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

Abstract

Question: 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. grana tense, 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 speciesspecific, 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.Keywords: Abiotic stress; Facilitation mechanism; Herbivory pressure; Mediterranean mountain; Nurse shrub; Sapling survival; Species-specific effect.Nomenclature: Castroviejo et al. (1986Castroviejo et al. ( -2001 for tree species, and Molero-Mesa et al. (1992) for shrub species.Abbreviations: PPFD = Photosynthetic photon flux density; RII = Relative interaction index; VPD = Vapour pressure deficit. IntroductionThe abundance, performance and spatial distribution of plant species are highly linked to the strength and sign of the interactions involving them in their communities (Roughgarden & Diamond 1986;Brown et al. 2001). In 1994, Bertness & Callaway proposed a conceptual model suggesting that strong positive interactions should be particularly common in communities developing under high abiotic stress and in communities with high consumer pressure, whereas competition should be the dominant interaction under mild physical conditions and low consumer pressure. Since then, a large number of empirical studies have supported this hypothesis by reporting that positive interactions are the rule under severe ...

Large-Scale Regeneration Patterns of Pinus nigra Subsp. salzmannii: Poor Evidence of Increasing Facilitation Across a Drought Gradient

“…The presence of rocks was associated to poor-sandy soils, steep slopes, and open canopies (data not shown). Therefore, and beyond the fact that rocks diminish the surface available for seed germination, the negative correlation found between seedling density and rockiness could be a consequence of water stress due to a combination of both inferior edaphic conditions, and high evaporation under low canopy cover [8]. Canopy cover consequently appeared as the main ecological factor determining seedling establishment in P. nigra stands.…”

“…Consequently, summer drought might be the major limiting factor of recruitment throughout the species distribution area. This impression was further sustained by the facts that, first, overgrazing does not seem to be responsible for the lack of P. nigra regeneration, although herbivore pressure is characteristically high in Mediterranean forests [8,50]. Second, although data did not show significant evidences of recent silvicultural treatments (a disturbance that usually triggers regeneration in managed stands), values of stand basal area were within those more suitable for the establishment of new trees [51,52].…”

“…High irradiance increases evaporative demands and further reduces soil-water availability, therefore, there are obvious reasons to explain the increased seedling mortality observed in open areas [5][6][7][8]. In poor light environments, the eventual death of P. nigra seedlings because of desiccation can be justified by at least two reasons.…”

“…Seedlings are highly vulnerable to drought stress, and Mediterranean-type ecosystems are characterized by a sharp seasonality of rainfall and temperature, with hot-dry summers [4]. Consequently, seedling mortality is found to be exacerbated both during the summer season and in open areas, where high irradiance increases evaporative demands and further reduces soil-water availability [5][6][7][8]. Studies focused on the recruitment dynamics of Mediterranean tree species have found increasing seedling mortality during the summer, compared with other periods of the year, and in open areas, compared with sites located under the canopy of woody plants [9,10].…”

“…Studies focused on the recruitment dynamics of Mediterranean tree species have found increasing seedling mortality during the summer, compared with other periods of the year, and in open areas, compared with sites located under the canopy of woody plants [9,10]. Thus, Mediterranean tree species generally benefit from being planted in the proximity of shrubs during reforestation work, as most shrub species may buffer against high temperatures and radiation [8]. Similarly, the risks of excessive evapotranspiration, overheating, and photo-inhibition are ameliorated under the cover of tree canopies [2,10,11].…”