Moving forward on facilitation research: response to changing environments and effects on the diversity, functioning and evolution of plant communities

Abstract

Once seen as anomalous, facilitative interactions among plants and their importance for community structure and functioning are now widely recognized. The growing body of modelling, descriptive and experimental studies on facilitation covers a wide variety of terrestrial and aquatic systems throughout the globe. However, the lack of a general body of theory linking facilitation among different types of organisms and biomes and their responses to environmental changes prevents further advances in our knowledge regarding the evolutionary and ecological implications of facilitation in plant communities. Moreover, insights gathered from alternative lines of inquiry may substantially improve our understanding of facilitation, but these have been largely neglected thus far.Despite over 15 years of research and debate on this topic, there is no consensus on the degree to which plant-plant interactions change predictably along environmental gradients (i.e. the stressgradient hypothesis), and this hinders our ability to predict how plant-plant interactions may affect the response of plant communities to ongoing global environmental change. The existing controversies regarding the response of plant-plant interactions across environmental gradients can be reconciled when clearly considering and determining the species-specificity of the response, the functional or individual stress type, and the scale of interest (pairwise interactions or community-level response). Here, we introduce a theoretical framework to do this, supported by multiple lines of empirical evidence. We also discuss current gaps in our knowledge regarding how plant-plant interactions change along environmental gradients. These include the existence of thresholds in the amount of species-specific stress that a benefactor can alleviate, the linearity or non-linearity of the response of pairwise interactions across distance from the ecological optimum of the beneficiary, and the need to explore further how frequent interactions among multiple species are and how they change across different environments. We review the latest advances in these topics and provide new approaches to fill current gaps in our knowledge. We also apply our theoretical framework to advance our knowledge on the evolutionary aspects of plant facilitation, and the relative importance of facilitation, in comparison with other ecological processes, for maintaining ecosystem structure, functioning and dynamics. We build links between these topics and related fields, such as ecological restoration, woody encroachment, invasion ecology, ecological modelling and biodiversity-ecosystem-functioning relationships. By identifying commonalities and insights from alternative lines of research, we further advance our understanding of facilitation and provide testable hypotheses regarding the role of (positive) biotic interactions in the maintenance of biodiversity and the response of ecological communities to ongoing environmental changes. Europe PMC Funders Group

Restoring forests: What constitutes success in the twenty-first century?

“…These studies show positive effects of biotic interactions with facilitation (Perea and Gil 2014;Soliveres et al 2014), habitat complexity associated with biological legacies (Stanturf et al 2014), or micro-topography (Questad et al 2014). This knowledge is essential to design more efficient restoration projects with reduced inputs.…”

Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub‐canopy plants in an arid desert

“…An increasing number of studies have attempted to integrate facilitation into empirical and theoretical research (Callaway et al 2002, Brooker et al 2008, Filazzola and Lortie 2014 and ecological restorations (Gómez-Aparicio et al 2004, but see Noumi et al 2015). However, the effect of increasing environmental stress on the net outcome of plant-plant interactions remains a subject of debate (Michalet et al 2006, Maestre et al 2009, Holmgren and Scheffer 2010, Kikvidze et al 2011, Soliveres et al 2015.…”

“…As a result, the SGH has been refined to indicate that positive interactions prevail under moderate environmental stress but are weakened, or even reversed under higher stress levels (Michalet et al 2006, Holmgren and Scheffer 2010, Malkinson and Tielbörger 2010, Liancourt et al 2017, O'Brien et al 2017. As suggested by a variety of ecologists (Butterfield 2009, Armas et al 2011, He et al 2013, Michalet et al 2014, Soliveres et al 2015, Liancourt et al 2017, the apparently opposing views of the original and recent versions of the SGH may stem from inter-study differences in the nature of stress gradients (resource or non-resource stress), life-history of response species (stress tolerant or competitively inferior), scale of study focus (community or species level), component of stress factors (single or multiple stresses), or whether the observed gradient is complete. Even though a large number of studies from various ecosystems have explored the SGH, empirical tests of the hypothesis in extreme environments remains insufficient (but see Armas et al 2011, de Bello et al 2011, Castanho et al 2015, Lopez et al 2016, Liancourt et al 2017.…”

Journal of Oceanography and Marine Research

“…A second aspect of stress is its intensity. Although stress intensity is was included in theoretical approaches since about 20 years [50] there are only a few studies on ecosystem functioning effects manipulating simultaneously stress intensity and biodiversity [51]. Three of those studies [52][53][54] were conducted in terrestrial systems, while one [22] used microalgae.…”

“…Such an experiment would have been almost impossible with larger plant species. In general, the relationship between biodiversity and stress intensity regarding ecosystem functioning was identified as poorly studied [51]. A focus on the influence of biodiversity on ecosystem functioning at different stress intensity levels will contribute to maintain effective ecosystem functioning in future ecosystems.…”