Forest composition in Mediterranean mountains is projected to shift along the entire elevational gradient under climate change

Aim Species distribution models have been used frequently to assess the effects of climate change on mountain biodiversity. However, the value and accuracy of these assessments have been hampered by the use of low‐resolution data for species distributions and climatic conditions. Herein we assess potential changes in the distribution and community composition of tree species in two mountainous regions of Spain under specific scenarios of climate change using data with a high spatial resolution. We also describe potential changes in species distributions and tree communities along the entire elevational gradient. Location Two mountain ranges in southern Europe: the Central Mountain Range (central west of the Iberian Peninsula), and the Iberian Mountain Range (central east). Methods We modelled current and future distributions of 15 tree species (Eurosiberian, sub‐Mediterranean and Mediterranean species) as functions of climate, lithology and availability of soil water using generalized linear models (logistic regression) and machine learning models (gradient boosting). Using multivariate ordination of a matrix of presence/absence of tree species obtained under two Intergovernmental Panel on Climate Change (IPCC) scenarios (A2 and B2) for two different periods in the future (2041–70 and 2071–2100), we assessed the predicted changes in the composition of tree communities. Results The models predicted an upward migration of communities of Mediterranean trees to higher elevations and an associated decline in communities of temperate or cold‐adapted trees during the 21st century. It was predicted that 80–99% of the area that shows a climate suitable for cold–wet‐optimum Eurosiberian coniferous and broad‐leaved species will be lost. The largest overall changes were predicted for Mediterranean species found currently at low elevations, such as Pinus halepensis, Pinus pinaster, Quercus ilex ssp. ballota and Juniperus oxycedrus, with sharp increases in their range of 350%. Main conclusions It is likely that areas with climatic conditions suitable for cold‐adapted species will decrease significantly under climate warming. Large changes in species ranges and forest communities might occur, not only at high elevations within Mediterranean mountains but also along the entire elevational gradient throughout this region, particularly at low and mid‐elevations. Mediterranean mountains might lose their key role as refugia for cold‐adapted species and thus an important part of their genetic heritage.     

Projecting potential future shifts in species composition of European urban plant communities

Aim

Urban floras are composed of species of different origin, both native and alien, and with various traits and niches. It is likely that these species will respond to the ongoing climate change in different ways, resulting in future species compositions with no analogues in current European cities. Our goal was to estimate potential shifts in plant species composition in European cities under different scenarios of climate change for the 21st century.

Location

Europe.

Methods

Potential changes in the distribution of 375 species currently growing in 60 large cities in Southern, Central and Western Europe were modelled using generalized linear models and four climate change projections for two future periods (2041–2060 and 2061–2080). These projections were based on two global climate models (CCSM4 and MIROC‐ESM) and two Representative Concentration Pathways (2.6 and 8.5).

Results

Results were similar across all climate projections, suggesting that the composition of urban plant communities will change considerably due to future climate change. However, even under the most severe climate change scenario, native and alien species will respond to climate change similarly. Many currently established species will decline and others, especially annuals currently restricted to Southern Europe, will spread to northern cities. In contrast, perennial herbs, woody plants and most species with temperate continental and oceanic distribution ranges will make up a smaller proportion of future European urban plant communities in comparison with the present communities.

Main conclusions

The projected 21st century climate change will lead to considerable changes in the species composition of urban floras. These changes will affect the structure and functioning of urban plant communities.

     

Equilibrium of vegetation and climate at the European rear edge. A reference for climate change planning in mountainous Mediterranean regions

Mediterranean mountains harbour some of Europe’s highest floristic richness. This is accounted for largely by the mesoclimatic variety in these areas, along with the co-occurrence of a small area of Eurosiberian, Boreal and Mediterranean species, and those of Tertiary Subtropical origin. Throughout the twenty-first century, we are likely to witness a climate change-related modification of the biogeographic scenario in these mountains, and there is therefore a need for accurate climate regionalisations to serve as a reference of the abundance and distribution of species and communities, particularly those of a relictic nature. This paper presents an objective mapping method focussing on climate regions in a mountain range. The procedure was tested in the Cordillera Central Mountains of the Iberian Peninsula, in the western Mediterranean, one of the ranges occupying the largest area of the Mediterranean Basin. This regionalisation is based upon multivariate analyses and upon detailed cartography employing 27 climatic variables. We used spatial interpolation of data based on geographic information. We detected high climatic diversity in the mountain range studied. We identified 13 climatic regions, all of which form a varying mosaic throughout the annual temperature and rainfall cycle. This heterogeneity results from two geographically opposed gradients. The first one is the Mediterranean-Euro-Siberian variation of the mountain range. The second gradient involves the degree of oceanicity, which is negatively related to distance from the Atlantic Ocean. The existing correlation between the climatic regions detected and the flora existing therein enables the results to be situated within the projected trends of global warming, and their biogeographic and ecological consequences to be analysed.     

Water-energy relationships shape the phylogenetic diversity of terricolous lichen communities in Mediterranean mountains: Implications for conservation in a climate change scenario

Lichens are symbiotic organisms sensitive to climate change and susceptible to a severe decline in diversity, especially in high elevation environments that are already threatened. In this study, we focused on water-energy relationships derived from climatic variables and phylogenetic diversity indices of terricolous lichen communities occurring on a representative Mediterranean mountain. We hypothesized that the variation of precipitation and temperature and their interaction along the altitudinal gradient will shape the phylogenetic diversity and structure of lichen communities. Our results reveal that dry and arid conditions lead to a strong loss in phylogenetic diversity with consequent impoverishment of high elevation lichen communities under a climate change scenario. The interaction between variables, reflecting water-energy relationships with phylogenetic and community diversity patterns, suggests that in a future climate change scenario, the novel climatic conditions may reduce the capability of the species to survive harsher conditions, and Mediterranean mountains may face a severe loss of genetic diversity in a climate change scenario.     

Declining population trends of European mountain birds

Mountain areas often hold special species communities, and they are high on the list of conservation concern. Global warming and changes in human land use, such as grazing pressure and afforestation, have been suggested to be major threats for biodiversity in the mountain areas, affecting species abundance and causing distribution shifts towards mountaintops. Population shifts towards poles and mountaintops have been documented in several areas, indicating that climate change is one of the key drivers of species’ distribution changes. Despite the high conservation concern, relatively little is known about the population trends of species in mountain areas due to low accessibility and difficult working conditions. Thanks to the recent improvement of bird monitoring schemes around Europe, we can here report a first account of population trends of 44 bird species from four major European mountain regions: Fennoscandia, UK upland, south‐western (Iberia) and south‐central mountains (Alps), covering 12 countries. Overall, the mountain bird species declined significantly (?7%) during 2002–2014, which is similar to the declining rate in common birds in Europe during the same period. Mountain specialists showed a significant ?10% decline in population numbers. The slope for mountain generalists was also negative, but not significantly so. The slopes of specialists and generalists did not differ from each other. Fennoscandian and Iberian populations were on average declining, while in United Kingdom and Alps, trends were nonsignificant. Temperature change or migratory behaviour was not significantly associated with regional population trends of species. Alpine habitats are highly vulnerable to climate change, and this is certainly one of the main drivers of mountain bird population trends. However, observed declines can also be partly linked with local land use practices. More efforts should be undertaken to identify the causes of decline and to increase conservation efforts for these populations.     

Diversity of Rich Fen Vegetation and Related Plant Specialists in Mountain Refugia of the Iberian Peninsula

In temperate mountains, fens have been reported as relict habitats subject to geographical fragmentation and broad climatic gradients, but few studies have analyzed the influence of these factors on plant diversity. Here we investigate the effect of isolation on the vegetation diversity of rich fens (Caricion davallianae) in the mountains of the Iberian Peninsula, the distribution limit of these habitats in south-western Europe. We used plot-based vegetation data from the Pyrenees and the Cantabrian mountain range to evaluate their regional species-pool, occurrence of specialists, beta-diversity and the effect of geo-climatic variables on their species-richness and species-composition. We found a lower ratio of rare specialists in the Pyrenees than in the Cantabrian range, but similar estimates in the species pools, total species-richness per plot and beta-diversity. The isolation of the two mountain regions resulted in different species assemblages best predicted by summer precipitation and bedrock types, showing region-based differences in the response of vegetation and plant specialists to the environment. The tighter correlation between local climate and diversity estimates in the Cantabrian range suggests relict character of rich fens in that region, where climatic conditions have restricted local distribution of formerly more widely distributed specialists. Although there is no relevant evidence of vegetation impoverishment in that region, historical isolation has probably resulted in the existence of fragmentary plant communities. We conclude that fen vegetation may experience long-time persistence in climatically sub-optimal mountain refugia, but related plant specialists may be sensitive to climatic changes and subject to the extinction of local populations.     

Habitat preferences of the bank voleMyodes glareolus in a Mediterranean mountain range

The bank voleMyodes glareolus Schreber, 1780 is a widely distributed rodent in Europe, being numerically dominant in small mammal communities living in temperate woodlands. However, it becomes scarce in southern Europe (Mediterranean area) where it reaches the southernmost limit of its distribution range. We studied the habitat preferences of bank voles in 9 plots in a transitional area between Mediterranean and Eurosiberian regions within a Mediterranean mountain. During the study period we captured 1919 small mammals of 9 species, including 287 bank voles (14.95%). Mean density ranged from zero individuals per plot (1.1 ha) at the boreo-subalpine scrubland to 10.27 ± 2.84 (SE) at a Mediterranean river woodland. Statistical path analysis was used to investigate relationships between mean bank vole density and climate and vegetation structure measured within plots. The variables selected by the structural equation model were those related to forest structure, like tree cover and height, dead vegetation, moss, and rock cover. Habitat moisture was also important (microclimatic conditions). Mean climate conditions (and elevation) did not have any significant effect on mean bank vole density, and no significant association with understorey vegetation (eg shrub and herbaceous cover) was observed. Our results pointed out that bank voles were habitat specialists in our study area, being more abundant and frequent in moist woodlands, and rare or absent in shrublands and grasslands.     

Identifying refugia from climate change using coupled ecological and genetic data in a transitional Mediterranean‐temperate tree species

Populations occurring in areas of overlap between the current and future distribution of a species are particularly important because they can represent “refugia from climate change”. We coupled ecological and range‐wide genetic variation data to detect such areas and to evaluate the impacts of habitat suitability changes on the genetic diversity of the transitional Mediterranean‐temperate tree Fraxinus angustifolia. We sampled and genotyped 38 natural populations comprising 1006 individuals from across Europe. We found the highest genetic diversity in western and northern Mediterranean populations, as well as a significant west to east decline in genetic diversity. Areas of potential refugia that correspond to approximately 70% of the suitable habitat may support the persistence of more than 90% of the total number of alleles in the future. Moreover, based on correlations between Bayesian genetic assignment and climate, climate change may favour the westward spread of the Black Sea gene pool in the long term. Overall, our results suggest that the northerly core areas of the current distribution contain the most important part of the genetic variation for this species and may serve as in situ macrorefugia from ongoing climate change. However, rear‐edge populations of the southern Mediterranean may be exposed to a potential loss of unique genetic diversity owing to habitat suitability changes unless populations can persist in microrefugia that have facilitated such persistence in the past.     

Potential effects of climate change on the distribution of Scarabaeidae dung beetles in Western Europe

Dung beetles are indispensable in pasturelands, especially when poor efficiency of earthworms and irregular rainfall (e.g. under a Mediterranean climate) limit pad decomposition. Although observed and projected species range shifts and extinctions due to climate change have been documented for plants and animals, little effort has focused on the response of keystone species such as the scarab beetles of dung beetle decomposers. Our study aims to forecast the distribution of 37 common Scarabaeidae dung beetle species in France, Portugal and Spain (i.e. more than half of the western European Scarabaeidae fauna) in relation to two climate change scenarios (A2 and B1) for the period leading to 2080. On average, 21 % of the species should change in each 50-km UTM grid cell. The highest faunistic turnover rate and a significant increase in species richness are expected in the north of the study area while a marked impoverishment is expected in the south, with little difference between scenarios. The potential enrichment of northern regions depends on the achievement of the northward shift of thermophilous species, and climate change is generally likely to reduce the current distribution of the majority of species. Under these conditions, the distribution of resource—i.e. the extent and distribution of pastures—will be a key factor limiting species’ responses to climate change. The dramatic abandonment of extensive grazing across many low mountains of southern Europe may thus represent a serious threat to dung beetle distribution changes.     

Mountain landscapes offer few opportunities for high‐elevation tree species migration

Climate change is anticipated to alter plant species distributions. Regional context, notably the spatial complexity of climatic gradients, may influence species migration potential. While high‐elevation species may benefit from steep climate gradients in mountain regions, their persistence may be threatened by limited suitable habitat as land area decreases with elevation. To untangle these apparently contradictory predictions for mountainous regions, we evaluated the climatic suitability of four coniferous forest tree species of the western United States based on species distribution modeling (SDM) and examined changes in climatically suitable areas under predicted climate change. We used forest structural information relating to tree species dominance, productivity, and demography from an extensive forest inventory system to assess the strength of inferences made with a SDM approach. We found that tree species dominance, productivity, and recruitment were highest where climatic suitability (i.e., probability of species occurrence under certain climate conditions) was high, supporting the use of predicted climatic suitability in examining species risk to climate change. By predicting changes in climatic suitability over the next century, we found that climatic suitability will likely decline, both in areas currently occupied by each tree species and in nearby unoccupied areas to which species might migrate in the future. These trends were most dramatic for high elevation species. Climatic changes predicted over the next century will dramatically reduce climatically suitable areas for high‐elevation tree species while a lower elevation species, Pinus ponderosa, will be well positioned to shift upslope across the region. Reductions in suitable area for high‐elevation species imply that even unlimited migration would be insufficient to offset predicted habitat loss, underscoring the vulnerability of these high‐elevation species to climatic changes.     

Buffered climate change effects in a Mediterranean pine species: range limit implications from a tree-ring study

Within-range effects of climatic change on tree growth at the sub-regional scale remain poorly understood. The aim of this research was to use climate and radial-growth data to explain how long-term climatic trends affect tree growth patterns along the southern limit of the range of Pinus nigra ssp. salzmannii (Eastern Baetic Range, southern Spain). We used regional temperature and precipitation data and measured sub-regional radial growth variation in P. nigra forests over the past two centuries. A dynamic factor analysis was applied to test the hypothesis that trees subjected to different climates have experienced contrasting long-term growth variability. We defined four representative stand types based on average temperature and precipitation to evaluate climate–growth relationships using linear mixed-effect models and multi-model selection criteria. All four stand types experienced warming and declining precipitation throughout the twentieth century. From the onset of the twentieth century, synchronised basal-area increment decline was accounted for by dynamic factor analysis and was related to drought by climate–growth models; declining basal-area increment trends proved stronger at lower elevations, whereas temperature was positively related to growth in areas with high rainfall inputs. Given the contrasting sub-regional tree-growth responses to climate change, the role of drought becomes even more complex in shaping communities and affecting selection pressure in the Mediterranean mountain forests. Potential vegetation shifts will likely occur over the dry edge of species distributions, with major impacts on ecosystem structure and function.     

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.     

Lags in the response of mountain plant communities to climate change

Rapid climatic changes and increasing human influence at high elevations around the world will have profound impacts on mountain biodiversity. However, forecasts from statistical models (e.g. species distribution models) rarely consider that plant community changes could substantially lag behind climatic changes, hindering our ability to make temporally realistic projections for the coming century. Indeed, the magnitudes of lags, and the relative importance of the different factors giving rise to them, remain poorly understood. We review evidence for three types of lag: “dispersal lags” affecting plant species’ spread along elevational gradients, “establishment lags” following their arrival in recipient communities, and “extinction lags” of resident species. Variation in lags is explained by variation among species in physiological and demographic responses, by effects of altered biotic interactions, and by aspects of the physical environment. Of these, altered biotic interactions could contribute substantially to establishment and extinction lags, yet impacts of biotic interactions on range dynamics are poorly understood. We develop a mechanistic community model to illustrate how species turnover in future communities might lag behind simple expectations based on species’ range shifts with unlimited dispersal. The model shows a combined contribution of altered biotic interactions and dispersal lags to plant community turnover along an elevational gradient following climate warming. Our review and simulation support the view that accounting for disequilibrium range dynamics will be essential for realistic forecasts of patterns of biodiversity under climate change, with implications for the conservation of mountain species and the ecosystem functions they provide.     

Differences between tree species seedling and adult altitudinal distribution in mountain forests during the recent warm period (1986–2006)

Spatial fingerprints of climate change on tree species distribution are usually detected at latitudinal or altitudinal extremes (arctic or alpine tree line), where temperatures play a key role in tree species distribution. However, early detection of recent climate change effects on tree species distribution across the overall temperature gradient remains poorly explored. Within French mountain forests, we investigated altitudinal distribution differences between seedling (≤50 cm tall and >1 yr old) and adult (>8 m tall) life stages for 17 European tree taxa, encompassing the entire forest elevation range from lowlands to the subalpine vegetation belt (50–2250 m a.s.l.) and spanning the latitudinal gradient from northern temperate to southern Mediterranean forests. We simultaneously identified seedlings and adults within the same vegetation plots. These twin observations gave us the equivalent of exactly paired plots in space with seedlings reflecting a response to the studied warm period (1986–2006) and adults reflecting a response to a former and cooler period. For 13 out of 17 species, records of the mean altitude of presence at the seedling life stage are higher than that at the adult life stage. The low altitudinal distribution limit of occurrences at the seedling life stage is, on average, 29 m higher than that at the adult life stage which is significant. The high altitudinal distribution limit also shows a similar trend but which is not significant. Complementary analyses using modelling techniques and focusing on the optimum elevation (i.e. the central position inside distribution ranges) have confirmed differences between life stages altitudinal distribution. Seedlings optima are mostly higher than adults optimum, reaching, on average, a 69 m gap. This overall trend showing higher altitudinal distribution at the seedling life stage in comparison to the adult one suggests a main driver of change highly related to elevation, such as climate warming that occurs during the studied period. Other drivers of change that could play an important role across elevation or act at more specific scales are also discussed as potential contributors to explain our results.     

Contrasting tree‐ring growth to climate responses of Abies alba toward the southern limit of its distribution area

Silver fir Abies alba is an indigenous tree species present in many southern European mountain forests. Its distribution area and its adaptive capacity to climate variability, expressed in tree‐ring growth series, make it a very suitable target species for studying responses to climate particularly in a complex area like the Mediterranean basin where significant changes are expected. We used a set of 52 site chronologies (784 trees) in the Italian Alps and Apennines (38.1°– 46.6°N and 6.7°– 16.3°E) and temperature and precipitation monthly data for the period 1900–1995. Principal component analyses of the tree‐ring site network was applied to extract common modes of variability in annual radial growth among the chronologies. Climate/growth relationships and their stationarity and consistency over time were computed by means of correlation and moving correlation functions. Tree‐ring chronologies show a clear distinction between the Alpine and the Mediterranean sites and a further separation of the Alpine region in western and eastern sectors. Accordingly, we found different transient and contrasting regional responses in time with the trends found in the Mediterranean sites marking a relaxation of some of the major climate limiting factors recorded prior to the last decades. Species’ sensitivity to global change may result in distinct spatial responses reflecting the complexity of the Mediterranean climate, with large differences between various areas of the basin. It is still unclear if these contrasting tree‐ring growth to climate responses of Abies alba are due to the corresponding separation between the Alpine and Mediterranean climate modes, the atmospheric CO₂ fertilization effect, the environmentally most fitted genetic pools of the southern fir ecotypes or a combination of all factors. Climate–growth analysis based on a wide site network and on long‐term weather records confirmed to be excellent tools to detect spatial and temporal variability of species’ responses to climate.     

Early signs of range disjunction of submountainous plant species: an unexplored consequence of future and contemporary climate changes

Poleward and upward species range shifts are the most commonly anticipated and studied consequences of climate warming. However, these global responses to climate change obscure more complex distribution change patterns. We hypothesize that the spatial arrangement of mountain ranges and, consequently, climatic gradients in Europe, will result in range disjunctions. This hypothesis was investigated for submountainous forest plant species at two temporal and spatial scales: (i) under future climate change (between 1950–2000 and 2061–2080 periods) at the European scale and (ii) under contemporary climate change (between 1914–1987 and 1997–2013 periods) at the French scale. We selected 97 submountainous forest plant species occurring in France, among which distribution data across Europe are available for 25 species. By projecting future distribution changes for the 25 submountainous plant species across Europe, we demonstrated that range disjunction is a likely consequence of future climate change. To assess whether it is already taking place, we used a large forest vegetation‐plot database covering the entire French territory over 100 years (1914–2013) and found an average decrease in frequency (?0.01 ± 0.004) in lowland areas for the 97 submountainous species – corresponding to a loss of 6% of their historical frequency – along with southward and upward range shifts, suggesting early signs of range disjunctions. Climate‐induced range disjunctions should be considered more carefully since they could have dramatic consequences on population genetics and the ability of species to face future climate changes.     

Projected Range Contractions of European Protected Oceanic Montane Plant Communities: Focus on Climate Change Impacts Is Essential for Their Future Conservation

Global climate is rapidly changing and while many studies have investigated the potential impacts of this on the distribution of montane plant species and communities, few have focused on those with oceanic montane affinities. In Europe, highly sensitive bryophyte species reach their optimum occurrence, highest diversity and abundance in the north-west hyperoceanic regions, while a number of montane vascular plant species occur here at the edge of their range. This study evaluates the potential impact of climate change on the distribution of these species and assesses the implications for EU Habitats Directive-protected oceanic montane plant communities. We applied an ensemble of species distribution modelling techniques, using atlas data of 30 vascular plant and bryophyte species, to calculate range changes under projected future climate change. The future effectiveness of the protected area network to conserve these species was evaluated using gap analysis. We found that the majority of these montane species are projected to lose suitable climate space, primarily at lower altitudes, or that areas of suitable climate will principally shift northwards. In particular, rare oceanic montane bryophytes have poor dispersal capacity and are likely to be especially vulnerable to contractions in their current climate space. Significantly different projected range change responses were found between 1) oceanic montane bryophytes and vascular plants; 2) species belonging to different montane plant communities; 3) species categorised according to different biomes and eastern limit classifications. The inclusion of topographical variables in addition to climate, significantly improved the statistical and spatial performance of models. The current protected area network is projected to become less effective, especially for specialised arctic-montane species, posing a challenge to conserving oceanic montane plant communities. Conservation management plans need significantly greater focus on potential climate change impacts, including models with higher-resolution species distribution and environmental data, to aid these communities'' long-term survival.     

Rapid altitudinal migration of mountain plants in Taiwan and its implications for high altitude biodiversity

Mountain systems throughout the globe are characterized by high levels of species richness and species endemism. Biodiversity, however, is not distributed evenly with altitude, but often declines from mid to high altitudes. Conversely, endemic species may be over‐represented at high altitudes. Upward elevational range shifts of mountain species have been reported in response to ongoing changes in climate, yet the reports are dominated by studies on woody species and mountains at high latitudes. We investigated spatial and temporal changes in the mountain biodiversity in the subtropical island of Taiwan, based on historical survey and resurvey data during the period 1906–2006. We found that upper altitudinal limits of mountain plant distributions have risen by ca 3.6 m yr^?1 during the last century, in parallel with rising temperatures in the region. Although species, genus, and family richness decline with altitude, ca 55% of species at the highest altitudes are endemic to the island. Given the steep decline in land area with increasing elevation, these high altitude areas are disproportionately important for plant biodiversity when richness and endemism are standardized by available land area. We argue that the distributional shift that we report, in combination with the altitudinal distribution of plant diversity, is likely to pose a major threat to high mountain species of this highly biodiverse island, a threat that is becoming increasingly evident for high mountain plants throughout the globe.     

Soil microbial communities along elevational gradients in the Madrean Sky Islands

The Madrean Sky Islands are mountain ranges isolated by a ‘desert sea’. This area is a biodiversity hotspot currently threatened by climate change. Here, we studied soil microbial communities along elevational gradients in eight Madrean Sky Islands in southeastern Arizona (USA). Our results showed that while elevational microbial richness gradients were weak and not consistent across different mountains, soil properties strongly influenced microbial community composition (overall composition and the abundance of key functional groups) along elevational gradients. In particular, warming is associated with a higher abundance of soil-borne fungal plant pathogens that concomitantly might facilitate upward elevational shifts of plant species released from negative plant–soil feedbacks. Furthermore, projected warming and drought in the area aggravated by anthropogenic nitrogen deposition on mountain tops (and thus, decreasing nitrogen limitation) can enhance a shift from ectomycorrhizal to arbuscular mycorrhizal fungi. Overall, these results indicate that climate change effects on plant–soil interactions might have profound ecosystem consequences.