29. McKnight, EA; Swanson, H; Brahney, J; Hik, DS. (2021) The physical and chemical limnology of Yukon’s largest lake, Lhù’ààn Mân’ (Kluane Lake), prior to the 2016 ‘A’ą̈y Chù’ diversion.Arctic ScienceThe physical and chemical limnology of Yukon’s largest lake, Lhù’ààn Mân’ (Kluane Lake), prior to the 2016 ‘A’ą̈y Chù’ diversion
Limnology, Yukon Climate Change, Lhu'aan Man' (Kluane Lake); 'A'ay Chu' (Slims River); physical limnology; climate change; large northern lakes
Despite increasing evidence that large northern lakes are rapidly changing due to climate change, descriptive baseline studies of their physicochemical properties are largely lacking, limiting our ability to detect or predict change. This study represents a comprehensive scientific assessment of the limnology of Yukon’s largest lake: Lhù’ààn Mân’ (Kluane Lake), an important waterbody for local and First Nation communities, and key habitat for trout and salmon. Water sample and instrument data generated throughout 2015 describe distinct regions within the lake and their respective seasonal variability. A deep, glacially-influenced southern basin was characterized by cold, turbid, poorly stratified, unproductive, and nutrient-poor conditions; a shallower northwestern region (Tthe Kaala Daagur (Brooks/Little Arm)) was warmer, fully mixed, and more productive; a northeast region (’Ùha K’ènji (Talbot/Big Arm)) was clear and stratified with intermediate depth, temperature, productivity, and nutrient concentrations; and a central region had intermediate physicochemical conditions relative to the other three. This variability demonstrates the need for adequate spatial (within lake) and temporal (between seasons) monitoring of large northern lakes. In 2016, glacier recession within the watershed resulted in diversion of the lake’s primary inflow (‘A’ą̈y Chù’ (Slims River)). Our results, when used together with Indigenous knowledge, form a historical reference that enables assessments of the potential ecological consequences to Lhù’ààn Mân’.Link DOI
28. Brachmann, CG; Hernandez-Ramirez, G; Hik, DS. (2020) CH4 uptake along a successional gradient in temperate alpine soils.Biogeochemistry 147: 109-123 CH4 uptake along a successional gradient in temperate alpine soils
Columbia mountains; Methane; Soil fluxes; Alpine; Deglaciated terrain, treelike
The effects of climate change appear to be amplified in mountains compared with lowland areas, with rapid changes in plant community composition, soil properties, and increased substrate for biological development following retreat of glaciers. Associated soil gaseous fluxes in alpine ecosystems contribute to the global balance of greenhouse gases, but methane and carbon dioxide soil fluxes and their controls are not well known. We used a dynamic closed-chamber method to measure methane and carbon dioxide fluxes along a successional gradient during the peak growing season in the North Selkirk Mountains, British Columbia, Canada. Soil physico-chemical properties, vegetation cover, and topographic variables were quantified to determine mechanisms influencing these fluxes. Mean methane uptake ranged from - 155 mu g CH4-C m(- 2) h(-1) in well vegetated sites to zero in recently deglaciated terrain. Soil total carbon (TC) and water content were the primary drivers of methane uptake. Sites with TC greater than 4% and moisture below 0.22 water fraction by volume (w.f.v) corresponded to the strongest methane sinks. Increased vegetation cover and relatively drier soil conditions, anticipated with future climate change, suggest that methane uptake may increase in these alpine ecosystems. DOI
27. Williamson, SN; Zdanowicz, C; Anslow, FS; Clarke, GKC; Copland, L; Danby, RK; Flowers, GE; Holdsworth, G; Jarosch, AH; Hik, DS. (2020) Evidence for Elevation-Dependent Warming in the St. Elias Mountains, Yukon, Canada.J. Clim. 33: 3253-3269 Evidence for Elevation-Dependent Warming in the St. Elias Mountains, Yukon, Canada
The climate of high midlatitude mountains appears to be warming faster than the global average, but evidence for such elevation-dependent warming (EDW) at higher latitudes is presently scarce. Here, we use a comprehensive network of remote meteorological stations, proximal radiosonde measurements, downscaled temperature reanalysis, ice cores, and climate indices to investigate the manifestation and possible drivers of EDW in the St. Elias Mountains in subarctic Yukon, Canada. Linear trend analysis of comprehensively validated annual downscaled North American Regional Reanalysis (NARR) gridded surface air temperatures for the years 1979-2016 indicates a warming rate of 0.028 degrees C a(-1) between 5500 and 6000 m above mean sea level (MSL), which is similar to 1.6 times larger than the global-average warming rate between 1970 and 2015. The warming rate between 5500 and 6000 m MSL was similar to 1.5 times greater than the rate at the 2000-2500 m MSL bin (0.019 degrees C a(-1)), which is similar to the majority of warming rates estimated worldwide over similar elevation gradients. Accelerated warming since 1979, measured by radiosondes, indicates a maximum rate at 400 hPa (similar to 7010 m MSL). EDW in the St. Elias region therefore appears to be driven by recent warming of the free troposphere. MODIS satellite data show no evidence for an enhanced snow albedo feedback above 2500 m MSL, and declining trends in sulfate aerosols deposited in high-elevation ice cores suggest a modest increase in radiative forcing at these elevations. In contrast, increasing trends in water vapor mixing ratio at the 500-hPa level measured by radiosonde suggest that a longwave radiation vapor feedback is contributing to EDW. DOI
25. Tiusanen, M; Huotari, T; Hebert, PDN; Andersson, T; Asmus, A; Bety, J; Davis, E; Gale, J; Hardwick, B; Hik, D; Korner, C; Lanctot, RB; Loonen, MJJE; Partanen, R; Reischke, K; Saalfeld, ST; Senez-Gagnon, F; Smith, PA; Sulavik, J; Syvanpera, I; Urbanowicz, C; Williams, S; Woodard, P; Zaika, Y; Roslin, T. (2019) Flower-visitor communities of an arcto-alpine plant-Global patterns in species richness, phylogenetic diversity and ecological functioning.Mol. Ecol. 28 Flower-visitor communities of an arcto-alpine plant-Global patterns in species richness, phylogenetic diversity and ecological functioning
arctic ecology; DNA barcoding; Dryas; ecosystem functioning; flower visitor; pollination
Pollination is an ecosystem function of global importance. Yet, who visits the flower of specific plants, how the composition of these visitors varies in space and time and how such variation translates into pollination services are hard to establish. The use of DNA barcodes allows us to address ecological patterns involving thousands of taxa that are difficult to identify. To clarify the regional variation in the visitor community of a widespread flower resource, we compared the composition of the arthropod community visiting species in the genus Dryas (mountain avens, family Rosaceae), throughout Arctic and high-alpine areas. At each of 15 sites, we sampled Dryas visitors with 100 sticky flower mimics and identified specimens to Barcode Index Numbers (BINs) using a partial sequence of the mitochondrial COI gene. As a measure of ecosystem functioning, we quantified variation in the seed set of Dryas. To test for an association between phylogenetic and functional diversity, we characterized the structure of local visitor communities with both taxonomic and phylogenetic descriptors. In total, we detected 1,360 different BINs, dominated by Diptera and Hymenoptera. The richness of visitors at each site appeared to be driven by local temperature and precipitation. Phylogeographic structure seemed reflective of geological history and mirrored trans-Arctic patterns detected in plants. Seed set success varied widely among sites, with little variation attributable to pollinator species richness. This pattern suggests idiosyncratic associations, with function dominated by few and potentially different taxa at each site. Taken together, our findings illustrate the role of post-glacial history in the assembly of flower-visitor communities in the Arctic and offer insights for understanding how diversity translates into ecosystem functioning. DOI PubMed
24. Barrio, IC; Hik, DS; Thorsson, J; Svavarsdottir, K; Marteinsdottir, B; Jonsdottir, IS. (2018) The sheep in wolf's clothing? Recognizing threats for land degradation in Iceland using state-and-transition models.Land Degrad. Dev. 29: 1714-1725 The sheep in wolf's clothing? Recognizing threats for land degradation in Iceland using state-and-transition models
adaptive monitoring; land management; rangeland; sheep grazing
Land degradation and extensive soil erosion are serious environmental concerns in Iceland. Natural processes associated with a harsh climate and frequent volcanic activity have shaped Icelandic landscapes. However, following human settlement and the introduction of livestock in the ninth century, the extent of soil erosion rapidly escalated. Despite increased restoration and afforestation efforts and a considerable reduction in sheep numbers during the late 20th century, many Icelandic rangelands remain in poor condition. A deeper understanding of the ecology of these dynamic landscapes is needed, and state-and-transition models (STMs) can provide a useful conceptual framework. STMs have been developed for ecosystems worldwide to guide research, monitoring, and management but have been used at relatively small spatial scales and have not been extensively applied to high-latitude rangelands. Integrating the best available knowledge, we develop STMs for rangelands in Iceland, where sheep grazing is often regarded as a main driver of degradation. We use STMs at a countrywide scale for 3 time periods with different historical human influence, from presettlement to present days. We also apply our general STM to a case-study in the central highlands of Iceland to illustrate the potential application of these models at scales relevant to management. Our STMs identify the set of possible states, transitions and thresholds in these ecosystems, and their changes over time and suggest increasing complexity in recent times. This approach can help identify important knowledge gaps and inform management efforts and monitoring programmes, by identifying realistic and achievable conservation and restoration goals. DOI
23. Bjorkman, AD; Myers-Smith, IH; Elmendorf, SC; Normand, S; Ruger, N; Beck, PSA; Blach-Overgaard, A; Blok, D; Cornelissen, JHC; Forbes, BC; Georges, D; Goetz, SJ; Guay, KC; Henry, GHR; HilleRisLambers, J; Hollister, RD; Karger, DN; Kattge, J; Manning, P; Prevey, JS; Rixen, C; Schaepman-Strub, G; Thomas, HJD; Vellend, M; Wilmking, M; Wipf, S; Carbognani, M; Hermanutz, L; Levesque, E; Molau, U; Petraglia, A; Soudzilovskaia, NA; Spasojevic, MJ; Tomaselli, M; Vowles, T; Alatalo, JM; Alexander, HD; Anadon-Rosell, A; Angers-Blondin, S; te Beest, M; Berner, L; Bjork, RG; Buchwal, A; Buras, A; Christie, K; Cooper, EJ; Dullinger, S; Elberling, B; Eskelinen, A; Frei, ER; Grau, O; Grogan, P; Hallinger, M; Harper, KA; Heijmans, MMPD; Hudson, J; Hulber, K; Iturrate-Garcia, M; Iversen, CM; Jaroszynska, F; Johnstone, JF; Jorgensen, RH; Kaarlejarvi, E; Klady, R; Kuleza, S; Kulonen, A; Lamarque, LJ; Lantz, T; Little, CJ; Speed, JDM; Michelsen, A; Milbau, A; Nabe-Nielsen, J; Nielsen, SS; Ninot, JM; Oberbauer, SF; Olofsson, J; Onipchenko, VG; Rumpf, SB; Semenchuk, P; Shetti, R; Collier, LS; Street, LE; Suding, KN; Tape, KD; Trant, A; Treier, UA; Tremblay, JP; Tremblay, M; Venn, S; Weijers, S; Zamin, T; Boulanger-Lapointe, N; Gould, WA; Hik, DS; Hofgaard, A; Jonsdottir, IS; Jorgenson, J; Klein, J; Magnusson, B; Tweedie, C; Wookey, PA; Bahn, M; Blonder, B; van Bodegom, PM; Bond-Lamberty, B; Campetella, G; Cerabolini, BEL; Chapin, FS; Cornwell, WK; Craine, J; Dainese, M; de Vries, FT; Diaz, S; Enquist, BJ; Green, W; Milla, R; Niinemets, U; Onoda, Y; Ordonez, JC; Ozinga, WA; Penuelas, J; Poorter, H; Poschlod, P; Reich, PB; Sande, B; Schamp, B; Sheremetev, S; Weiher, E. (2018) Plant functional trait change across a warming tundra biome.Nature 562 Plant functional trait change across a warming tundra biome
The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature-trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming. DOI PubMed
22. Myers-Smith, IH; Hik, DS. (2018) Climate warming as a driver of tundra shrubline advance.J. Ecol. 106: 547-560 Climate warming as a driver of tundra shrubline advance
age distributions; alpine; climate; high-latitude; range expansion; recruitment; shrub; tundra; willow (Salix); Yukon
1. Climate warming is predicted to alter ecological boundaries in high-latitude ecosystems including the elevational or latitudinal extent of tall shrubs in Arctic and alpine tundra. Over 60 studies from 128 locations around the tundra biome have investigated shrub expansion in tundra ecosystems; however, only six studies test whether shrublines are actually advancing up hill-slopes or northward into tundra where tall shrubs are currently absent. 2. We test the hypothesis that willow shrublines have expanded to higher elevations in relation to climate across a 50 x 50 km area in the Kluane Region of the southwest Yukon Territory, Canada by surveying of 379 shrubs at 14 sites and sampling of 297 of the surveyed shrubs at 10 sites. We compared growth and recruitment to climate variables to test the climate sensitivity of shrub increase using annual radial growth analysis, age distributions and repeat field surveys to estimate the current rate of shrubline advance. 3. We found consistent and increasing rates of recruitment of alpine willows, with estimates of faster advancing shrublines on shallower hill-slopes. Mortality was extremely low across the elevation gradient. Aspect, elevation and species identity did not explain variation in recruitment patterns, suggesting a regional factor, such as climate, as the driver of the observed shrubline advance. 4. Annual radial growth of willows was best explained by variation in summer temperatures, and recruitment pulses by winter temperatures. Measured recruitment rates are similar to 20 +/- 5 individuals per hectare per decade (M +/- SE) and measured rates of increased shrub cover of similar to 5 +/- 1% per decade (M +/- SE) measured at the Pika Camp site between field surveys in 2009 and 2013. Our results suggest that shrubline will continue to advance over the next 50 years, if growing conditions remain suitable. However, if future conditions differ between summer and winter seasons, this could lead to contrasting trajectories for recruitment vs. growth, and influence the vegetation change observed on the landscape. 5. Synthesis. Our findings in the context of a review of the existing literature indicate that elevational and latitudinal shrublines, like treelines, are advancing in response to climate warming; however, the trajectories of change will depend on the climate drivers controlling recruitment vs. growth. DOI
21. Williamson, SN; Anslow, FS; Clarke, GKC; Gamon, JA; Jarosch, AH; Hik, DS. (2018) Spring warming in Yukon mountains is not amplified by the snow albedo feedback.Sci Rep 8 Spring warming in Yukon mountains is not amplified by the snow albedo feedback
alpine
Decreasing spring snow cover may amplify Arctic warming through the snow albedo feedback. To examine the impact of snowmelt on increasing temperature we used a 5,000 m elevation gradient in Yukon, Canada, extending from valley-bottom conifer forests, through middle elevation tundra, to high elevation icefields, to compare validated downscaled reanalysis air temperature patterns across elevational bands characterized by different patterns of spring snowmelt. From 2000 to 2014 we observed surface warming of 0.01 degrees C/a.1,000 m in May (0.14 degrees C/a at 1,000 m to 0.19 degrees C/a at 5,000 m), and uniform cooling of 0.09 degrees C/a in June at all elevations. May temperature trends across elevationally dependent land cover types were highly correlated with each other despite large variations in albedo and snow cover trends. Furthermore, a clear dependency of infrared skin temperature on snow cover mediated albedo decline was observed in tundra, but this was insufficient to influence average diurnal air temperature. We observed negative June temperature trends which we attribute to increasing daytime cloud cover because albedo and snow cover trends were unchanging. We conclude that 8-day and monthly averaged Spring air temperature trends are responding to a synoptic external forcing that is much stronger than the snow albedo feedback in sub-Arctic mountains. DOI
20. Roslin, T; Hardwick, B; Novotny, V; Petry, WK; Andrew, NR; Asmus, A; Barrio, IC; Basset, Y; Boesing, AL; Bonebrake, TC; Cameron, EK; Dattilo, W; Donoso, DA; Drozd, P; Gray, CL; Hik, DS; Hill, SJ; Hopkins, T; Huang, S; Koane, B; Laird-Hopkins, B; Laukkanen, L; Lewis, OT; Milne, S; Mwesige, I; Nakamura, A; Nell, CS; Nichols, E; Prokurat, A; Sam, K; Schmidt, NM; Slade, A; Slade, V; Suchankova, A; Teder, T; van Nouhuys, S; Vandvik, V; Weissflog, A; Zhukovich, V; Slade, EM. (2017) Higher predation risk for insect prey at low latitudes and elevations.Science 356: 742-744 Higher predation risk for insect prey at low latitudes and elevations
Biotic interactions underlie ecosystem structure and function, but predicting interaction outcomes is difficult. We tested the hypothesis that biotic interaction strength increases toward the equator, using a global experiment with model caterpillars to measure predation risk. Across an 11,660-kilometer latitudinal gradient spanning six continents, we found increasing predation toward the equator, with a parallel pattern of increasing predation toward lower elevations. Patterns across both latitude and elevation were driven by arthropod predators, with no systematic trend in attack rates by birds or mammals. These matching gradients at global and regional scales suggest consistent drivers of biotic interaction strength, a finding that needs to be integrated into general theories of herbivory, community organization, and life-history evolution. DOI
19. Williamson, SN; Hik, DS; Gamon, JA; Jarosch, AH; Anslow, FS; Clarke, GKC; Rupp, TS. (2017) Spring and summer monthly MODIS LST is inherently compared to air temperature in snow covered sub-Arctic mountains.Remote Sens. Environ. 189: 14-24 Spring and summer monthly MODIS LST is inherently compared to air temperature in snow covered sub-Arctic mountains
MODIS LST; Downscaled NARR; SNAP; Snow cover; Cloud cover; Cryosphere: alpine
Satellite-derived land surface temperature (skin temperature) provides invaluable information for data-sparse high elevation and Arctic regions. However, the relationship between satellite-derived clear-sky skin temperature and various downscaled air temperature products for snow covered sub-Arctic alpine regions remain poorly understood, such that trend analysis or air temperature product integration is difficult. We compared monthly average air temperatures from two independent downscaled temperature products to MODIS Land Surface Temperature (LST) and air temperature at nine meteorological stations situated above tree-line in the southwest Yukon, Canada, between May and August 2008 for a full range of snow cover fractions. We found that both down scaled products generally agreed with LST for the low elevation, snow-free, vegetation classes. However, a systematic cold bias in Average LST emerged for snow fractions greater than approximately 40%, and this bias increased in magnitude as snow cover increased. In these situations the downscaled air temperatures were 57 degrees C warmer than Average LST for snow fractions of >90%, and this pattern was largely independent of the number of measurements of LST within a month. Maximum LST was similar to average air temperatures for high snow fractions, but Minimum LST was colder by 10 degrees C or more for all snow fractions. Consequently, the average of Maximum and Minimum LST produces the cold bias, compared to air temperature, for high snow cover fractions. Air temperature measured at nine meteorological monitoring stations located between elevations of 1408-2690 m, on land cover classes Barren, Sparsely Vegetated or Permanent Snow and Ice, confirmed the cold bias results when incorporating Minimum LST in monthly averages. For snow fractions of <40% the RMSE for all of the temperature products was <2.5 degrees C when compared to station air temperature and all biases were positive and <2.0 degrees C. For snow fractions of >40%, the average LST bias became strongly negative at -4.5 degrees C, and the RMSE increased to 6.1 degrees C, whereas the downscaled products bias and RMSE were similar to those from snow fractions of <40%. A weak warm bias for all the temperature products occurred for small snow fractions over non-forested land cover classes. Downscaled air temperature fields show physically real differences from Average LST in spring and summer, caused by snow cover and the interplay of Maximum and Minimum LST. These findings indicate that the integration of MODIS 1ST with downscaled air temperature products or local air temperature requires the incorporation of snow cover. (C) 2016 Elsevier Inc. All rights reserved. DOI
18. Barrio, IC; Bueno, CG; Gartzia, M; Soininen, EM; Christie, KS; Speed, JDM; Ravolainen, VT; Forbes, BC; Gauthier, G; Horstkotte, T; Hoset, KS; Hoye, TT; Jonsdottir, IS; Levesque, E; Morsdorf, MA; Olofsson, J; Wookey, PA; Hik, DS. (2016) Biotic interactions mediate patterns of herbivore diversity in the Arctic.Glob. Ecol. Biogeogr. 25: 1108-1118 Biotic interactions mediate patterns of herbivore diversity in the Arctic
Biodiversity; biotic interactions; predator-prey; species richness; trophic interactions; tundra
AimUnderstanding the forces shaping biodiversity patterns, particularly for groups of organisms with key functional roles, will help predict the responses of ecosystems to environmental changes. Our aim was to evaluate the relative role of different drivers in shaping the diversity patterns of vertebrate herbivores, a group of organisms exerting a strong trophic influence in terrestrial Arctic ecosystems. This biome, traditionally perceived as homogeneous and low in biodiversity, includes wide variation in biotic and physical conditions and is currently undergoing major environmental change. LocationThe Arctic (including the High Arctic, Low Arctic and Subarctic) MethodsWe compiled available data on vertebrate (birds and mammals) herbivore distribution at a pan-Arctic scale, and used eight variables that represent the most relevant hypotheses for explaining patterns of species richness. We used range maps rasterized on a 100kmx100km equal-area grid to analyse richness patterns of all vertebrate herbivore species combined, and birds and mammalian herbivores separately. ResultsOverall, patterns of herbivore species richness in the Arctic were positively related to plant productivity (measured using the normalized difference vegetation index) and to the species richness of predators. Greater species richness of herbivores was also linked to areas with a higher mean annual temperature. Species richness of avian and mammalian herbivores were related to the distance from the coast, with the highest avian richness in coastal areas and mammalian richness peaking further inland. Main conclusionsHerbivore richness in the Arctic is most strongly linked to primary productivity and the species richness of predators. Our results suggest that biotic interactions, with either higher or lower trophic levels or both, can drive patterns of species richness at a biome-wide scale. Rapid ongoing environmental changes in the Arctic are likely to affect herbivore diversity through impacts on both primary productivity and changes in predator communities via range expansion of predators from lower latitudes. DOI
17. Bueno, CG; Williamson, SN; Barrio, IC; Helgadottir, A; Hik, DS. (2016) Moss Mediates the Influence of Shrub Species on Soil Properties and Processes in Alpine Tundra.PLoS One 11 Moss Mediates the Influence of Shrub Species on Soil Properties and Processes in Alpine Tundra
alpine plants
In tundra ecosystems, bryophytes influence soil processes directly and indirectly through interactions with overstory shrub species. We experimentally manipulated moss cover and measured seasonal soil properties and processes under two species of deciduous shrubs with contrasting canopy structures, Salix planifolia pulchra and Betula glandulosa-nana complex. Soil properties (seasonal temperature, moisture and C:N ratios) and processes (seasonal litter decomposition and soil respiration) were measured over twelve months. Shrub species identity had the largest influence on summer soil temperatures and soil respiration rates, which were higher under Salix canopies. Mosses were associated with lower soil moisture irrespective of shrub identity, but modulated the effects of shrubs on winter soil temperatures and soil C:N ratios so that moss cover reduced differences in soil winter temperatures between shrub species and reduced C:N ratios under Betula but not under Salix canopies. Our results suggest a central role of mosses in mediating soil properties and processes, with their influence depending on shrub species identity. Such species-dependent effects need to be accounted for when forecasting vegetation dynamics under ongoing environmental changes. DOI
16. Williamson, SN; Barrio, IC; Hik, DS; Gamon, JA. (2016) Phenology and species determine growing-season albedo increase at the altitudinal limit of shrub growth in the sub-Arctic.Glob. Change Biol. 22: 3621-3631 Phenology and species determine growing-season albedo increase at the altitudinal limit of shrub growth in the sub-Arctic
albedo; alpine tundra; Betula; growing season; phenology; Salix
Arctic warming is resulting in reduced snow cover and increased shrub growth, both of which have been associated with altered land surface-atmospheric feedback processes involving sensible heat flux, ground heat flux and biogeochemical cycling. Using field measurements, we show that two common Arctic shrub species (Betula glandulosa and Salix pulchra), which are largely responsible for shrub encroachment in tundra, differed markedly in albedo and that albedo of both species increased as growing season progressed when measured at their altitudinal limit. A moveable apparatus was used to repeatedly measure albedo at six precise spots during the summer of 2012, and resampled in 2013. Contrary to the generally accepted view of shrub-covered areas having low albedo in tundra, full-canopy prostrate B. glandulosa had almost the highest albedo of all surfaces measured during the peak of the growing season. The higher midsummer albedo is also evident in localized MODIS albedo aggregated from 2000 to 2013, which displays a similar increase in growing-season albedo. Using our field measurements, we show the ensemble summer increase in tundra albedo counteracts the generalized effect of earlier spring snow melt on surface energy balance by approximately 40%. This summer increase in albedo, when viewed in absolute values, is as large as the difference between the forest and tundra transition. These results indicate that near future (<50 years) changes in growing-season albedo related to Arctic vegetation change are unlikely to be particularly large and might constitute a negative feedback to climate warming in certain circumstances. Future efforts to calculate energy budgets and a sensible heating feedback in the Arctic will require more detailed information about the relative abundance of different ground cover types, particularly shrub species and their respective growth forms and phenology. DOI
15. Myers-Smith, IH; Elmendorf, SC; Beck, PSA; Wilmking, M; Hallinger, M; Blok, D; Tape, KD; Rayback, SA; Macias-Fauria, M; Forbes, BC; Speed, JDM; Boulanger-Lapointe, N; Rixen, C; Levesque, E; Schmidt, NM; Baittinger, C; Trant, AJ; Hermanutz, L; Collier, LS; Dawes, MA; Lantz, TC; Weijers, S; Jorgensen, RH; Buchwal, A; Buras, A; Naito, AT; Ravolainen, V; Schaepman-Strub, G; Wheeler, JA; Wipf, S; Guay, KC; Hik, DS; Vellend, M. (2015) Climate sensitivity of shrub growth across the tundra biome (vol 5, pg 887, 2015).Nat. Clim. Chang. 5 Climate sensitivity of shrub growth across the tundra biome (vol 5, pg 887, 2015)
14. Patil, VP; Karels, TJ; Hik, DS. (2015) Ecological, Evolutionary and Social Constraints on Reproductive Effort: Are Hoary Marmots Really Biennial Breeders?PLoS One 10 Ecological, Evolutionary and Social Constraints on Reproductive Effort: Are Hoary Marmots Really Biennial Breeders?
Biennial breeding is a rare life-history trait observed in animal species living in harsh, unproductive environments. This reproductive pattern is thought to occur in 10 of 14 species in the genus Marmota, making marmots useful model organisms for studying its ecological and evolutionary implications. Biennial breeding in marmots has been described as an obligate pattern which evolved as a mechanism to mitigate the energetic costs of reproduction (Evolved Constraint hypothesis). However, recent anecdotal evidence suggests that it is a facultative pattern controlled by annual variation in climate and food availability (Environmental Constraint hypothesis). Finally, in social animals like marmots, biennial breeding could result from reproductive competition between females within social groups (Social Constraint hypothesis). We evaluated these three hypotheses using mark-recapture data from an 8-year study of hoary marmot (Marmota caligata) population dynamics in the Yukon. Annual variation in breeding probability was modeled using multi-state markrecapture models, while other reproductive life-history traits were modeled with generalized linear mixed models. Hoary marmots were neither obligate nor facultative biennial breeders, and breeding probability was insensitive to evolved, environmental, or social factors. However, newly mature females were significantly less likely to breed than older individuals. Annual breeding did not result in increased mortality. Female survival and, to a lesser extent, average fecundity were correlated with winter climate, as indexed by the Pacific Decadal Oscillation. Hoary marmots are less conservative breeders than previously believed, and the evidence for biennial breeding throughout Marmota, and in other arctic/alpine/antarctic animals, should be re-examined. Prediction of future population dynamics requires an accurate understanding of life history strategies, and of how life history traits allow animals to cope with changes in weather and other demographic influences. DOI
13. Trefry, SA; Hik, DS. (2010) Variation in pika (Ochotona collaris, O-princeps) vocalizations within and between populations.Ecography 33: 784-795 Variation in pika (Ochotona collaris, O-princeps) vocalizations within and between populations
Understanding geographic call variation can resolve evolutionary and behavioural questions, yet the factors influencing divergent acoustic signals remain poorly understood in mammals. We explored call variation between collared pikas in Yukon and Alaska and American pikas in Alberta, and between individuals within a population of collared pikas. Classification trees were used to determine the extent of call divergence between populations and the elements of calls driving these differences. Pika populations had significant differences in call structure, and individual pikas were classified to their correct populations with up to 94% accuracy. To investigate possible mechanisms responsible for interspecific variability, we tested the acoustic adaptation hypothesis by using a playback experiment to explore whether American and collared pika calls transmit with less degradation across their own species' habitat than the habitat of their congener. We found no support for the acoustic adaptation hypothesis. Geographic call variation in these two species of pikas likely reflects genetic divergence, and may be a result of separate evolutionary histories. We calculated the potential for individual coding for both time and frequency measurements of calls. High frequency harmonics showed greater between- than within-individual variation, and may act as sources of information regarding individual identity. DOI
12. Mitchell, MGE; Cahill, JF; Hik, DS. (2009) Plant interactions are unimportant in a subarctic-alpine plant community.Ecology 90: 2360-2367 Plant interactions are unimportant in a subarctic-alpine plant community
Artemisia norvegica; Carex consimilis; community structure; competition; facilitation; importance; intensity; plant interactions; regression trees; subarctic-alpine; Yukon, Canada
We investigated whether plant interaction intensity in a subarctic-alpine meadow is important for determining community structure and species abundance. Using two common species as phytometers, we measured interaction intensity using a neighbor removal approach. Eight biotic and abiotic variables known to influence species abundance and community structure were measured, with regression trees used to examine how plant interactions and the biotic and abiotic variables were related to species evenness, richness, and phytometer spatial cover. A range of interactions was present, with both strong competition and facilitation present over small-scale abiotic and biotic gradients. Despite the variation in interaction intensity, it was generally unrelated to either community structure or phytometer cover. In other words, plant interactions were intense in many cases but were not important to community structure. This may be due to the prevalence of clonal species in this system and the influence of previous year's interactions on plant survival and patterns of community structure. These results also suggest how conflicting theories of the role of competition in unproductive environments may be resolved. Our findings suggest that plant interactions may be intense in reducing individual growth, while simultaneously not important in the context of community structure. Plant interactions need to be viewed and tested relative to other factors and stresses to accurately evaluate their importance in plant communities, with continued differentiation between the intensity of plant interactions and their relative importance in communities. DOI
10. Cornelissen, JHC; van Bodegom, PM; Aerts, R; Callaghan, TV; van Logtestijn, RSP; Alatalo, J; Chapin, FS; Gerdol, R; Gudmundsson, J; Gwynn-Jones, D; Hartley, AE; Hik, DS; Hofgaard, A; Jonsdottir, IS; Karlsson, S; Klein, JA; Laundre, J; Magnusson, B; Michelsen, A; Molau, U; Onipchenko, VG; Quested, HM; Sandvik, SM; Schmidt, IK; Shaver, GR; Solheim, B; Soudzilovskaia, NA; Stenstrom, A; Tolvanen, A; Totland, O; Wada, N; Welker, JM; Zhao, XQ. (2007) Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes.Ecology Letters 10: 619-627 Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes
alpine; carbon; circum-arctic; global change; growth form; litter turnover; mass loss; vegetation change
Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition. DOI
9. Danby, RK; Hik, DS. (2007) Variability, contingency and rapid change in recent subarctic alpine tree line dynamics.J. Ecol. 95: 352-363 Variability, contingency and rapid change in recent subarctic alpine tree line dynamics
climate change; dendroecology; ecotones; forest-tundra; non-linearity; Picea glauca; Salix glauca; stand dynamics; timberline; Yukon: alpine
Boundaries between forest and tundra ecosystems, tree lines, are expected to advance in altitude and latitude in response to climate warming. However, varied responses to 20th century warming suggest that in addition to temperature, tree line dynamics are mediated by species-specific traits and environmental conditions at landscape and local scales. We examined recent tree line dynamics at six topographically different, but climatically similar, sites in south-west Yukon, Canada. Dendroecological techniques were used to reconstruct changes in density of the dominant tree species, white spruce (Picea glauca), and to construct static age distributions of willow (Salix spp.), one of two dominant shrub genera. Data were analysed to identify periods and rates of establishment and mortality and to relate these to past climate. Tree line elevation and stand density increased significantly during the early to mid 20th century. However, this change was not uniform across sites. Spruce advanced rapidly on south-facing slopes and tree line rose 65-85 m in elevation. Tree line did not advance on north-facing slopes, but stand density increased 40-65%. Differences observed between aspects were due primarily to the differential presence of permafrost. Additional variability among sites was related to slope and vegetation type. Results were less conclusive for willow, but evidence for an advance was found at two sites. Increases in stand density were strongly correlated with summer temperatures. The period of rapid change coincided with a 30-year period of above average temperatures, beginning in 1920. The highest correlations were obtained using a forward average of 30-50 years, supporting the hypothesis that tree line dynamics are controlled more by conditions influencing recruitment than by establishment alone. The changes observed at several sites are suggestive of a threshold response and challenge the notion that tree lines respond gradually to climate warming. Overall, the results provide further evidence to support the idea that the pattern and timing of change is contingent on local, landscape, and regional-scale factors, as well as species' biology. DOI
8. Danby, RK; Hik, DS. (2007) Responses of white spruce (Picea glauca) to experimental warming at a subarctic alpine treeline.Glob. Change Biol. 13: 437-451 Responses of white spruce (Picea glauca) to experimental warming at a subarctic alpine treeline
climate change; experimental warming; open-top chamber; Picea glauca; seedling growth; timberline; treeline dynamics; Yukon: alpine
From 2001 to 2004 we experimentally warmed 40 large, naturally established, white spruce [Picea glauca (Moench) Voss] seedlings at alpine treeline in southwest Yukon, Canada, using passive open-top chambers (OTCs) distributed equally between opposing north and south-facing slopes. Our goal was to test the hypothesis that an increase in temperature consistent with global climate warming would elicit a positive growth response. OTCs increased growing season air temperatures by 1.8 degrees C and annual growing degree-days by one-third. In response, warmed seedlings grew significantly taller and had higher photosynthetic rates compared with control seedlings. On the south aspect, soil temperatures averaged 1.0 degrees C warmer and the snow-free period was nearly 1 month longer. These seedlings grew longer branches and wider annual rings than seedlings on the north aspect, but had reduced Photosystem-II efficiency and experienced higher winter needle mortality. The presence of OTCs tended to reduce winter dieback over the course of the experiment. These results indicate that climate warming will enhance vertical growth rates of young conifers, with implications for future changes to the structure and elevation of treeline contingent upon exposure-related differences. Our results suggest that the growth of seedlings on north-facing slopes is limited by low soil temperature in the presence of permafrost, while growth on south-facing slopes appears limited by winter desiccation and cold-induced photoinhibition. DOI
7. Koh, S; Hik, DS. (2007) Herbivory mediates grass-endophyte relationships.Ecology 88: 2752-2757 Herbivory mediates grass-endophyte relationships
endophytic fungi; Festuca altaica; herbivory; mutualism; Neotyphodium; optimal defense theory; subarctic; symbiosis
Endophytic fungi are plant symbionts living asymptomatically within plant tissues. Neotyphodium spp., which are asexual vertically transmitted systemic fungal endophytes of cool-season grasses, are predicted to be plant mutualists. These endophytes increase host plant resistance to environmental stresses and/or increase the production of alkaloid-based herbivore deterrents. The ubiquity of this defense mutualism is unclear, and a variety of alternative mechanisms may explain the observed variation in infection rates, levels of deterrence, and the maintenance of asexual endophytes in grass populations. We found that grass-endophyte interactions are variable and ordered along an herbivory gradient in an undisturbed subarctic alpine ecosystem. Native grass populations in grazed sites had significantly greater frequency of Neotyphodium infection compared to ungrazed sites. Tillers from grazed sites had significantly higher hyphal densities compared to ungrazed sites. The ability of grass-Neotyphodium constituents to deter vertebrate herbivory in natural systems is thought to be rare. In grazed meadows, we showed that endophyte infection resulted in the deterrence of grazing by native vertebrate herbivores. However, the same herbivores did not distinguish between infected and uninfected grass harvested from ungrazed areas. These results demonstrate that the relationship between vertically transmitted endophytes and grasses in the alpine tundra vary greatly within populations. This may be based in part on defense mutualism and is consistent, under varying levels of herbivory, with the predictions of optimal defense theory. DOI
6. Kyle, CJ; Karels, TJ; Davis, CS; Mebs, S; Clark, B; Strobeck, C; Hik, DS. (2007) Social structure and facultative mating systems of hoary marmots (Marmota caligata).Mol. Ecol. 16: 1245-1255 Social structure and facultative mating systems of hoary marmots (Marmota caligata)
hoary marmot; Marmota caligata; mating system; microsatellites; parentage; social structure
Mate-choice theory predicts different optimal mating systems depending on resource availability and habitat stability. Regions with limited resources are thought to promote monogamy. We tested predictions of monogamy in a social rodent, the hoary marmot (Marmota caligata), at the northern climatic extreme of its distribution. Mating systems, social structure and genetic relationships were investigated within and among neighbouring colonies of marmots within a 4 km(2) valley near Kluane National Park, Yukon, Canada, using 21 microsatellite loci. While both monogamous and polygynous populations of hoary marmots have been observed in the southern reaches of this species' range; northern populations of this species are thought to be predominantly monogamous. Contrary to previous studies, we did not find northern hoary marmot social groups to be predominantly monogamous; rather, the mating system seemed to be facultative, varying between monogamy and polygyny within, as well as among, social groups. These findings reveal that the mating systems within colonies of this species are more flexible than previously thought, potentially reflecting local variation in resource availability. DOI
5. Morrison, SF; Hik, DS. (2007) Demographic analysis of a declining pika Ochotona collaris population: linking survival to broad-scale climate patterns via spring snowmelt patterns.J. Anim. Ecol. 76: 899-907 Demographic analysis of a declining pika Ochotona collaris population: linking survival to broad-scale climate patterns via spring snowmelt patterns
collared pika; fertility; life-table response experiment; Pacific Decadal Oscillation; snowmelt
1. Demographic analysis is essential in order to determine which factors, such as survival, fertility and other life-history characteristics, have the greatest influence on a population's rate of growth (lambda). 2. We used life-table response experiments (LTREs) to assess the relative importance of survival and fertility rates for an alpine lagomorph, the collared pika Ochotona collaris, using 12 years (1995-2006) of census data. The LTRE analysis was repeated for each of three subpopulations within the main study site that were defined by aspect (east, west and south). 3. Across the entire study site, the survival and fertility of adults contributed 35.6 and 43.5%, respectively, to the variance observed in the projected population growth rate, V(lambda), whereas juvenile survival contributed 20.9%. Adult survival and fertility contributed approximately equal amounts for each subpopulation when considered separately, although their rank order varied spatially. 4. Adult survival across the entire site was positively correlated to the Pacific Decadal Oscillation (PDO) with a time lag of 1 year, and was uncorrelated to adult density. The PDO was negatively correlated to the timing of spring snowmelt at our site, implicating the importance of earlier spring conditions and plant phenology on the subsequent winter survival of adults and therefore, population growth. 5. When subpopulations were analysed separately, survivals and fertilities were variously correlated to lagged PDO and adult densities, but the patterns varied spatially. Therefore, the mechanisms underlying V(lambda) can vary substantially over relatively short distances. DOI
4. Franken, RJ; Hik, DS. (2004) Influence of habitat quality, patch size and connectivity on colonization and extinction dynamics of collared pikas Ochotona collaris.J. Anim. Ecol. 73: 889-896 Influence of habitat quality, patch size and connectivity on colonization and extinction dynamics of collared pikas Ochotona collaris
area; isolation; metapopulation; occupancy; persistence
1. The effects of habitat quality, patch size and connectivity between patches on patterns of local extinction and colonization of collared pikas were studied over 7 years in alpine meadows in the south-west Yukon. 2. Although adult population size independently had a significant influence on patch extinction, its influence was minimal when other variables were included in generalized linear models. Instead, an index of habitat quality and the connectivity of a patch were found to be the best predictors of pika extinction. 3. Similarly, patch connectivity only partly explained the recolonization of talus patches by pikas. Other patch characteristics, including aspect, amount of vegetation within the patch and an index of habitat quality based on survival probability of pikas also had a significant influence on recolonization. 4. These results suggest that the influence of patch quality on local extinction and recolonization need to be more fully incorporated into metapopulation models. DOI
3. Mcintire, EJB; Hik, DS. (2002) Grazing history versus current grazing: leaf demography and compensatory growth of three alpine plants in response to a native herbivore (Ochotona collaris).J. Ecol. 90: 348-359 Grazing history versus current grazing: leaf demography and compensatory growth of three alpine plants in response to a native herbivore (Ochotona collaris)
alpine meadows; collared pika; Erigeron humilis; grazing history; herbivory gradient; Kobresia myosuroides; Ochotona collaris; Oxytropis nigrescens
1 We measured leaf births, leaf deaths and leaf length of three alpine perennial species, Kobresia myosuroides, Erigeron humilis and Oxytropis nigrescens, from sites with different grazing histories (strong or weak) in response to two levels of current season grazing (present or absent) by collared pikas (Ochotona collaris), a small lagomorph, in the south-west Yukon. 2 All three species appeared to tolerate the removal of 58-61% of summer leaf production under natural conditions. Grazing history, which was defined by the location of plants located either < 2 m or > 6 m from boulderfields with a history of occupation by pikas, was the most significant factor determining shifts in leaf births and leaf deaths following herbivory. 3 The only detectable influence of current season herbivory for any measured species was a reduction of leaf length of Kobresia. 4 A comparison of historically grazed with historically ungrazed plants indicated several changes in leaf demography and morphology. Kobresia leaves were generally shorter and had higher rates of production of new leaves. Oxytropis had higher rates of new leaf production. Erigeron had fewer leaf births throughout the summer, but showed a large and highly significant delay in the timing of leaf senescence. 5 These responses can be largely understood as strategies to avoid the predictable intensive late season foraging that is characteristic of pikas. Morphological mechanisms allow these species to tolerate and, more importantly for the herbivore, persist under heavy and chronic grazing. DOI
2. HIK, DS; JEFFERIES, RL; SINCLAIR, ARE. (1992) FORAGING BY GEESE, ISOSTATIC UPLIFT AND ASYMMETRY IN THE DEVELOPMENT OF SALT-MARSH PLANT-COMMUNITIES.J. Ecol. 80: 395-406 FORAGING BY GEESE, ISOSTATIC UPLIFT AND ASYMMETRY IN THE DEVELOPMENT OF SALT-MARSH PLANT-COMMUNITIES
ARCTIC SALT-MARSHES; EDAPHIC PROCESSES; GRAZING LAWNS; LESSER SNOW GOOSE; MULTIPLE-STATE COMMUNITIES
1. The response of different salt-marsh plant communities to grazing by lesser snow geese and isostatic uplift was examined at La Perouse Bay, Manitoba on the Hudson Bay coast. Results are based on direct experimental manipulation of swards and previously published floristic and biomass data. 2. At the seaward end of the salt-marsh the vegetation is composed of Puccinellia phryganodes and Carex subspathacea. This assemblage of species is also present in the upper marsh, but only as a consequence of intense grazing by geese. Grazed swards of these species persist over a period of at least 10 years. 3. When swards are protected from grazing in the upper marsh, rapid changes in floristic composition of vegetation occur. There is a substantial increase in above-ground biomass and the vegetation is dominated by Calamagrostis deschampsiodes, Festuca rubra and dicotyledonous plants. When these swards are grazed the species composition does not revert to the Puccinellia-Carex assemblage. 4. In the presence of grazing this change from the Puccinellia-Carex assemblage to the Calamagrostis-Festuca assemblage occurs gradually as a consequence of changes in edaphic conditions associated with isostatic uplift. The geese delay the rate of vegetational development associated with the effects of isostatic uplift but they cannot arrest it. 5. Both types of plant assemblage may become extinct locally as a result of grubbing by the geese for roots and rhizomes of graminoid species. Habitat conditions are sufficiently altered following grubbing that the process of succession recommences from bare sediment. 6. The results of changes in floristic composition, above-ground biomass and replacement rates of vegetation are examined in relation to predictions of multiple-state models which emphasize the directional asymmetry in the development of plant communities. DOI
