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Alpine habitats are regarded as particularly vulnerable to the effects of climate change. On one hand, global warming is supposed to contributes to alpine environments becoming less stressful. On the other hand, altered snowpack due to warmer temperatures can intensify the stress in these habitats. The presence of non‐native plants on some of these habitats is due to the facilitative effects exerted by native nurse plants, becoming an additional threat. According to the stress gradient hypothesis the importance of facilitative interactions with nurse species is expected to diminish as environmental harshness decreases due to climate change yet remains important if climate change heightens the stress in alpine habitats. However, the responses also depend on climate change's impact on the nurse species. We conducted an experimental warming experiment in the Andes of the central Chile to assess the effects of warmer temperatures on the growth, reproduction and photochemical efficiency of the cushion nurse plant Azorella madreporica. Further we performed a cushion removal experiment involving three native species (Gamocarpha ventosa, Nocaccea magellanica and Rytidosperma pictum) and two non‐native species (Cerastium arvense and Taraxacum officinale) to assess whether facilitative interactions changed with warming. We expected that under warmer conditions facilitation continued be important for the native species but decreased for the non‐natives as the latter are abundant at warmer low elevations. We found that warmer conditions increased the photochemical efficiency and growth of the nurse cushions. Removal of cushions resulted in high mortality rates for all species. However, under warmer conditions, native species exhibited lower survival rates, whereas non‐natives showed no significant changes compared to control groups. In summary, warmer temperatures were beneficial for the nurses maintaining the importance of facilitative interactions for native species, but not for the non‐natives. [ABSTRACT FROM AUTHOR]

Ecology Letters 16: 478 - 86. 7 Callaway, R. M. 1995. Functional Ecology 28: 75 - 86. 31 Michalet, R., C. Schöb, S. Xiao, L. Zhao, T. Chen, L. An, and R. M. Callaway. Functional Ecology 30: 79 - 87. 32 Michalet, R., S. Xiao, B. Touzard, D. S. Smith, L. A. Cavieres, R. M. Callaway, and T. G. Whitham. Keywords: alpine; competition; cushion plants; dendrochronology; facilitation; growth rings; stress EN alpine competition cushion plants dendrochronology facilitation growth rings stress 1 5 5 05/03/23 20230501 NES 230501 "Your eyes can deceive you, don't trust them.". [Extracted from the article]

Aims Positive interactions are defined as non-trophic interactions where at least one of the interacting species is benefited in terms of fitness and the other remains unaffected. Nevertheless, the bidirectional feedbacks between species may be positive, neutral or negative. Thus, if facilitated species induce negative effects on their 'nurses', the assumed definition of positive interactions could be reconsidered. Methods We assessed if ecological interactions between cushions of Azorella madreporica and their facilitated species are positive. Specifically, we tested if cover of facilitated species has any costs for cushion plants from an ecophysiological perspective, and if these costs increase with the amount of cover of facilitated species. In addition, through pathway analysis and correlations, we assessed if cover and richness of facilitated species have a direct and/or indirect effect on the fitness of cushion plants. Important Findings We found that facilitated plant species induced a significant cost for their nurses (cushion plants), and this cost increases with cover of the facilitated species. Additionally, the facilitated species exert a strong direct negative effect on the cushion's fitness and a moderate indirect negative cost evident through the nutrient status and physiological performance of cushion plants. We thus contribute evidence that positive interactions between high mountain cushion plants of central Chile and their 'facilitated' species may be an artifact more than a fact, especially when bidirectional effects are considered; contrasting with the majority of studies that document only one side of the interaction. [ABSTRACT FROM AUTHOR]

Las plantas en cojín que crecen en ambientes de alta montaña modifican microclimáticamente su entorno, generando microhábitats favorables para el establecimiento de otras especies, actuando como nodrizas. Varios estudios han demostrado que los cojines contienen una mayor riqueza de especies que los espacios abiertos. Sin embargo, hasta ahora, no se ha evaluado el efecto de los cojines sobre otros parámetros comunitarios como la abundancia de individuos, la diversidad y la equitatividad. En este trabajo se analiza el efecto que posee Oreopolus glacialis, una planta en cojín de la cordillera andina, sobre la riqueza, abundancia, diversidad y equitatividad de la comunidad vegetal en un ambiente de alta montaña ubicado a 1.900 m en Chile central. Se determino la riqueza y abundancia de especies que crecen tanto sobre cojines de O. glacialis como en los espacios abiertos entre cojines. Los resultados mostraron que la riqueza de especies y la abundancia de individuos por unidad de área es mayor dentro de los cojines que fuera de ellos. Sin embargo, los valores de diversidad y equitatividad sobre O. glacialis fueron menores que fuera de ellos debido a la presencia de algunas especies dominantes. Análisis particulares de la frecuencia y abundancia de cada especie mostraron que no todas las especies son afectadas de la misma manera por la presencia de este cojín, por lo que O. glacialis actuaría como nodriza sólo para una parte limitada de los componentes de la comunidadCushion plants growing in high mountain habitats can modify the microclimate within their canopy generating microhabitats more favorable for the recruitment of other plant species, acting as nurse plants. Although several studies have shown that species richness is higher within cushions than outside them, no attempts have been made in order to assess the effect of cushions on other community aspects such as total and individual abundance, diversity and evenness. In this study we analyzed the effect of Oreopolus glacialis -a cushion plant from the South American Andes- on the species richness, total and individual abundance, diversity and evenness of a plant community located in a high mountain environment at 1,900 m of altitude in central Chile. Species richness and abundance were determined both within and outside O. glacialis cushions. Results showed that, per area, species richness and individual abundance were higher within cushions. In contrast, diversity and evenness within O. glacialis cushions were lower than outside. This apparent incongruence was caused by some species being highly dominant within cushions. Analyses of the frequency and abundance of each species showed that not all the species were affected in the same way by the cushions, with only four species growing more frequently within cushions. This suggests that nurse effect of O. glacialis is a species-specific phenomenon

While there is a large consensus that plant–plant interactions are a crucial component of the response of plant communities to the effects of climate change, available data remain scarce, particularly in alpine systems. This represents an important obstacle to making consistent predictions about the future of plant communities. Here, we review current knowledge on the effects of climate change on facilitation among alpine plant communities and propose directions for future research. In established alpine communities, while warming seemingly generates a net facilitation release, earlier snowmelt may increase facilitation. Some nurse plants are able to buffer microenvironmental changes in the long term and may ensure the persistence of other alpine plants through local migration events. For communities migrating to higher elevations, facilitation should play an important role in their reorganization because of the harsher environmental conditions. In particular, the absence of efficient nurse plants might slow down upward migration, possibly generating chains of extinction. Facilitation–climate change relationships are expected to shift along latitudinal gradients because (1) the magnitude of warming is predicted to vary along these gradients, and (2) alpine environments are significantly different at low vs. high latitudes. Data on these expected patterns are preliminary and thus need to be tested with further studies on facilitation among plants in alpine environments that have thus far not been considered. From a methodological standpoint, future studies will benefit from the spatial representation of the microclimatic environment of plants to predict their response to climate change. Moreover, the acquisition of long-term data on the dynamics of plant–plant interactions, either through permanent plots or chronosequences of glacial recession, may represent powerful approaches to clarify the relationship between plant interactions and climate change.

Nurse plant facilitation is a commonly reported plant–plant interaction and is an important factor influencing community structure in stressful environments. Cushion plants are an example of alpine nurse plants that modify microclimatic conditions within their canopies to create favourable environments for other plants. In this meta-analysis, the facilitative effects of cushion plants was expanded from previous syntheses of the topic and the relative strength of facilitation for other plants and for arthropods were compared globally.The abundance, diversity, and species presence/absence effect size estimates were tested as plant responses to nurse plants and a composite measure was tested for arthropods. The strength of facilitation was on average three times greater for arthropods relative to all plant responses to cushions. Plant species presence, i.e., frequency of occurrence, was not enhanced by nurse-plants. Cushion plants nonetheless acted as nurse plants for both plants and arthropods in most alpine contexts globally, and although responses by other plant species currently dominate the facilitation literature, preliminary synthesis of the evidence suggests that the potential impacts of nurses may be even greater for other trophic levels.

Positive interactions have started to gain a place as important drivers of community structure and biological diversity. Defined as non-trophic interspecific interactions that increase the average individual fitness of one species, by definition, the presence of one plant species enhances the chances that another species co-occur in the same place, indicating that positive interactions may determine biological diversity. However, this has been poorly explored., The majority of the studies addressing community-level consequences of facilitation have compared the diversity of the plant assemblages growing within nurses vs. those growing outside them, reporting contrasting results among them. Nonetheless, nurses and their alternative microhabitats (open areas among nurses) are part of the same community. Thus, if nurses allow for the persistence of species that otherwise would be excluded from the community, a net increase in the species diversity at the entire community level will be generated even though nurse plants contained fewer species than open areas., Here, we conducted a bibliographic search using the ISI Web of Knowledge data base and reviewed the literature conducted on alpine plant communities where assessments of the diversity of plants growing within and outside a nurse species were available. In most cases nurse species substantially increased species richness at the community level, despite the fact that in some cases, they contained lower species numbers than open areas. Nurse species enhanced species richness more in systems with impoverished local diversity, suggesting that facilitative interactions in alpine habitats act as an insurance policy that sustains diversity under very harsh conditions. [ABSTRACT FROM AUTHOR]

In facilitative interactions, the beneficiary feedback effect ( BFE) has been defined as the effect of beneficiary species (facilitated species) on their benefactor. BFEs have been shown to be dependent on environmental conditions and the composition of the beneficiary community. In alpine cushion systems, BFEs are more negative with more abundant, diverse and phylogenetically aggregated communities of beneficiary species., We tested the hypothesis that the functional composition of the beneficiary communities correlates with the direction and strength of BFE received by alpine cushion benefactors and specifically that a more negative BFE would occur with increasing density of graminoids and a more positive BFE would occur with increasing density of forbs and legumes. Additionally, we predicted that the negative BFE of graminoids would increase with increasing summer aridity., We used a data base of alpine cushion communities from 30 sites throughout the world to assess the overall relationship between the composition of beneficiary communities and the total flower density of cushion benefactors, and its variation with increasing drought. Additionally, in order to assess more precisely the role of the functional composition of the beneficiary communities on BFE in a very dry site with cushion benefactors exhibiting contrasting functional compositions of beneficiary communities, we also designed a field study in the Qilian Shan mountain range (China). At this site with a highly continental climate, we compared the number of flowers and fruits of different phenotypes of the alpine cushion species Thylacospermum caespitosum hosting numerous graminoids, numerous forbs or very few beneficiary species., In the intercontinental study, we found a negative relationship between graminoids and cushion benefactor flower density but no effect of other functional groups. The negative BFE of graminoids increased with increasing summer drought. In the dry Qilian Shan range, we found both a negative effect of graminoids on total flower density and a positive effect of forbs on flower density and fruit set., Our study indicates that the context dependence of BFE may be partially explained by the composition of beneficiary communities and in particular the negative effect of graminoids. [ABSTRACT FROM AUTHOR]

Facilitation has been proposed to be a fundamental mechanism for plant coexistence, being particularly important in highly stressful environments such as alpine environments. In this type of environment, species called “cushion plants” can ameliorate the stressful conditions, acting as nurses for other plants. Of the several mechanisms proposed in the positive-interactions framework, plant–microorganism interaction may be one of the most common, but least documented. Here we show that the presence of endophytes isolated from the roots of cushion plants Laretia acaulis can play a fundamental role in the establishment, performance and survival of both native and exotic plant seedlings that are known to be facilitated by the cushion species. To test this, we measured survival and growth of two native and one invasive species at 3200 m in the Andes of Central Chile. Plants were grown inside artificial cushions filled with native soil, with or without sterilization, and with or without fungal endophytic inoculation to evaluate the role of fungal endophytes on survival and growth. In addition, we conducted a second experiment in a greenhouse with the invasive species to evaluate the effect of fungal endophytic infection/association on plant ecophysiological performance, dry biomass and seed output. Overall, our results showed a strong positive effect of fungal endophytes on the survival and growth of both native and invasive species. Moreover, maximum quantum efficiency (Fv/Fm), biomass accumulation and seed production were enhanced in the invasive species when soils were inoculated with endophytes. Thus, facilitation by root endophytic fungi on native and invasive alpine plants could determine survival and establishment in this harsh environment. Several studies have shown that direct facilitation by cushion plants in alpine environments improves the performance and fitness of both native and exotic plants. Our results suggest that there are indirect effects, mediated by microorganism associations that may also help to explain the successful establishment of native and invasive species in these environments. If indirect plant–plant facilitation through root fungal endophytes proves to be a widespread phenomenon in alpine ecosystems, it could be a key component in the structuring of plant communities in those stressful environments. [ABSTRACT FROM AUTHOR]

Questions Are variable effects of different phenotypes of foundation plant species on subordinates across contrasting habitat conditions (with varying stress and disturbance levels) due to either varying environmental conditions or heritable differences in traits between phenotypes? To evaluate the contribution of environmental effects we quantified the effects of contrasting phenotypes of a foundation legume shrub on their subordinate species across exposure (drought stress) and grazing conditions. Location A sub-alpine xerophytic community of western Mount Lebanon, Lebanon. Methods For two phenotypes of the spiny cushion shrub Onobrychis cornuta, a facilitative phenotype that occurs in concave topographies (mesic soil) and a competitive phenotype that occurs in convex topographies (xeric soil), we quantified cushion traits, environmental conditions and subordinate plant species abundances (within and outside the cushions) for the two phenotypes in northern (low stress) and southern (high stress) exposures, and with and without grazing. Relative interaction index ( RII) for subordinate species richness and abundance was calculated in the eight treatment combinations and a correspondence analysis ( CA) was conducted on species composition. Results Drought stress exacerbated phenotypic effects in southern exposure, with loose phenotypes being more facilitative and tight phenotypes more competitive than in northern exposure. This was related to both changes in cushion traits of the two phenotypes and to an increase in the pool of subordinate species sensitive to cushion effects. In contrast, grazing increased cushion positive effects of both phenotypes through the occurrence of indirect facilitation, with loose phenotypes becoming more facilitative and tight phenotypes less competitive. This was due both to changes in cushion traits of the two phenotypes and to their shared spiny phenotype limiting grazing effects. Conclusions Because increasing stress from northern to southern exposure did not increase competitive effects of either phenotype, and because the cessation of grazing did not cancel out differences in facilitative effects between phenotypes, neither drought stress nor grazing disturbance appeared to be the main drivers of the observed phenotypic effects on subordinate species across habitats. We conclude that differences in phenotypic effects of this legume shrub are very likely due to heritable differences in traits between phenotypes. [ABSTRACT FROM AUTHOR]

While there is a large consensus that plant-plant interactions are a crucial component of the response of plant communities to the effects of climate change, available data remain scarce, particularly in alpine systems. This represents an important obstacle to making consistent predictions about the future of plant communities. Here, we review current knowledge on the effects of climate change on facilitation among alpine plant communities and propose directions for future research. In established alpine communities, while warming seemingly generates a net facilitation release, earlier snowmelt may increase facilitation. Some nurse plants are able to buffer microenvironmental changes in the long term and may ensure the persistence of other alpine plants through local migration events. For communities migrating to higher elevations, facilitation should play an important role in their reorganization because of the harsher environmental conditions. In particular, the absence of efficient nurse plants might slow down upward migration, possibly generating chains of extinction. Facilitation-climate change relationships are expected to shift along latitudinal gradients because (1) the magnitude of warming is predicted to vary along these gradients, and (2) alpine environments are significantly different at low vs. high latitudes. Data on these expected patterns are preliminary and thus need to be tested with further studies on facilitation among plants in alpine environments that have thus far not been considered. From a methodological standpoint, future studies will benefit from the spatial representation of the microclimatic environment of plants to predict their response to climate change. Moreover, the acquisition of long-term data on the dynamics of plant-plant interactions, either through permanent plots or chronosequences of glacial recession, may represent powerful approaches to clarify the relationship between plant interactions and climate change. [ABSTRACT FROM AUTHOR]

Positive interactions between species are known to play an important role in the structure and dynamics of alpine plant communities. The balance between negative and positive interactions is known to shift along spatial and temporal gradients, with positive effects prevailing over negative ones as the environmental stress increases. Thus, this balance is likely to be affected by climate change. We hypothesized that increases in temperature (a global warming scenario) should decrease the importance of positive interactions for the survival and growth of alpine plant species. To test this hypothesis, we selected individuals of the native grass species Hordeum comosum growing within the nurse cushion species Azorella madreporica at 3,600 m.a.s.l. in Los Andes (Chile), and performed nurse removal and seedling survival experiments under natural and warmer conditions. For warmer conditions, we used open-top chambers, which increased the temperature by 4 °C. After two growing seasons, we compared the effect of nurse removal on the survival, biomass, and photochemical efficiency of H. comosum individuals under warmer and natural conditions. Nurse removal significantly decreased the survival, biomass, and photochemical efficiency of H. comosum, demonstrating the facilitative effects of nurse cushions. Seedling survival was also enhanced by cushions, even under warmer conditions. However, warmer conditions only partially mitigated the negative effects of nurse removal, suggesting that facilitative effects of cushions do not wane under warmer conditions. Thus, facilitative interactions are vital to the performance and survival of alpine species, and these positive interactions will continue to be important in the warmer conditions of the future in high-alpine habitats. [ABSTRACT FROM AUTHOR]

1. Although the consequences of facilitation at individual and population levels are well known, the community-level consequences of these processes have received much less attention. In particular, the importance of facilitation in determining richness at the entire community level has seldom been evaluated. 2. In this study, we sampled 11 alpine plant communities along the southern Andes in South America, spanning from tropical (25°S) to sub-antarctic latitudes (55°S). Plant communities were dominated by cushion plants, a particular growth form that acts as a nurse plant for other plant species. Through rarefaction curves, we assessed the effectiveness of community sampling and estimated the number of species present within and outside cushions. Non-metric Multidimensional Scaling ordinations (NMDS) were used to assess differences between the species assemblages growing within and outside cushions. Finally, samples from cushions and open areas were combined in a single matrix accounting for the difference in cover between both microhabitats, and through rarefaction curves we assessed how many more species are added to the community due to the presence of cushions. 3. Samples taken within cushions always contained more species than equivalent samples from open areas. However, the magnitude of this difference varied among communities. NMDS ordination indicated that cushions generate species assemblages structurally different from those found in open areas. Inclusion of samples from cushion and open areas in synthetic analyses – where differences in cover were accounted for – indicated that the presence of cushions consistently increased species richness at the entire community level. The magnitude of these increases in species richness varied with habitat severity, with lower values at both extremes of the environmental severity gradient. 4. Synthesis. Facilitative interactions with cushion nurse plants along the high Andes of southern South America changed plant assemblage structure and increased species richness at the entire community level, indicating that facilitative interactions are pivotal in maintaining the diversity of these harsh environments. [ABSTRACT FROM AUTHOR]

1 Ecosystem engineers are organisms that change abiotic conditions in ways that affect the performance and distribution of other species, including exotics. Several mechanisms have been proposed for the successful establishment of exotic plants in natural communities, but the positive effects that native engineer species may have on the distribution and performance of exotic plants remain unknown. 2 In this study, we propose that amelioration of extreme abiotic conditions by ecosystem engineers can make stressful habitats invadable by exotic plant species, with larger positive effects on the performance of exotic plants as environmental harshness increases. We tested this hypothesis by assessing the effects of a high-Andean ecosystem engineer, the cushion plant Azorella monantha, which is known to create habitat patches where environmental conditions are less extreme than in the surrounding habitats, on the distribution and the performance of two exotic plant species, field chickweed ( Cerastium arvense) and common dandelion ( Taraxacum officinale), along a an elevation gradient in the Andes of central Chile. 3 We measured and compared the abundance, biomass and survival of both exotic species within and outside cushion habitat patches at three elevations (3200 m, 3400 m and 3600 m), and evaluated whether the effects of A. monantha varied across elevations. 4 The results indicated that cushion plants positively impact the performance of both exotics, and have greater facilitative effects at higher elevations. Indeed, at the higher elevation site, C. arvense was only detected within A. monantha patches, suggesting that cushions may expand the distribution range of exotics. These results suggest that ecosystem engineering by native species could promote biological invasions in harsh environments, leading to higher abundances of invaders than those expected in the absence of engineers. 5 Given the conspicuousness of ecosystem engineering in nature, we suggest that exotic species eradication programmes might be less successful by not taking into account the facilitative effects of native engineer species on invaders. Further, we suggest that the recent proposals to use engineer species in ecosystem restoration should be aware of their potential role in promoting invasions. [ABSTRACT FROM AUTHOR]

• In alpine habitats, positive interactions among plants tend to increase with elevation as a result of altitudinal increase in environmental harshness. However, in mountains located in arid zones, lower elevations are also stressful because of scarce availability of water, suggesting that positive interactions may not necessarily increase with elevation. • Here we analysed the spatial association of plant species with the nurse cushion plant Laretia acaulis at two contrasting elevations, and monitored the survival of seedlings of two species experimentally planted within and outside cushions in the semiarid Andes of central Chile. • Positive spatial associations with cushions were more frequent at lower elevations. Species growing at the two elevations changed the nature of their association with cushions from neutral or negative at higher elevations to positive at lower elevations. Survival of seedlings was higher within cushions, particularly at lower elevations. • The increased facilitation by cushions at lower elevations seems to be related to provision of moisture. This result suggests that cushion plants play a critical role in structuring alpine plant communities at lower elevations, and that climatic changes in rainfall could be very relevant for persistence of plant communities. [ABSTRACT FROM AUTHOR]

Abstract: Positive interactions among native plant species are common in alpine habitats, particularly those where one species (nurse plant) generates microclimatic conditions that are more benign than the surrounding environment, facilitating the establishment of other species. Nonetheless, these microclimatic conditions could facilitate the establishment of non-native species as well. A conspicuous component of the alien alpine flora of the central Chilean Andes is the perennial herb Taraxacum officinale agg. (dandelion). In contrast to other alien species that are restricted to human-disturbed sites, T. officinale is frequently observed growing within native plant communities dominated by cushion plants. In this study we evaluated if T. officinale is positively associated with the cushion plant Azorella monantha. Via seedling survival experiments and gas-exchange measurements we also assessed the patterns of facilitation between cushions and dandelions, and explore the potential mechanisms of invasion by dandelions. T. officinale grows spatially positively associated with cushions of A. monantha. Survival of seedlings, as well as their net-photosynthetic rates and stomatal conductance, were higher within cushions than in open areas away from them, suggesting that the microclimatic modifications generated by this native cushion facilitates the establishment and performance of a non-native invasive species. Our results, as well as other recent studies, highlight the role of native communities in facilitating rather than constraining non-native plant invasions, particularly in stressful habitats such as alpine environments. [Copyright &y& Elsevier]