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

Abstract

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 opalusssp.grana‐tense, Quercus ilex, Pinus nigra ssp. salzmaniiandP. sylvestrisvar.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 ofAcer opalusandQuercus ilexthree and twofold, respectively, but having a minor effect on thePinusspecies. Herbivory damage was also species‐specific, being much higher forAcer opalusthan for any other species. Shrubs did not protect saplings of any species against ungulates. Thus, allAcersaplings (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.