Background and Rationale
Climate Change and Shrub Dominance
Although climate change is a well-known and well-documented global phenomenon, the Canadian territories have experienced greater warming than more southern parts of the country and are expected to continue warming at a higher rate (Bush and Lemmen (editors), 2019). This warming in mountainous regions is causing montane and high-elevation species to shifting their habitat ranges farther up mountains to occupy previously inhospitable areas (Figure 1) (Chen et al., 2011; Freeman et al., 2018), and many studies show that shrubs are among the taxa becoming more dominant at higher elevations (Danby et al., 2010; Myers-Smith et al., 2011; Rundqvist et al., 2011; Formica et al., 2014; Weijers et al., 2018).
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Figure 1: Pictogram depicting the upslope movement of montane (brown) and subalpine (green) species.
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Shrubs are defined as woody-stemmed vegetation less than 10 m tall, and are common features on mountains. In the subalpine, shrubs predominantly grow to medium (1-2 m) and tall (2-10 m) heights (Figure 2). However, a defining feature of alpine tundra habitats at the tops of mountains is the lack of tall vegetation, and in these habitats shrubs typically occur as low (0.5-1 m) or very low/dwarf varieties (0.15-0.5 m) (Figure 3) (Flynn and Francis, 2016). In response to warming temperatures, subalpine habitats are becoming increasingly dominated by shrubs, and alpine habitats are contracting as shrubs grow taller and farther up slopes (Barrado et al., 2020). For example, percent cover of willows in alpine areas of the Rocky Mountains in Colorado increased exponentially from 1946-2008 through both clonal expansion and seed dispersal (Formica et al., 2014), and similar cover expansion of birch species was observed in the Scandes from 1976-2010 (Rundqvist et al., 2011). Southwestern Yukon mountain habitats increased in shrub density, richness, diversity, stem size (Danby et al., 2010), and cover (Myers-Smith and Hik, 2017), changes that are associated with warming climate.
This upslope march of shrubs is causing the herbaceous plant and lichen dominated alpine habitat to contract and shrink in size. Alpine tundra habitats in Europe are predicted to decrease by up to 48% with only a 1.5°C average temperature increase (Barrado et al., 2020), and arctic regions could experience a 52% increase in woody vegetation by 2050 (Pearson et al., 2006). Rapid shrub expansion is expected to continue and increase as the climate warms (Breshears, 2008; Myers-Smith and Hik, 2017; Post et al., 2009), and alpine specialists are faced with habitat loss and species extirpation as this contraction forces them to seek cooler temperatures and appropriate habitats (Chen et al., 2011; Freeman et al., 2018). |
Figure 2: Example of subalpine habitats dominated by medium and tall shrubs.
Figure 3: Example of alpine habitat dominated by low and dwarf vegetation. Pictured: C. Lisa Mahon and Lena Ware.
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High-elevation Bird Vulnerability
Many bird species live, breed, and feed in shrub habitats (e.g. Fox Sparrow (Passerella ilica), Golden-crowned Sparrow (Zonotrichia atricapilla), Gray-cheeked Thrush (Catharus minimus), and American Tree Sparrow (Spizelloides arborea)), using the shrubs for nest concealment, cover from predators, and food from seeds, buds, and insects both in the shrubs themselves and leaf litter (Weckstein et al., 2020) (Figure 4). These birds may be affected differently by increasing shrub dominance. For example, Fox Sparrows may be associated with greater shrub height and density due to an increase in leaf litter where these birds primarily forage for insects, or an increase in nest site availability (Weckstein et al., 2020). However, alpine specialists and breeders such as Ptarmigan (Lagopus spp.) and American Pipits (Anthus rubescens) that nest at ground level and feed on low growing flowers, buds, and berries (Montgomerie and Holder, 2020) may face habitat loss as shrubs progress to higher elevations and would be negatively associated with elements of shrub dominance (Figure 5). Understanding how different bird species may be affected by shrubification can offer information about species of conservation concern as the climate continues to warm and shrubs expand farther up mountains.
Research Objectives
The overall objective of this research is to assess the vulnerability or security of high-elevation birds to shrubification and habitat loss due to increasing shrub dominance on mountains. I will address this objective though these specific objectives:
- Assess the relationships between subalpine and alpine bird species with shrub dominance (as measured by shrub height, density, and stem count) and shrub species.
- Identify high-elevation species or species groups are most at risk (i.e. losers) or most likely to benefit (i.e. winners) from shrubification in a changing climate.
References
Barredo JI, Mauri A, Caudullo G. 2020. Alpine tundra contraction under future warming scenarios in Europe. Atmosphere 11:689, DOI: 10.3390/atmos11070698
Breshears DD, Huxman TE, Adams HD, Zou CB, Davison JE. 2008. Vegetation synchronously leans upslope as climate warms. PNAS 105(33):11591-11592.
Bush, E. and Lemmen, D.S., editors (2019): Canada’s Changing Climate Report; Government of Canada, Ottawa, ON. 444 p.
Chen I, Hill JK, Roy DB, Thomas CD. 2011. Rapid range shifts of species associated with high levels of climate warming. Science. 333(6045), 1024-1026. DOI: 10.1126/science.1206432
Danby RK, Koh S, Hik DS, Price LW. 2011. Four decades of plant community change in the alpine tundra of southwest Yukon, Canada. AMBIO 40:660-671, DOI: 10.1007/s13280-011-0172-2
Flynn, N. and Francis. S., editors. 2016. Yukon Ecological and Landscape Classification and Mapping Guidelines. Version 1.0. Whitehorse (YT): Environment Yukon, Department of Environment, Government of Yukon.
Formica A, Farrer EC, Ashton IW, Suding KN. 2014. Shrub expansion over the past 62 years in Rocky Mountain alpine tundra: Possible causes and consequences. Arctic, Antarctic, and Alpine Research 46:3, 616-631, DOI: 10.1657/1938-4246-46.3.616
Freeman BG, Lee-Yaw JA, Sunday JM, Hargreaves AL. 2018. Expanding, shifting, and shrinking: The impact of global warming on species’ elevational distributions. Global Ecology and Biogeography. 27:1268–1276. DOI: 10.1111/geb.12774.
Montgomerie R and Holder K. 2020. Rock Ptarmigan (Lagopus muta), version 1.0. In Birds of the World (S. M. Billerman, B. K. Keeney, P. G. Rodewald, and T. S. Schulenberg, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.rocpta1.01
Myers-Smith IH and Hik DS. 2017. Climate warming as a driver of tundra shrubline advance. Journal of Ecology 106(2):547-560, DOI: 10.1111/1365-2745.12817
Nogués-Bravo D, Araújo MB, Errea MP and Martínez-Rica JP. 2007. Exposure of global mountain systems to climate warming during the 21st century. Global Environmental Change 17:420-428.
Pearson RG, Philips SJ, Loranty MM, Beck PSA, Damoulas T, Knight SJ, Goetz SJ. 2013. Shifts in Arctic vegetation and associated feedbacks under climate change. Nature Climate Change. 3:673-677. DOI:10.1038/NCLIMATE1858
Post E, Forchhammer MC, Syndonia Bret-Harte M, Callaghan TV, Christensen TR, Elberling B, Fox AD, Gilg O, Hik DS, Hoye TT, Ims RA, Jeppesen E, Klein DR, Madsen J, McGuire AD, Rysgaard S, Schindler DE, Stirling I, Tamstorf MP, Tyler NCJ, van der Wal R, Welker J, Wookey PA, Martin Schmidt NM and Aastrup P. 2009. Science 325:1355-1258.
Plummer DA, Caya D, Frigon A, Côté H, Giguère M, Paquin D, Biner S, Harvey R, de Elia R. 2006. Climate and climate change over North America as simulated by the Canadian RCM. Journal of Climate. 19:3112-3132. DOI:10.1175/JCLI3769.1
Rundqvist S, Hedenås H, Sandtröm A, Emanuelsson U, Eriksson H, Jonasson C, Callaghan TV. 2011. Tree and shrub expansion over the past 34 years at the tree-line near Abisko, Sweden. AMBIO 40:683-692, DOI: 10.1007/s13280-011-0174-0
Wang Q, Fan A, and Wang M. 2015. Evidence of high-elevation amplification versus Arctic amplification. Scientific Reports 6:19219.
Weckstein JD, Kroodsma DE, and Faucett RC. 2020. Fox Sparrow (Passerella iliaca), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.foxspa.01
Weijers S, Pape R, Löffler J and Myers-Smith IH. 2018. Contrasting shrub species respond to early summer temperatures leading to correspondence of shrub growth. Environmental Research Letters 13:034005, DOI: https://doi.org/10.1088/1748-9326/aaa5b8.
Breshears DD, Huxman TE, Adams HD, Zou CB, Davison JE. 2008. Vegetation synchronously leans upslope as climate warms. PNAS 105(33):11591-11592.
Bush, E. and Lemmen, D.S., editors (2019): Canada’s Changing Climate Report; Government of Canada, Ottawa, ON. 444 p.
Chen I, Hill JK, Roy DB, Thomas CD. 2011. Rapid range shifts of species associated with high levels of climate warming. Science. 333(6045), 1024-1026. DOI: 10.1126/science.1206432
Danby RK, Koh S, Hik DS, Price LW. 2011. Four decades of plant community change in the alpine tundra of southwest Yukon, Canada. AMBIO 40:660-671, DOI: 10.1007/s13280-011-0172-2
Flynn, N. and Francis. S., editors. 2016. Yukon Ecological and Landscape Classification and Mapping Guidelines. Version 1.0. Whitehorse (YT): Environment Yukon, Department of Environment, Government of Yukon.
Formica A, Farrer EC, Ashton IW, Suding KN. 2014. Shrub expansion over the past 62 years in Rocky Mountain alpine tundra: Possible causes and consequences. Arctic, Antarctic, and Alpine Research 46:3, 616-631, DOI: 10.1657/1938-4246-46.3.616
Freeman BG, Lee-Yaw JA, Sunday JM, Hargreaves AL. 2018. Expanding, shifting, and shrinking: The impact of global warming on species’ elevational distributions. Global Ecology and Biogeography. 27:1268–1276. DOI: 10.1111/geb.12774.
Montgomerie R and Holder K. 2020. Rock Ptarmigan (Lagopus muta), version 1.0. In Birds of the World (S. M. Billerman, B. K. Keeney, P. G. Rodewald, and T. S. Schulenberg, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.rocpta1.01
Myers-Smith IH and Hik DS. 2017. Climate warming as a driver of tundra shrubline advance. Journal of Ecology 106(2):547-560, DOI: 10.1111/1365-2745.12817
Nogués-Bravo D, Araújo MB, Errea MP and Martínez-Rica JP. 2007. Exposure of global mountain systems to climate warming during the 21st century. Global Environmental Change 17:420-428.
Pearson RG, Philips SJ, Loranty MM, Beck PSA, Damoulas T, Knight SJ, Goetz SJ. 2013. Shifts in Arctic vegetation and associated feedbacks under climate change. Nature Climate Change. 3:673-677. DOI:10.1038/NCLIMATE1858
Post E, Forchhammer MC, Syndonia Bret-Harte M, Callaghan TV, Christensen TR, Elberling B, Fox AD, Gilg O, Hik DS, Hoye TT, Ims RA, Jeppesen E, Klein DR, Madsen J, McGuire AD, Rysgaard S, Schindler DE, Stirling I, Tamstorf MP, Tyler NCJ, van der Wal R, Welker J, Wookey PA, Martin Schmidt NM and Aastrup P. 2009. Science 325:1355-1258.
Plummer DA, Caya D, Frigon A, Côté H, Giguère M, Paquin D, Biner S, Harvey R, de Elia R. 2006. Climate and climate change over North America as simulated by the Canadian RCM. Journal of Climate. 19:3112-3132. DOI:10.1175/JCLI3769.1
Rundqvist S, Hedenås H, Sandtröm A, Emanuelsson U, Eriksson H, Jonasson C, Callaghan TV. 2011. Tree and shrub expansion over the past 34 years at the tree-line near Abisko, Sweden. AMBIO 40:683-692, DOI: 10.1007/s13280-011-0174-0
Wang Q, Fan A, and Wang M. 2015. Evidence of high-elevation amplification versus Arctic amplification. Scientific Reports 6:19219.
Weckstein JD, Kroodsma DE, and Faucett RC. 2020. Fox Sparrow (Passerella iliaca), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.foxspa.01
Weijers S, Pape R, Löffler J and Myers-Smith IH. 2018. Contrasting shrub species respond to early summer temperatures leading to correspondence of shrub growth. Environmental Research Letters 13:034005, DOI: https://doi.org/10.1088/1748-9326/aaa5b8.