Most studies of invasive species have been in highly modified, lowland environments, with comparatively little attention directed to less disturbed, high‐elevation environments. However, increasing evidence indicates that plant invasions do occur in these environments, which often have high conservation value and provide important ecosystem services. Over a thousand non‐native species have become established in natural areas at high elevations worldwide, and although many of these are not invasive, some may pose a considerable threat to native mountain ecosystems. Here, we discuss four main drivers that shape plant invasions into high‐elevation habitats: (1) the (pre‐)adaptation of non‐native species to abiotic conditions, (2) natural and anthropogenic disturbances, (3) biotic resistance of the established communities, and (4) propagule pressure. We propose a comprehensive research agenda for tackling the problem of plant invasions into mountain ecosystems, including documentation of mountain invasion patterns at multiple scales, experimental studies, and an assessment of the impacts of non‐native species in these systems. The threat posed to high‐elevation biodiversity by invasive plant species is likely to increase because of globalization and climate change. However, the higher mountains harbor ecosystems where invasion by non‐native species has scarcely begun, and where science and management have the opportunity to respond in time.

Nonnative species richness typically declines along environmental gradients such as elevation. It is usually assumed that this is because few invaders possess the necessary adaptations to succeed under extreme environmental conditions. Here, we show that nonnative plants reaching high elevations around the world are not highly specialized stress tolerators but species with broad climatic tolerances capable of growing across a wide elevational range. These results contrast with patterns for native species, and they can be explained by the unidirectional expansion of nonnative species from anthropogenic sources at low elevations and the progressive dropping out of species with narrow elevational amplitudes-a process that we call directional ecological filtering. Independent data confirm that climatic generalists have succeeded in colonizing the more extreme environments at higher elevations. These results suggest that invasion resistance is not conferred by extreme conditions at a particular site but determined by pathways of introduction of nonnative species. In the future, increased direct introduction of nonnative species with specialized ecophysiological adaptations to mountain environments could increase the risk of invasion. As well as providing a general explanation for gradients of nonnative species richness and the importance of traits such as phenotypic plasticity for many invasive species, the concept of directional ecological filtering is useful for understanding the initial assembly of some native floras at high elevations and latitudes.altitudinal gradient | dispersal | invasibility | nestedness | Rapoport effect S everal factors are known to shape species richness patterns along elevational gradients, notably energetic constraints on primary productivity and species-area relationships (1, 2). However, these factors are often highly correlated, making it difficult to assign causality, especially because species richness patterns are the result of both contemporary and historical ecological and evolutionary forces. High-elevation floras are typically composed of species with narrow climatic ranges and specialized ecophysiological adaptations to low temperatures, such as low stature, slow growth rates, and freezing resistance (3). Because richness gradients emerge from the overlap of individual species ranges, some authors have generated null models for richness patterns by assuming that species ranges are placed at random within a bounded elevational domain (4, 5). This usually produces a mid-domain effect, with richness peaking at mid-elevations where the overlap of species ranges is greatest. Indeed, such mid-elevation peaks do occur, and at least some of them can be explained by the overlap at ecotones of species adapted to different parts of the gradient (6).Although there is a long tradition of studies on elevational richness patterns of native species, little is known about similar phenomena in nonnative species. Nearly 1,000 nonnative plant species have been recorded from mountains throughout t...

Aim To investigate how species richness and similarity of non-native plants varies along gradients of elevation and human disturbance.Location Eight mountain regions on four continents and two oceanic islands. MethodsWe compared the distribution of non-native plant species along roads in eight mountainous regions. Within each region, abundance of plant species was recorded at 41-84 sites along elevational gradients using 100-m 2 plots located 0, 25 and 75 m from roadsides. We used mixed-effects models to examine how local variation in species richness and similarity were affected by processes at three scales: among regions (global), along elevational gradients (regional) and with distance from the road (local). We used model selection and information criteria to choose best-fit models of species richness along elevational gradients. We performed a hierarchical clustering of similarity to investigate human-related factors and environmental filtering as potential drivers at the global scale. ResultsSpecies richness and similarity of non-native plant species along elevational gradients were strongly influenced by factors operating at scales ranging from 100 m to 1000s of km. Non-native species richness was highest in the New World regions, reflecting the effects of colonization from Europe. Similarity among regions was low and due mainly to certain Eurasian species, mostly native to temperate Europe, occurring in all New World regions. Elevation and distance from the road explained little of the variation in similarity. The elevational distribution of non-native species richness varied, but was always greatest in the lower third of the range. In all regions, non-native species richness declined away from roadsides. In three regions, this decline was steeper at higher elevations, and there was an interaction between distance and elevation. Main conclusionsBecause non-native plant species are affected by processes operating at global, regional and local scales, a multi-scale perspective is needed to understand their patterns of distribution. The processes involved include global dispersal, filtering along elevational gradients and differential establishment with distance from roadsides.

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Current analyses and predictions of spatially explicit patterns and processes in ecology most often rely on climate data interpolated from standardized weather stations. This interpolated climate data represents long-term average thermal conditions at coarse spatial resolutions only. Hence, many climate-forcing factors that operate at fine spatiotemporal resolutions are overlooked. This is particularly important in relation to effects of observation height (e.g. vegetation, snow and soil characteristics) and in habitats varying in their exposure to radiation, moisture and wind (e.g. topography, radiative forcing or cold-air pooling). Since organisms living close to the ground relate more strongly to these microclimatic conditions than to free-air temperatures, microclimatic ground and near-surface data are needed to provide realistic forecasts of the fate of such organisms under anthropogenic climate change, as well as of the functioning of the ecosystems they live in. To fill this critical gap, we highlight a call for temperature time series submissions to SoilTemp, a geospatial database initiative compiling soil and near-surface temperature data from all over the world. Currently, this database contains time series from 7,538 temperature sensors from 51 countries

1Mountain ecosystems have been less adversely affected by invasions of non-native plants than 2 most other ecosystems, partially because most invasive plants in the lowlands are limited by 3 climate and cannot grow under harsher high-elevation conditions. However, with ongoing 4 climate change, invasive species may rapidly move upwards and threaten mid, and then high 5 elevation mountain ecosystems. We evaluated this threat by modeling the current and future 6 habitat suitability for 48 invasive plant species in Switzerland and New South Wales, Australia. 7Both regions had contrasting climate interactions with elevation, resulting in possible different 8 responses of species distributions to climate change. Using a species distribution modeling 9 approach that combines data from two spatial scales, we built high-resolution species distribution 10 models (≤ 250 m) that account for the global climatic niche of species and also finer variables 11 depicting local climate and disturbances. We found that different environmental drivers limit the 12 elevation range of invasive species in each of the two regions, leading to region-specific species 13 responses to climate change. The optimal suitability for plant invaders is predicted to markedly 14 shift from the lowland to the montane or subalpine zone in Switzerland, whereas the upward shift 15 is far less pronounced in New South Wales where montane and subalpine elevations are already 16 suitable. The results suggest that species most likely to invade high elevations in Switzerland 17 will be cold-tolerant, whereas species with an affinity to moist soils are most likely to invade 18 higher elevations in Australia. Other plant traits were only marginally associated with elevation 19limits. These results demonstrate that a more systematic consideration of future distributions of 20 invasive species is required in conservation plans of not yet invaded mountainous ecosystems. 21

Aim: We investigated patterns of species richness and community dissimilarity along elevation gradients using globally replicated, standardized surveys of vascular plants. We asked how these patterns of diversity are influenced by anthropogenic pressures (road construction and non-native species). Location: Global.Time period: 2008-2015.Major taxa studied: Vascular plants.Methods: Native and non-native vascular plant species were recorded in 943 plots along 25 elevation gradients, in nine mountain regions, on four continents. Sampling took place in plots along and away from roads. We analysed the effects of elevation and distance from road on species richness patterns and community dissimilarity (beta-diversity), and assessed how non-native species modified such elevational diversity patterns.Results: Globally, native and total species richness showed a unimodal relationship with elevation that peaked at lower-mid elevations, but these patterns were altered along roads and due to non-native species. Differences in elevational species richness patterns between regions Global Ecol Biogeogr. 2018;27:667-678.wileyonlinelibrary.com/journal/geb disappeared along roadsides, and non-native species changed the patterns' character in all study regions. Community dissimilarity was reduced along roadsides and through non-native species. We also found a significant elevational decay of beta-diversity, which however was not affected by roads or non-native species.Main conclusions: Idiosyncratic native species richness patterns in plots away from roads implicate region-specific mechanisms underlying these patterns. However, along roadsides a clearer elevational signal emerged and species richness mostly peaked at mid-elevations. We conclude that both roads and non-native species lead to a homogenization of species richness patterns and plant communities in mountains. K E Y W O R D S alien, altitude, beta-diversity, elevational decay, exotic, homogenization, hump-shaped pattern, roadsides, species replacement, species turnover

Roadways are increasingly recognized as common points of entry for non-native species into natural habitats in mountainous areas. Studies were conducted within the Greater Yellowstone Ecosystem from 2003 to 2007 to evaluate (1) landscape scale patterns of non-native plant richness along roadways, and (2) local scale factors influencing native and nonnative plant richness and cover, and surrogate nonnative plant (SNP) emergence in an invaded habitat. At the landscape scale, non-native plant richness decreased with increased elevation and increased distance from the road, and was positively correlated to the proportion of plots with signs of disturbance. Non-native plant richness also varied by habitat type: sagebrush steppe had the highest and alpine the lowest. At the local scale, in sagebrush steppe, SNP emergence was negatively associated with increased distance from the road, and percent cover of litter was positively associated with SNP emergence. The proportion of non-native plant cover and richness decreased, while the proportion of native cover and native species richness increased with distance from road. Our study suggests that landscape scale variables such as elevation and habitat type influence non-native plant success, and that while local conditions adjacent to the road may be favorable for non-native plants, factors which vary at the local scale can also effect non-native plant establishment away from the roadside. This highlights the need for studies to evaluate multiple scales when assessing patterns and processes driving non-native plant invasions, and suggests that sagebrush steppe may be resistant to invasion as long as it remains undisturbed.