Microclimate buffering and fertility island formation during Juniperus communis ontogenesis modulate competition–facilitation balance

Abstract

Aims: Facilitation has been reported in a wide range of plant communities, with evidence of positive interactions between beneficiary and nurse plants shifting during their ontogenetic development. This study explored the hypothesis that shrubs of Juniperus communis subsp. nana (thereafter Juniperus) play a crucial role in the successional sequence of plant communities acting as nurse for different species, but only after reaching a certain size. In addition, we examined whether plant-plant interaction changes during ontogenesis of these shrubs in the presence of contrasting disturbance regimes in terms of substrate stability. Location: Sibillini National Park, central Italy. The vegetation is semi-natural dry grasslands (92/43/EEC Habitats Directive: Habitat 6210 -Festuco-Brometalia).Methods: Field measurements were carried out to assess the effects of Juniperus on (1) the distribution of co-existing vascular species, (2) the above-and belowground microclimate, and (3) changes in soil fertility and hydrology.Results: The capacity of Juniperus shrubs to facilitate heterospecific plants considerably increases during its ontogenetic development, i.e. small shrubs mainly compete for resources with local vegetation, whereas large shrubs act as nurse plants for herbaceous and especially for woody species. The facilitation effect was slight, albeit significantly higher in the disturbed area than in the more stable one. Juniperus was able to promote the formation of an island of fertility under its canopy by accumulating a considerable amount of organic matter, N, P, Ca 2+ , Mg 2+ and K + in a few decades. Juniperus shrubs improve soil hydrological properties and mitigate the daily range of soil temperature, reducing the exposure of co-existing plants to high temperatures and water loss through soil evaporation, particularly during the growing periods in spring and summer.Conclusions: This study demonstrates that biogenic amelioration of soil quality, coupled with the mitigation of below-canopy microclimatic conditions, control the establishment and growth of co-existing plants during Juniperus shrub development.

Size-Mediated Interaction between a Cushion Species and Other Non-cushion Species at High Elevations of the Hengduan Mountains, SW China

“…These intra-specific changes in facilitation intensity could have been the result of size-dependent amelioration of physical and chemical properties (particularly soil nutrients) in the upper soil layer by cushions. A similar size–age related interaction has also been reported in a semi-arid environment, where plant diversity in the understory of R. sphaerocarpa and Juniperus communis was found to increase with shrub size/age, probably due to greater heterogeneity under the larger canopy (Pugnaire et al, 1996; Allegrezza et al, 2016). In addition, one previous study of cushions in the same region as ours attributed interspecific differences ( Arenaria polytrichoides vs. Potentilla articulata ) in intensity of facilitation to distinct capacities for improving soil nutrients of two phylogenetically distant species but which shared similar cushion morphology (Chen et al, 2014).…”

“…nana ) species has been attributed to a higher decomposition rate due to greater microbial activity and/or particular chemical or physical characteristics of litter under their canopies (Pugnaire et al, 1996; Allegrezza et al, 2016). Cushion species at high elevations have evolved their remarkable compact structure to trap and accumulate leaf litter and consequently create a constrained nutrition cycle via litter input (Körner, 2003).…”

“…In all of these studies, the shift from competition to facilitation was related to changes in the ambient environmental conditions surrounding both benefactor and beneficiary species (but see Maestre et al, 2009). Recently, some studies have suggested that the interplay between facilitation and competition can shrift during the ontogenetic life cycle of both nurse and beneficiary species (e.g., Pugnaire et al, 1996; Miriti, 2006; Reisman-Berman, 2007; Soliveres et al, 2010; Allegrezza et al, 2016). For instance, in a semi-arid habitat in Spain, Pugnaire et al (1996) reported that plant diversity in the understory beneath a leafless leguminous shrub, Retama sphaerocarpa , increased with the shrub’s age and size.…”

“…We therefore argue that a small and not well-developed A. polytrichoides cushion is likely to provide negligible nurse effects on other plant species. It has been shown that facilitation by nurse plants can be strongly influenced by plant size, with larger individuals exerting a stronger positive impact on modification of surrounding environments and consequently on the growth of their neighbors (Allegrezza et al, 2016). With canopy development, fertility beneath the cushion should increase such that its facilitative effects should also increase.…”

Windstorm disturbance triggers multiple species invasion in an urban Mediterranean forest

“…The study area is characterized by a Mediterranean climate with alternation of cold-wet (autumn-winter) and hot-dry (spring-summer) seasons, with very short wet-to-dry and dry-to-wet transition stages. Several studies highlighted that Mediterranean summer drought establishes microclimatic conditions particularly severe for the regeneration of shrub and tree species (Allegrezza et al 2016). In such conditions, the forest canopy buffers the local microclimatic severity by favoring a decrease of air temperature and a simultaneous increase of air relative humidity during the hottest hours of the summer days, thus contributing to mitigate vapor pressure deficit, leaf temperature and transpiration losses.…”