Editing IPCC:AR6/WGII/Cross-Chapter-Paper-5 (section)

=== CCP5.2.1 Ecosystems and Ecosystem Services ===

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Changes in climate over short distances in mountains are reflected in large ecological gradients. AR5 reported new evidence that plant species of mid and low elevations were starting to colonise higher elevations in mountains. Since AR5, new studies have been published (e.g., [[#Steinbauer--2018|Steinbauer et al., 2018]] ; [[#Payne--2020|Payne et al., 2020]] ), including in some previously less well studied areas such as the Andes (e.g., [[#Morueta-Holme--2015|Morueta-Holme et al., 2015]] ; [[#Báez--2016|Báez et al., 2016]] ) and parts of Asia (e.g., [[#Telwala--2013|Telwala et al., 2013]] ; [[#Artemov--2018|Artemov, 2018]] ). There is now ''high confidence'' that many plant species’ distributions have shifted to higher elevations in recent decades, consistent with climatic warming (Sections 2.4.2, 10.4.2.1.1, 13.3.1.1). In recent years publications have also started to show similar trends in some animal species, including birds ( [[#Freeman--2018|Freeman et al., 2018]] ; [[#Bani--2019|Bani et al., 2019]] ; [[#Lehikoinen--2019|Lehikoinen et al., 2019]] ) and snails ( [[#Baur--2013|Baur and Baur, 2013]] ). Other climatic variables besides temperature can also affect elevational limits of species ( [[IPCC:Wg2:Chapter:Chapter-2#2.4.2|Section 2.4.2]] ) and sometimes in ways that contrast with temperature, for example increasing precipitation can allow some species to occur at lower elevations in dry climates ( [[#Crimmins--2011|Crimmins et al., 2011]] ; [[#Coals--2018|Coals et al., 2018]] ). [[#Tsai--2015|Tsai et al. (2015)]] reported large changes in the montane bird community in Taiwan, which they link to changes in weather patterns, including more severe typhoons. Changes in the amplitude and frequency of bank vole population waves in the Ilmen Nature Reserve in the Middle Urals can be linked to longer frost-free periods ( [[#Kiseleva--2020|Kiseleva, 2020]] ).

There are interactions with land use, for example a decrease in forest cover can exacerbate the effects of rising temperatures ( [[#Guo--2018|Guo et al., 2018]] ). In contrast, Bhatta et al. (2018) showed a downward shift of species assemblages in Langtang National Park, Nepal, most likely related to interactions with land use, especially reduced grazing. Where glaciers retreat, new areas become available for pioneer species to colonise and new communities to form ( [[#Cuesta--2019|Cuesta et al., 2019]] ; [[#Hock--2019|Hock et al., 2019]] ; [[#Muhlfeld--2020|Muhlfeld et al., 2020]] ). The risk of extreme events such as wildfire, drought, floods and landslips is increasing in a wide range of places as a result of climate change, and the evidence of the disturbance they cause to ecosystems has grown in recent decades (Section 2.3.1, Box CCP5.1). The impacts of such extreme events may be greater than those of incremental changes.

For species at lower elevations, mountains may represent refugia to which species can retreat. In this respect, Elsen et al. (2018) highlighted the importance of protecting areas along elevational gradients. This applies to freshwater and terrestrial habitats with mountain streams acting as potential refugia ( [[#Isaak--2016|Isaak et al., 2016]] ). In contrast, species restricted to the highest elevations are increasingly at risk, including from competition with colonising species ( [[#Britton--2016|Britton et al., 2016]] ; [[#Winkler--2016|Winkler et al., 2016]] ). Mountain-top species are often separated from potential new habitats by large areas with unsuitable climates, and tropical mountain species often have particularly narrow thermal tolerance and limited dispersal capacity ( [[#Polato--2018|Polato et al., 2018]] ).

The risks posed by non-native species may increase with climate change ( [[#Carboni--2018|Carboni et al., 2018]] ; [[#Shrestha--2018|Shrestha et al., 2018]] ; [[#Thapa--2018|Thapa et al., 2018]] ). [[#Koide--2017|Koide et al. (2017)]] found that non-native plant species on Hawaii were moving to higher elevations, whereas native species’ distributions were retracting at their lower elevational limit. [[#Dainese--2017|Dainese et al. (2017)]] found that non-native plant species spread to higher elevations approximately twice as fast as native species. Following recent climate warming, invasive ''Phyllostachys edulis'' and ''Phyllostachys bambusoides'' (Poaceae) bamboo species in Japan have shifted northwards and upslope in the last three decades ( [[#Takano--2017|Takano et al., 2017]] ). New evidence has shown that variations in microclimate, with topography and cold groundwater seeps, can provide micro-refugia small areas of locally suitable conditions where cold-adapted species can survive ( [[#Bramer--2018|Bramer et al., 2018]] ; [[#Muhlfeld--2020|Muhlfeld et al., 2020]] ) ( [[IPCC:Wg2:Chapter:Chapter-2#2.6.2|Section 2.6.2]] ). Some alpine species have thrived in recent years, and the range of microclimates may partly explain this ( [[#Rumpf--2018|Rumpf et al., 2018]] ).

Treeline elevation is linked to temperature ( [[#Paulsen--2014|Paulsen and Körner, 2014]] ) but may also be affected by water supply ( [[#Sigdel--2018|Sigdel et al., 2018]] ; [[#Lu--2021|Lu et al., 2021]] ) and land management. A recent summary of treeline shifts worldwide found that 67% of studied alpine treelines had shifted upwards while 33% remained stable (based on 142 published studies), and 88.8% of the 143 undisturbed alpine treelines across the Northern Hemisphere had shifted upwards ( [[#Hansson--2021|Hansson et al., 2021]] ; [[#Lu--2021|Lu et al., 2021]] ). Since AR5, new evidence of shifting treeline ecotones has emerged for a wide variety of species in different locations, including in Siberia ( [[#Pospelova--2017|Pospelova et al., 2017]] ), various parts of the Ural Mountains ( [[#Shiyatov--2015|Shiyatov and Mazepa, 2015]] ; [[#Zolotareva--2017|Zolotareva and Zolotarev, 2017]] ; [[#Sannikov--2018|Sannikov et al., 2018]] ), in the Canadian Rocky Mountains ( [[#Trant--2020|Trant et al., 2020]] ) and the Himalaya ( [[#Tiwari--2015|Tiwari and Joshi, 2015]] ; [[#Chakraborty--2016|Chakraborty et al., 2016]] ; [[#Gaire--2016|Gaire, 2016]] ; [[#Yadava--2017|Yadava et al., 2017]] ). Recent studies of treelines that have not or hardly shifted include those in the Himalaya ( [[#Singh--2015|Singh et al., 2015]] ; [[#Sigdel--2018|Sigdel et al., 2018]] ), eastern Tibetan Plateau ( [[#Wang--2020|Wang et al., 2020]] ) and the Andes ( [[#Lutz--2014|Lutz et al., 2014]] ). Migration rates are not proceeding as fast as warming rates, implying other processes also limit treeline ecotone response (e.g., [[#Sigdel--2020|Sigdel et al., 2020]] ; [[#Lu--2021|Lu et al., 2021]] ).

Whether treeline shifts occur, and if so at what rate, depends on a range of factors, including land use (especially livestock grazing and fire), species interactions, wildfires and climatic stress factors (wind, frost, drought, excess or shortage of snow) interacting with tree population processes (viable seed production, dispersal, seedling establishment, clonal propagation, growth, dieback, mortality). Differences in treeline shifts between north- and south-facing slopes have been demonstrated in the Rocky Mountains ( [[#Elliott--2015|Elliott and Cowell, 2015]] ). [[#Grigorieva--2018|Grigorieva and Moiseev (2018)]] showed that significant factors limiting the number of seedlings and shoots are the snow depth, the topsoil temperature dependent on it and the degree of competition from the parental tree stand and grass–shrub vegetation. In addition, land use and management exert an influence in many mountains around the world. [[#Suwal--2016|Suwal et al. (2016)]] found that elevational shifts in Himalayan silver fir in Nepal were larger when areas were protected from management. Similarly, [[#Lutz--2014|Lutz et al. (2014)]] found faster treeline shifts in the Peruvian Andes in protected areas than that in other areas, where cattle grazing and fires are more frequent. Treeline ecotones can also change independently of climate change if land use changes ( [[#Vitali--2019|Vitali et al., 2019]] ; [[#Körner--2020|Körner, 2020]] ).

Changes in community composition are also happening within ecosystem types. Duque et al. (2015) showed a change in the composition of northern Andean forests, and [[#Feeley--2013|Feeley et al. (2013)]] showed such a change in that of forests up to 2800 m in Costa Rica. In both cases the proportion of species adapted to warmer conditions increased, driven primarily by patterns of mortality, indicating that the changes in composition are mostly via range retractions, rather than range shifts or expansions. An analysis of 200 forest inventory plots in the Andes likewise indicated a widespread, though not ubiquitous, thermophilisation of tree species’ composition ( [[#Fadrique--2018|Fadrique et al., 2018]] ). Within a period of 8 years (2003–2010), significant shifts in communities of vascular plants, butterflies and birds were found in Switzerland ( [[#Roth--2014|Roth et al., 2014]] ). At lower elevations, communities of all species groups changed towards warm-dwelling species, corresponding to an average uphill shift of 8 m, 38 m and 42 m in plant, butterfly and bird communities respectively. However, rates of community change decreased with elevation in plants and butterflies, while bird communities shifted towards warm-dwelling species at all elevations ( [[#Roth--2014|Roth et al., 2014]] ).

Changes in mountain biodiversity and ecosystems have a wide range of impacts on ecosystem services and effects on people. Some mountain ecosystems, particularly those with peatlands or forests, are important carbon stores, and climate change presents a risk to these in some locations ( [[#Dwire--2018|Dwire et al., 2018]] ) (Sections 2.4.3.8, 2.4.4.4 and 2.4.4.5). [[#Palomo--2017|Palomo (2017)]] identified a wide range of threats to the lives, livelihoods and culture of mountain people as a consequence of the impacts of climate change on ecosystems. However, impacts are very heterogeneous between locations, even within the same region and ecosystem type (e.g., mountain forests in Europe) ( [[#Mina--2017|Mina et al. (2017)]] and are not necessarily all negative. In addition to changes in services, other impacts on humans from a changing climate may be mediated through species and ecosystems, for example changes in vector distribution shifting disease incidence into higher elevation areas ( [[#Escobar--2016|Escobar et al., 2016]] ).

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