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Bosreservaat Jansheideberg (hallerbos): monitoring van de dendrometrische gegevens en de vegetatie in steekproefcirkels en een kernvlakte

Onderzoeksoutput: Boek/rapportRapporten van het Instituut voor Natuur- en BosonderzoekOnderzoek

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Originele taal-2Nederlands
UitgeverijInstituut voor Natuur- en Bosonderzoek
Aantal pagina's186
StatusGepubliceerd - 2007

Publicatie series

Naam Rapporten van het Instituut voor Natuur- en Bosonderzoek
UitgeverijInstituut voor Natuur- en Bosonderzoek: Geraardsbergen
Nr.INBO.R.2007.43

Abstract

Jansheideberg is a 26.5 ha unmanaged forest reserve, embedded in the forest complex of Hallerbos (559 ha), located 15 km southwest of Brussels. Most of the forest reserve (19.1 ha) was appointed in 1996, and in 2003 it was enlarged with a 7.4 ha beech stand. The last thinnings were executed in 1994, but the valley in the northern part of the forest reserved is unmanaged since 1983. After the installation of the forest reserve, a Scotch pine stand and most of the larches which were planted in the stands with broadleaved species, were cut in order to eliminate exotic tree species.

Most of the forest reserve Jansheideberg is located on a plateau, with dry silt soils with no groundwater table within reach of the vegetation. In the northeast of the forest reserve, a small valley with a spring brooklet is found. Soil types in this valley are wet alluvial soils and peat soils. The slopes of this valley are acid and sandy, as the eolian silt is erdoded. Due to this process, colluvial soils are found below the slopes.

Forest vegetation of Jansheideberg is well-developed (saturated), with many ancient woodland species indicating a long and continuous cover by woodland. Vegetation on the silt plateau is dominated by Hyacinthoïdes non-scripta and is classified as an Endymio-Carpinetum, with Athyrium filix-femina and Milium effusum as characteristic species. On the sandy slopes, a Fago-Quercetum with Convallaria majalis, Maianthemum bifolium, and Pteridium aquilinum is found. In the valley, the spring brooklet is accompanied by an Alno-Padion community, with locally a typical spring wood community (Carici remotae-Fraxinetum). Colluvial soils, which are characterised by a facies of Allium ursinum, can be classified as an Endymio-Carpinetum allietosum.

Hallerbos is an ancient woodland and the forest reserve Jansheideberg is located in a part which was probably permanently covered by forest since early medieval times. The area of the forest reserve was managed by the noble family of Arenberg from 1650 to 1918 – with a short intermission after the French revolution, when the forest was confiscated by the state. The forest traditionally was managed as a coppice or a coppice with standards. Management plans of the beginning of the 20th century indicate that the coppice on the plateau and slopes was dominated by Robinia pseudoacacia and Castanea sativa. After large-scale clearcuttings during WWI, Hallerbos was converted to a high forest dominated by broadleaved species between 1930 and 1943. Few relics of the former coppice woodland, such as large boles of Castanea sativa, are found within the forest reserve and were mapped as special structures. The systematic conversion on a short timespan from a coppice with standards to a broadleaved forest, offers an excellent oportunity to compare forest dynamics of evenaged Beech and oak stands on silt soils. For this reason, and for the reason that it represents a forest type which is relatively scarce in Europe (Endymio-Carpinetum), Jansheideberg was incorporated into the monitoring programme.

The forest reserve was monitored for the first time in 2004. It included an inventory of the trees, shrubs and herbs in 31 grid-based plots and in a 0,98 ha core area. We also repeated an inventory of living trees and shrubs in 14 grid-based plots, which were inventoried in 1987 as a support for the management plan.

The total stem number, basal area and living volume calculated by means of the 31 circular plots, respectively amounted to 583 per ha, 30.7 m2/ha and 423.6 m3/ha. The most important tree species with regard to the living volume, were Fagus sylvatica and Quercus robur + Q. petraea, both accounting for approximately 75% of the living volume. Other species (Larix decidua, Fraxinus excelsior, Acer pseudoplatanus, Castanea sativa) only accounted for less than 10% of the total living volume. The largest trees, with DBH values up to 70 cm, were relic trees of Larix decidua which were originally planted in rows within the stands of broadleaved species, when the forest was converted to a high forest. As a consequence of this thourough conversion, no coppiced trees were found within the inventoried plots.

Fagus and Quercus also were the dominant species regarding the stem number, but stem number of Acer pseudoplanatus was almost equal to that of the former two species. However, Acer trees have small dimensions compared to Fagus and Quercus trees which dominate the upperstorey. Quercus robur + Q. petraea as well as Fagus sylvatica have a median DBH between 30 and 35 cm, with the DBH distribution of the latter being more dispersed, as both thicker and smaller Fagus trees were frequently found. The DBH distribution of Acer pseudoplatanus has a shape which is typical for a colonizing tree species, with many small individuals and only few large ones. In fact, Acer is the only tree species abundantly regenerating in the forest reserve. Mapping the inventory results revealed that Acer pseudoplatanus was dominant in the understorey of stands with light-demanding tree species, Quercus spp. in the first place. The understorey of the Fagus stands was far less dense and regeneration of any tree species, including Acer pseudoplatanus, was scarce.

The 14 circular plots, which were inventoried in 1987 for the first time, were revisited in 2006. Results confirmed that the establishment of Acer pseudoplatanus in Jansheideberg is a recent phenomenon.There are indications that this species was selectively cut before Jansheideberg was appointed as a forest reserve, in order to preserve the flowering aspect of the bluebell (Hyacinthoïdes non-scripta), in particular in Fagus stands. The stem number of Fagus sylvatica and Quercus robur declined between 1987 and 2006, which is mainly caused by thinnings, as the amount of dead wood is still very low in the forest reserve.

Dead wood only amounted to 7.2 m3 per ha, when calculated for the 31 circular plots. This estimated value increased to a value of 11.1 m3 per ha, when the coarse dead wood outside the circular plots was taken into account as well. Many of this dead trees originated from initial fellings to reduce the number of exotic trees. This is a low value, compared to the other forest reserves which were monitored so far. The dead wood is still fresh or intermediately decomposed, indicating a recent origin.

The core area was not located in a homogeneous part of the forest reserve, which is the standard approach, but was positioned on the border of a Fagus and a Quercus stand (both planted in 1935), in order to study spatial aspects of future forest dynamics in both stands. The dendrometric characteristics of both stands are in agreement with the results found for the forest reserve as a whole: Fagus trees were growing faster than Quercus trees, resulting after 69 years into an average height difference of 10 m and a difference of the median DBH values between 5 and 10 cm. As a result, the living volume of the Fagus stand amounted to 150% of the value calculated for the Quercus stand (561 m3 per ha versus 368 m3 per ha).

Dead wood was scarce in the core area, the total volume was estimated at 4.2 m3 per ha, which is a very low value. Most of it were fragments and small dead trees located in the Quercus stand. The dead wood in the Fagus stand (0.6 m3 per ha) originated from stumps, which are relics of the former forest management. A dead Larix tree, which was felled to close down a footpath on the border between both stands, accounted for 1.6 m3 (38% of the total necromass in the core area).

There was a remarkable difference of the forest vegetation in both parts of the core area, which can be explained by soil characteristics. The topsoil is less acid in the Fagus stand than in the Quercus stand, with median pH-CaCl2 values of 3.3 and 3.1 respectively. This can explain the low frequency in the Quercus stand of species which are sensitive to acidification, such as Anemone nemorosa, Milium effusum, Athyrium filix-femina and Hedera helix. The only species with a preference for the oak stand, is Pteridium aquilinum. As a result, species diversity is much higher in the Fagus stand than in the Quercus stand. Walleyn et al. (2006) observed a similar pattern for the diversity of fungi in the core area.

Kriging maps of soil pH and plant-available P in the core area, do not show a gradual change, indicating a natural gradient, but a discontinuity at the stand border. Further research is necessary to determine if this is caused by a tree species effect, or by differences in the former management of both stands (which is, as far as known, not the case).

An important goal of the monitoring research in Janseheideberg is to determine the future equilibrium between the three most frequent tree species on the silt plateau: Quercus robur + Q. petraea, Fagus sylvatica and Acer pseudoplatanus. We can already make some prognoses based on the results of the first inventory. As the trees are of an intermediate age (69 years) and the forest reserve was managed until 10 years ago, only minor and gradual changes are expected in the nearby future. Competition between trees in the overstorey will increase, which will cause a natural reduction of the stem number and a gradual increase of the dead wood volume. However, in stands dominated by light-demanding tree species (mostly Quercus robur and Q. petraea), where the understorey is dominated by Acer pseudoplatanus, Acer which will grow into the overstorey within a few decades. Since Acer pseudoplatanus is a more competitive climax species than Quercus robur + petraea, this might cause a dyback of the latter tree species. Establishment of Acer pseudoplatanus in Fagus stands is limited by the dense canopy of the overstorey trees and it is assumed that gaps are necessary to promote establishment of Acer pseudoplatanus.

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