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Population growth has been closely associated with agricultural production, since the first famine predicted by Malthus (1798) up to the Green Revolution of the past century. Today, we continue to face increasing demand for food and crop production (Tilman et al., 2011). Considering the combined caloric or protein content of the 275 major crops used directly as human foods or as livestock and fish feeds, Tilman et al. (2011) forecast a 100% increase in global demand for crops from 2005 to 2050. Meeting this demand with the lowest impact on the environment could be achieved by sustainable intensification of existing cropland with reduced land clearing (Tilman et al., 2011; Fischer and Connor, 2018).

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BACKGROUND AND AIMS: Cluster roots (CRs) constitute a special root adaptation that enables plants to take up nutrients, especially phosphorus (P), from soils with low nutrient availability, including recent volcanic deposits. It is unclear, however, how CR species interact with non-cluster root-bearing (NCR) species, and how substrates' fertility modulates potential interactions. METHODS: We experimentally assessed the net interaction between CR and NCR species using two substrates of contrasting fertility: nutrient-rich nursery mix and tephra (low P availability). We planted seedlings of two southern South American (SSA) Proteaceae, CR species and two NCR Nothofagus species in pairs (conspecifics and heterospecifics) and as singles. We analysed the effect of seedling neighbours on survival, growth performance (e.g. total biomass and leaf area) and leaf and substrate nutrient concentrations (including manganese, a proxy for P-acquisition efficiency through CR activity) using the relative interaction index. KEY RESULTS: After three growing seasons, we found that (1) Proteaceae species had fewer CRs and lower CR biomass and grew less in the tephra than in the nursery substrate; (2) Nothofagus species did not improve their survival and growth in the presence of Proteaceae species in any substrate; (3) contrary to Nothofagus, Proteaceae species improved their growth more when planted with any neighbour (including conspecifics) than when planted alone, which was accompanied by a significant accretion of leaf P; and (4) the presence of a neighbour increased the final nitrogen and P concentrations in the nursery substrate, regardless of species identity. CONCLUSIONS: CRs provide Proteaceae a competitive advantage over NCR species at the seedling stage, which may have important consequences for species coexistence and community structuring. The investigated SSA Proteaceae, which have not evolved in nutrient-impoverished soils, as have their relatives in south-western Australia and South Africa, improve their growth when cultivated in pairs, especially in nutrient-rich substrates.

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Positive interactions in plant communities are under-reported in subtropical systems most likely because they are not identified as stressful environments. However, environmental factors or disturbance can limit plant growth in any system and lead to stressful conditions. For instance, salinity and low nutrient and water availability generate a gradient of stressful conditions in coastal systems depending on distance to shore. In a tropical coastal system in SE Brazil, we aimed to assess whether Guapira opposita, a shrub common in restinga environments, acted as nurse involved in ecological succession and which factors influenced its facilitation process. We sampled perennial species above 10 cm in height under the canopy of 35 G. opposita individuals and in neighbouring open areas. Shrub height, canopy area and distance to freshwater bodies were measured in the field, and distance to the ocean was obtained from aerial images. In addition, we measured the distance to the closest forest patch as a potential source of seeds. Plant abundance and species richness were higher under the canopy of G. opposita than in open areas. Facilitation by G. opposita was mainly determined by shrub height, which had a positive relationship with woody and bromeliads abundance and species richness while there was no relationship with the other factors. Overall, our data evidence that tropical environments may be highly stressful for plants and that nurse species play a key role in the regeneration of restinga environments, where their presence is critical to maintain ecosystem diversity and function.

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The elevational range of the alpine cushion plant Laretia acaulis (Apiaceae) comprises a cold upper extreme and a dry lower extreme. For this species, we predict reduced growth and increased non-structural carbohydrate (NSC) concentrations (i.e. carbon sink limitation) at both elevational extremes. In a facilitative interaction, these cushions harbor other plant species (beneficiaries). Such interactions appear to reduce reproduction in other cushion species, but not in L. acaulis. However, vegetative effects may be more important in this long-lived species and may be stronger under marginal conditions. We studied growth and NSC concentrations in leaves and stems of L. acaulis collected from cushions along its full elevational range in the Andes of Central Chile. NSC concentrations were lowest and cushions were smaller and much less abundant at the highest elevation. At the lowest elevation, NSC concentrations and cushion sizes were similar to those of intermediate elevations but cushions were somewhat less abundant. NSC concentrations and growth did not change with beneficiary cover at any elevation. Lower NSC concentrations at the upper extreme contradict the sink-limitation hypothesis and may indicate that a lack of warmth is not limiting growth at high-elevation. At the lower extreme, carbon gain and growth do not appear more limiting than at intermediate elevations. The lower population density at both extremes suggests that the regeneration niche exerts important limitations to this species' distribution. The lack of an effect of beneficiaries on reproduction and vegetative performance suggests that the interaction between L. acaulis and its beneficiaries is probably commensalistic.

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A proposed refinement to the stress-gradient hypothesis requires consideration of the strategies of the interacting species and the characteristics of the stress factors. While the strength and direction of these interactions can be predicted for different ecosystems, this idea remains largely untested in the field. We performed a manipulative field experiment complemented with a descriptive study to test the predictions in a natural setting that represents the extreme end of a precipitation gradient. There, wind driven desiccation and water availability are the main stressors (non-resource and resource-based stresses, respectively). We evaluated the interaction between the shrub and grasses that are dominant in the Patagonian steppe. The species had differences in morpho-functional traits and drought tolerance that fit into the C-S axis of Grime's strategies. We experimentally separated root zones to limit direct competition for soil moisture and reduce the resource-based stress on grasses. We also manipulated the distance to shrubs to evaluate non-resource stress amelioration by canopies (e.g., sun and wind) on grasses. Finally, we evaluated the distribution of naturally established C and S grasses in the neighborhood of C and S shrubs to infer process-pattern relationships. Our growth data coincide to a large degree to the predictions. We found positive effects on the growth of beneficiaries when stress was non-resource based and when strategies differed (i.e., Cshrub -Sgrass and Sshrub -Cgrass ). We also found strong negative effects when the abiotic stress was driven by water, particularly on C grasses. Additionally, shrubs only increased the survival of grasses when strategies differed (i.e., Cshrub -Sgrass and Sshrub -Cgrass ). Our manipulative and descriptive study supported previous results that showed that stress-tolerant species are important for the persistence of competitive species at high stress. While the applicability and generality of these predictions remains to be tested with more field experiments, some ecological factors, such as stress types and species traits, can explain much of the variation in how dominant shrubs and grasses interact in this extreme arid environment. Moreover, this framework could be extended to specifically test the importance of facilitation under different levels of stress.

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Recent applications of coexistence theory to plant invasions posit that non-natives establish in resident communities through either niche differences or traits conferring them with fitness advantages, the former being associated with coexistence and the latter with dominance and competitive exclusion. Plant-soil feedback is a mechanism that is known to explain both coexistence and dominance. In a system where natives and non-natives appear to coexist, we explored how plant-soil feedbacks affect the performance of nine native and nine non-native ruderal species-the prevalent life-history strategy among non-natives-when grown alone and with a phytometer. We also conducted field samplings to estimate the abundance of the 18 species, and related feedbacks to abundances. We found that groups of native and non-native ruderals displayed similar frequencies of negative, positive, and neutral feedbacks, resulting in no detectable differences between natives and non-natives. Likewise, the phytometer exerted comparable negative impacts on native and non-native plants, which were unchanged by plant-soil feedbacks. Finally, feedbacks explained plant abundances only after removing one influential species which exhibited strong positive feedbacks but low abundance. Importantly, however, four out of five species with negative feedbacks were rare in the field. These findings suggest that soil feedbacks and plant-plant interactions do not confer an advantage to non-native over native species, but do contribute to the observed coexistence of these groups in the system. By comparing natives and non-natives with overlapping abundances and strategies, our work broadens understanding of the consequences of plant-soil feedbacks in plant invasion and, more generally, coexistence within plant communities.

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Empirical studies in salt marshes, arid, and alpine systems support the hypothesis that facilitation between plants is an important ecological process in severe or 'stressful' environments. Coastal dunes are both abiotically stressful and frequently disturbed systems. Facilitation has been documented, but the evidence to date has not been synthesized. We did a systematic review with meta-analysis to highlight general research gaps in the study of plant interactions in coastal dunes and examine if regional and local factors influence the magnitude of facilitation in these systems. The 32 studies included in the systematic review were done in coastal dunes located in 13 countries around the world but the majority was in the temperate zone (63%). Most of the studies adopt only an observational approach to make inferences about facilitative interactions, whereas only 28% of the studies used both observational and experimental approaches. Among the factors we tested, only geographic region mediates the occurrence of facilitation more broadly in coastal dune systems. The presence of a neighbor positively influenced growth and survival in the tropics, whereas in temperate and subartic regions the effect was neutral for both response variables. We found no evidence that climatic and local factors, such as life-form and life stage of interacting plants, affect the magnitude of facilitation in coastal dunes. Overall, conclusions about plant facilitation in coastal dunes depend on the response variable measured and, more broadly, on the geographic region examined. However, the high variability and the limited number of studies, especially in tropical region, indicate we need to be cautious in the generalization of the conclusions. Anyway, coastal dunes provide an important means to explore topical issues in facilitation research including context dependency, local versus regional drivers of community structure, and the importance of gradients in shaping the outcome of net interactions.

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