Aim: Climate change is expected to impact mountain biodiversity by shifting species ranges and the biomes they shape. The extent and regional variation in these impacts are still poorly understood, particularly in the highly biodiverse Andes. Regional syntheses of climate change impacts on vegetation are pivotal to identify and guide research priorities. Here we review current data, knowledge and uncertainties in past, present and future climate change impacts on vegetation in the Andes.Location: Andes.Taxon: Plants.Methods: We (i) conducted a literature review on Andean vegetation responses to past and contemporary climatic change, (ii) analysed future climate projections for different elevations and slope orientations at 19 Andean locations using an ensemble of model outputs from the Coupled Model Intercomparison Project 5, and (iii) calculated changes in the suitable climate envelope area of Andean biomes and compared these results to studies that used species distribution models.Results: Future climatic changes (2040–2070) are projected to be stronger at high‐elevation areas in the tropical Andes (up to 4°C under RCP 8.5), while in the temperate Andes temperature increases are projected to be up to 2°C. Under this worst‐case scenario, temperate deciduous forests and the grasslands/steppes from the Central and Southern Andes are predicted to show the greatest losses of suitable climatic space (30% and 17%–23%, respectively). The high vulnerability of these biomes contrasts with the low attention from researchers modelling Andean species distributions. Critical knowledge gaps include a lack of an Andean wide plant checklist, insufficient density of weather stations at high‐elevation areas, a lack of high‐resolution climatologies that accommodates the Andes' complex topography and climatic processes, insufficient data to model demographic and ecological processes, and low use of palaeo data for distribution modelling.Main conclusions: Climate change is likely to profoundly affect the extent and composition of Andean biomes. Temperate Andean biomes in particular are susceptible to substantial area contractions. There are, however, considerable challenges and uncertainties in modelling species and biome responses and a pressing need for a region‐wide approach to address knowledge gaps and improve understanding and monitoring of climate change impacts in these globally important biomes.

It is largely unknown how South America’s Andean forests affect the global carbon cycle, and thus regulate climate change. Here, we measure aboveground carbon dynamics over the past two decades in 119 monitoring plots spanning a range of >3000 m elevation across the subtropical and tropical Andes. Our results show that Andean forests act as strong sinks for aboveground carbon (0.67 ± 0.08 Mg C ha⁻¹y⁻¹) and have a high potential to serve as future carbon refuges. Aboveground carbon dynamics of Andean forests are driven by abiotic and biotic factors, such as climate and size-dependent mortality of trees. The increasing aboveground carbon stocks offset the estimated C emissions due to deforestation between 2003 and 2014, resulting in a net total uptake of 0.027 Pg C y⁻¹. Reducing deforestation will increase Andean aboveground carbon stocks, facilitate upward species migrations, and allow for recovery of biomass losses due to climate change.

As global climate change leads to warmer and dryer conditions in the central Andes, alpine plant communities are forced to upward displacements following their climatic niche. Species range shifts are predicted to have major impacts on alpine communities by reshuffling species composition and abundances. Using a standardized protocol, we surveyed alpine plant communities in permanent plots on four high Andean summits in NW Argentina, which range from 4,040 to 4,740 m a.s.l. After a baseline survey in 2006–2008, we resurvey the same plots in 2012, and again in 2017. We found a significant decrease in plant cover, species richness, and diversity across the elevation gradient in the three censuses and a strong decrease in soil temperature along the elevation gradient. We found a high plant community turnover (37%–49%) among censuses, differentiating according to summits and aspects; major changes of community turnover were observed in the lowest summit (49%) and on the northern (47%) and western (46%) aspects. Temporal patterns in community changes were represented by increases in plant cover in the highest summit, in species richness in the lower summit, and in diversity (Shannon index) in the four summits, over time, together with increase in small herbs and non‐tussock grasses. We suggest that the observed trend in plant community dynamics responds to short‐term temperature and precipitation variability, which is influenced by El Niño Southern Oscillation (ENSO), and due to time lags in plant community response, it may take much longer than one decade for the observed trends to become stables and statistically significant. Our study provides an important foundation for documenting more profound changes in these subtropical alpine plant communities as global climate change continues.

Aim: Climate change is transforming mountain summit plant communities worldwide, but we know little about such changes in the High Andes. Understanding large‐scale patterns of vegetation changes across the Andes, and the factors driving these changes, is fundamental to predicting the effects of global warming. We assessed trends in vegetation cover, species richness (SR) and community‐level thermal niches (CTN) and tested whether they are explained by summits' climatic conditions and soil temperature trends.Location: High Andes.Time period: Between 2011/2012 and 2017/2019.Major taxa studied: Vascular plants.Methods: Using permanent vegetation plots placed on 45 mountain summits and soil temperature loggers situated along a ~6800 km N‐S gradient, we measured species and their relative percentage cover and estimated CTN in two surveys (intervals between 5 and 8 years). We then estimated the annual rate of changes for the three variables and used generalized linear models to assess their relationship with annual precipitation, the minimum air temperatures of each summit and rates of change in the locally recorded soil temperatures.Results: Over time, there was an average loss of vegetation cover (mean = −0.26%/yr), and a gain in SR across summits (mean = 0.38 species m²/yr), but most summits had significant increases in SR and vegetation cover. Changes in SR were positively related to minimum air temperature and soil temperature rate of change. Most plant communities experienced shifts in their composition by including greater abundances of species with broader thermal niches and higher optima. However, the measured changes in soil temperature did not explain the observed changes in CTN.Main conclusions: High Andean vegetation is changing in cover and SR and is shifting towards species with wider thermal niche breadths. The weak relationship with soil temperature trends could have resulted from the short study period that only marginally captures changes in vegetation through time.

Dispersal is a key ecological process that influences plant community assembly. Therefore, understanding whether dispersal strategies are associated with climate is of utmost importance, particularly in areas greatly exposed to climate change. We examined alpine plant communities located in the mountain summits of the tropical Andes across a 4,000‐km latitudinal gradient. We investigated species dispersal strategies and tested their association with climatic conditions and their evolutionary history. We used dispersal‐related traits (dispersal mode and growth form) to characterize dispersal strategies for 486 species recorded on 49 mountain summits. Then we analysed the phylogenetic signal of traits and investigated the association between dispersal traits, phylogeny, climate and space using structural equation modelling and fourth‐corner analysis together with RLQ ordination. A median of 36% species in the communities was anemochorous (wind‐dispersed) and herbaceous. This dispersal strategy was followed by the barochory‐herb combination (herbaceous with unspecialized seeds, dispersed by gravity) with a median of 26.3% species in the communities. The latter strategy was common among species with distributions restricted to alpine environments. While trait states were phylogenetically conserved, they were significantly associated with a temperature gradient. Low minimum air temperatures, found at higher latitudes/elevations, were correlated with the prevalence of barochory and the herb growth form, traits that are common among Caryophyllales, Brassicaceae and Poaceae. Milder temperatures, found at lower latitudes/elevations, were associated with endozoochorous, shrub species mostly from the Ericaceae family. Anemochorous species were found all along the temperature gradient, possibly due to the success of anemochorous Compositae species in alpine regions. We also found that trait state dominance was more associated with the climatic conditions of the summit than with community phylogenetic structure. Although the evolutionary history of the tropical Andean flora has also shaped dispersal strategies, our results suggest that the environment had a more predominant role. Synthesis. We showed that dispersal‐related traits are strongly associated with a gradient of minimum air temperatures in the Andes. Global warming may weaken this key filter at tropical alpine summits, potentially altering community dispersal strategies in this region and thus, plant community structure and composition.