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

Abstract

Current conceptual models predict that an increase in stress shifts interactions between plants from competitive to facilitative; hence, facilitation is expected to gain in ecological importance with increasing stress. Little is known about how facilitative interactions between plants change with increasing biotic stress, such as that incurred by consumer pressure or herbivory (i.e. disturbance sensu Grime). In grazed ecosystems, the presence of unpalatable plants is reported to protect tree saplings against cattle grazing and enhance tree establishment. In accordance with current conceptual facilitation-stress models, we hypothesised a positive relationship between facilitation and grazing pressure. We tested this hypothesis in a Weld experiment in which tree saplings of four diVerent species (deciduous Fagus sylvatica, Acer pseudoplatanus and coniferous Abies alba, Picea abies) were planted either inside or outside of the canopy of the spiny nurse shrub Rosa rubiginosa in enclosures diVering in grazing pressure (low and high) and in exclosures. During one grazing season we followed the survival of the diVerent tree saplings and the level of browsing on these; we also estimated browsing damage to the nurse shrubs. Shrub damage was highest at the higher grazing pressure. Correspondingly, browsing increased and survival decreased in saplings located inside the canopy of the shrubs at the high grazing pressure compared to the low grazing pressure. Saplings of both deciduous species showed a higher survival than the evergreens, while sapling browsing did not diVer between species. The relative facilitation of sapling browsing and sapling survival -i.e. the diVerence between saplings inside and outside the shrub canopy -decreased at high grazing pressure as the facilitative species became less protective. Interestingly, these Wndings do not agree with current conceptual facilitationstress models predicting increasing facilitation with abiotic stress. We used our results to design a conceptual model of facilitation along a biotic environmental gradient. Empirical studies are needed to test the applicability of this model. In conclusion, we suggest that current conceptual facilitation models should at least consider the possibility of decreasing facilitation at high levels of stress.

Influences de la sylviculture sur le risque de dégâts biotiques et abiotiques dans les peuplements forestiers

“…Conifer-inhabiting bark beetles are able to perceive non-host volatiles released by angiosperm trees and to use them to avoid non-suitable habitats (Zhang and Schlyter, 2004). In mixed stands host tree saplings can also be protected against mammalian grazers by non-host, nurse plants in the form of physically (thorns) or chemically (toxins) defensive species (Smit et al, 2007). Third, damage by generalist insect or mammalian herbivores on a focus tree species may also be reduced in tree mixtures that contain other, more palatable, secondary tree species, which would be exploited first.…”

Effect of livestock grazing in the partitions of a semiarid plant–plant spatial signed network

“…We assume that with the appearing of livestock, some species 303 disappeared from the community while several species that do not associate to others 304 established, reducing the number of associations per species. The decrease in the number of 305 partitions with stocking rate can be explained by the preferential feeding of grazers on palatable 306 plants growing without protection through 'nurse' plants, which leads to a selection on 307 association strategies (Smit et al, 2009;Smit et al, 2007). Both the establishment of non-308 associative species and simplification of association strategies were favored by an increase in 309 stocking rate, particularly at the site with highest stocking rate (G4).…”

“…These findings can be explained by a 313 reduction of facilitative effects in plant community due to high herbivore pressures. The intense 314 grazing caused severe damage to 'nurses', which could not act as proper shelters against grazing 315 and/or aridity (Graff et al, 2007; Smit et al, 2007). 316…”

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

“…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.…”