Nurse effect of Bolax gummifera cushion plants in the alpine vegetation of the Chilean Patagonian Andes

Abstract. It has been proposed that in the harsh arctic and alpine climate zones, small microtopographic variations that can generate more benign conditions than in the surrounding environment could be perceived as safe sites for seedling recruitment. Cushion plants can modify wind pattern, temperature and water availability. Such modifications imply that cushion plants could act as ‘nurse plants’ facilitating the recruitment of other species in the community. This effect should be more evident under stressful conditions. We tested these hypotheses comparing the number of species that grow inside and outside Bolax gummifera cushions at two elevations (700 and 900 m a.s.l.) in the Patagonian Andes of Chile (50°S). At both elevations, and in equivalent areas, the number of species was registered within and outside cushions. A total of 36 and 27 plant species were recorded either within or outside B. gummifera cushions at 700 and 900 m a.s.l., respectively. At 700 m a.s.l., 33 species were recorded growing within cushions and 29 outside them, while at 900 m a.s.l. these numbers were 24 and 13 respectively. At both elevations there were significantly more species growing within than outside cushions, and the proportion of species growing within cushions increased with elevation. Thus there is a nurse effect of cushion plants and it is more evident at higher elevations. Shelter from wind and increased soil water availability seem to be the factors that increase plant recruitment within cushions.     

Positive interactions between alpine plant species and the nurse cushion plant Laretia acaulis do not increase with elevation in the Andes of central Chile

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.     

Nurse effect of the native cushion plant Azorella monantha on the invasive non-native Taraxacum officinale in the high-Andes of central Chile

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.     

Positive associations between the cushion plant Arenaria polytrichoides (Caryophyllaceae) and other alpine plant species increase with altitude in the Sino‐Himalayas

Question: Does the facilitative effect of cushion plants increase with elevation as a result of increases in environmental harshness? Does this hypothesis apply in the Sino‐Himalayan Mountains? Location: Lakaka Pass on the Baima Snow Mountains (28°20′N, 99°05′E), SW China. Methods: We evaluated the spatial association of several plant species with the cushion plant Arenaria polytrichoides (Caryophyllaceae) at two elevations (4500 m and 4700 m) in the study site and monitored temperature, moisture and nutritional status of soil beneath and outside the cushions. Results: While 14 species grow more frequently associated with the cushions at the higher elevation, at the lower site only three species were positively associated with cushions. Eleven of the species that occurred at both elevations changed their spatial association from neutral or negative with cushions at the lower site to positive at the higher elevation site. Substrate temperatures were rather similar between the cushions and areas of bare ground. Cushions maintained higher moisture than areas of bare ground at both elevations. Soils beneath cushions contained significantly more available nitrogen and potassium compared to open areas at the higher elevation. Conclusions: Our results show that facilitation by A. polytrichoides cushions increases with elevation in the Sino‐Himalayan region. This facilitation effect of A. polytrichoides cushions is probably due to the improved nutrient availability provided by cushion plants in the higher elevation, and these conditions probably permit increased plant recruitment, growth and survival.     

Testing the Stress-Gradient Hypothesis at the Roof of the World: Effects of the Cushion Plant Thylacospermum caespitosum on Species Assemblages

Many cushion plants ameliorate the harsh environment they inhabit in alpine ecosystems and act as nurse plants, with significantly more species growing within their canopy than outside. These facilitative interactions seem to increase with the abiotic stress, thus supporting the stress-gradient hypothesis. We tested this prediction by exploring the association pattern of vascular plants with the dominant cushion plant Thylacospermum caespitosum (Caryophyllaceae) in the arid Trans-Himalaya, where vascular plants occur at one of the highest worldwide elevational limits. We compared plant composition between 1112 pair-plots placed both inside cushions and in surrounding open areas, in communities from cold steppes to subnival zones along two elevational gradients (East Karakoram: 4850–5250 m and Little Tibet: 5350–5850 m). We used PERMANOVA to assess differences in species composition, Friedman-based permutation tests to determine individual species habitat preferences, species-area curves to assess whether interactions are size-dependent and competitive intensity and importance indices to evaluate plant-plant interactions. No indications for net facilitation were found along the elevation gradients. The open areas were not only richer in species, but not a single species preferred to grow exclusively inside cushions, while 39–60% of 56 species detected had a significant preference for the habitat outside cushions. Across the entire elevation range of T. caespitosum, the number and abundance of species were greater outside cushions, suggesting that competitive rather than facilitative interactions prevail. This was supported by lower soil nutrient contents inside cushions, indicating a resource preemption, and little thermal amelioration at the extreme end of the elevational gradient. We attribute the negative associations to competition for limited resources, a strong environmental filter in arid high-mountain environment selecting the stress-tolerant species that do not rely on help from other plants during their life cycle and to the fact the cushions do not provide a better microhabitat to grow in.     

Fine nurse variations explain discrepancies in the stress‐interaction relationship in alpine regions

Despite a large consensus on increasing facilitation among plants with increasing stress in alpine regions, a number of different outcomes of interaction have been observed, which impedes the generalisation of the ‘stress‐gradient hypothesis’ (SGH). With the aim to reconcile the different viewpoints on the stress‐interaction relationship in alpine environments we hypothesized that fine nurse variations within a single life form (cushion) may explain this pattern variability. To test this hypothesis, we compared the magnitude of the stress‐interaction relationship in a single study area with that observed in existing studies involving cushions, worldwide. We characterized the nurse effects of cushions on the whole plant community at inter‐specific, intra‐specific and intra‐individual levels along a stress gradient in the dry, alpine tropics of Bolivia (4400 m, 4700 m and 4900 m a.s.l). Using a relative index of interaction (RII) we included our data in a meta‐analysis on the nurse effects of cushions along alpine gradients, worldwide. At inter‐specific level, the loose cushion Pycnophyllum was a better nurse than the compact Azorella compacta. However, at intra‐individual level facilitation was higher at the periphery than at the centre of cushions, exceeding in magnitude the variation observed at inter‐specific level. This pattern was associated with higher minimum temperature and lower mortality at the periphery of cushions. The net effects of cushions on plant communities became more positive at higher elevation, corroborating the SGH. Within our single site in Bolivia, fine morphological nurse variations captured a similar variability in the stress‐interaction relationship as that observed in a subset of studies on cushions on a worldwide scale. This suggests that fine variations in nurse traits, in general those not considered in protocols dealing with facilitation or in restoration/conservation management plans, explain in part the current discrepancies among SGH studies in alpine regions.     

Carbohydrate reserves in the facilitator cushion plant <Emphasis Type="Italic">Laretia acaulis</Emphasis> suggest carbon limitation at high elevation and no negative effects of beneficiary plants

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.     

Variation of biomass and morphology of the cushion plant Androsace tapete along an elevational gradient in the Tibetan Plateau

Alpine ecosystems are among those biomes that are most vulnerable to climate change. Cushion plants are an important life form of alpine ecosystems and will likely play a critical role for the resilience of these habitats to climate change. We studied cushion size distribution and different measures of the compactness of cushions (biomass and rosette density, leaf area index) of the cushion plant, Androsace tapete along an elevational gradient from 4500 to 5200 m a.s.l. in the Nyainqentanglha Mountains of the central Tibetan Plateau. Cushion size distribution, total cover, and compactness of cushions varied substantially along the elevational gradient. At the driest site at low elevation we found the lowest total cushion cover, a particularly high proportion of very small cushions, and the most compact cushions (highest rosette and biomass densities, and leaf area index (LAI) per cushion). Our results indicate that in the semi‐arid Tibetan Plateau water availability is the more important climate factor than temperature affecting cushion plant traits and morphology.     

Variation and response to experimental warming in a New Zealand cushion plant species

Cold adapted plants, such as cushion plants, may be particularly sensitive to climate warming because of their compact growth form and high branch density. In the oceanic southern hemisphere, cushion communities tend to have large range distributions at low latitudes (sea level to low alpine), thus providing an opportunity to test the effects of temperature on plant morphology and reproduction across gradients. Using Donatia novae‐zelandiae as a model species, we compared the leaf morphology, reproduction and responses to warming. Two low‐alpine sites (Maungatua (880 m a.s.l.), Blue Mountains (1000 m a.s.l.)) and two sea‐level sites (Waituna 1 (0 m a.s.l.), Waituna 2 (0 m a.s.l.)) in South Island, New Zealand were used. Donatia novae‐zelandiae cushions differed significantly between the high‐elevation and sea‐level sites both morphologically and in terms of reproduction. High‐elevation cushions produced more flowers (threefold more flowers per plant) and seeds (sevenfold more seeds per capsule) than at sea level, but leaves were larger at sea level (in length and specific leaf area). The cushions were also twice as compact at the high‐elevation sites. After two growing seasons of artificial warming, seed production (35%), leaf length (7%) and width (13%), and specific leaf area (63%) significantly decreased in D. novae‐zelandiae plants; flower production was not significantly affected. Cushion plant morphology and reproduction were significantly affected by environmental drivers at their establishment sites, but all populations responded negatively to artificial warming of 1–3°C. Many cushion plants are considered keystone species because of their propensity to facilitate the growth and establishment of other plant species, the inferred negative effects of global warming on cushion plant species may have a cascading effect on other alpine plant groups.     

Biogenic habitat creation affects biomass–diversity relationships in plant communities

Biogenic habitat creation refers to the ability of some organisms to create, maintain or destroy habitats. These habitat changes affect species diversity of natural communities, but it remains to be elucidated if this process also affects the link between ecosystem functions and species diversity. Based on the widely accepted positive relationships between ecosystem functions and species diversity, we hypothesize that these relationships should be different in biogenically created habitat patches as compared to unmodified habitat patches. We tested this hypothesis by assessing the effects of a high-Andean cushion plant, Azorella madreporica, which creates habitat patches with different environmental conditions than in the surrounding open areas with reduced vegetation cover. We used observational and experimental approaches to compare the plant biomass–species richness relationships between habitat patches created by A. madreporica cushions and the surrounding habitat without cushion plants. The observational assessment of these relationships was conducted by counting and collecting plant species within and outside cushion patches. In the experiment, species richness was manipulated within and outside cushion patches. The cushion plant itself was not included in these approaches because we were interested in measuring its effects. Results of both approaches indicated that, for a given level of species richness, plant biomass within cushions was higher than in the surrounding open areas. Furthermore, both approaches indicated that the shape of plant biomass–species richness curves differed between these habitat types. These findings suggest that habitat modifications performed by A. madreporica cushions would be positively affecting the relationships between ecosystem functions and species diversity.     

Spatial patterns in cushion‐dominated plant communities of the high Andes of central Chile: How frequent are positive associations?

Question: In stressful abiotic environments positive plant interaction is expected to be a frequent and an important process driving community composition and structure. In the high Andes in central Chile, the cushion plant Azorella madreporica dominates plant communities and appears to benefit the assemblage of species that grows within it. However, there are also many other species that grow outside this nurse cushion plant, which may or may not interact with this species. What is the prevailing type of spatial associations among the plant species that are not growing inside the nurse plant? What is the type of interactions between cushion plants and those species growing outside them? Location: Molina River basin (33°20'S, 70°16’ W, 3600 m a.s.l.), in the Andes of central Chile, ca. 50 km east of Santiago. Methods: Two accurate mapping plots of individual plants of different species were located at two summits (Franciscano and Tres Puntas sites). The spatial distributions and associations between species growing outside cushions and within cushions at each site were estimated by point‐pattern analyses using the univariate and bivariate transformations of Ripley's K‐functions. Results: We found both positive and, especially, negative spatial associations (8 out of 12 species in Franciscano site) between A. madreporica cushions and plants growing outside them. However, most of the species showed positive spatial associations among them. The variation in spatial association was site‐specific and also depended on the type of plants involved. Adesmia spp., the second most abundant non‐cushion species, displayed negative associations with cushions and positive associations with other species growing outside cushions. Conclusions: Our study suggests very complex interactions among species, which ranged from positive to negative, and are also affected by abiotic environmental conditions.     

The best for the guest: high Andean nurse cushions of Azorella madreporica enhance arbuscular mycorrhizal status in associated plant species

Positive interactions between cushion plant and associated plants species in the high Andes of central Chile should also include the effects of fungal root symbionts. We hypothesized that higher colonization by arbuscular mycorrhizal (AM) fungi exists in cushion-associated (nursling) plants compared with conspecific individuals growing on bare ground. We assessed the AM status of Andean plants at two sites at different altitudes (3,200 and 3,600 m a.s.l.) in 23 species, particularly in cushions of Azorella madreporica and five associated plants; additionally, AM fungal spores were retrieved from soil outside and beneath cushions. 18 of the 23 examined plant species presented diagnostic structures of arbuscular mycorrhiza; most of them were also colonized by dark-septate endophytes. Mycorrhization of A. madreporica cushions showed differences between both sites (68% and 32%, respectively). In the native species Hordeum comosum, Nastanthus agglomeratus, and Phacelia secunda associated to A. madreporica, mycorrhization was six times higher than in the same species growing dispersed on bare ground at 3,600 m a.s.l., but mycorrhiza development was less cushion dependent in the alien plants Cerastium arvense and Taraxacum officinale at both sites. The ratio of AM fungal spores beneath versus outside cushions was also 6:1. The common and abundant presence of AM in cushion communities at high altitudes emphasizes the importance of the fungal root symbionts in such situations where plant species benefit from the microclimatic conditions generated by the cushion and also from well-developed mycorrhizal networks.     

Distinguishing colonisation modes from spatial structures in populations of the cushion plant Azorella madreporica in the high‐Andes of central Chile

Abstract We developed multiple a priori hypotheses to link the observed spatial patterns with colonisation processes in the high alpine cushion plant, Azorella madreporica. We conducted this study in the Molina River basin (33°20′ S, 70°16′ W, 3600 m a.s.l.), in the Andes of central Chile, approximately 50 km east of Santiago. We mapped and measured size (as a surrogate for age) of individual cushions in two populations and used a standard spatial analytical tool (semivariograms) to test our alternative a priori hypotheses related to colonisation mode of the cushion species. In both populations, the size distribution of A. madreporica reflected a negative exponential or inverse‐J pattern, typical of uneven‐aged populations, where most of the cushions belonged to relatively smaller size classes, in effect, a regular success in the establishment of seedlings, where all size classes of cushions were represented in the population. The results were site‐specific, where best‐fit semivariograms for spatial cushion's size distribution suggested a gradual colonisation in one population and an episodic colonisation in the other population. Microsite distribution proved to be homogeneous at both sites. Thus, the study of the spatial explicit size‐age population distribution of an alpine species provides valuable information about the frequency, magnitude and site variation of the reproductive pulses in these harsh environments.     

Between-year variation in seed weights across altitudes in the high-alpine plant <Emphasis Type="Italic">Eritrichium nanum</Emphasis>

Seed weight is a prominent life history trait of plants affecting dispersal, establishment, and survival. In alpine environments, the few studies investigating the effect of elevation on seed weight within species have mainly detected a decrease in seed weight with increasing elevation. This relationship is generally attributed to the adverse climate at high elevations. In order to test this hypothesis, we analyzed seed weight variation across altitudes (2,435–3,055 m a.s.l.) in two consecutive years that differed in weather conditions in the high-alpine cushion plant Eritrichium nanum. We found a significant reduction in seed weight with increasing elevation in both years, but in the growing season with more adverse weather conditions, the reduction was more substantial than in the more favorable year. We conclude that alpine plants may be able to produce well-developed seeds at low elevations in almost all years, independent of weather conditions, whereas reproduction through seeds is potentially limited to years of favorable weather at high elevation.     

Cushions of Thylacospermum caespitosum (Caryophyllaceae) do not facilitate other plants under extreme altitude and dry conditions in the north-west Himalayas

Background

Cushion plants are commonly considered as keystone nurse species that ameliorate the harsh conditions they inhabit in alpine ecosystems, thus facilitating other species and increasing alpine plant biodiversity. A literature search resulted in 25 key studies showing overwhelming facilitative effects of different cushion plants and hypothesizing greater facilitation with increased environmental severity (i.e. higher altitude and/or lower rainfall). At the same time, emerging ecological theory alongside the cushion-specific literature suggests that facilitation might not always occur under extreme environmental conditions, and especially under high altitude and dryness.

Methods

To assess these hypotheses, possible nursing effects of Thylacospermum caespitosum (Caryophyllaceae) were examined at extremely high altitude (5900 m a.s.l.) and in dry conditions (precipitation <100 mm year⁻¹) in Eastern Ladakh, Trans-Himalaya. This is, by far, the highest site, and the second driest, at which the effects of cushions have been studied so far.

Key Results

In accordance with the theoretical predictions, no nursing effects of T. caespitosum on other alpine plants were detected. The number and abundance of species were greater outside cushions than within and on the edge of cushions. None of the 13 species detected was positively associated with cushions, while nine of them were negatively associated. Plant diversity increased with the size of the area sampled outside cushions, but no species–area relationship was found within cushions.

Conclusions

The results support the emerging theoretical prediction of restricted facilitative effects under extreme combinations of cold and dryness, integrating these ideas in the context of the ecology of cushion plants. This evidence suggests that cases of missing strong facilitation are likely to be found in other extreme alpine conditions.     

Nest microhabitats and tree size mediate shifts in ant community structure across elevation in tropical rainforest canopies

Declines or mid-elevation peaks in invertebrate diversity with elevation are often attributed to climate and geometric constraints. However, vegetation structure may also drive diversity patterns, especially for tree-dwelling species, via its effects on microhabitat use and competitive interactions. Here we investigate these effects on the diversity and community structure of tree-nesting ants over elevation. We exhaustively sampled ant nests in 1254 trees within continuous plots of primary rainforest at low (200 m a.s.l.), mid (900 m a.s.l.) and high (1800 m a.s.l.) elevation in Papua New Guinea. Ant diversity, nest abundance and tree occupancy peaked at mid-elevation. Although host tree diversity also peaked at mid-elevation, there was low specialisation of ant species to tree species at all elevations. Mid-elevation trees hosted more species, more nests and a greater diversity of nest types than trees of a similar size at low or high elevation. Tree size and nest microhabitat use were the strongest predictors of species composition, explaining twice as much of the variability in the communities than elevation. At mid to high elevation there were proportionally fewer large nests than in the lowlands, with an increase in smaller nests in live hollow twigs and epiphytes. There was high species turnover between elevations, and between trees within elevations. Species co-occurrence patterns within trees differed with tree size, and with elevation. In large trees species tended to co-occur at random at low and high elevation, but co-occurred more often than expected by chance at mid elevation, indicating an elevational shift in competitive interactions. We conclude that the more extreme diurnal temperatures at higher elevations, combined with increased epiphyte availability, drive ants to nest in more insulated microhabitats. This results in smaller colony sizes and a decrease in interspecific competition, thereby boosting species co-existence at mid elevation.     

Impacts of ecosystem engineers on community attributes: effects of cushion plants at different elevations of the Chilean Andes

Ecosystem engineers are organisms able to modulate environmental forces and, hence, may change the habitat conditions for other species. In so doing, ecosystem engineers may affect both species richness and evenness of communities and, in consequence, change species diversity. If these changes in community attributes are related to the magnitude of the habitat changes induced by the engineers, it seems likely that engineer species will have greater effects on diversity in sites where they cause larger habitat changes. We addressed this issue by evaluating the effects of three alpine cushion plants on species richness, evenness, and diversity of high-Andean plant communities. Given that the difference in microclimatic conditions between cushions and the external environment increases with elevation, we proposed that these organisms should have greater effects on community attributes at higher than at lower elevation sites. Results showed that the three cushion species had positive effects on species richness, diversity, and evenness of plant communities. It was also observed that the magnitude of these effects changed with elevation: positive effects on species richness and diversity increased towards upper sites for the three cushions species, whereas positive effects on evenness increased with elevation for one cushion species but decreased with elevation for other two cushion species. These results suggest that the presence of cushions is important to maintain plant diversity in high-Andean communities, but this positive effect on diversity seems to increase as the difference in environmental conditions between cushions and the external environment increases with elevation.     

Facilitative interactions do not wane with warming at high elevations in the Andes

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.     

Phylogenetic distance among beneficiary species in a cushion plant species explains interaction outcome

Determining which drivers lead to a specific species assemblage is a central issue in community ecology. Although many processes are involved, plant–plant interactions are among the most important. The phylogenetic limiting similarity hypothesis states that closely related species tend to compete stronger than distantly related species, although evidence is inconclusive. We used ecological and phylogenetic data on alpine plant communities along an environmental severity gradient to assess the importance of phylogenetic relatedness in affecting the interaction between cushion plants and the whole community, and how these interactions may affect community assemblage and diversity. We first measured species richness and individual biomass of species growing within and outside the nurse cushion species, Arenaria tetraquetra. We then assembled the phylogenetic tree of species present in both communities and calculated the phylogenetic distance between the cushion species and its beneficiary species, as well as the phylogenetic community structure. We also estimated changes in species richness at the local level due to the presence of cushions. The effects of cushions on closely related species changed from negative to positive as environmental conditions became more severe, while the interaction with distantly related species did not change along the environmental gradient. Overall, we found an environmental context‐dependence in patterns of phylogenetic similarity, as the interaction outcome between nurses and their close and distantly‐related species showed an opposite pattern with environmental severity.