Unpalatable plants facilitate tree sapling survival in wooded pastures

Abstract

Summary 1.In endangered wooded pasture ecosystems established tree saplings are frequently found in spatial association with protective structures, suggesting nurse effects. This associational resistance is thought to be a driving force behind tree regeneration in wooded pastures. Experimental evidence for associational resistance is, however, scarce. We studied the effects of unpalatable plants on tree sapling performance in a wooded pasture ecosystem in Switzerland and tested the associational resistance hypothesis experimentally. 2. We transplanted 600 Picea abies saplings of two size classes (5 cm and 10 cm) in plots: (i) with unpalatable plants (either Gentiana lutea or Cirsium acaule , representing a chemically and mechanically defended species, respectively); (ii) with one of both unpalatable plants clipped to the ground; and (iii) without any unpalatable plants. Grazing intensity was measured once in each plot and tree sapling performance was followed for 2 consecutive years. 3. Grazing intensity was significantly higher in plots without unpalatable plants, and significantly higher in plots with Cirsium than with Gentiana . Correspondingly, sapling survival (21·5% and 10·6% after 1 and 2 grazing years, respectively) was significantly higher near unpalatable plants, and significantly higher in plots with Gentiana than with Cirsium . Large tree saplings survived significantly better than small ones, and depended less on the unpalatable plants for survival. 4. Except for saplings planted near Gentiana , sapling removal by cows and horses was the major cause of death, the extent varying between unpalatable species and treatments. 5. The growth in height of the saplings was unaffected by the treatments. 6. Synthesis and applications. This study shows that unpalatable plants can enhance tree regeneration in wooded pastures. Sapling survival was significantly higher near unpalatable plants where grazing intensity was significantly lower. Protection against cattle is particularly important for small saplings. These results have important management implications for the endangered and disappearing wooded pastures in western Europe. Transplanting tree saplings near unpalatable plants could be an alternative reforestation technique in intensively grazed wooded pastures. On the other hand, removal of unpalatable plants and shrubs might prevent or slow down undesired tree encroachment in less intensively grazed areas.

Nurse plants, tree saplings and grazing pressure: changes in facilitation along a biotic environmental gradient

“…This clearly demonstrates associational resistance, i.e. reduced herbivory by association with a defended neighbour (Smit et al 2006). We also found that grazing pressure exerted a great eVect on this facilitative process.…”

“…Recent studies show that establishment of woody species in grazed systems can be facilitated by nurse plants in the form of physically (thorns) or chemically (toxins) defensive species that protect tree saplings against large herbivores (Bakker et al 2004;Rousset and Lepart 2000;Smit et al 2005Smit et al , 2006). This process is also known as associational avoidance, associational resistance or defence guilds (Atstatt and Odowd 1976;Milchunas and Noy-Meir 2002).…”

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

“…Within this high structural diversity, a rich biodiversity in both plant and animal species can develop (Olff et al 1999;Vera 2000). Several studies show that low-intensity grazing plays an essential role in maintaining the rich structure and associated biodiversity in these systems (Olff et al 1999;Bakker et al 2004;Bakker et al 2006;Smit et al 2006a). Introducing large herbivores is therefore an increasingly used method throughout Europe since the last two decades (Van Uytvanck et al 2008a).…”

“…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).…”

Distribution and socio-ecological impacts of the invasive alien cactus Opuntia stricta in eastern Africa

“…Significantly, more men reported eating O. stricta than women (v 2 = 4.02 (df = 1); p = 0.044) as they are likely to spend more time in the rangelands herding their livestock (Tangka et al 2000). Respondents mentioned that a lot of time and effort is needed to remove the small barbs (glochids) from the fruit and that it could only be eaten in moderation otherwise it protect other plant species from browsing animals (Smit et al 2005(Smit et al , 2006. According to Smit et al (2007) an ''extremely toxic and well-defended nurse plant'' would provide significant protection to associated recruits.…”