Bosreservaat De Heirnisse: monitoring van de dendrometrische gegevens en de vegetatie in steekproefcirkels en een kernvlakte

A 46 ha unmanaged part of the forest reserve Heirnisse, which has a total surface area of 76 ha, was incorporated in the monitoring programme and inventoried for the first time in 2004. It included an inventory of the trees, shrubs and herbs in 68 grid-based plots and in a 0,98 ha core area. The part of the forest reserve which was monitored, was left unmanaged since the acquirement of the forest by the government in 1992. Contrary to other forest reserves, exotic tree species (mostly Prunus serotina and Quercus rubra) were cut or girdled after the first inventory. This resulted in a clearcut area of 2 – 2.5 ha. As the results represent the situation before this management, an estimation was made to correct the data to the actual situation (see below).

The forest reserve Heirnisse mostly is located within the Moervaart valley, a broad alluvial plain which originated by natural drainage of a late-glacial lake in which gyttja was formed. The southern part of the forest reserve is located outside the Moervaart valley, on a moist but acid sandy soil. Forest vegetation on this part is an unsaturated Quercion, which will probably develop towards a Violo-Quercetum. The calcareous part located in the Moervaart valley, actually is an unsaturated Alno-Padion community, with fragments of Alnion vegetation along the numerous ditches. In the northern part of the forest reserve, some recently abandoned grasslands, which are covered by a facies of Carex spp., are spontaneously colonized by trees (Quercus robur, Salix spp., Fraxinus excelsior…).

The area was property of the Baudeloo abbey from medieval times up to the 18th century and was in use as a grassland. Large parts were afforested in the 18th century, which was accompanied by the construction of ridges and furrows and possibly also included an exploitation of the gyttja. The whole area, except some parcels in the northwest, was reclaimed for farmland use in the 19th century but reforested after a few decades. Forest in the alluvial parts was managed as a coppice in which poplar cultivars were planted. On the sandy soils in the south, mostly high forest was found, including a Pinus nigra stand, a Quercus robur stand and a stand with Quercus rubra and Prunus serotina.

The total stem number, basal area and living volume calculated by means of the 68 circular plots, respectively amounted to 1270 per ha, 24.7 m²/ha and 189 m³/ha.The most important tree species with regard to the living volume, were: Populus x euramericana, Alnus spp. (incana and glutinosa), Quercus robur and Betula spp. (pendula and pubescens). The largest DBH-values (75-80 cm) were measured on Populus x euramericana trees, planted in between the former coppice. Most frequent species are Betula spp. and Alnus spp., which have been coppiced and therefore have several shoots per bole. Regenerating species were, in decreasing order of frequency: Prunus serotina (locally abundant), Acer pseudoplatanus, Fraxinus excelsior, Alnus incana and Betula pubescens. Seedlings of Fraxinus excelsior were restricted to the calcareous part of the forests, while Acer was frequently found on sandy soils. These findings indicate that in the long run, Fraxinus excelsior can become dominant on calcareous soils in the north, while Acer can dominate on moist sandy soils in the south. These forecasts assume that the hydrology of the area is unaffected (see below) and that regeneration by Quercus rubra and Prunus serotina is inhibited.

The total necromass of forest reserve Heirnisse, based on the inventory before the felling or girdling of exotic species, amounted to 22.3 m³ per ha. After this management, the necromass increased to 35 m³ per ha, which is a high value for a forest with a low living stock. Before this management, most of the necromass (80%) was accounted for by Populus x euramericana, in an advanced state of decomposition. Dead wood fragments of other species were numerous, but dimensions were small as they originated from a natural reduction of the number of shoots on coppice boles.

As a consequence of the variation in soil conditions and habitats, a high number of vascular plant species was observed in the forest reserve. Many hygrophilous species were present in the grid-based plots, including waterplants (Typha latifolia), species specific for Alnion woodland (Solanum dulcamara, Scutellaria galericulata) and light-demanding species preferring abandoned grassland (Cirsium oleraceum, Eupatorium cannabinum, Juncus inflexus). On acidic soils, the herbaceous vegetation was less diverse, Rubus caesius was absent, while Dryopteris dilatata, D. carthusiana and Rubus fruticosus aggl. dominated. Ancient woodland species were scarce as only 10 species are mentioned in European lists (Hermy et al. 1999) and only three species are ancient woodland species in Flanders (Honnay et al. 1998).

The core area was located in the west of forest reserve Heirnisse, on a gradient from calcareous, alluvial soils to sandy, relatively dry soils. This is illustrated by maps with the area of distribution of Rubus caesius, Rubus fruticosus and seedlings of Fraxinus excelsior. By contrast, the tree layer is homogenous and dominated by Alnus incana, which originated from coppice, and Populus x euramericana (DBH 55-80) which were planted along the forest path in the north. Both species have an equal living volume and the total living volume in the core area amounted to 269 m³ per ha, which is a higher value than the average for the forest reserve as a whole. This also applies to the stem number , equalling 1703 per ha, but the necromass (15.7 m³ per ha) is below the average value recorded for the whole forest reserve. Approximately 2/3 of the necromass in te core area is accounted for by Populus x euramericana, 1/3 by small fragments of Alnus incana.

The dendrometric characteristics indicate that the forest reserve Heirnisse is in an increment phase characterised by a high stem number, with small dimensions of the trees and a low living volume. The total aboveground biomass (sum of living and dead volume) is very low compared to other forest reserves which were monitored. The ratio of dead wood volume (mostly poplar cultivars) to the total aboveground biomass is an average value. The present site conditions, with moist but not wet soils during the growing season, allow a much higher aboveground biomass. Without changes in the site conditions, the living biomass will gradually increase, but the dead wood volume will not and even can decrease by rapid decomposition of the lying dead poplars. As a consequence of these changes, the contribution of dead wood to the total aboveground biomass will gradually decrease.

These forecasts are based on the present site conditions. However, hydrological conditions of the area are controlled by a pumping-engine. A rise of the groundwater level can introduce a shift from an Alno-Padion to an Alnion vegetation and cause a dyback of tree species which are sensitive to a permanent high groundwater level, such as Fraxinus excelsior, Acer pseudoplatanus, Quercus robur, Populus x euramericana. This scenario implicates a strong but temporary increase of the necromass and a decline of the living biomass, but the magnitude of this ‘catastrophe’ is determined by speed and the rise of the groundwater level (in particular in the vegetation season).