Dusan Rozenbergar Department of Forestry and Renewable Forest Resources, University of Ljubljana (SI)
Tihomir Rugani Department of Forestry and Renewable Forest Resources, University of Ljubljana (SI)
Dejan Firm Department of Forestry and Renewable Forest Resources, University of Ljubljana (SI)
Old-growth forest reserves in Slovenia: the past, present, and future
Slovenia has a small number of old-growth forest remnants, as well as many forest reserves approaching old-
growth conditions. In this paper, we describe some of the basic characteristics of these old-growth remnants
and the history of their protection in Slovenia. We then trace the long-term development of research in these
old-growth remnants, with a focus on methodological changes. We also review some of the recent findings from
old-growth research in Slovenia and discuss future research needs. The conceptual understanding of how these
forests work has slowly evolved, from thinking of them in terms of stable systems to more dynamic and unpre-
dictable ones due to the influence of natural disturbances and indirect human influences. In accordance with
this thinking, the methods used to study old-growth forests have changed from descriptions of stand structure
to studies that address natural processes and ecosystem functions.
Keywords: Balkan, Central Europe, primeval forest, virgin forest, old-growth forest
doi: 10.3188/szf.2012.0240
* Vecna Pot 83, SI-1000 Ljubljana, E-mail tom.nagel@bf.uni-lj.si
B
y providing a reference for understanding nat-
ural pattern and process, old-growth forest ec-
osystems have long been valued by foresters
and ecologists alike. In addition to scientific insight,
the innate value of old-growth forests, including the
ecosystem services they provide and the unique ar-
ray of biodiversity they hold, raises significant chal-
lenges regarding their conservation and manage-
ment. These issues are particularly important in the
temperate zone of Europe, where silvicultural sys-
tems based on patterns and processes found in old-
growth are still in need of quantitative information,
yet very few old-growth ecosystems remain and ex-
tant remnants are often influenced by a number of
indirect anthropogenic disturbances. Nevertheless,
the remaining old-growth forests in the temperate
zone of Europe have a long history of protection and
have been thoroughly studied (e.g. Korpel' 1995, Pe-
terken 1996, Splechtna & Gratzer 2005).
Similar to other countries in the Balkan re-
gion, Slovenia has several old-growth remnants that
escaped histor ica l logging, of ten due to the individ-
ual efforts of visionary foresters. These forests are at
the heart of Slovenia’s long tradition of ecologically
based forest management, and have been a focal
point of re s e a r c h for a ha l f - c e nt u r y. Du r i ng this ti me
period, the view of how these forests work has slowly
evolved, from thinking of them in terms of stable
systems to more dynamic and unpredictable ones
(Splechtna & Gratzer 2005). In line with this
thinking, the methods used to study old-growth for-
ests have changed from descriptions of vegetation
and st a nd structure (e.g. Hoceva r et al 1980) to st u d-
ies that address natural processes and ecosystem
functions (e.g. Firm et al 2009).
The goal of this paper is to provide an overview
of old-growth forest research in Slovenia. We begin
the paper with a description of the historical devel-
opment of old-growth forests in Slovenia. This is fol-
lowed by a description of the extant network of old-
growth forest reserves. We then trace the historical
development of research methodologies used to study
old-growth forests in the region. Finally, we provide
a brief overview of more recent research findings from
different old-growth forests in Slovenia, and conclude
with a discussion of future research directions.
Historical development of old-growth
forest reserves in Slovenia
There is a long tradition of old-growth forest
protection and research in Slovenia. Already in 1892,
an extensive area of old-growth forest in the Dinaric
241
CONNAISSANCES
Schweiz Z Forstwes 163 (2012) 6: 240–246
Mountains of southeastern Slovenia was protected
under the forest management plan for the region
(Hufnagel 1893). The first inventories of forest com-
position and structure were carried out at that time.
Duri ng the 195 0 s, the r e wa s a live l y di s c ussio n ab out
the importance of nature reserves for life sciences in
general and for the science and practice of forestry
in particular (Wraber 1952). By 1973 the area of pro-
tected old-growth forest remnants encompassed
343ha. In the same decade, a large project focused
on the protection of new forest reserves – “old-growth
forests for the future” – was started in Slovenia. It
was led by Professor Dusan Mlinsek and inspired by
the naturalistic movement of silviculture professors,
especially Hans Leibundgut (Leibundgut 1959). The
result of the project was a network of 173 protected
forest reserves covering an area of 9,040ha, which
included the previously protected old-growth re-
serves as well as several new ones, making a total of
fourteen old-growth forest remnants with a cumu-
lative area of approximately 500 ha (Mlinsek et al
1980). In the meantime, detailed guidelines for re-
search and maintenance of forest reserves were de-
velop ed (Anko et al 1976). Comp rehe nsive stud ies of
individual old-growth remnants were published as
mo nograph s (e.g. Hocevar et al 1985, Turk et al 1985,
Hartman 1987). More recently, re-privatization of
forests that followed the independence of Slovenia
in 1991 introduced minor changes to the forest re-
serve network, but the network of old-growth forests
remained unaffected. Today there are 170 forest re-
serves in Slovenia covering approximately 9,600 ha,
which is about 0.8% of the total forest area in Slov-
enia (Figure 1).
Old-growth forests in Slovenia
Most of the old-growth reserves in Slovenia (of-
ficially refer red to as “virgin forests” in Slovenia) are
small (typically < 50 ha) remnants dominated by
mixtures of Abies alba and Fagus sylvatica, but there
are some exceptions (Table 1). Of the 14 old-growth
reserves, most are located at upper elevations of the
Dinaric Mountain range, predominantly on lime-
stone parent material (Figure 1). In this region, A.alba
and F. sylvatica are the dominant species, but less
shade tolerant tree species, including Acer pseudopla-
tanus, Ulmus glabra, Fraxinus excelsior, and Picea abies,
are also present depending on the site conditions and
dist urbanc e hi s tory. The largest an d most resea rc he d
old-growth remnants in the region are Rajhenavski
Rog (Nr. 7 in figure 1), Pecka (Nr. 8), and Krokar
(Nr. 3). Seve r a l smaller remnants (Strmec [Nr.4], Kop a
[Nr. 6], Gorjanci-Trdinov vrh [Nr. 9], Ravna gora
[Nr.10 ], and Bu kov vrh [Nr. 1]) are mainly dominate d
by F. sylvatica. The only old-growth P.abies remnant
in Slovenia, Prelesnikova kolisevka (Nr. 5), is located
in a karst sinkhole at the foothills of the Dinaric re-
gion where a po ol of cold - a i r (temperature inve r si on)
is commonly formed. The Zdrocle reserve (Nr. 2), lo-
cated on the southeastern side of Sneznik, the high-
est peak in the Dinaric Mountains of Slovenia, is the
la r gest re serve on paper, but parts of the reser ve were
managed in the past, so stands with old-growth con-
ditions form a smaller area. Zdrocle occurs on rather
extreme karst geological conditions, characterized
by abundant sinkholes and limestone outcroppings,
where F. sylvatica is the most abundant species.
The remaining old-growth reserves occur out-
side the Dinaric Mountain region. They include two
F. sylvatica dominated remnants in the Haloze re-
gion of eastern Slovenia, known as Donacka gora
(Nr. 12) and Belinovec (Nr. 13), as well as Sumik
Fig 1 Locations of old-growth forests (stars) and forest reserves (black circles) in Slovenia.
Numbered stars correspond to the list of reserves in Table 1.
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9
11
12 13
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ITALY
AUSTRIA
CROATIA
Kranj Celje
Koper
Maribor
Ljubljana
Novo mesto
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Tab 1 Basic characteristics of old-growth forest reserves in Slovenia. * Approximate size of
the old-growth area. The actual sizes of the reserves that contain the old-growth areas
are often larger than the values reported here because they include buffer zones.
Number in
Figure 1
Old-growth reserve Size
(ha)*
Dominant tree species Altitude
(m a.s.l.)
1Bukov vrh 8Fagus sylvatica 1,2 50–1,314
2Zdrocle 157 F. sylvatica, Picea abies
1,300 –1,477
3Krokar 75 F. sylvatica, Abies alba 750 –1,190
4Strmec 16 A. alba, F. sylvatica 820–940
5Prelesnikova kolisevka 3P. abies 425 – 475
6Kopa 13 F. sylvatica 980–1,080
7Rajhenavski Rog 51 A. alba, F. sylvatica 800–920
8Pecka 60 A. alba, F. sylvatica795 –910
9Gorjanci-Trdinov vrh 23 F. sylvatica 990 –1,15 0
10 Ravna gora 16 F. sylvatica 890
11 Krakovo 40 Quercus robur 152–153
12 Donacka gora 28 F. sylvatica 600–800
13 Belinovec 4F. sylvatica 600–700
14 Sumik 20
F. sy l vat i ca, A. alba , P. abi e s
80 0 –1,150
Total area 514
242 WISSEN Schweiz Z Forstwes 163 (2012) 6: 240–246
(Nr.14), a mixed Abies-Fagus-Picea remnant in the
Pohorje Mountain range in northeastern Slovenia
and Krakovo (Nr. 11), a lowland Quercus robur dom-
inated floodplain forest between the Krka and Sava
rivers in southeastern Slovenia. In addition to the
remnants under official protection, there are several
old-growth remnants on state land that are currently
unprotected, such as the Lucka Bela forest in the
Kamniske Alps, as well as forest reserves that have
old-growth conditions that are not on the list of old-
growth remnants, such as the Menina-Strojnik for-
est reserve in the Kamniske Alps.
Methodological development of
old-growth research
The methods used to study old-growth forests
have changed over the past century in Slovenia,
partly because of the paradigm shift in ecology em-
phasizing change over stability, but also because
researchers are asking different types of questions
today than they did fifty years ago. Additionally,
modern research tools (i.e. remote sensing, lidar,
light sensors, etc.) have opened up a world of new
possibilities to forest researchers.
In Slovenia, much like other parts of Central
Europe, early research in old-growth forests in the
20
th
century was focused on describing vegetation
and stand structure. Many of the initial investigations
in old-growth remnants consisted of very detailed in-
ventories of site conditions, flora, fungi, and lichens
(e.g. Hocevar et al 1980, Hocevar et al 1995). It is im-
portant to note that early forest resea rcher s generally
worked under the assumption that old-growth forests
were stable ecosystems in terms of species composi-
tion and structure at a stand scale, whereby on a par-
ticular site type a specific climax forest would develop
and reach a stable end point (Clem ents 1916). The ba-
sis of the climax concept, which originated in North
America, was very similar to the phytosociological
study of vegetation that developed in Europe (Braun-
Blanquet 1964). Therefore, early researchers working
in Slovene old-growth forests spent much time de-
scribing and mapping phytosociological associa-
tions, and this traditional approach is still carried on
to d ay (e.g. Ac cet to 20 02, Mar i ncek & Ma r i nsek 20 0 9).
Much effort was also invested in studying the
dynamics of these old-growth remnants by mapping
patches of forest in different development phases (e.g.
Turk et al 1985, Hartman 1987, Kovac 1999, Bonci-
na 2000). This development cycle approach encom-
passed all stages of development, from regeneration
in gaps to senescence of old trees, and was generally
thou g ht to be dr i ven by cont inuo u s , endogeno u s mor-
tality of canopy trees. In most of the old-growth for-
ests in Slovenia, developmental phases were mapped
at least once during the past 30 years, and selected
reserves ha ve been mapped severa l times. In additio n
to mapping development phases, full inventories of
tree diameter, both living and dead, were carried out
every 10 years since around 1980 in selected old-
growth forests. Combined with repeated inventories
of developmental phases, these “full callipering” da-
tasets provide valuable insight into long-term changes
in stand structure and composition (e.g. Diaci et al
2008b, Diaci et al 2010). Long-term monitoring of
old-growth reserves is also carried out in a network
of permanent research plots. These plots were estab-
lished in most old-growth reserves about 30 years ago
largely due to the forward thinking of professor Du-
san Mlinsek. The plots generally range in size from
0.25 to 1 ha, and all trees > 5 cm dbh are tagged and
numbered. Monitoring growth and mortality in the
plots is carried out approximately every 10 years (e.g.
Rozenbergar 2000). Mlinsek also initiated studies on
reg e neration ecolog y, in c ludi ng st ud ies on sapling ar-
chitecture and browsing impact (Mlinsek 1967, Mlin-
sek & Zupancic 1974).
More recently, research in old-growth forests
in Slovenia has diversified and moved from descrip-
tions of vegetation and stand structure toward re-
search that examines various ecological processes
and ecosystem functions. For example, many stud-
ies have focused on the role of coarse woody debris,
including studies on quantity and quality (Kraigher
et al 2002, Christensen et al 2005, Debeljak 2006)
and the importance of dead wood for vascular plants
(Kutnar et al 2002), bryophytes (Odor & Van Dort
2002), and fungi (Piltaver et al 2002, Grebenc et al
2004, Odor et al 2006). Researc h has als o been done
on the importance of old-growth structural condi-
tions for fauna, especially birds (Perusek 1992,
Boncina 2000) and insects (Floren & Gogala 2002).
Another major research theme focuses on under-
standing the role of canopy openings in old-growth
fo r e sts, including the inf luence of gaps on wa ter bal-
ance (Vilhar et al 2005), mycorrhizal communities
(Grebenc et al 2009), and tree regeneration (Rozen-
bergar et al 20 07, Diaci et al 2008a, Mi k a c et al 200 8).
Finally, in order to move past the early ideas of sta-
bility in old-growth, recent studies have emphasized
the role of disturbance processes in driving stand
dynamics. This research has generally employed
three different approaches, which include measur-
ing characteristics of canopy gaps (Zeibig et al 2005),
observations of recent higher intensity disturbance
eve nts (M a rinsek & Diaci 20 04, Nage l & Diac i 20 0 6 ),
and dendroecological investigations of disturbance
history (Nagel et al 2007, Firm et al 2009).
Recent research findings
In the following section, we highlight some of
the re c e nt resea rc h find i ng s on the dy namics of old -
243
connaissances
Schweiz Z Forstwes 163 (2012) 6: 240–246
growth forests in Slovenia. A significant advance-
ment in our understanding of old-growth dynamics
in Slovenia has come from studies on natural distur-
bance. While traditional models of old-growth for-
est dynamics in Central Europe recognized that
larger disturbances occurred, they generally under-
emphasized their role relative to continuous, small-
scale endogenous mortality processes (Splechtna &
Gratzer 2005). Therefore, old-growth forests were
thought to exist in a dynamic equilibrium, charac-
terized by a shifting small-scale patchwork of differ-
ent development stages (e.g. Korpel' 1995). One of
the major findings that has emerged from research
in old-growth Fagus-Abies forests in both the Alps
and the Dinaric Mountains is that intermediate se-
verity wind disturbances occur on top of the back-
ground of small-scale gap dynamics (Nagel & Diaci
2006, Nagel et al 2007, Firm et al 2009; Figure 2).
These events may have a long-lasting influence on
stand structure and composition, particularly with
regard to maintaining less shade tolerant species
(Marinsek & Diaci 2004, Nagel et al 2010). Conse-
quently, this recent research has expanded the tra-
ditional site-controlled phytosociological view of
forest composition toward a standpoint that high-
lights the interplay between site and disturbance in
controlling species composition.
Another focus of recent research is the influ-
ence of novel disturbances related to anthropogenic
activities on old-growth forest dynamics. These proc-
esses include high levels of browsing damage because
ungulate populations are maintained at a high den-
sity (through regulated hunting and supplemental
feeding) and the decline of A. alba that occur r e d du r-
ing th e late 20 th ce ntury in the Dinaric mount a i n re -
gion . The s e st ud ie s sh ow that the intera c tion of th e s e
two processes has led to a shift from A. alba to F. syl-
vatica in some Slovene old-growth stands (Rozenber-
gar et al 2007, Diaci et al 2010; Figure 3).
Finally, the last major research theme has fo-
cused on the influence of light on various aspects of
tree regeneration. This body of work includes stud-
ie s on st a n d level patterns of rege n e r ation in relatio n
to cano p y st r ucture (N agel et al 20 06), fine -scale pat-
terns of light and regeneration within gaps (Diaci &
Thormann 2002, Rozenbergar et al 2007, Mikac et
al 2008), and studies on the influence of light on
architectural characteristics of regeneration (Debel-
jak & Mli nsek 1998, Diaci & Kozjek 2005, Rozenber-
gar 2007). An interesting finding of the latter stud-
ies is that low levels of light (diffuse light < 5–10%)
increase the probability of plagiotropic growth for
F.sylvatica seedlings, but at later development stages,
individuals either recover from this growth form or
are outcompeted by individuals with straighter
growth forms (Figure 4). Research has also compared
the light conditions in gaps of similar size found
within old-growth and managed forests (Diaci et al
2005). Generally, there is less light in old-growth for-
ests, which may be attributed to taller trees, non
thinned stands at gap borders, and the presence of
a highly structured layer of advanced regeneration
Fig 2 Coarse woody
debris and dense beech
regeneration resulting
from a storm in 1983
that caused intermedi-
ate severity damage to
the Pecka old-growth
forest in the Dinaric
Mountains (left). Loca-
tions of windthrown
trees from the two
storm events in 1983
and 2004 in Pecka
(right; original source:
Nagel & Diaci 2006).
800
825
850
875
900
900
875
850
PECKA FOREST RESERVE
200 meters
1983 windthrow
2004 windthrow
N
244 WISSEN Schweiz Z Forstwes 163 (2012) 6: 240–246
in old-growth. Taken together, these regeneration
studies suggest that modern close-to-nature silvicul-
ture should not just mimic old-growth gap size dis-
tribut ions, but should pay close attention to the gap
microclimate, particularly light conditions.
Future directions
Old growth forest ecosystems are undeniably
imp or tant fo r pr e s e r v ing biodive r s it y and ma int a in-
ing a va riety of ecosystem functions. Likewise, they
are also crucial for providing valuable insights into
nat ural fu nc t i oni ng of forest ecos y s tems, which was
recognized decades ago by forest scientists working
in old-growth forests in Europe (Leibundgut 1978,
Mlinsek 1986). The “reference” conditions provided
by old growth are central for both developing eco-
logically based silvicultural systems and for assess-
ing the long-term effects of forest management
(Wraber 1952, Leibundgut 1959, Foster et al 1996,
Frelich et al 2005). More recently, old-growth forests
have received special attention because they serve
as ideal test beds to examine subtle changes in for-
est dynamics caused by globa l climate change . The r e
is increasing evidence that global climate change is
causing widespread drought induced mortality in
forests worldwide (van Mantgem et al 2009, Allen et
al 2010). Long term monitoring of mortality in old-
growth forests provides estimates of background
rates of mortality, which if tracked over long time
period s , ca n y ie ld im p o r tant in for m at i on on ch a nges
in mortality rates.
A necessary prerequisite to using old-growth
stands as reference conditions, however, is to eval-
uate their “old-growthness”, or the degree of old-
growth attributes developed or maintained in a for-
est stand (Bauhus et al 2009). For example, some
old-growth remnants in Slovenia have all the com-
mon structural attributes associated with old-growth
forests and have very little evidence of past anthro-
pogenic disturbance (e.g. Rajhenavski Rog, Pecka),
while othe rs stil l appear to be transitioning into the
old-growth phase (e.g. Krakovo, Krokar). Although
histo r i c re cords show that no ma n agement oc c urred
in these areas for the past century, it is very likely
that they are still recovering from ancient land-use
practices, such as timber harvesting or forest graz-
ing. They may also be recovering from historical nat-
ural disturbances, as is the case in the Lucka bela
old-growth remnant (Firm et al 2009). Therefore, it
would be warranted to evaluate the naturalness or
“old-growthness” of old-growth remnants and other
non-managed forest reserves in Slovenia. Such an as-
sessment would require an interdisciplinary ap-
proach, ranging from analysis of historical records
to dendroecological and paleoecological analyses.
We also see a need for improved systematic
monitoring of the current network of old-growth for-
ests in Slovenia, particularly regarding the network
of permanent research plots. Currently, many of the
permanent research plots have not been monitored
for more than 10 years and some old-growth rem-
nant s do not have suff ic ient plo t coverage . The r e fore,
in collaboration with the Slovenian Forest Service,
the Department of Forestry, University of Ljubljana,
is cu r r e nt ly set t i n g up a sy s t e m to monito r plots every
5 years, renov ate old plots, and to establ i s h new plots
in several old-growth forest reserves. Along these
same lines, it would be beneficial to unify monitor-
ing strategies in old-growth forests across interna-
Fig 3 An ongoing process in some old-growth Fagus-Abies reserves in Slovenia is a shift
toward F. sylvatica dominance caused by the interaction of A. alba decline and intense
deer browsing on A. alba regeneration. The photograph shows a typical stand in Rajhen-
avski Rog with standing dead A. alba trees and dense F. sylvatica regeneration.
Fig 4 Proportion of plagiotropic, non-straight, and straight individuals of F. sylvatica
measured on the same plots in 2000 and 2005 in the Rajhenavski Rog old-growth forest
reserve. During this time period the average relative diffuse radiation increased from 6 to
10% and the height of individuals from 60 to 160 cm. Note the increase in the proportion
of straight stems over the five-year period.
%
20
40
60
80
100
0
120
2000
Year
2005
Straight Non-straight Plagiotropic
245
CONNAISSANCES
Schweiz Z Forstwes 163 (2012) 6: 240–246
tional borders. If monitoring approaches would be
consistent over a large network of permanent plots
in old-growth remnants throughout the temperate
zone of Europe, this would be a powerful tool to as-
sess long-term changes in tree mortality and carbon
storage in the context of global climate change.
Finally, research in old-growth forest reserves
deserves more attention and recognition from within
the forestry profession and the general public. In
particular, despite the long history of “close-to-na-
ture” silv icult ure in Slovenia, ther e is still a ne ed for
knowledge transfer from old-growth forest research
to silvicultural practice (Brang 2005). This could be
accomplished through various forms, such as publi-
cations in professional journals, involvement of pro-
fessional foresters in old-growth research, and joint
workshops. n
Submitted: 5 May 2011, accepted (with review): 26 January 2012
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Urwaldreservate in Slowenien: gestern,
heute und morgen
Slowenien verfügt über einige wenige Urwaldreste und über
viele Waldreservate, die sich dem Urwaldzustand annähern.
In diesem Aufsatz beschreiben wir einige Charakteristiken
di ese r Ur wal dre s te un d die Ge sch ichte ihre r Untersc hut z s tel -
lung. Wir zeichnen dann die lange Entwicklung der Forschung
in den Urwaldresten nach, wobei wir dabei ein Schwerge-
wicht auf die methodischen Änderungen legen. Wir geben
auch einen Überblick über neue Ergebnisse der Urwaldfor-
schung in Slowenien und diskutieren den zukünftigen For-
schungsbedarf. Das Verständnis, wie sich diese Wälder ent-
wickeln, wandelte sich langsam; sie wurden zunächst als
stabile, dann aber zunehmend als dynamische und unvor-
hersehbare Systeme angesehen, als Folge von natürlichen
Störungen und indirekten menschlichen Einflüssen. Entspre-
chend haben sich die Methoden zur Erforschung der Urwäl-
der gewandelt – von Beschreibungen der Bestandesstruktur
zu Studien über natürliche Prozesse und Ökosystemfunktio-
nen.
Les réserves de forêts vierges en Slovénie:
le passé, le présent et l’avenir
La Slovénie a un nombre restreint de forêts vierges, cepen-
dant elle possède un grand nombre de réserves forestières
qui s’approchent d’un état ancien. Dans cet article, nous dé-
crivons certaines des caractéristiques fondamentales de ces
forêts vierges et l’histoire de leur protection en Slovénie. Par
la suite, nous retraçons le développement à long terme de la
recherche dans ces forêts en mettant l’accent sur les change-
ments méthodologiques. Nous examinons aussi quelques-
unes des conclusions de recherches récentes portant sur ces
forêts et discutons des besoins de recherche futurs. La com-
préhension conceptuelle de la façon dont ces écosystèmes
forestiers fonctionnent a lentement évolué, passant de sys-
tè m e s st a b les à plus dyna m i q ues et impr é v isible s, à caus e no -
tamment de l’influence des perturbations naturelles et des
influences indirectes anthropiques. Conformément à cette
pensée, les méthodes utilisées pour étudier les forêts vierges
ont évolué, passant de descriptions générales de la structure
du peuplement à des études plus poussées qui portent sur
les processus naturels et sur les fonctions de l’écosystème.
