Spontaneous re-establishment of natural structure and related biodiversity in a previously managed beech forest in Belgium after 20 years of non intervention
Kris Vandekerkhove, Luc De Keersmaeker, Hans Baeté and Ruben Walleyn Institute for Forestry and Game Management, Gaverstraat 4, B-9500 Geraardsbergen, Belgium Kris.vandekerkhove@lin.vlaanderen.be; luc.dekeersmaeker@lin.vlaanderen.be;
hans.baete@lin.vlaanderen.be; ruben.walleyn@lin.vlaanderen.be
Abstract
A network of strict forest reserves is currently being installed in Flanders (Belgium). Their main objectives are nature conservation and scientific research. Belgium is densely populated and has a long history of intensive forest management, so no natural forests are left. Even the forest reserves are composed of man-made, semi-natural forest stands. In the strict reserves, an intensive monitoring programme is performed in order to study the spontaneous development of the woody and herbal layer when managed forests are left for free development. The methodology involves measurements in a grid of nested circular sample plots, combined with an intensive full inventory in a core zone. The results for typical dendrometric and structural variables (total woody biomass, necromass, gap size and distribution) from the forest reserve of Kersselaerspleyn (230 year old, man-made beech forest, 20 years of non-intervention) are compared to other beech forest reserves in Europe, both natural and man-made. It is remarkable to see how quickly the stands from Kersselaerspleyn have developed towards a dynamic equilibrium in total living and dead biomass, comparable to that in natural beech stands. The analysis shows that the spon- taneous development of a man-made forest to a more natural state can be relatively fast, provided that the original stand is old enough to reach an age where natural decay normally starts to occur.
For beech stands, this can be the case for stands of 200 years and older. The consequences of this development on species richness and the composition of mosses, vascular plants, saproxylic invertebrates and fungi were also studied: 48 plant species, 55 mosses, 244 species of fungi and over 100 species of saproxylic invertebrates have up to now been registered on the dead wood, including many rare indicator species of valuable old-growth sites.
Keywords: natural dynamics, strict forest reserve, biodiversity, fungi, mosses, vascular plants, invertebrates
1 Introduction
Flanders has a long history of dense population and intensive land use. This has resulted in a low forest index: only 10 % of the land is covered with forest. Of these forests, only a small fraction (15 %) is considered to be ancient woodland: land-use shifts and landscape inver- sions have taken place, based on changing economic climate and needs for forest products, resulting in de- and reforestations. Moreover, all forests have been used in an intensive way, resulting in high proportions of even-aged plantations: more than 70 % of the forest consists of even-aged stands, that are less than 80 year old (AFDELING BOS EN GROEN 2001).
Natural forests are therefore non-existent in Flanders, and even the proportion of ancient semi-natural woodland is limited.
The remaining forests in Flanders represent an important element for nature conser- vation. Therefore a network of nature and forest reserves is being set up in order to protect and develop natural values in forests. The network includes both managed and strict reserves (left unmanaged for spontaneous development). The first forest reserves were des- ignated in 1995 and, since then, a network of approximately 1700 ha of strict forest reserves have been established. In a European context, strict forest reserves are mainly conceived for the study of spontaneous forest dynamics (PARVIAINENet al.2000). In Flanders they ad- ditionally have an equally important nature conservation function (VANDEKERKHOVE
1998). This double goal is also reflected in the research programme in the reserves. It consists of a detailed programme monitoring the spontaneous developments of the forest structure and vegetation, and additional research on specific species groups in order to evaluate the influence of these spontaneous developments on the natural value of the site.
In this paper findings on forest structure are presented for a specific site consisting of an overmature beech forest after 20 years of non-intervention. The consequences for biodiversity are illustrated for a number of specific organism groups.
2 Material and methods
2.1 The study siteThe forest reserve ‘Kersselaerspleyn’ covers an area of 98 ha in the centre of the large ancient woodland of Sonian Forest (4300 ha), located on the southern border of the city of Brussels. The actual forest stand developed on a thick, severely lessivated, acid loamy topsoil of Quarternary niveo-eolian origin, covering an impermeable clay layer on top of a cal- careous sandy layer both of Tertiary marine origin, resulting in a Pseudogley soil. Soils are classified as Luvisols and Albeluvisols (FAO 1998). The dominating forest type is Milio- Fagetum (NOIRFALISE1984).
The forest reserve consists of two distinct areas. The main area is a dominated by beech- stands (Fagus sylvatica)of about 150 years old. Other tree species intermixed include ash (Fraxinus excelsior), oak (Quercus robur and Quercus petraea) and Scots pine (Pinus sylvestris).It has been left unmanaged only since 1995 and includes relatively low amounts of dead wood, as windblown and other dead trees were removed within the regular manage- ment of the stand.
An area of about 15 ha, however, is much older, and consists mainly of a homogeneous beech stand, planted in about 1775 (VANDENBERGEet al.1990a; 1990b). It has been left unmanaged since 1983, when a first storm blew down a number of trees.
2.2 Setup of the survey: monitoring procedure for strict reserves
The standard methodology for the monitoring of strict forest reserves in Flanders was devel- oped based on experience and methods used in neighbouring countries (PETERKEN and BACKMEROFF 1988; KOOP 1989; ALBRECHT 1990; ALTHOFF et al. 1993; Projektgruppe Naturwaldreservate 1993; MOUNTFORDet al.1999), the guidelines of the European COST- action E4 (HOCHBICHLERet al.2000) and our own preliminary studies (VANDEKERKHOVE
et al.2003a; VANDENMEERSSCHAUTet al.2000). It is based on permanent plots, combining a grid of sample plots with a core area (Fig. 1). The overall aim is to monitor the development of the vegetation in forests that are deliberately left unmanaged. The combined sampling
NESTED SAMPLE PLOTS ON GRID POINTS 70 m 140 m
CORE AREADimensions (DBH and height) and positions of trees and shrubs in 98 plots of 10m x 10m: living trees with DBH >5cm and height >1m (IUFRO-classes are also determined) Standing dead wood with DBH >5cm and lying dead wood with diameter >10cm and lenght >1m (decomposition stadium is also determined) Frequency of tree regeneration in 98 plots of 10m x 10m: Counting regeneration with DBH <5cm in 4 height classes: 0-30, 30-50, 50-200 and 200cm Vegetation sampling in 98 plots of 10m x 10m: Londo scale Fish-eye photographs alternating on grid points (totalling 60) Soil samples alternating on grid points, including one row outside the core area (totalling 80) Monitoring of fungi in 10 plots of the central transect (10 m x 100 m) 1. Circular plot with r = 18m (1018 m2): Living trees (DBH>40cm) with determination of IUFRO-classes, dead wood with diameter > 10 cm and length or height > 1 m, determination of decomposition stadium 2. Circular plot with r = 9m (254 m2 ): Living trees and shrubs (DBH>5cm) with determination of IUFRO- classes, dead trees with DBH > 5cm, including determination of decomposition stadium 1. Circular plot with r = 4,5m (64 m2): counting regeneration with DBH < 5cm in 2 height classes: 50-200cm and >200cm 2. Circular plot with r = 2,25m (16 m2): counting regeneration with diameter < 5cm in 2 height classes: 0-30cm en 30–50cm On the grid point: Fish-eye photograph, positioned photographs and soil sample Vegetation samples in 16m x 16m plot (N-S orientation)
Dimensions (DBH and height of a selection of trees) and positions of trees and shrubs in circular plots: Numbers of seedlings and regeneration in:
WHOLE FOREST RESERVEMapping of vegetation and special structures 50 m Fig.1.Schematic representation of the sampling design in strict forest reserves in Flanders:a combination of a grid of nested sampling plots and a detailed study of a core area of approximately 1 ha.
should provide knowledge of the development of the reserve as a whole, as well as better insight into the processes triggering the dynamics.
The grid system consists of circular nested plots of 1000 m2, covering 20 % of the total area. In these sample plots basic measurements are done on the woody vegetation and herbaceous layer. The plot design and the methodology of the measurements are identical to the ones used in the Flemish Forest Inventory, thus allowing immediate comparison. The systematic grid of circular plots are mainly to be considered as pure samples, allowing an overview of the development of the area as a whole.
The need for more detailed datasets in order to allow analysis of the processes and inter- actions triggering the development is covered by the information from the core area.
The standard dimensions of the core area are 70 × 140 m, which is in accordance with the SILVI-STAR method developed by KOOP(1989). All trees were identified, positioned and measured, vegetation and regeneration were mapped in detail in 10 m ×10 m subplots, and soil and light conditions were analysed. The detailed information provided here on ground cover, tree positions, crown parameters, soil properties, light measurements, etc. allow for instance a robust analysis of multilateral influences between soil, woody layer and ground vegetation.
All measurements are stored electronically: all plot and tree positions were measured by Total Station, and the data were imported into a GIS-environment, allowing geographical analysis. The measurements of vegetation and dendrometry are to be repeated at least every 10 years. Analysis and typology of the soil, and reconstruction of management and land-use history were performed once at the beginning of the monitoring.
The methodology is flexible in the sense that it can be adjusted to local conditions, so long as the minimum requirements are respected. In the forest reserve Kersselaerspleyn (Sonian Forest), for example, the size of the circular plots was expanded to 2000 m2as tree dimensions and distances are extraordinarily high in this site. Size extension was necessary in order to comply with the basic principles of KRAMERand AKÇA (1982) and SPURR
(1952), who pointed out that an individual plot should contain at least 20 trees per plot.
By widening the grid to 100 × 100 m, the sample size of 20 % could be respected. Also the core area was considerably larger than required, covering 7.5 ha instead of 1 ha (DE
KEERSMAEKERet al.2002). This extension was based on the fact that detailed dendrometric information was available for this area from 1987 and 1990 (VANDENBERGEet al.1990a, 1990b), providing opportunities for interesting detailed comparisons.
2.3 Additional specific inventories according to standardised protocols and methodologies
In addition to the standard monitoring procedures, some detailed research on specific organisms linked to dead wood was performed. The species richness of epiphytic vascular plants, mosses and fungi was studied on 200 dead beech trees, equally distributed over differ- ent diameter and decay classes. Species composition and richness was analysed using stan- dardised protocols (VANDEKERKHOVEet al.2003b; WALLEYNand VANDEKERKHOVE2002;
VANDORTand VANHEES2002).
A preliminary inventory of saproxylic invertebrates was also performed using a set of standardised trapping techniques including pitfalls, stem-eclector and window-traps.
Coleoptera (43 families), Empididae, Dolichopodidae and Aranea were studied (VERSTEIRT
et al.2000; HEIRBAUTet al.2001).
3 Results
3.1 Forest dynamics after 20 years of non-intervention
Most of the reserve (80 ha) received its status of strict reserve in 1995. Until that time, it was managed in an extensive way, using nature-based silviculture. The total average biomass is about 638 m2/ha, with a total necromass of 36 m2.
The development of the living and dead biomass of the 230 year old beech stand (the core zone of the research) is given in Table 1. In 1986, the stand already had a high total volume, as it consisted of an overmature, homogeneous closed beech stand. Also the total amount of dead wood was already relatively important, considering the very short period of non-intervention (3 years). At the time of installation, the area already contained a number of large dead trees. The hurricanes of 1990 blew down more than 50 large beech trees, re- sulting in a rapid increase in the amount of dead wood, and causing a temporary decline in the living biomass. At that time, the stand was considered to be in its degradation phase (VANDENBERGEet al.1990a; 1990b). However, over the last 10 years, living biomass rapidly increased, as the remaining beeches showed high increments, resulting in a total living biomass that clearly surpasses the amounts before the important windblows. The stand is still accumulating both total and living biomass, and is thus still in an aggradation phase (DE KEERSMAEKERet al.2002).
Table 1. Development of the total living and dead biomass (m2/ha) for a 7.5 ha core zone of the forest reserve of Kersselaerspleyn as calculated from three consecutive detailed inventories.
1986 1990 2000
Living biomass 715 654 793
Dead biomass 28 99 139
Total 743 753 932
The high standing volume of the stand is primarily explained by the individual tree dimen- sions. There is a high proportion of over-mature, big trees, as illustrated in the diameter distribution of the stand (Fig. 2). Some trees reach diameters of over 150 cm and total tree heights of 48 to 51 meters. The average stand height is well over 40 m. This figure is much higher than expected on a lessivated acid loess and indicates that the trees extract their nutrients from the deeper more fertile tertiary layers.
The gap size and distribution is quite diverse: next to the small gaps, created by individual and small groups of trees, there are some larger windthrow areas, caused by the 1990 storms.
One is over 1 ha in size, the other about 3000 m2. Contrary to what we expected, newly fallen trees since 1990 are not necessarily linked to these existing gaps, but are randomly distrib- uted over the area.
2.2 Consequences for biodiversity: vascular plants, mosses and fungi on dead wood
In the reserve, epiphytic vascular plants, mosses and fungi were recorded on 203 selected dead trees representing more than 1200 m2of coarse woody debris. An equal distribution in the selection of the logs over the different diameter and decay classes was not possible, due to the low presence of decay phases 5 and 6 (see Table 2). There is an overrepresentation of heavy logs of decay stage 3, as most of the objects were fully-grown trees, uprooted during the storms of 1990.
The inventory of vascular plants on the logs and uprootings resulted in a total of 48 plant species (VANDORT and VANHEES2002). Except for a few seedlings, almost no vascular plants occur on freshly fallen logs. However, already starting from decay phase 2, but es- pecially from phase 3 onwards, many species start to grow on the decaying logs. Table 2 suggests that the total species number decreases after decay stage 4. However, this decrease is mainly due to the lower volume of investigated logs. Data on plant species on the logs was compared to the detailed dataset of ground vegetation in the same area (DEKEERSMAEKER
et al.2002). Of a total of 85 plant species found within the core zone, 15 species were exclus- ively found on logs and/or root-plates and uprooting systems (VANDEKERKHOVE et al.
2003b).
A total of 55 species of bryophytes were found on the logs (VANDORTand VANHEES
2002). Most common species of mosses were Brachythecium rutabulum, Hypnum cupres- siforme, Lophocolea heterophylla and Brachythecium salebrosum. Species richness over different decay stages is presented in Table 2. Again decay classes 3 and 4 appear the most species rich (keeping in mind the overrepresentation of these classes). Compared to vascular Fig. 2. Diameter distribution of the trees over 30 cm DBH, in the core zone of the forest reserve of Kersselaerspleyn: comparison of the situation in 1986 and 2000.
0 20 40 60 80 100 120 140 160
Diameter classes (cm)
# of trees
1986 2000
30–39 50–59 70–79 90–99 110–119 130–139 >149
40–49 60–69 80–89 100–109 120–129 140–149
plants, the colonisation by mosses is faster, with already high numbers of species at decay phase 1. Of the relatively high total number of species, at least 20 were exclusive to the logs (VANDEKERKHOVEet al. 2003b). Notable findings were Drepanocladus uncinatus, Rhyti- diadelphus loreus, Zygodon viridissimus, Nowellia curvifoliaand Ptilium pulcherrimum(VAN
DORTand VANHEES2002).
Table 2. Total number of vascular plant and moss species found on coarse woody debris in the forest reserve Kersselaerspleyn – distribution over the different decay-phases (ordinal scale based on presence of bark, branches, softness and surface of wood and shape of trunk, developed in the Nat-Man- project;
ODORet al.2004) 1= fresh; 5–6 = strongly decayed.
Decay-phase 1 2 3 4 5–6 Total
Number of logs 21 24 105 35 10 203
Number of vascular plant species 4 20 35 37 17 48
Number of moss species 21 26 45 40 22 55
Up to now, 244 species of fungi have been identified on the logs. This figure is quite compar- able to the species richness in other reserves in Europe, even compared to some genuine primary beech forests. Also the quality of the species found is quite high: at least 31 species are considered threatened or rare in Flanders, and 16 species are listed on the list of indicator species for conservation value, developed by HEILMANN-CLAUSEN and CHRISTENSEN
(2000).
2.3 Saproxylic invertebrates
The study revealed more than 100 species, which is quite limited compared to other similar research in Germany (KÖHLER 1996, 1999, 2000). However, at least 30 of these are con- sidered rare or red-listed in Germany and UK (HYMAN1992; KÖHLER1996, 1999, 2000) and two species (Carabus auronitens and Strictoleptura scuttellata) are included in the list of indicator species for sites of European importance for saproxylic invertebrates (SPEIGHT
1989).
3 Discussion
After 20 years of non intervention, the forest stand covering the core area of the forest reserve Kersselaerspleyn has developed remarkably, with high figures for both total above- ground biomass and total necromass. In fact these figures come into the range of genuine primary beech forests, as described in a number of Central European beech forest reserves.
Figure 3 compares some basic results on living and dead biomass from Kersselaerspleyn and other Belgian beech reserves with a number of natural and semi-natural strict beech reserves in Europe, using data from CHRISTENSEN and HAHN (2003), KORPÉL (1997a, 1997b) KEITELand HOCKE(1997) and CLERCKXet al.(1995).
All recently installed, previously managed forests in the Netherlands and Belgium are situated in the lower left corner of the graph. The forest reserves of Central Europe are grouped within the boundaries of the range for Primary Beech Forests set by KOOP(1989)
and represented by the grey rectangle in the figure. Some forest reserves in Germany and Denmark, already installed several decades ago, but still originating from managed forest stands, occupy an intermediate position. The core zone of the forest reserve of Kerssel- aerspleyn shows a remarkable development, represented in the Figure by the white inter- linked diamonds. Already at its installation as a reserve, the site had high figures for total biomass, but relatively low amounts of dead wood. Due to the storms of 1990, there was a sudden increase in necromass, already positioning the site within the range of the primary beech forests. Over the last 10 years, the reserve has made a further shift both in total and dead biomass, positioning it in-between some of the most renowned beech reserves in Europe.
However, if we look in more detail, there are still important differences between this site and the genuine primary beech forests.
Instead of the typical inverse J-shaped curve of a natural forest, the diameter distribution is a typical curve for even-aged stands (normal distribution). Even after 230 years, it still shows this typical characteristic of a man-made plantation. Although the stand is quite open, there is also a lack of regeneration. This is a problem all over the forest, and is probably linked to the high predation of seeds and seedlings by wood pigeons (Columba palumbus),
Fig. 3. Comparison of total aboveground biomass and total dead biomass for a number of strict beech forest reserves in Europe (B = Belgium; NL = The Netherlands; D = Germany; DK = Denmark; F = France; SK = Slovakia; CZ = Czech Republic; SLO = Slovenia; PBF = Primary Beech Forests). The interlinked diamonds show the evolution of the core zone in the strict reserve of Kersselaerspleyn over the last 15 years.
0 100 200 300 400 500 600
0 200 400 600 800 1000 1200
Total aboveground biomass
total dead biomass
B NL D DK F SK CZ SLO Kers.
Range of PBF
squirrels and roedeer, the extreme acid topsoil, and carbon dioxide-intoxication and drought stress in summer both linked to the specific soil profile (DEVOSet al.2004). There is also no equal distribution of dead wood over all decay phases: young and intermediate decay phases are over-represented, coinciding with the wind-throw events; old decay phases are almost absent (WALLEYNand VANDEKERKHOVE2002).
Finally, also the gap-size distribution is atypical for natural dynamics in beech forests: the size of the two large gaps in the core zone, one covering 3000 m2and the other over 1 ha, is quite uncommon. In most reserves in Europe, gaps of over 1000 m2are exceptional: most gaps are less than 500 m2(MOUNTFORD2001; HAHNand EMBORG2001; WIJDEVENand VANHEES2001; DIACIand ZEIBIG 2001). The appearance of larger gaps is also seen in Fontainebleau (PONTAILLERet al.1997). This might illustrate slight differences in dynamics in Atlantic compared to Continental beech forests: storms and windblows are considered to be a more important factor triggering the dynamics in the forests of the Western-European plain (PARVIAINENet al.2000; PONTAILLERet al. 1997). However, recent analysis of gap- sizes in Beech reserves all over Europe show little differences in dynamics between bio- geographical regions: in both Continental and Atlantic beech forests, small gaps are the rule, with gaps of over 1000 m2being exceptional but not impossible. Larger windthrows, compar- able to and even larger than the ones observed in Kersselaerspleyn though have been observed, also in Central Europe (DIACIand ZEIBIG2001). Coincidence and stand history are probably more likely to explain the occurrence of large gaps. In strict reserves, originating from even-aged man-made stands, like in Kersselaerspleyn, such events are indeed more likely to occur.
The spontaneous development of the site, resulting in large amounts of dead wood has had an important influence on the biological diversity of the reserve. The results for epiphytic plants, mosses and saproxylic fungi suggest that the dead wood component has an important and often underestimated role in the conservation and development of botanical richness in forests. High amounts of dead wood, composed of all dimensional classes and decomposition stages, perform as a suitable substrate for often specialised species of vascular plants, fungi and mosses.
When comparing the number of species of fungi to available inventories of other reserves in Europe, the forest reserve Kersselaerspleyn takes an intermediate position, with figures comparable to Fontainebleau (France) and Zofinski Prales (Czech Republic) (WALLEYN
and VANDEKERKHOVE2002). When using the newly developed European indicator list of saproxylic fungi (CHRISTENSENet al.2004) and new inventories from European top-sites, the reserve is repositioned well below the European top sites (6 species, compared to 10–16 in the top sites), but still showing important potential. Especially species linked to the oldest decay phases and some rare heart rotting species are still missing (WALLEYN and VANDEKERKHOVE2004).
On saproxylic invertebrates, the study revealed about 100 species, which is quite limited compared to findings from other similar research sites in Germany (KÖHLER1996, 1999, 2000). For saproxylic invertebrates continuity in the supply of dead wood and connectivity and size of forest areas are important factors for survival (SPEIGHT1989). The results indeed suggest that the fragmentation and history of intensive use led to the species richness being somewhat eroded. Still the reserve harbours important numbers of rare and specialized species, including German and UK red listed species and indicator species of European interest. As the reserve is situated in the largest patch of ancient woodland in Flanders, which was also primarily used as a royal hunting ground in the past, rare species with specific requirements of dead wood appear to have managed to survive under unfavourable con- ditions. Installing strict reserves and increasing the quantity and quality of dead wood in such potentially valuable sites will provide possibilities for recovery of these communities.
4 Conclusions
It is striking to see how quickly the old man-made stand from Kersselaerspleyn has devel- oped towards a dynamic equilibrium in which total living and dead biomass and gap dis- tributions are roughly comparable to natural beech stands. This development shows that spontaneous development of a man-made forest to a more natural state can indeed be quite fast, provided that the original stand is old enough, reaching an age where natural decay normally starts to occur. For beech this appears to be the case for stands reaching ages of over 200 years.
The species diversity on coarse woody debris was also investigated for specific groups of organisms: epiphytic mosses and vascular plants, saproxylic invertebrates and fungi. The results prove that species richness and the composition of saproxylic organisms in recently installed strict reserves, originating even from man-made stands, can be comparable to other reserves in Europe with a long history of non-intervention, provided that the site is an ancient forest site (forest continuity) and old overmature stands are included in the reserve.
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Accepted April 1st, 2005
