De Levende Natuur nummer 4 van 2011 (English summary)


DLN 2011-4

Large grazers steer the development of new woodlands on former agricultural land

J. Van Uytvanck

Low intensity grazing with large herbivores is increasingly used to steer the development of mosaic landscapes and woodland expansion on former agricultural land. We studied the germination, survival, growth, browsing and establishment patterns of woody species in grassland and abandoned arable land.
Our findings show that in grassland tree establishment occurs rather slow and in a spatial association with scrub and tall herb patches. Fast growing spiny shrubs like Rubus sp. offer protection against grazing for palatable saplings, allowing establishment and growing out beyond the browse-line (mainly Fraxinus, Alnus, Quercus, Salix). On former arable land, establishment is different. We mainly found a massive and fast colonisation of grazing tolerant or resistant pioneer tree species (Salix, Betula). However, for both grassland and former arable land we could detect thresholds for grazing pressure. A threshold for regeneration on former pastures was found around 130 grazing days ha-1 y-1 (= ±0.35 AU ha-1 y-1); on former arable land around 180 grazing days ha-1 y-1 (= ±0.50 AU ha-1 y-1). Less than 0.5% of established trees will grow out.
Modelling crown cover increment over 100 years, based on demographic models and assessed tree numbers growing above the browsing-line, revealed that steering grazing pressure could result in a variety of woodland types (in terms of landscape openness). However, on former arable land a grazing pressure >125-150 grazing days ha-1 y-1 is needed to prevent complete closure of the forest within 100 years; on former pastures, only a very low grazing density (<25 grazing days ha-1 y-1) would lead to closed forest. Results show that the early successional stages on grazed former agricultural land are most important in directing the development of new woodland landscapes.

Groundwater level, site conditions and species composition of the vegetation

J. Runhaar, M.H. Jalink & R.P. Bartholomeus

Groundwater regime strongly influences plant growth. However, the relationship between groundwater level and species composition of the vegetation is always indirect, through operational site factors such as oxygen supply, moisture supply, acidity and nutrient supply.

How groundwater regime affects these operational site factors depends upon conditional factors such as soil type, climate, surface water levels and groundwater chemistry.

How groundwater level influences oxygen stress and moisture stress, and how this affects the occurrence of xerophytes, mesophytes and hygrophytes, is relatively well known. Oxygen stress is the main determinant for the occurrence of hygrophytes. Oxygen stress occurs in sites with shallow groundwater tables. In The Netherlands the mean spring groundwater level is a good predictor for the occurrence of hygrophytes (75% variance explained on the basis of a nonlinear relationship). The moisture stress not only depends on groundwater level, but also on soil texture and organic matter content. The number of days with a soil water potential near wilting point, as calculated with a hydrological model for the unsaturated zone (SWAP), is a good predictor for the abundance of xerophytes  (90% variance explained on the basis of a nonlinear relationship).

Groundwater level also influences the acidity and the nutrient availability of a site. However, these relationships are very indirect and influenced by many other factors.
In the assessment of critical groundwater ranges per vegetation type use is made of both knowledge of underlying processes and of empirical data on the groundwater levels in reference sites.

Bog orchid: floristic treasure under threat

The number of localities of the Bog orchid (Hammarbya paludosa) in The Netherlands has declined sharply during recent decades. In 2010, the Bog orchid was only present in larger numbers in rich fens with Sphagnum-dominated reedlands, situated in the low-lying holocene peatland areas. In the pleistocene part of the country the species is almost extinct due to dessication and drastic lowering of the water tables. Although no specific restoration projects for Bog orchid exist, the holocene peat areas are nowadays the best suitable places for restoration of the former habitats. Key factors are regular mowing of the vegetation and the conservation or realisation of a mesotrophic water system. To restore former dessicated peat-habitats a shallow type of sud cutting (scraping) was successful at some localities. In peat-meadow systems incidental and short-term grazing in autumn can also be successful. In the pleistocene areas long term projects should aim at restoration of lowered water tables and tackling the effects of dessication and eutrophication; in the holocene peat areas long term projects should aim at creating new possibilities for terrestrialisation (succession of open water bodies to ‘dry’ land).

Species signpost the way to effective nature restoration

W.C.E.P. Verberk

Understanding the match between species and their environment requires a thorough knowledge on both the biology of the species and the functioning of the landscape. Such an understanding greatly increases the information derived from species inventories, which can be used to better guide management decisions. Three different approaches to obtain knowledge on the match between species and their landscape exist: a comparative approach, the use of life-history strategies and an experimental approach. These approaches are best used in conjunction. Three examples highlight how these approaches can yield new insights in the causal mechanisms underlying species occurrence and the identification of the key processes in a given situation. These new insights can be translated directly to recommendations for improving the effectiveness of restoration practices.

Observing flowers in Zwanenwater dune reserve during 140 years

A. Barendregt, C. ten Haaf, D. van Lunsen & R. Roos

Northwards in the dunes of Holland is the nature reserve Zwanenwater that includes many wet dune slacks and the largest dune lakes of Europe. Descriptions of the flora in 1870-1890 can be compared with present species. Most former species appear to be present today, explained by the continuation of the conditions, poor in nutrients and high in water levels. Historical data indicate that the mean water tables did not change after 1932. At the same time the system as a whole experienced succession, resulting in expanding willow shrubs. In the southern parts of the area, some species from primary dune slacks maintain, although the impact of the sea is absent for more than a century. Just the dynamics in the water tables might explain the presence of these species.