Rare self-facilitation in terrestrial plants as compared to aquatic sessile organisms: empirical evidences and causal mechanisms

Abstract

Species-specificity of facilitative interactions is ecologically relevant for community organization and dynamics. The prevalence of heterospecific facilitation which is found between higher plants seems an evolutionary paradox since beneficiary heterospecifics can compete with, and even replace, their nurse. An extensive review on facilitative interactions for both higher plants and sessile aquatic organisms is presented. The study reports on the occurrence of positive interactions between conspecifics and heterospecifics, in relation to different growth forms and facilitative mechanisms in terrestrial and aquatic environments. Four hypotheses are considered to explain the observed rarity of conspecific compared to heterospecific facilitation: 1) the occurrence of plant-induced unsuitable recruitment conditions (negative conspecific plant-soil feedback and hump-shaped recruitment distribution), 2) higher competition levels between conspecifics, 3) the prevalence of non-facilitated regeneration niche for nurses, and, 4) differences in the research effort. Self-facilitation is reported much more rarely in terrestrial vegetation (5.3% of 1554 cases of facilitiative interactions), than in aquatic ecosystems (36.2%, n = 130). In absolute terms, far more occurrences of heterospecific facilitative interactions are reported for all growth forms. However, when the occurrences of facilitative interactions are expressed as a percentage of the total con-and heterospecific interactions, annual and perennial herbaceous nurse plants show prevailing conspecific interactions, while woody (trees and shrubs) nurse species mostly show heterospecific facilitation. Increase of soil nutrient fertility, improvement of above-ground microclimate, associational refuge and seed trapping are the most common mechanisms of heterospecific interactions. Differently, conspecific facilitation is mostly due to improved soil biotic conditions, changes in fire regimes and reduction of heterospecific competition. Given the frequently reported occurrence of non-facilitated regeneration niche for nurse species, conspecific negative feedback and hump-shaped recruitment distribution in terrestrial plants, these processes are suggested as significantly contributing to explain the observed rarity of conspecific facilitation.

Assessing occurrence, specificity, and mechanisms of plant facilitation in terrestrial ecosystems

“…The extensive data set compiled by Bonanomi et al (2010) was updated including articles published between 1909 and 2011. Data were obtained from 539 articles with a total of 2,080 reported cases of facilitation among higher plants.…”

“…In recent years, the significance of environmentally mediated positive, non-trophic species interactions (i.e., facilitation) in shaping natural plant communities has been re-evaluated (reviews in Callaway 1995;Brooker et al 2008). Facilitative interactions have been reported for a variety of ecosystems worldwide, including deserts (Yeaton 1978), Mediterranean mountains (Gómez-Aparicio et al 2004), freshwater and saline wetlands (Bertness and Hacker 1994), temperate (Bonanomi et al 2010), and alpine ecosystems (Callaway et al 2002). The importance of facilitation as a cohesive force driving the organization of ecological communities has been reevaluated (e.g., Bruno et al 2003) focusing on the intensity of abiotic stressors (Maestre et al 2006), the underlying mechanisms (Holzapfel and Mahall 1999) and the consequences for community species diversity (Hacker and Gaines 1997) by such positive interactions.…”

“…In this context, the specificity of facilitation has been considered a crucial issue (Callaway 1998), ranging from species-specificity, with facilitation occurring between either conspecifics or heterospecifics (Bonanomi et al 2010), to broader levels, as in the case of functional guilds when different growth forms are compared (Gómez-Aparicio 2009). The classification of plant structural and functional traits has been attempted for a long time by plant ecologists (Warming 1909) and the subdivision based on plant size, architecture and life span in trees, shrubs, perennial and annual herbaceous plants, cacti and vines has been early and widely accepted.…”

“…In the frame of facilitation studies, this approach has been proven useful in predicting plant occurrences as either nurse or beneficiary (Gómez-Aparicio 2009). In a recent review, based on 1,521 case studies, Bonanomi et al (2010) reported that shrubs were the most common nurse growth form, followed by trees, herbaceous perennials, cacti, and annual plants. However, the variation of growth forms occurrences, as nurse and beneficiary, across different ecosystems remains to be clarified.…”

“…In the first case, it is well known that nitrogen-fixing plants lead to enhanced local nutrient recycling and organic matter accumulation (Belsky 1994), thus creating the so called ''islands of fertility'' (Schlesinger et al 1996) which produce a widespread facilitating effect (Callaway 2007;Bonanomi et al 2010). In the case of fruit type, Valiente-Banuet et al (2006) reported that, in Mediterranean ecosystems, the establishment of species bearing fleshy fruits was much more dependent on facilitative interactions compared to other plants.…”