Facilitation consequences for reproduction of the benefactor cushion plant Laretia acaulis along an elevational gradient: costs or benefits?

Environmental stress may favour facilitative interactions among plants but whether these interactions are positive for the benefactor and how this depends on stress factors, remains to be determined. We studied the effect of beneficiary cover and biomass on reproduction of the benefactor cushion plant Laretia acaulis (Apiaceae) in the central Chilean Andes during three years. Study sites were situated along an elevational gradient at 2600, 2800, 3000 and 3150 m a.s.l. This range comprises a cold‐ and a drought‐stress gradient, with moisture increasing and temperature decreasing with elevation. We studied the effect of natural gradients in beneficiary cover and of experimental cover removal on cushion flower and fruit production. Beneficiary cover had a negative effect on flower production but not on infructescence and fruit densities or fruit weights. A positive effect of beneficiaries on the fraction of flowers converted into fruits was detected for hermaphrodite cushions. The effect of beneficiary cover on flowering was independent of elevation or cushion gender, although these latter factors explained most of the variation. Removing the aboveground parts of the beneficiaries positively affected flowering at 2800 m a.s.l. but not at the other elevations. Our results suggest negative effects of facilitation on L. acaulis flowering, but these are neutralized in fruit production. Surprisingly, this conclusion holds along the entire elevational or stress gradient. This suggests that this system of facilitation is evolutionarily stable and not very sensitive to environmental change. It remains to be tested, however, whether facilitation affects fitness via growth and long‐term survival in these slow‐growing alpine cushions.     

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.     

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.     

A field test of the stress‐gradient hypothesis along an aridity gradient

Aims: The stress‐gradient hypothesis (SGH) predicts how plant interactions change along environmental stress gradients. We tested the SGH in an aridity gradient, where support for the hypothesis and the specific shape of its response curve is controversial. Location: Almería, Cáceres and Coimbra, three sites in the Iberian Peninsula that encompass the most arid and wet habitats in the distribution range of a nurse shrub species –Retama sphaerocarpa L. (Boiss) – in Europe. Methods: We analysed the effect of Retama on its understorey plant community and measured the biomass and species richness beneath Retama and in gaps. We estimated the frequency (changes in species richness), importance and intensity of the Retama effects, and derived the severity–interaction relationship pattern, analysing how these interaction indices changed along this aridity gradient. Results and conclusions: The intensity and frequency of facilitation by Retama increased monotonically with increasing environmental severity, and the importance tended to have a similar pattern, overall supporting the SGH. Our data did not support predictions from recent revisions of the SGH, which may not apply to whole plant communities like those studied here or when interactions are highly asymmetrical. Facilitation by Retama influenced community composition and species richness to the point that a significant fraction of species found at the most arid end of the gradient were only able to survive beneath the nurse shrub, whereas some of these species were able to thrive in gaps at more mesic sites, highlighting the ecological relevance of facilitation by nurse species in mediterranean environments, especially in the driest sites.     

Nurse species and indirect facilitation through grazing drive plant community functional traits in tropical alpine peatlands

Facilitation among plants mediated by grazers occurs when an unpalatable plant extends its protection against grazing to another plant. This type of indirect facilitation impacts species coexistence and ecosystem functioning in a large array of ecosystems worldwide. It has nonetheless generally been understudied so far in comparison with the role played by direct facilitation among plants. We aimed at providing original data on indirect facilitation at the community scale to determine the extent to which indirect facilitation mediated by grazers can shape plant communities. Such experimental data are expected to contribute to refining the conceptual framework on plant–plant–herbivore interactions in stressful environments. We set up a 2‐year grazing exclusion experiment in tropical alpine peatlands in Bolivia. Those ecosystems depend entirely on a few, structuring cushion‐forming plants (hereafter referred to as “nurse” species), in which associated plant communities develop. Fences have been set over two nurse species with different strategies to cope with grazing (direct vs. indirect defenses), which are expected to lead to different intensities of indirect facilitation for the associated communities. We collected functional traits which are known to vary according to grazing pressure (LDMC, leaf thickness, and maximum height), on both the nurse and their associated plant communities in grazed (and therefore indirect facilitation as well) and ungrazed conditions. We found that the effect of indirectly facilitated on the associated plant communities depended on the functional trait considered. Indirect facilitation decreased the effects of grazing on species relative abundance, mean LDMC, and the convergence of the maximum height distribution of the associated communities, but did not affect mean height or cover. The identity of the nurse species and grazing jointly affected the structure of the associated plant community through indirect facilitation. Our results together with the existing literature suggest that the “grazer–nurse–beneficiary” interaction module can be more complex than expected when evaluated in the field.     

Shift from facilitative to neutral interactions by the cushion plant Silene acaulis along a primary succession gradient

Background

The stress‐gradient hypothesis predicts a shift from facilitative to competitive plant interactions with decreasing abiotic stress. This has been supported by studies along elevation and temperature gradients, but also challenged by the hypothesis of a facilitation collapse at extremely harsh sites. Although facilitation is known to be important in primary succession, few studies have examined these hypotheses along primary succession gradients.

Aim

To examine whether there is a relationship between the presence of the circumpolar cushion plant Silene acaulis and other species, and if so, whether there is a shift between positive and negative interactions along a primary succession gradient in a glacier foreland.

Location

Finse, southern Norway.

Methods

We examined the performance of the common alpine forb Bistorta vivipara, species richness of vascular plants, bryophytes and lichens, and the number of seedlings and fertile vascular plants in S. acaulis cushions, and control plots without S. acaulis, along a succession gradient with increasing distance from a glacier front, and thus decreasing abiotic stress. To examine if S. acaulis cushions modify the abiotic environment, we recorded soil temperature, moisture, organic content and pH in cushions and control plots.

Results

Bistorta vivipara performed better, as shown by bigger leaves in S. acaulis cushions compared to control plots in the harshest part of the gradient close to the glacier. There were few differences in B. vivipara performance between cushion and control plots in the more benign environment further away from the glacier. This suggests a shift from facilitative to mainly neutral interactions by S. acaulis on the performance of B. vivipara with decreasing abiotic stress. A trend, although not significant, of higher vascular species richness and fertility inside S. acaulis cushions along the whole gradient, suggests that S. acaulis also facilitates community‐level species richness. The causal mechanism of this facilitation is likely that the cushions buffer extreme temperatures.

Conclusions

Our results support the stress‐gradient hypothesis for the relationship between the cushion plant S. acaulis and the performance of a single species along a primary succession gradient in a glacier foreland. S. acaulis also tended to increase vascular plant species richness and fertility regardless of stress level along the gradient, suggesting facilitation at the community level. We found no collapse of facilitation at the most stressful end of the gradient in this alpine glacier foreland.     

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 Azorella monantha (Apiaceae) and alpine plant species in the Chilean Patagonian Andes

Low growing, compact cushion plants are a common and often dominant life form in temperate and subpolar alpine habitats. The cushion life-form can modify wind patterns, temperature and water availability and thus cushion species could be expected to act as nurse-plants facilitating the establishment of other alpine plant species on their surfaces. It has been suggested that the nurse effect should be most pronounced under more stressful environmental conditions, as found with increasing elevation in the alpine. One of the approaches used to detect the nurses has been the study of spatial associations among species, in which extreme clumping within or beneath one species has been interpreted as evidence of nursing. We characterized microclimatic conditions (soil and air temperature) within and outside cushions of Azorella monantha at two elevations (700 m a.s.l., corresponding to an elevation just above treeline, and 900 m a.s.l., corresponding to the upper limit of the cushion belt zone) on Cerro Diente in the Patagonian alpine of southern South America (50° S) and recorded all plant species growing upon cushions of various sizes and for paired sampling areas of equivalent sizes outside cushions. At 5 cm depth, soil temperature was slightly higher under cushions than under bare ground, but only significantly so at 900 m. Air temperature at ground level was significantly higher in the cushion microhabitat at both 700 m and 900 m, with the difference being more exaggerated at the highest elevation. At 700 m, a total of 27 species were recorded growing within cushions as compared to 29 outside cushions. At 900 m the corresponding numbers were 34 and 18. At the highest elevation, significantly more species grow within cushions than for equal areas outside cushions. Here moreover, 17 (48.6%) species grew preferentially within cushions, with eight of the latter being limited to the cushion microhabitat at this elevation. However, at 700 m there was no significant difference in species richness in the two microhabitats, and only one species (3.1%) grew preferentially on cushions. Considering individual species, nine occurring at both elevations showed non-preferential recruitment on cushions at 700 m, but significantly higher frequencies on cushions at 900 m. Results suggest striking altitudinal variation in the association with Azorella monantha on Cerro Diente, ranging from a very strong at 900 m to near absence at 700 m. Milder air and soil temperatures, shelter from wind, and greater water availability within cushions as opposed to outside cushions are discussed as possible factors favoring strong plant recruitment on cushions at higher elevations in the harsh Patagonian alpine environment. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Tempo and mode of vegetation dynamics over 50 years in a New Zealand alpine cushion / tussock community

Abstract. Question: What are the tempo and mode of long‐term succession and of demographic processes in an alpine community, especially: tenacity, transition patterns, predictions, growth of individual cushion plants, cyclic succession, spatial patterns? Location: A low‐alpine mixed cushion /turf /snow‐tussock / shrub community in southeastern New Zealand. Methods: The distribution of seven plant cover‐types was recorded at 1024 fixed points in an 8 m × 8 m plot at approximately decade intervals for 50 years. The diameters of eight Donatia novae‐zelandiae cushions were monitored. Results: The process was essentially first‐order Markovian. There was a change in transition frequencies about 1980. The tenacity of the two major cover types — cushion and turf — was high, but that of cushion decreased about 1980 as some of its area was taken over by turf. The original informal prediction of 1955 that the cushion/turf would increase proved to be correct, probably because of paludification of the site. A prediction of 1987 made from observed transitions that cushion would dominate over turf has proved untrue because of a change in the transition probabilities in the 1980s, of unknown cause. There is a ten‐fold range in diameter growth rates among the eight cushions measured, but the mean rate of 5.3 mm.a^‐1 is similar to that reported from other alpine and arctic sites. As cushions aged, turf colonized their centres, and in two cases new cushions colonized into this turf: consistent with cyclic succession. The pattern of transitions was compatible with a general interpretation of cyclic succession, but not definitive. Conclusions: Change is slow in this alpine community, and tenacity high. The change in transition frequencies about 1980, the invasion of individual cushions, and the decrease in spatial autocorrelation all suggest that cushions established on the site as a result of the clearance of woody vegetation after 1400 AD. Paludification may be causing some loss of tussock grass. A tendency for the cushions to break up, and the shallow peat accumulated below them, may indicate that they are the first generation of cushions on the site. Though these cushions are breaking up, other cushions are establishing, and cushions will continue to be an important part of the vegetation dynamics which may be part of a cyclic succession.     

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.     

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.     

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.     

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.     

Phenotypic effects of the nurse Thylacospermum caespitosum on dependent plant species along regional climate stress gradients

Contrasting phenotypes of alpine cushion species have been recurrently described in several mountain ranges along small‐scale topography gradients, with tight competitive phenotypes in stressful convex topography and loose facilitative phenotypes in sheltered concave topography. The consistency of phenotypic effects along large‐scale climate stress gradients have been proposed as a test of the likely genetic bases of the differences observed at small‐scale. Inversely, plastic phenotypic effects are more likely to vanish at some points along climate stress gradients. We tested this hypothesis for two phenotypes of the alpine cushion species Thylacospermum caespitosum at four points along regional gradients of cold and drought stress in northwest China. We measured the traits of the two cushion phenotypes and quantified their associated plant communities and environmental variables along the regional temperature and aridity gradients. Cushion height, convexity and stem density overall showed significant effect of phenotypes. Difference in tightness of cushions between phenotypes was consistent across climate conditions, whereas differences in cushion convexity and height between phenotypes increased with increasing cold stress. Phenotypic effects on species richness and abundance were consistent along both climate gradients but not effects on species composition, while there were no phenotypic effects on environmental variables. Additionally, RII (relative interaction index) curves were linear along the drought gradient but unimodal along the temperature gradient, likely due to the occurrence of contrasting species pools at the different sites. We conclude that the consistency of phenotypic effects of T. caespitosum was high for species richness and abundance and mainly explained by differences in interference mediated by likely heritable differences in cushion tightness. Additionally, our study shows that the shapes of the relationship between plant responses to neighbours and environmental stresses are not necessarily driven by niche‐based deterministic factors.     

Two alternatives to the stress‐gradient hypothesis at the edge of life: the collapse of facilitation and the switch from facilitation to competition

New evidence demonstrates that facilitation plays a crucial role even at the edge of life in Maritime Antarctica. These findings are interpreted as support for the stress‐gradient hypothesis (SGH) – a dominant theory in plant community ecology that predicts that the frequency of facilitation directly increases with stress. A recent development to this theory, however, proposed that facilitation often collapses at the extreme end of stress and physical disturbance gradients. In this paper, we clarify the current debate on the importance of plant interactions at the edge of life by illustrating the necessity of separating the two alternatives to the SGH, namely the collapse of facilitation, and the switch from facilitation to competition occurring in water‐stressed ecosystems. These two different alternatives to the SGH are currently often amalgamated with each other, which has led to confusion in recent literature. We propose that the collapse of facilitation is generally due to a decrease in the effect of the nurse plant species, whilst the switch from facilitation to competition is driven by environmental conditions and strategy of the response species. A clear separation between those two alternatives is particularly crucial for predicting the role of plant–plant interactions in mediating species responses to global change.     

Extending the stress‐gradient hypothesis – is competition among animals less common in harsh environments?

The role of positive interactions has become widely accepted as a mechanism shaping community dynamics. Most empirical evidence comes from plant communities and sessile marine organisms. However, evidence for the relative role of positive interactions in organizing terrestrial animal communities is more limited, and a general framework that includes positive interactions among animals is lacking. The ‘stress gradient hypothesis’ (SGH) developed by plant ecologists predicts that the balance between positive and negative interactions will vary along gradients of biotic and abiotic stress, with positive interactions being more important in stressful environments. Paralleling the SGH, stress gradients for terrestrial herbivores could be equated to inverse primary productivity gradients, so we would expect positive interactions to prevail in more stressful, low productivity environments. However, this contradicts the typical view of terrestrial animal ecology that low primary productivity systems will foster intense competition for resources among consumers. Here we use alpine herbivores as a case study to test one of the predictions of the SGH in animal communities, namely the prevalence of positive interactions in low productivity environments. We identify potential mechanisms of facilitation and review the limited number of examples of interspecific interactions among alpine herbivores to assess the role of positive and negative interactions in structuring their communities. A meta‐analysis showed no clear trend in the strength and direction of interactions among alpine herbivores. Although studies were biased towards reporting significant negative inter actions, we found no evidence of competition dominating in harsh environments. Thus, our results only partially support the SGH, but directly challenge the dominant view among animal ecologists. Clearly, a sound theoretical framework is needed to include competition, positive and neutral interactions as potential mechanisms determining the structure of animal communities under differing environmental conditions, and the stress‐gradient hypothesis can provide a solid starting point.     

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.