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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. [ABSTRACT FROM AUTHOR]

Background:As the climate warms, plant interactions between shrubs and conifer seedlings may affect migration of boreal trees into alpine areas; however these interactions have not been widely tested across conifer life stages. Aims:Determine the role of shading byBetula glandulosashrubs on earlyPicea marianarecruitment in the forest-tundra ecotone of the Mealy Mountains, Labrador (Canada). Methods:Four shrub treatments were established with varying degrees of shade (unaltered, trimmed, cleared, shaded). Emergence and growth ofP. marianain treatment plots were compared to control plots without shrubs. Results:Seedling emergence was facilitated; seedlings beneath shrubs emerged at greater levels (16% ± 5.1%) than in the control (0.5% ± 0.3%); however, evidence of facilitation post-emergence was weak, likely due to stronger interactions with seedbed species at this stage. Vertical growth of young seedlings was greatest in control and shaded plots, but needle production significantly decreased with increased shade suggesting that seedlings may compete with shrubs for light early in life. Conclusions:Although facilitation has been previously implicated as an important mechanism in stressful environments, our study indicates that the early life stages of some conifers at the tree line are not strongly facilitated by shrubs as has been previously suggested for adults. [ABSTRACT FROM PUBLISHER]

1. Facilitation allows some plant species to occupy environments that they are otherwise unable to inhabit, potentially leading to greater fitness, greater productivity or abundance, and range expansions. However, we know little about the costs incurred by facilitators or how variation in resource allocation by facilitators influences effects and costs. 2. Gynodioecy provides an opportunity to explore the effects of facilitation and potential costs because females and hermaphrodites differ in resource allocation patterns. We explored whether environmental stress and gender-specific facilitation influence species interactions in an alpine plant community. We investigated the degree of facilitation and correlative associated costs for the gynodioecious alpine plant, Silene acaulis, at two elevations (2317 and 2560 m) in the northern Rocky Mountains of Montana, USA. 3. Hermaphroditic Silene individuals supported a greater number of plant species and individual plants than female Silene (hermaphrodites = 4.2 ± 0.3, females = 3.5 ± 0.2; hermaphrodites = 11.5 ± 1.0, females = 9.1 ± 1.1). Facilitative effects on species richness significantly increased with elevation (2.1 ± 1.6 species at the low site vs. 3.2 ± 1.8 species at the high site), but abundance and percentage cover did not. 4. Silene fitness at the high elevation site (2560 m) was reduced compared to the lower site (2317 m) as measured by flower and seed production. Female flower production decreased by 40%, the number of seeds per fruit by 11.6% and leaf size by 24%. 5. Increases in the percentage cover of beneficiaries reduced the number of flowers per Silene plant, indicating a cost of facilitating other species, and this cost was slightly greater for females. However, seed size for hermaphrodites and leaf length for females increased with the percentage cover of beneficiaries, suggesting a mutual benefit of harbouring other species. 6. Synthesis. Our results show that gender can affect the balance between competitive and facilitative interactions. Stronger facilitation by hermaphroditic Silene acaulis, coupled with a lower cost of harbouring beneficiaries, suggests that life-history traits and related patterns of resource allocation can influence the facilitative effects of a species. [ABSTRACT FROM AUTHOR]

As climate warms, abiotic factors, as well as plant community and biodiversity structure, may constrain or promote the movement of ecotone boundaries. Our study sought to examine how plant communities change across the tree-line ecotone of the Mealy Mountains in Labrador, Canada. We established eight transects (50-100 m in length) along an elevation gradient in three tree-line zones (forest, forest-tundra, and alpine-tundra) and recorded all species and cover of vegetation in contiguous 1 × 1 m quadrats. Companion abiotic parameters of nutrients and soil temperature were also measured. The absence of abrupt changes in important soil nutrients and growing season temperatures suggests that these factors do not limit tree species establishment beyond the current tree line. Vegetation cover and richness, however, were highly variable and in some cases changed non-linearly across the tree-line ecotone. Tree cover and species density generally decreased with elevation, while some field layer species (< 25 cm in height) increased; the latter change seems to be influenced by ground shrubs rather than herbaceous species. As expected, transects separated by the greatest difference in elevation were the least similar (higher beta diversity/species turnover); however, species turnover between the forest and forest-tundra transects was higher than it was between forest-tundra and alpine-tundra transects, even though the latter were separated by a greater elevation. Community structure and species turnover vary greatly across a tree line with the greatest differences between the forest and the forest-tundra, suggesting a biotic or abiotic barrier. While our ability to predict how the tree line will respond to continued climate change is complicated by these patterns in plant communities, the potential barriers investigated and others identified will be a useful focus for future studies. [ABSTRACT FROM AUTHOR]

A chart is presented that lists the changes in plant species communities at Subarctic Alpine tree line in Labrador, Canada.

Foundation species can change plant community structure by modulating important ecological processes such as community assembly, yet this topic is poorly understood. In alpine systems, cushion plants commonly act as foundation species by ameliorating local conditions. Here, we analyze diversity patterns of species' assembly within cushions and in adjacent surrounding open substrates (83 sites across five continents) calculating floristic dissimilarity between replicate plots, and using linear models to analyze relationships between microhabitats and species diversity. Floristic dissimilarity did not change across biogeographic regions, but was consistently lower in the cushions than in the open microhabitat. Cushion plants appear to enable recruitment of many relatively stress-intolerant species that otherwise would not establish in these communities, yet the niche space constructed by cushion plants supports a more homogeneous composition of species than the niche space beyond the cushion's influence. As a result, cushion plants support higher adiversity and a larger species pool, but harbor assemblies with lower β-diversity than open microhabitats. We conclude that habitats with and without dominant foundation species can strongly differ in the processes that drive species recruitment, and thus the relationship between local and regional species diversity. [ABSTRACT FROM AUTHOR]

Facilitative effects of some species on others are a major driver of biodiversity. These positive effects of a benefactor on its beneficiary can result in negative feedback effects of the beneficiary on the benefactor and reduced fitness of the benefactor. However, in contrast to the wealth of studies on facilitative effects in different environments, we know little about whether the feedback effects show predictable patterns of context dependence., We reanalyzed a global data set on alpine cushion plants, previously used to assess their positive effects on biodiversity and the nature of the beneficiary feedback effects, to specifically assess the context dependence of how small- and large-scale drivers alter the feedback effects of cushion-associated (beneficiary) species on their cushion benefactors using structural equation modelling., The effect of beneficiaries on cushions became negative when beneficiary diversity increased and facilitation was more intense. Local-scale biotic and climatic conditions mediated these community-scale processes, having indirect effects on the feedback effect. High-productivity sites demonstrated weaker negative feedback effects of beneficiaries on the benefactor., Our results indicate a limited impact of the beneficiary feedback effects on benefactor cushions, but strong context dependence. This context dependence may help to explain the ecological and evolutionary persistence of this widespread facilitative system. [ABSTRACT FROM AUTHOR]

Facilitative interactions are defined as positive effects of one species on another, but bidirectional feedbacks may be positive, neutral, or negative. Understanding the bidirectional nature of these interactions is a fundamental prerequisite for the assessment of the potential evolutionary consequences of facilitation., In a global study combining observational and experimental approaches, we quantified the impact of the cover and richness of species associated with alpine cushion plants on reproductive traits of the benefactor cushions., We found a decline in cushion seed production with increasing cover of cushion-associated species, indicating that being a benefactor came at an overall cost. The effect of cushion-associated species was negative for flower density and seed set of cushions, but not for fruit set and seed quality. Richness of cushion-associated species had positive effects on seed density and modulated the effects of their abundance on flower density and fruit set, indicating that the costs and benefits of harboring associated species depend on the composition of the plant assemblage., Our study demonstrates 'parasitic' interactions among plants over a wide range of species and environments in alpine systems, and we consider their implications for the possible selective effects of interactions between benefactor and beneficiary species. [ABSTRACT FROM AUTHOR]

Interactions among species determine local-scale diversity, but local interactions are thought to have minor effects at larger scales. However, quantitative comparisons of the importance of biotic interactions relative to other drivers are rarely made at larger scales. Using a data set spanning 78 sites and five continents, we assessed the relative importance of biotic interactions and climate in determining plant diversity in alpine ecosystems dominated by nurse-plant cushion species. Climate variables related with water balance showed the highest correlation with richness at the global scale. Strikingly, although the effect of cushion species on diversity was lower than that of climate, its contribution was still substantial. In particular, cushion species enhanced species richness more in systems with inherently impoverished local diversity. Nurse species appear to act as a 'safety net' sustaining diversity under harsh conditions, demonstrating that climate and species interactions should be integrated when predicting future biodiversity effects of climate change. [ABSTRACT FROM AUTHOR]