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.     

Presence of cushion plants increases community diversity in the high equatorial Andes

Cushion plants are a common growth form in the equatorial páramo vegetation and their surfaces are often colonized by other plants. This paper analyzes the effect of the cushion plants on the community diversity at 4650 m on the eastern slope of the Iliniza volcano in Ecuador. Ninety sample plots of 1 m² size were located in the study area and were divided into 25 subplots in which presence and abundance of plant species was recorded. The community diversity was expressed as species richness, Simpson's diversity index, and evenness. Correlation between the cushion species and the composition of the colonists was measured with the CCA ordination analysis, correlation between the cushion cover and community diversity was measured by means of correlation analysis. Randomized species–area curves were used to compare richness of plant communities with and without the cushions. A total of 32 species were found including five cushion plants. Most species preferred to grow on the cushion surface whereas only a few species were able to colonize open ground. Species richness and Simpson's index were significantly correlated to the cushion area but no correlation was found for evenness. The cushions were usually composed of more than one species which hampered the examination of the cushion–colonist specific relationships. Nevertheless, cushions of Azorella and Arenaria seemed to provide more favorable habitat for colonization than the other cushion species. Comparison with an earlier study made on Iliniza indicates that the presence of the cushions significantly increases the richness of the plant community.     

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.     

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.     

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.     

Leaf δ13C reflects ecosystem patterns and responses of alpine plants to the environments on the Tibetan Plateau

Leaf δ¹³C is an indicator of water-use efficiency and provides useful information on the carbon and water balance of plants over longer periods. Variation in leaf δ¹³C between or within species is determined by plant physiological characteristics and environmental factors. We hypothesized that variation in leaf δ¹³C values among dominant species reflected ecosystem patterns controlled by large-scale environmental gradients, and that within-species variation indicates plant adaptability to environmental conditions. To test these hypotheses, we collected leaves of dominant species from six ecosystems across a horizontal vegetation transect on the Tibetan Plateau, as well as leaves of Kobresia pygmaea (herbaceous) throughout its distribution and leaves of two coniferous tree species ( Picea crassifolia, Abies fabri ) along an elevation gradient throughout their distribution in the Qilian Mountains and Gongga Mountains, respectively. Leaf δ¹³C of dominant species in the six ecosystems differed significantly, with values for evergreen coniferous13C values of the dominant species and of K. pygmaea were negatively correlated with annual precipitation along a water gradient, but leaf δ¹³C of A. fabri was not significantly correlated with precipitation in habitats without water-stress. This confirms that variation of δ¹³C between or within species reflects plant responses to environmental conditions. Leaf δ¹³C of the dominant species also reflected water patterns on the Tibetan Plateau, providing evidence that precipitation plays a primary role in controlling ecosystem changes from southeast to northwest on the Tibetan Plateau.     

Species richness,vegetation structure,and floristic composition of woody plants along the elevation gradient of Mt. Meru,Tanzania

Understanding the change in vegetation composition along elevational gradients is critical for species conservation in a changing world. We studied the species richness, tree height, and floristic composition of woody plants along an elevation gradient of protected habitats on the eastern slope of Mount Meru and analyzed how these vegetation variables are influenced by the interplay of temperature and precipitation. Vegetation data were collected on 44 plots systematically placed along five transects spanning an elevational gradient of 1600 to 3400 m a.s.l. We used ordinary linear models and multivariate analyses to test the effect of mean annual temperature and precipitation on woody plant species richness, tree height, and floristic composition. We found that species richness, mean tree height, and maximum tree height declined monotonically with elevation. Models that included only mean annual temperature as an explanatory variable were generally best supported to predict changes in species richness and tree height along the elevation gradient. We found significant changes in woody plant floristic composition with elevation, which were shaped by an interaction of mean annual temperature and precipitation. While plant communities consistently changed with temperature along the elevation gradient, levels of precipitation were more important for plant communities at lower than for those at higher elevations. Our study suggests that changes in temperature and precipitation regimes in the course of climate change will reshape elevational gradients of diversity, tree height, and correlated carbon storage in ecosystems, and the sequence of tree communities on East African mountains.     

Co-ordination between xylem anatomy,plant architecture and leaf functional traits in response to abiotic and biotic drivers in a nurse cushion plant

Background and AimsPlants in dry Mediterranean mountains experience a double climatic stress: at low elevations, high temperatures coincide with water shortage during summer, while at high elevations temperature decreases and water availability increases. Cushion plants often act as nurses by improving the microclimate underneath their canopies, hosting beneficiary species that may reciprocally modify their benefactors’ microenvironment. We assess how the nurse cushion plant Arenaria tetraquetra subsp. amabilis adjusts its hydraulic system to face these complex abiotic and biotic constraints.MethodsWe evaluated intra-specific variation and co-ordination of stem xylem anatomy, leaf functional traits and plant architecture in response to elevation, aspect and the presence of beneficiary species in four A. tetraquetra subsp. amabilis populations in the Sierra Nevada mountains, southern Spain.Key ResultsXylem anatomical and plant architectural traits were the most responsive to environmental conditions, showing the highest mutual co-ordination. Cushions were more compact and had smaller, more isolated conductive vessels in the southern than in the northern aspect, which allow minimization of the negative impacts of more intense drought. Only vessel size, leaf mass per area and terminal branch length varied with elevation. Nurse cushions co-ordinated plant architecture and xylem traits, having higher canopy compactness, fewer leaves per branch and fewer, more isolated vessels than non-nurse cushions, which reflects the negative effects of beneficiary plants on nurse water status. In non-nurse cushions, plant architecture co-ordinated with leaf traits instead. The interacting effects of aspect and elevation on xylem traits showed that stress due to frost at high elevation constrained xylem anatomy in the north, whereas stress due to drought had a parallel effect in the south.ConclusionsTrait co-ordination was weaker under more demanding environmental conditions, which agrees with the hypothesis that trait independence allows plants to better optimize different functions, probably entailing higher adjustment potential against future environmental changes.     

基于OTCs模拟增温方式探讨气候变暖对青藏高原草地生态系统的影响

开顶式生长室(OTCs)增温实验是研究全球气候变化与陆地生态系统关系的主要方法之一,已广泛应用于青藏高原地区。该文通过对近些年国内外研究文献的回顾,分别从植物物候、群落结构、生物量和土壤方面综合分析青藏高原草地生态系统对OTCs模拟增温实验的响应。研究发现:增温使群落返青期提前、枯黄期延迟,生长季延长;有利于禾本科植物的生长;高寒草甸地下生物量分配格局向深层转移;高寒草地生态系统对模拟增温的响应存在不确定性,受到地域、群落类型和实验时间的影响;在增温条件下,降雨和冻土融化引起的土壤水分变化通过调控生态系统的物候、生产力、土壤等途径控制着生态系统对气候变暖的响应。并在此基础上,提出了将来应着重研究的几个方面。     

Plant leaf traits,height and biomass partitioning in typical ephemerals under different levels of snow cover thickness in an alpine meadow

Aims In the cold life zones, snow cover is a comprehensive environmental factor that directly influences soil temperature, soil water content, light and nutrient availability. Plants in these zones develop a series of unique mechanisms involving phenological characteristics, reproductive strategies, physiology and morphology to adapt to environmental changes. This paper is focused on the responses of plant leaf traits, height and biomass partitioning to variations in snow cover thickness, in order to better understand the responses of plant functional traits and specific adaptation strategies under global climate change scenarios. Methods Three transects were established along a gradient of snow cover in an alpine meadow of Mt. Kaka, in the eastern Qinghai-Xizang Plateau. Primula purdomii, Pedicularis kansuensis and Ranunculus tanguticus, which are three widely distributed and dominant ephemerals, were sampled and studied, particularly at their blooming stages. Plant height, specific leaf area (SLA) and biomass partitioning were measured accordingly. Important findings The values of SLA in Pedicularis kansuensis and R. tanguticus were relatively greater under better soil conditions; it was smaller in Primula purdomii with thick snow cover. The relationship between aboveground biomass and belowground biomass in Primula purdomii was allometric at sites with both thick and thin snow cover. No significant relationships were found between aboveground biomass and belowground biomass in Pedicularis kansuensis and R. tanguticus at some individual sites. However, when samples of the three species were pooled, the relationships between aboveground biomass and belowground biomass were allometric at all sites, which did not support isometric scaling hypothesis. In addition, on sites with either thick or thin snow cover, aboveground biomass had greater rate of accumulation than belowground biomass; whereas on sites with medium snow cover, the rate of biomass accumulation was greater for belowground component than aboveground component. Functional traits and biomass variables were better correlated in Primula purdomii and Pedicularis kansuensis than in R. tanguticus.     

不同雪被厚度下典型高山草地早春植物叶片性状、株高及生物量分配的研究

在高寒生命带, 雪被作为重要的综合环境因子, 影响着植物的生理生态特征、种群动态及群落演替等过程, 进而作用于生态系统的功能与服务。通过在青藏高原东缘高寒草甸设置厚雪、中雪和浅雪3个地段, 选取早春开花的常见种紫罗兰报春(Primula purdomii)、甘肃马先蒿(Pedicularis kansuensis)、高原毛茛(Ranunculus tanguticus), 研究了三种植物株高、叶片性状和生物量分配随雪被厚度的变化规律, 以及三者之间的关系。结果表明: 甘肃马先蒿和高原毛茛在生境状况较好的地段比叶面积相对较高, 紫罗兰报春由于具有根状茎和肉质根, 在厚雪地段比叶面积相对较小; 关于单个物种的地上-地下生物量的关系, 紫罗兰报春在厚雪和浅雪地段为完全一致的异速生长关系, 而甘肃马先蒿和高原毛茛在个别地段并无显著相关关系。总体而言, 三种植物整体样本的地上-地下生物量在不同雪被地段均为异速生长关系, 不支持等速生长假说, 并且地上生物量能够很好地解释地下生物量的变异; 功能性状和生物量指标间的相关性, 在紫罗兰报春和高原毛茛表现较好, 而在甘肃马先蒿表现较弱。植物对环境变化的适应具有一定的规律, 同时也表现出物种特异性。今后的研究中, 应注重构建植物适应环境变化的功能性状谱, 以更好地理解全球变化背景下植物功能性状的响应及其适应策略。     

Leaf litter of a dominant cushion plant shifts nitrogen mineralization to immobilization at high but not low temperature in an alpine meadow

Aims

We evaluated the effects of temperature and addition of leaf litter of Androsace tapete MaximWe–a dominant cushion plant species of alpine meadows on the Tibetan Plateau–on carbon (C) and nitrogen (N) mineralization, microbial biomass C (MBC) and N (MBN).

Methods

A laboratory incubation experiment with and without cushion plant litter addition was conducted for 112 days at three temperature regimes (?1, 5 and 11 °C). C and net N mineralization were simultaneously measured during the incubation period.

Results

C and N mineralization were affected by interactions between litter addition and temperature. Litter addition increased C mineralization and MBN but shifted N mineralization to immobilization at higher temperature. The positive relationship between net N mineralization and MBC and MBN was shifted to a negative one through cushion plant litter addition. Cushion plant litter also changed the relationship between C mineralization and net N mineralization from insignificantly positive to significantly negative.

Conclusions

These findings indicate that low temperature in winter could be important for alpine plants because low temperature can increase net N mineralization and supply plants with available N for their growth in the early growing season. During the growing season, climate warming–either directly through a temperature effect or indirectly through triggering increased cushion plant litter production–might lead to stronger competition for N between alpine plants and microorganisms.     

青藏高原高寒草甸物种多样性的海拔梯度分布格局及对地上生物量的影响

植物物种多样性在海拔梯度上的变化规律以及物种多样性与生产力的关系是生态学研究的热点, 至今还没有得出一般性规律。本文以青海省海南藏族自治州贵德县的拉脊山(36°21′ N, 101°27′ E, 海拔3,389-3,876 m)和果洛藏族自治州的玛沁县军牧场山体(34°22′ N, 100°30′ E, 海拔4,121-4,268 m)为研究对象, 对植物高度、盖度、地上生物量和物种多样性随海拔高度的变化进行调查和统计分析, 以探讨青藏高原高寒草甸的物种多样性和地上生物量在海拔梯度上的变化规律及两者的关系。结果表明: (1)两条山体样带上地上生物量与物种多样性随海拔的变化规律一致: 随着海拔的升高, 地上生物量线性降低; Shannon-Wiener指数、Simpson指数和物种丰富度都呈单峰曲线, 在中间海拔最大, 而Pielou指数随海拔的升高线性增加。结合目前针对青藏高原高寒草甸的研究数据, 发现物种丰富度随海拔高度的变化均呈单峰曲线, 说明随着海拔的升高物种多样性先升高后降低可能是青藏高原物种多样性分布的普遍规律。(2)地上生物量与物种多样性的关系在两条山体样带上表现一致: 地上生物量随Shannon- Wiener指数、Simpson指数和Pielou指数的升高而线性降低, 但与物种丰富度不相关。综合两条山体样带所有样方数据, 发现地上生物量与Shannon-Wiener指数、Simpson指数不相关, 而随物种丰富度的升高线性增加。结合目前在青藏高原的相关研究数据, 发现地上生物量与物种丰富度呈S型曲线(logistic model)。     

Cushion plants as critical pioneers and engineers in alpine ecosystems across the Tibetan Plateau

Cushion plants are widely representative species in the alpine ecosystem due to their vital roles in influencing abiotic and biotic environments, ecological succession processes, and ecosystem engineering. Importantly, cushion plants, such as Androsace L. and Arenaria L., are considered to be critical pioneers of ecosystem health, restoration, and sustainability across the Tibetan Plateau. This is because cushion plants (a) show tenacious vitality and can modify regional climates, substrates, and soil nutrients in extreme environments; (b) facilitate relationships with the surroundings and maintain the diversity of aboveground and belowground communities; and (c) are highly sensitive to environmental changes and thus can indicate grassland ecosystem health and resilience in the context of global change.     

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.     

Biological traits are correlated with elevational distribution range of eastern Tibetan herbaceous species

Plants that share the same habitat often exhibit similar biological traits. As climatic conditions change with increasing elevation, biological traits are expected to vary among plant species with temperate endemic, alpine endemic, or widespread elevational distributions. On a northeast Tibetan flora including temperate (1,850–2,800 m a.s.l.), subalpine (2,800–3,400 m a.s.l.), and alpine (3,400–4,150 m a.s.l.) vegetations, we analyzed patterns of change for ten traits of 326 annual and perennial herbaceous species using both univariate and multivariate analyses. The traits selected for this study fit in different groups, i.e., traits related to light competition, photosynthesis, reproduction, defense, and dispersal. We found that plant shape (plant height and leaf distribution along stem) and leaf traits (specific leaf area and leaf area) were significant predictors of plants’ elevational distributions, suggesting an important role for a trade-off between light competition and biomass costs in support structures, and between photosynthetic efficiency and leaf defense. Moreover, species with a broad distribution had significantly higher seed germination and shorter first germination time, as compared to species with narrow distributions, which indicates that rapid regeneration may be crucial for widespread species. However, dispersal-related traits may not be the main factor in shaping plants’ elevational distribution because no significant difference was detected in wind- and animal-dispersal ability. In addition, annual species potentially may have different strategies and adaptive mechanisms because we detected no differences in trait related to the elevational distributions.     

A simulation approach to determine statistical significance of species turnover peaks in a species-rich tropical cloud forest

Use of β-diversity indices in the study of spatial distribution of species diversity is hampered by the difficulty of applying significance tests. To overcome this problem we used a simulation approach in a study of species turnover of ferns, aroids, bromeliads, and melastomes along an elevational gradient from 1700 m to 3400 m in a species-rich tropical cloud forest of Bolivia. Three parameters of species turnover (number of upper/lower elevational species limits per elevational step, Wilson–Shmida similarity index between adjacent steps) were analysed. Significant species turnover limits were detected at 2000 (± 50) m and 3050 m, which roughly coincided with the elevational limits of the main vegetation types recognized in the study area. The taxon specificity of elevational distributions implies that no single plant group can be used as a reliable surrogate for overall plant diversity and that the response to future climate change will be taxon-specific, potentially leading to the formation of plant communities lacking modern analogues. Mean elevational range size of plant species was 490 m (± 369). Elevational range sizes of terrestrial species were shorter than those of epiphytes. We conclude that our simulation approach provides an alternative approach for assessing the statistical significance of levels of species turnover along ecological gradient without the limitations imposed by traditional statistical approaches.     

Recruitment of the high elevation cushion plant Arenaria polytrichoides is limited by competition,thus threatened by currently established vegetation

Cushion plants have been confirmed to be keystone species of alpine ecosystems. However, the adaptive strategies responsible for their recruitment and persistence remain largely unclear. Also, the effect of competition by species already established in surrounding vegetation is not known. We first assessed population size and the frequency distribution of individuals of different size in communities of the keystone cushion Arenaria polytrichoides Edgew. along an elevational gradient in southwestern China. Furthermore, in controlled experiments the effects of soil quality, light availability, and competition by other species on seedling recruitment and survival was tested. At lower elevation individuals were larger (elder), but population size (density) was lower compared to higher elevation; similarly, adults and old individuals were relatively more frequent at low elevation, whereas juvenile and small individuals were more frequent at higher elevations. Seedling recruitment differed depending on soil origin with seeds sown in soils from low elevation having a higher germination percentage and seedling survival when grown without competition. Competition by other species delayed germination but did not influence the final germination percentage, but it increased seedling mortality and reduced their survival. Full light accelerated the germination process, but did not affect final germination percentage and seedling mortality. Results suggest that the recruitment and persistence of A. polytrichoides is strongly affected by competition, whereas temperature and soil quality are less important. We speculate that when populations of cushion A. polytrichoides are likely to be exposed to higher competition due to environmental changes, this species is in danger of being weeded out.     

Indication of antagonistic interaction between climate change and erosion on plant species richness and soil properties in semiarid Mediterranean ecosystems

We analyzed the consequences of climate change and the increase in soil erosion, as well as their interaction on plant and soil properties in semiarid Mediterranean shrublands in Eastern Spain. Current models on drivers of biodiversity change predict an additive or synergistic interaction between drivers that will increase the negative effects of each one. We used a climatic gradient that reproduces the predicted climate changes in temperature and precipitation for the next 40 years of the wettest and coldest end of the gradient; we also compared flat areas with 20° steep hillslopes. We found that plant species richness and plant cover are negatively affected by climate change and soil erosion, which in turn negatively affects soil resistance to erosion, nutrient content and water holding capacity. We also found that plant species diversity correlates weakly with plant cover but strongly with soil properties related to fertility, water holding capacity and resistance to erosion. Conversely, these soil properties correlate weaker with plant species cover. The joint effect of climate change and soil erosion on plant species richness and soil characteristics is antagonistic. That is, the absolute magnitude of change is smaller than the sum of both effects. However, there is no interaction between climate change and soil erosion on plant cover and their effects fit the additive model. The differences in the interaction model between plant cover and species richness supports the view that several soil properties are more linked to the effect that particular plant species have on soil processes than to the quantity and quality of the plant cover and biomass they support. Our findings suggest that plant species richness is a better indicator than plant cover of ecosystems services related with soil development and protection to erosion in semiarid Mediterranean climates.     

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.