Does extreme environmental severity promote plant facilitation? An experimental field test in a subtropical coastal dune

Abstract

The stress gradient hypothesis (SGH) postulates how the balance between plant competition and facilitation shifts along environmental gradients. Early formulations of the SGH predicted that facilitation should increase monotonically with stress. However, a recent theoretical refinement of the SGH postulates stronger facilitation under moderate stress, followed by a decreasing role of facilitation in the most severe environments. We conducted field experiments along the most severe part of a coastal dune gradient in southeast Brazil to test the effect of stress on the intensity and importance of the net interactions between two tree species. First, we compared the performance of distinct life stages of Ternstroemia brasiliensis in the presence and absence of Guapira opposita adults along a beach-to-inland gradient, a gradient of environmental severity. To test the effect of one stress factor in particular, we also manipulated water availability, a limiting resource due to the sandy soils. At the most severe part of the coastal gradient (i.e. closest to the seashore), both intensity and importance of the interaction between G. opposita and T. brasiliensis were negatively related to stress, with a pattern consistent across distinct life stages of the target species. However, the sign of the net interaction depended on the life stage of the target species. Our results provide empirical evidence that the role of facilitation tends to wane, leading to neutral or even negative net interactions between species as stress reaches its maximum, as predicted by the recent refinements of the SGH.

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

“…Such temporal variation in plant-plant interactions under extreme drought stress does not support the original SGH and, instead, is more consistent with the recent refined SGH. Both field studies in water-limited environments and meta-analyses at species and community levels have also demonstrated that the intensity and/or importance of positive effects diminish under extreme drought stress, or shift to neutral or even negative effects (Tielbörger and Kadmon 2000, Maestre and Cortina 2004, Soliveres et al 2013, Castanho et al 2015, Liancourt et al 2017, O' Brien et al 2017, but see Lopez et al 2016. Contrary to the SGH, the results of these studies are highly inconsistent, since many other factors (e.g.…”

“…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.…”

“…Therefore, understanding the relationships between rainfall fluctuation and species interactions in arid regions will provide information that can be used to predict the responses of vegetation to future rainfall changes (Soliveres et al 2013). Many previous studies that have investigated the effects of water regimes on plant-plant interactions have often selected different locations along drought gradients (Armas et al 2011, Granda et al 2012, Kidron and Gutschick 2013, Ziffer-Berger et al 2014 or used varying water levels in controlled environments (Maalouf et al 2012, Gao et al 2014, Grant et al 2014, Rysavy et al 2014, Castanho et al 2015. However, few reports have addressed the balance between competition and facilitation along drought gradients from the same community for multiple growing seasons (but see Tielbörger and Kadmon 2000, Abdallah and Chaieb 2012, Metz and Tielbörger 2016, O'Brien et al 2017.…”

Long‐term regeneration of a tropical plant community after sand mining

“…Interestingly, the species that are becoming dominants in the sand‐mined site are not those for what we have knowledge about their facilitative effects (i.e., nurse species) and this has implications for the conservation and restoration of these coastal sandy plains. Plant facilitation can have a crucial role in restoration ecology (Brooker et al., ) by positively affecting the emergence and survival of plant species (Gómez‐Aparicio, ) or increasing diversity at intermediary‐to‐high environmental severity conditions (Castanho et al., ; Michalet et al., ). However, facilitation will be irrelevant if the plant species known to have such potential positive effect in undisturbed sites are absent from restoring sites.…”

“…Positive interactions are common in restinga communities and explain, for example, the association between understory trees and isolated adult trees (Castanho, Oliveira, & Prado, ). Nevertheless, such positive effects can become neutral or even negative under extreme environmental conditions (Castanho, Oliveira, & Prado, ), being strongly dependent on life form (e.g., herbs, shrubs, trees, and climbers). Scarano et al.…”