De Levende Natuur nummer 2 van 2009 (English summary)


DLN 2009-2

Pond bats in Fryslân

J. Schut, D. Kuijper, A.J. Haarsma, J. Ouwehand, H.J.G.A. Limpens & D. van Dullemen

In the province of Fryslân five areas are designated as Special Areas of Protection (EC Habitat Directive) for the Pond bat (Myotis dasycneme). In this research the bat presence in four of these Natura 2000-areas is compared to four non Natura 2000 areas. The aim was to investigate whether the areas designated as Natura 2000 areas are indeed the most important areas. The highest bat presence was found in Alde Feanen and Groote Wielen (Natura 2000-areas), but also in the Slotermeer (non Natura 2000) a high bat presence was measured.
Pond bats use a network of day-time roosts and foraging areas, interconnected by commuting routes. Effective protection of roosts and routes is crucial for the protection of the Pond bat. In order to find day-time roosts and commuting routes, we tried to find commuting routes near the Alde Feanen and Slotermeeer. By following commuting bats with bat detectors and by using radio telemetry, we found 20 previously unknown roosts, housing over 2700 individual bats. Before this research, the roost sites of 770 Pond bats were known.
One of the uncertainties in Pond bat protection is the influence of artificial illumination on the bats. Therefore we did an experiment on this subject. The results of the experiment showed changes in behavior under the influence of light, mainly consisting of disturbed feeding behavior.

Old oak woods: new insights and opportunities

R.J. Bijlsma, J. den Ouden & H.N. Siebel

Two Natura 2000 woodland types occur on dry, acid soils in the Netherlands, namely Atlantic acidophilous beech forests with Ilex and sometimes Taxus in the shrub layer (H9120) and Old acidophilous oak woods with Quercus robur on sandy plains (H9190). The former occurs on relatively rich, loamy soils, the latter on the most infertile, sandy soils. The distinction between them is complicated by the fact that most old oak woods are former coppice on loamy soils. These forests are now invaded by beech. On the other hand, both beech and holly encroach upon nutrient poor cover sands and drift sands as well. Holly is a common garden escape and is spreading rapidly in all kinds of woodlands.
In order to clarify the distinction between the two types, their definitions, based on vegetation types, have been supplemented by soil and historical characteristics. Beech-oak forests (H9120) can only occur on non-podzolised, loamy soils, boulder clay, old river clay or acidic loess. Old oak woods (H9190) can be found only on podzolised, non-loamy soils or drift sands. Particular woodlands of both types must be present on the first topographical map of the Netherlands of about 1850 as (degraded) woodland or must have an age of at least 100 years. This corresponds to the EC-description of Beech-oak forests that mentions several characteristic ancient woodland vascular plant species.
As a consequence of the augmented definition, Beech-oak forests (H9120) are ancient woodlands in the usual sense. They occur close to settlements, are often surrounded by banks and ditches, have pits (loam, iron, gravel), veteran trees or coppice stools and are cut through by old roads and tracks. Bracken (Pteridium aquilinum) often dominates the herb layer. Other ancient woodland species are Maianthemum bifolium and Oxalis acetosella. Several species with persistent seed banks are characteristic as well, such as Hypericum pulchrum. Old oak woods (H9190), on the other hand, occur in the periphery of (former) heathlands or drift sands. They show relief, are often bordered by ridges of drift sand and lack banks and old roads. The herb layer is dominated by Vaccinium myrtillus. North-facing slopes contain northerly species such as Trientalis europaeus and the moss Dicranum majus.
The augmented definitions of both Natura 2000 types emphasize differences in geomorphology and topography. This opens opportunities for management. Old oaks woods can be considered as parts of heathland-woodland gradients where openness and conditions for the natural regeneration of oak are maintained by grazing. Beech-oak forests can occur in the upper, more base rich part of the same gradient or as ancient woodlands in larger afforestations. Here, management can focus on veteran trees, coarse dead wood, gap dynamics and natural rejuvenation of beech and holly.

A new view on old oaks

J. den Ouden, P. Copini & U.G.W. Sass-Klaassen

On the sandy areas in The Netherlands, oak trees may grow as clusters of multi-stemmed clones that extend up to 35 m in circumference. The size of these clones suggests a high age of the genetic individual. However, the estimate of age depends on the speed by which the clonal structure expands through time. We investigated how a number of these clusters on Wilde Kamp, a woodland area near Garderen, have originated by vegetative propagation.
Under coppice, the tree resprouts each time the stems are cut. Each new sprout grows from the base of the stump, and parts of the lower stem and upper root system die after each coppice cycle. Over time, a stool develops that expands outward. Since this is a slow process, high ages are inferred from large clones.
When branches become buried they can build their own root system and grow into independent stems. This layering leaves an unequivocal mark in the tree ring pattern and anatomy of buried stem wood, which can be dated with annual precision. Layering occurs when the base of the tree crown is covered by sand, or when low branches expand horizontally, bend to the ground and then get covered by litter and plants and so become slowly buried. Lateral expansion rate is much higher than in coppice, so large clones may be relatively young.
The oak clusters on Wilde Kamp did not grow on an old coppice stool, but the stems were connected by branches that had become underground woody structures after burial. The oldest wood was dated 1826. These clones originated as frequently grazed shrubs in an open habitat. The trees expanded by layering and were cut probably once in the early 20th century. Oak clusters are the legacy of the livestock grazing and open aspect of the woodland landscape.

Long-term effects of cutting on roadside grasslands and forest edges

R.J.W. van de Haterd, B. van den Hengel & P.J. Keizer

Roadside vegetation constitutes a significant amount of semi-natural grasslands in The Netherlands. Rijkswaterstaat (the Dutch directorate-general for public works and water management) is responsible for the management of major roads and waterways. Management aims at maximizing natural values within boundaries set by traffic safety and road use. Research in nature reserves has shown that cutting with removal of cuttings leads to a lower biomass and a higher biodiversity. Rijkswaterstaat wanted to investigate the long-term effects of cutting with removal in roadsides verges and establish the optimal cutting frequency for different soil types and sun exposure. In a 23-year experiment five different cutting frequencies (2x/year, 1x/year in spring, 1x/year in autumn, 1x/2years and never) were tested in the sun and in the shade, on a sandy soil with a Thero-Airion vegetation and on a clay soil with an Arrhenatherion elatioris. Despite hay-making, biomass slightly increased in all plots except the yearly cut plots on clay. Biomass increased faster if cutting frequency was lower. A higher cutting frequency yielded significantly more species, rare species and flowers. In the shade however, the benefits of cutting are relatively low and it is questionable whether these outweigh the costs. The optimal cutting frequency on clay is two times per year, on nutrient poor sandy soil cutting only once in autumn is enough.
Where roadside verges are wide enough, a development of a zone of shrubs and rough grassland near forest edges can be allowed. A comparison of such zones with reference plots where grassland management had continued showed that after 10 years a significant decrease of species numbers and of rare species had occurred (based on vascular plants). This trend was visible in shady and sunny plots, on sandy as well as clay soils. We therefore advise against this management practice on roadside verges where grassland values are high or potentially high.