Restoration of woodpasture on former agricultural land: The importance of safe sites and time gaps before grazing for tree seedlings

Abstract

Low intensity grazing Large herbivoresTree regeneration A B S T R A C TWoodpastures (open, grazed woodlands with a mosaic of grassland, shrub and tree patches) are of high biological and cultural value and have become a threatened ecosystem in Europe. Spontaneous tree regeneration in the presence of large herbivores, is an essential process for management and restoration of this structurally diverse habitat. We examined the suitability of five vegetation types (grasslands, ruderal vegetations, tall sedges, rush tussocks and bramble thickets), grazed by large herbivores, for tree regeneration.We hypothesized that bramble thickets and tall herb communities operate as safe sites for palatable tree species through the mechanism of associational resistance. We set up a field experiment with tree seedlings in grazed and ungrazed conditions and recorded mortality and growth of seedlings of two palatable tree species (Quercus robur and Fraxinus excelsior) during three growing seasons. In the same experiment, we studied the effect of a two year's initial time gap before grazing.Bramble thickets were suitable safe sites for survival and growth of seedlings of both species. Tall sedges, soft rush tussocks and ruderal vegetations with unpalatable or spiny species provided temporal protection, allowing seedlings to survive. Tree regeneration in livestock grazed grassland was highly constrained. Rabbits may undo the nursing effects of bramble thickets. The first year's survival is of major importance for the establishment of trees. Subsequent grazing affects growth rather than survival. A two year's initial time gap before grazing, had positive effects on survival, but did not enhance outgrowth of unprotected trees.

The larch wood pasture: structure and dynamics of a cultural landscape

“…Other studies performed in different environments revealed that unpalatable shrub thickets play as shelters protecting woody species from browsing (Smit et al 2009;Van Uytvanck et al 2008;Vera 2000). However, the encroachment of juniper thickets may reduce the area where larch recruits more effectively and grows faster than late-successional species.…”

“…Mayer et al (2006) suggested to hold down a grazing pressure threshold of 1 LUּ ha -1 or to provide time intervals without grazing (Van Uytvanck et al 2008). Heavy grazing pressure followed by periods of moderate grazing is often associated to the onset of tree regeneration in many sites (Mast et al 1997).…”

Tree-shrub associations in grazed woodlands: first rodents, then cattle?

“…Grazers are hypothesized to play a key role in this succession by damaging palatable tree saplings within grassland but not within spiny shrubs. Several empirical studies have indeed shown that unpalatable species-spiny or toxic plants-protect tree saplings from ungulate damage (Smit et al 2006a;Van Uytvanck et al 2008b), resulting in spatial associations between young palatable trees (e.g., pedunculate oak Quercus robur) and spiny or toxic plants (e.g., spiny blackthorn Prunus spinosa) (Bakker et al 2004;Smit et al 2005;Van Uytvanck et al 2008a). This process in which an unpalatable species facilitates a palatable species by reducing the amount of herbivory is called associational resistance (Hamback et al 2000;Milchunas and Noy-Meir 2002) and is considered an important factor in the proposed cyclic succession in grazed woodlands (Olff et al 1999).…”

Grazing and abandonment determine different tree dynamics in wood-pastures

“…Deciduous trees, however, have a greater potential for compensatory growth (Hester et al 2004;Vandenberghe et al 2007). The browsing pressure on palatable seedlings may be lower if they are protected by surrounding unpalatable vegetation (Smit et al 2006;VanUytvanck et al 2008). Thorny or toxic shrubs may facilitate the survival of palatable trees, which in turn will grow and outshade the shrubs (Olff et al 1999;Bakker et al 2004).…”

“…Thorny or toxic shrubs may facilitate the survival of palatable trees, which in turn will grow and outshade the shrubs (Olff et al 1999;Bakker et al 2004). Alternatively, time gaps (of a few years) in grazing can improve the growth of trees that regenerate abundantly in the absence of grazers (VanUytvanck et al 2008;VanUytvanck and Verheyen 2014).…”