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Facilitation of tree saplings by nurse plants: microhabitat amelioration or protection against herbivores?

AuthorsGómez Aparicio, Lorena ; Zamora Rodríguez, Regino; Castro Gutiérrez, Jorge; Hódar, José Antonio
KeywordsAbiotic stress
Facilitation mechanism
Herbivory pressure
Mediterranean mountain
Nurse shrub
Sapling survival
Species-specific effect
Issue DateJan-2008
PublisherOpulus Press
CitationJournal of Vegetation Science 19: 161-172 (2008)
AbstractQuestion: Positive interactions are predicted to be common in communities developing under high physical stress or high herbivory pressure due to neighbour amelioration of limiting physical and consumer stresses, respectively. However, when both stress sources meet in the same community, the relative importance of the two facilitation mechanisms is poorly understood. We ask: What is the relative importance of abiotic vs. biotic mechanisms of facilitation of tree saplings by shrubs in Mediterranean mountain forests? Location: Sierra Nevada, SE Spain (1800-1850 m a.s.l.) Methods: Saplings of four tree taxa (Acer opalus ssp. granatense, Quercus ilex, Pinus nigra ssp. salzmanii and P. sylvestris var. nevadensis) were planted following a 2 × 2 factorial design: two levels of herbivory (control and ungulate exclusion) and two microhabitats (under shrubs and in open areas). Sapling survival and growth were monitored for five years. Results: Shrubs had positive effects on sapling survival both in control and ungulate excluded plots. This effect was species-specific, with shrubs increasing the survival of Acer opalus and Quercus ilex three and twofold, respectively, but having a minor effect on the Pinus species. Herbivory damage was also species-specific, being much higher for Acer opalus than for any other species. Shrubs did not protect saplings of any species against ungulates. Thus, all Acer saplings (the most damaged species) suffered herbivory outside the exclosures, which largely reduced sapling height. Conclusions: Protection from abiotic stress (summer drought and winter frost) was much more relevant than protection from biotic stress (herbivory). However, we propose that the final balance between the two mechanisms can be expected to vary strongly between sites, depending on the relative magnitude of the different sources of stress and the intrinsic traits (e.g. palatability) of the species interacting.
Description12 pages, 4 figures, 4 tables, 73 references. We thank the Consejería de Medio Ambiente, Junta de Andalucía, and the direction of the National Park, for permission to work in Sierra Nevada. We are especially grateful to Empresa de Transformación Agraria S.A. (TRAGSA) for carrying out the experimental plantations. Jose M. Gómez, Sergio de Haro, Elena Baraza and Daniel García kindly helped with the field work. David Nesbitt looked over the English version of the manuscript.
Publisher version (URL)DOI: 10.3170/2007-8-18347
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