Global climate change and reindeer: effects of winter weather on the autumn weight and growth of calves

Reindeer/caribou (Rangifer tarandus), which constitute a biological resource of vital importance for the physical and cultural survival of Arctic residents, and inhabit extremely seasonal environments, have received little attention in the global change debate. We investigated how body weight and growth rate of reindeer calves were affected by large-scale climatic variability [measured by the North Atlantic Oscillation (NAO) winter index] and density in one population in central Norway. Body weights of calves in summer and early winter, as well as their growth rate (summer to early winter), were significantly influenced by density and the NAO index when cohorts were in utero. Males were heavier and had higher absolute growth than females, but there was no evidence that preweaning condition of male and female calves were influenced differently by the NAO winter index. Increasing NAO index had a negative effect on calves' body weight and growth rate. Increasing density significantly reduced body weight and growth rate of calves, and accentuated the effect of the NAO winter index. Winters with a higher NAO index are thus severe for reindeer calves in this area and their effects are associated with nutritional stress experienced by the dams during pregnancy or immediately after calving. Moreover, increased density may enhance intra-specific competition and limits food available at the individual level within cohorts. We conclude that if the current pattern of global warming continues, with greater change occurring in northern latitudes and during winter as is predicted, reduced body weight of reindeer calves may be a consequence in areas where winters with a high NAO index are severe. This will likely have an effect on the livelihood of many northern indigenous peoples, both economically and culturally.     

Non-linear feedback processes and a latitudinal gradient in the climatic effects determine green spruce aphid outbreaks in the UK

The role of climatic fluctuations in determining the dynamics of insect populations has been a classical problem in population ecology. Here, we use long-term annual data on green spruce aphid populations at nine localities in the UK for determining the importance of endogenous processes, local weather and large-scale climatic factors. We rely on diagnostic and modelling tools from population dynamic theory to analyse these long-term data and to determine the role of the North Atlantic Oscillation (NAO) and local weather as exogenous factors influencing aphid dynamics. Our modelling suggests that the key elements determining population fluctuations in green spruce aphid populations in the UK are the strong non-linear feedback structure, the high potential for population growth and the effects of winter and spring weather. The results indicate that the main effect of the NAO on green spruce aphid populations is operating through the effect of winter temperatures on the maximum per capita growth rate (R_m). In particular, we can predict quite accurately the occurrence of an outbreak by using a simple logistic model with weather as a perturbation effect. However, model predictions using different climatic variables showed a clear geographical signature. The NAO and winter temperature were best for predicting observed dynamics toward the southern localities, while spring temperature was a much better predictor of aphid dynamics at northern localities. Although aphid species are characterized by complex life-cycles, we emphasize the value of simple and general population dynamic models in predicting their dynamics.     

Effects of density-dependence and climate on the dynamics of a Svalbard reindeer population

To determine the main factors affecting the population dynamics of Svalbard reindeer, we analysed 21 yr of annual censuses, including data on population size, recruitment rate (calves per female) and mortality (number of deaths), from the Reindalen reindeer population. In accordance with previous studies on population dynamics of Svalbard reindeer, we found large inter-annual variation in population size, mortality and recruitment rates within the study area. Population size decreased in years with low recruitment rate as well as high winter mortality and vice versa. Apparently. the fluctuations were due to both direct density-dependent food limitation and variation in winter climate associated with high precipitation and icing of the feeding range. We found no delayed density-dependence or effect of climatic conditions during summer on the population dynamics. The mortality during die-off years was mainly of calves and very old individuals, indicating that the population was more vulnerable to high die oft in years following high recruitment rate. These results suggest an unstable interaction between the reindeer population and its food supply in these predator-free environments.     

Influences of the El Niño/Southern Oscillation and the North Atlantic Oscillation on avian productivity in forests of the Pacific Northwest of North America

To model the effects of global climate phenomena on avian population dynamics, we must identify and quantify the spatial and temporal relationships between climate, weather and bird populations. Previous studies show that in Europe, the North Atlantic Oscillation (NAO) influences winter and spring weather that in turn affects resident and migratory landbird species. Similarly, in North America, the El Niño/Southern Oscillation (ENSO) of the Pacific Ocean reportedly drives weather patterns that affect prey availability and population dynamics of landbird species which winter in the Caribbean. Here we show that ENSO‐ and NAO‐induced seasonal weather conditions differentially affect neotropical‐ and temperate‐wintering landbird species that breed in Pacific North‐west forests of North America. For neotropical species wintering in western Mexico, El Niño conditions correlate with cooler, wetter conditions prior to spring migration, and with high reproductive success the following summer. For temperate wintering species, springtime NAO indices correlate strongly with levels of forest defoliation by the larvae of two moth species and also with annual reproductive success, especially among species known to prey upon those larvae. Generalized linear models incorporating NAO indices and ENSO precipitation indices explain 50–90% of the annual variation in productivity reported for 10 landbird species. These results represent an important step towards spatially explicit modelling of avian population dynamics at regional scales.     

Spatial heterogeneity in climate change effects decouples the long‐term dynamics of wild reindeer populations in the high Arctic

The ‘Moran effect’ predicts that dynamics of populations of a species are synchronized over similar distances as their environmental drivers. Strong population synchrony reduces species viability, but spatial heterogeneity in density dependence, the environment, or its ecological responses may decouple dynamics in space, preventing extinctions. How such heterogeneity buffers impacts of global change on large‐scale population dynamics is not well studied. Here, we show that spatially autocorrelated fluctuations in annual winter weather synchronize wild reindeer dynamics across high‐Arctic Svalbard, while, paradoxically, spatial variation in winter climate trends contribute to diverging local population trajectories. Warmer summers have improved the carrying capacity and apparently led to increased total reindeer abundance. However, fluctuations in population size seem mainly driven by negative effects of stochastic winter rain‐on‐snow (ROS) events causing icing, with strongest effects at high densities. Count data for 10 reindeer populations 8–324 km apart suggested that density‐dependent ROS effects contributed to synchrony in population dynamics, mainly through spatially autocorrelated mortality. By comparing one coastal and one ‘continental’ reindeer population over four decades, we show that locally contrasting abundance trends can arise from spatial differences in climate change and responses to weather. The coastal population experienced a larger increase in ROS, and a stronger density‐dependent ROS effect on population growth rates, than the continental population. In contrast, the latter experienced stronger summer warming and showed the strongest positive response to summer temperatures. Accordingly, contrasting net effects of a recent climate regime shift—with increased ROS and harsher winters, yet higher summer temperatures and improved carrying capacity—led to negative and positive abundance trends in the coastal and continental population respectively. Thus, synchronized population fluctuations by climatic drivers can be buffered by spatial heterogeneity in the same drivers, as well as in the ecological responses, averaging out climate change effects at larger spatial scales.     

Main determinants of rodent population fluctuations in managed Central European temperate lowland forests

Whilst studies have shown that climatic (North Atlantic Oscillation (NAO)) and biotic (acorn production) factors influence rodent populations, mechanisms driving temporal and spatial fluctuation of rodent populations are understudied. This study evaluates relationships between the influence of environmental factors (biotic and abiotic) and phenotypic characteristics across two rodent feeding guilds (granivorous and non-granivorous species) represented by four species of rodents in Central Europe. We hypothesise that the relationship between acorn density and population growth rate are indirectly affected by climatic factors (winter NAO) and that these effects differ amongst herbivorous and granivorous species. In addition, we also tested whether effects of weather and competition on individual phenotype characteristic vary amongst mast and non-mast years. Rodent populations were estimated by catching individuals in snap traps during the growing season (from March to November) over a period of 9 years at three sites. The results of the generalised linear model provide evidence that acorn production best explained the population fluctuations. We therefore conclude that the between-year population fluctuations in rodent abundance were governed by density dependence and initiated primarily by acorn mast years. Auto-regressive models also revealed direct density dependence in combination with the direct effects of mast years. Therefore, strong intraspecific competition for food is likely in years following mast years. Our results also showed that abundance of non-granivorous species is mainly influenced by local weather conditions which could regulate food quality and abundance. On the other hand, population dynamics of granivorous species are caused directly by acorn density and indirectly by climatic condition influencing acorn production.     

Experimental icing affects growth,mortality, and flowering in a high Arctic dwarf shrub

Effects of climate change are predicted to be greatest at high latitudes, with more pronounced warming in winter than summer. Extreme mid‐winter warm spells and heavy rain‐on‐snow events are already increasing in frequency in the Arctic, with implications for snow‐pack and ground‐ice formation. These may in turn affect key components of Arctic ecosystems. However, the fitness consequences of extreme winter weather events for tundra plants are not well understood, especially in the high Arctic. We simulated an extreme mid‐winter rain‐on‐snow event at a field site in high Arctic Svalbard (78°N) by experimentally encasing tundra vegetation in ice. After the subsequent growing season, we measured the effects of icing on growth and fitness indices in the common tundra plant, Arctic bell‐heather (Cassiope tetragona). The suitability of this species for retrospective growth analysis enabled us to compare shoot growth in pre and postmanipulation years in icing treatment and control plants, as well as shoot survival and flowering. Plants from icing treatment plots had higher shoot mortality and lower flowering success than controls. At the individual sample level, heavily flowering plants invested less in shoot growth than nonflowering plants, while shoot growth was positively related to the degree of shoot mortality. Therefore, contrary to expectation, undamaged shoots showed enhanced growth in ice treatment plants. This suggests that following damage, aboveground resources were allocated to the few remaining undamaged meristems. The enhanced shoot growth measured in our icing treatment plants has implications for climate studies based on retrospective analyses of Cassiope. As shoot growth in this species responds positively to summer warming, it also highlights a potentially complex interaction between summer and winter conditions. By documenting strong effects of icing on growth and reproduction of a widespread tundra plant, our study contributes to an understanding of Arctic plant responses to projected changes in winter climatic conditions.     

Continuous and discrete extreme climatic events affecting the dynamics of a high-arctic reindeer population

Climate at northern latitudes are currently changing both with regard to the mean and the temporal variability at any given site, increasing the frequency of extreme events such as cold and warm spells. Here we use a conceptually new modelling approach with two different dynamic terms of the climatic effects on a Svalbard reindeer population (the Brøggerhalvøya population) which underwent an extreme icing event (“locked pastures”) with 80% reduction in population size during one winter (1993/94). One term captures the continuous and linear effect depending upon the Arctic Oscillation and another the discrete (rare) “event” process. The introduction of an “event” parameter describing the discrete extreme winter resulted in a more parsimonious model. Such an approach may be useful in strongly age-structured ungulate populations, with young and very old individuals being particularly prone to mortality factors during adverse conditions (resulting in a population structure that differs before and after extreme climatic events). A simulation study demonstrates that our approach is able to properly detect the ecological effects of such extreme climate events.     

Population dynamics ofClethrionomys glareolus andApodemus flavicollis: seasonal components of density dependence and density independence

Using long-term data on two forest rodent species [the bank voleClethrionomys glareolus (Schreber, 1780) and the yellow-necked mouseApodemus flavicollis (Melchior, 1834)] from the Białowieża Primeval Forest (E Poland), we decompose the annual density-dependent and density-independent structures into their seasonal components. For this purpose we adopt a state-space modelling approach explicitly incorporating sampling stochasticity. As density-independent factors we use the North Atlantic Oscillation (NAO) — a proxy variable for the overall climatic condition — and data on annual seed production. We find a weak effect of the NAO in the annual models for both species as well as during the winter in the seasonal model forC. glareolus. The effect of the NAO disappears, however, when seed-crops are incorporated into the models (for both the annual and the seasonal — suggesting that NAO primarily affects seed production). Seed production enters the models with a positive effect during the winter only, suggesting that the among-year variation in rodent density is primarily accounted for by differences in seed-production, particularly oak seeds. ForA. flavicollis, a slightly positive effect of hornbeam also appears in the summer dynamics. The obtained results are discussed on the basis of earlier studies on the same populations, on the same species studied elsewhere as well as on the basis of general ecological insight.

     

Winter North Atlantic Oscillation, temperature and ischaemic heart disease mortality in three English counties

As cold weather is an ischaemic heart disease (IHD) risk factor, year-to-year variations of the level of IHD mortality may be partly determined by inter-annual variations in winter climate. This paper investigates whether there is any association between the level of IHD mortality for three English counties and the winter North Atlantic Oscillation (NAO), which exerts a fundamental control on the nature of the winter climate over Western Europe. Correlation and regression analysis was used to explore the nature of the association between IHD mortality and a climate index (CI) that represents the interaction between the NAO and temperature across England for the winters 1974–1975 to 1989–1999. Statistically significant inverse associations between the CI and the level of IHD mortality were found. Generally, high levels of winter IHD mortality are associated with a negative CI, which represents winters with a strong negative phase of the NAO and anomalously low temperatures across England. Moreover, the nature of the CI in the early stages of winter appears to exert a fundamental control on the general level of winter IHD mortality. Because winter climate is able to explain a good proportion of the inter-annual variability of winter mortality, long-lead forecasting of winter IHD mortality appears to be a possibility. The integration of climate-based health forecasts into decision support tools for advanced general winter emergency service and capacity planning could form the basis of an effective adaptive strategy for coping with the health effects of harsh winters.     

Divergent patterns of impact of environmental conditions on life history traits in two populations of a long-distance migratory bird

Some areas have experienced recent dramatic warming due to climate change, while others have shown no change at all, or even recent cooling. We predicted that patterns of selection on life history would differ between southern and northern European populations of a long-distance migratory bird, the barn swallow Hirundo rustica, because global patterns of weather as reflected by large-scale weather phenomena such as the North Atlantic Oscillation (NAO) and the El Niño-Southern Oscillation (ENSO) have different effects on environmental conditions in different parts of the world frequented during the annual cycle. We investigated relationships between mean arrival date, dispersal rate and yearling survival rate among years, using two long-term population studies in Spain and Denmark. We found evidence of a difference in the effects of normalized difference vegetation index in North and West Africa on mean arrival date of male barn swallows, with the effect differing significantly between populations. Second, there was a significant interaction between ENSO and population on dispersal rate, showing that conditions in Africa during winter differentially affected dispersal in the two populations. Finally, the NAO index in winter had an effect on yearling survival that differed between populations. These findings highlight the divergent patterns of response to climate change among populations, and they suggest that climate change can differentially affect important life history traits with potential implications for maintenance of viable populations and gene flow among populations.     

High Female Mortality Resulting in Herd Collapse in Free-Ranging Domesticated Reindeer (Rangifer tarandus tarandus) in Sweden

Reindeer herding in Sweden is a form of pastoralism practised by the indigenous Sámi population. The economy is mainly based on meat production. Herd size is generally regulated by harvest in order not to overuse grazing ranges and keep a productive herd. Nonetheless, herd growth and room for harvest is currently small in many areas. Negative herd growth and low harvest rate were observed in one of two herds in a reindeer herding community in Central Sweden. The herds (A and B) used the same ranges from April until the autumn gathering in October–December, but were separated on different ranges over winter. Analyses of capture-recapture for 723 adult female reindeer over five years (2007–2012) revealed high annual losses (7.1% and 18.4%, for herd A and B respectively). A continuing decline in the total reindeer number in herd B demonstrated an inability to maintain the herd size in spite of a very small harvest. An estimated breakpoint for when herd size cannot be kept stable confirmed that the observed female mortality rate in herd B represented a state of herd collapse. Lower calving success in herd B compared to A indicated differences in winter foraging conditions. However, we found only minor differences in animal body condition between the herds in autumn. We found no evidence that a lower autumn body mass generally increased the risk for a female of dying from one autumn to the next. We conclude that the prime driver of the on-going collapse of herd B is not high animal density or poor body condition. Accidents or disease seem unlikely as major causes of mortality. Predation, primarily by lynx and wolverine, appears to be the most plausible reason for the high female mortality and state of collapse in the studied reindeer herding community.     

The Arctic Oscillation predicts effects of climate change in two trophic levels in a high-arctic ecosystem

During recent decades there has been a change in the circulation of atmospheric pressure throughout the Northern Hemisphere. These variations are expressed in the recently described Arctic Oscillation (AO), which has shown an upward trend (associated with winter warming in the eastern Arctic) during the last three decades. We analysed a 12‐year time series on growth of Cassiope tetragona (Lapland Cassiope) and a 21‐year time series on abundance of a Svalbard reindeer population. High values of the AO index were associated with reduced plant growth and reindeer population growth rate. The North Atlantic Oscillation index was not able to explain a significant proportion of the variance in either plant growth or reindeer population fluctuations. Thus, the AO index may be a better predictor for ecosystem effects of climate change in certain high‐arctic areas compared to the NAO index.     

Time series analysis of climate‐related factors and their impact on a red‐listed noble crayfish population in northern Sweden

1. Global climate change is predicted to raise water temperatures and alter flow regimes in northern river systems. Climate‐related factors might have profound impacts on survival, reproduction and distribution of freshwater species such as red‐listed noble crayfish (Astacus astacus) in its northern limit of distribution. 2. In this study, noble crayfish capture data over 27 years from the River Ljungan, Sweden, were examined. Time series of catch per unit effort (CPUE) were analysed in relation to the North Atlantic Oscillation (NAO) index, regional weather factors and water flow. CPUE was assumed to reflect differences in population size. Two models were constructed to explore the relative impact of different climate factors and density dependence on variability of catch sizes. 3. The most parsimonious model for CPUE time series, explaining 72% of the variance in CPUE, included density‐dependent population dynamics of the crayfish and climate or weather factors. The specific effect from density dependence in the model was 37%, while climate/weather factors contributed with 35% of the variation. The most important climate/weather factors are variations in NAO index and water flow. Temperature did not improve the model fit to capture data. 4. The best model was evaluated using independent data sets that gave correlations between model predictions and data ranging from 0.44 to 0.53. The density dependence shows a time lag of 1 year, while climate variables show time lags from 2 to 6 years in relation to CPUE, indicating effects on different cohorts of the crayfish population. 5. Both density dependence and climatic factors play a significant role in population fluctuations of noble crayfish. A 6‐year time lag for NAO index is puzzling but indicates that some as yet unidentified factors related to NAO might act on the juvenile stages of the population. Water flow shows a 2‐year lag to the CPUE, and high flow in the river may affect adult survival. The reasons for fluctuation of crayfish catches in response to climate need to be identified, and fishing quotas should consider the different cohort sizes because of variation in environment. Reintroduction programmes for crayfish need to consider effects of climate change when designing management strategies.     

Signatures of large-scale and local climates on the demography of white-tailed ptarmigan in Rocky Mountain National Park,Colorado, USA

Global climate change may impact wildlife populations by affecting local weather patterns, which, in turn, can impact a variety of ecological processes. However, it is not clear that local variations in ecological processes can be explained by large-scale patterns of climate. The North Atlantic oscillation (NAO) is a large-scale climate phenomenon that has been shown to influence the population dynamics of some animals. Although effects of the NAO on vertebrate population dynamics have been studied, it remains uncertain whether it broadly predicts the impact of weather on species. We examined the ability of local weather data and the NAO to explain the annual variation in population dynamics of white-tailed ptarmigan ( Lagopus leucurus) in Rocky Mountain National Park, USA. We performed canonical correlation analysis on the demographic subspace of ptarmigan and local-climate subspace defined by the empirical orthogonal function (EOF) using data from 1975 to 1999. We found that two subspaces were significantly correlated on the first canonical variable. The Pearson correlation coefficient of the first EOF values of the demographic and local-climate subspaces was significant. The population density and the first EOF of local-climate subspace influenced the ptarmigan population with 1-year lags in the Gompertz model. However, the NAO index was neither related to the first two EOF of local-climate subspace nor to the first EOF of the demographic subspace of ptarmigan. Moreover, the NAO index was not a significant term in the Gompertz model for the ptarmigan population. Therefore, local climate had stronger signature on the demography of ptarmigan than did a large-scale index, i.e., the NAO index. We conclude that local responses of wildlife populations to changing climate may not be adequately explained by models that project large-scale climatic patterns.     

Analysis of phenotypic change in relation to climatic drivers in a population of Soay sheep Ovis aries

Climatic change has frequently been identified as a key driver of change in biological communities. These changes can take the form of alterations to population dynamics, phenotypic characters, genetics and the life history of organisms and can have impacts on entire ecosystems. This study presents a novel investigation of how changes in a large scale climatic index, the North Atlantic Oscillation (NAO) can influence population dynamics and phenotypic characters in a population of ungulates. We use an integral projection model combined with actual climate change predictions to project future body size distributions for a population of Soay sheep Ovis aries. The climate change predictions used to direct our model projections were taken from published results of climate models, covering a range of different emissions scenarios. Our model results showed that for positive changes in the mean NAO large population declines occurred simultaneously with increases in mean body weight. The exact direction and magnitude of changes to population dynamics and character distributions were dependent on the greenhouse gas emissions scenario and model used to predict the NAO. This study has demonstrated how integral projection models can use outputs of climate models to direct projections of population dynamics and phenotypic character distributions. This approach allows the results of this study to be placed within current climate change research. The nature of integral projection models means that this methodology can be easily applied to other populations. The model can also be easily updated when new climate change predictions become available, making it a useful tool for understanding potential population level responses to climatic change. Synthesis Understanding how changes in climate affect biological communities is a key component in predicting the future form of populations. Utilising a novel approach that incorporates climatic drivers (in this instance the winter North Atlantic Oscillation) into an integral projection model framework, we predict future Soay sheep dynamics under specific climate change scenarios. Tracking quantitative trait distributions and life history metrics, our results predict declining population size and increasing body weight for an increasingly positive winter North Atlantic Oscillation index, as predicted by climate models. This has important implications for future wildlife management strategies and linking demographic responses to climate change.     

Influence of body mass and environmental conditions on winter mortality risk of a northern ungulate: Evidence for a late‐winter survival bottleneck

1. A relationship between winter weather and survival of northern ungulates has long been established, yet the possible roles of biological (e.g., nutritional status) and environmental (e.g., weather) conditions make it important to determine which potential limiting factors are most influential.
2. Our objective was to examine the potential effects of individual (body mass and age) and extrinsic (winter severity and snowmelt conditions) factors on the magnitude and timing of mortality for adult (>2.5 years old) female white‐tailed deer (Odocoileus virginianus [Zimmerman, 1780]) during February–May in the Upper Peninsula of Michigan, USA.
3. One hundred and fifty deer were captured and monitored during 2009–2015 in two areas with varying snowfall. February–May survival ranged from 0.24 to 0.89 (mean = 0.69) across years. Mortality risk increased 1.9% with each unit increase in cumulative winter severity index, decreased 8.2% with each cumulative snow‐free day, and decreased 4.3% with each kg increase in body mass. Age and weekly snow depth did not influence weekly deer survival. Predation, primarily from coyote (Canis latrans [Say, 1823]) and wolves (Canis lupus [L., 1758]), accounted for 78% of known‐cause mortalities.
4. Our results suggest that cumulative winter severity, and possibly to a lesser degree deer condition entering winter, impacted deer winter survival. However, the timing of spring snowmelt appeared to be the most influential factor determining late‐winter mortality of deer in our study. This supports the hypothesis that nutrition and energetic demands from weather conditions are both important to northern ungulate winter ecology. Under this model, a delay of several weeks in the timing of spring snowmelt could exert a large influence on deer survival, resulting in a survival bottleneck.

     

Combined Effects of the North Atlantic Oscillation and the Arctic Oscillation on Sea Surface Temperature in the Alborán Sea

We explored the possible effects of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) on interannual sea surface temperature (SST) variations in the Alborán Sea, both separately and combined. The probability of observing mean annual SST values higher than average was related to NAO and AO values of the previous year. The effect of NAO on SST was negative, while that of AO was positive. The pure effects of NAO and AO on SST are obscuring each other, due to the positive correlation between them. When decomposing SST, NAO and AO in seasonal values, we found that variation in mean annual SST and mean winter SST was significantly related to the mean autumn NAO of the previous year, while mean summer SST was related to mean autumn AO of the previous year. The one year delay in the effect of the NAO and AO on the SST could be partially related to the amount of accumulated snow, as we found a significant correlation between the total snow in the North Alborán watershed for a year with the annual average SST of the subsequent year. A positive AO implies a colder atmosphere in the Polar Regions, which could favour occasional cold waves over the Iberian Peninsula which, when coupled with precipitations favoured by a negative NAO, may result in snow precipitation. This snow may be accumulated in the high peaks and melt down in spring-summer of the following year, which consequently increases the runoff of freshwater to the sea, which in turn causes a diminution of sea surface salinity and density, and blocks the local upwelling of colder water, resulting in a higher SST.     

Ice encapsulation protects rather than disturbs the freezing lichen

Arctic and alpine terricolous lichens are adapted to harsh environments and are tolerant to extremely low temperatures when metabolically inactive. However, there are reports indicating that freezing can be lethal to metabolically active lichens. With a projected warmer and more unstable climate, winter precipitation at high latitudes will fall more frequently as rain, causing snowmelt and encapsulating terricolous lichens in ice or exposing them to large temperature fluctuations. Lichens are a major winter food source for reindeer in most parts of the circumpolar region. A laboratory experiment tested how three hydrated reindeer forage lichen species covered by snow, encapsulated in ice, or uncovered responded to storage at freezing temperatures and subsequent warming. Photosynthetic performance (maximal fluorescence of dark-adapted samples and net photosynthetic rates) was significantly lower in lichens not insulated by snow or ice, whereas there were few differences between the snow and ice treatments. It is suggested that snow and ice provide sufficiently moist environments to improve extracellular and reduce intracellular ice nucleation activity. Ice encapsulation, which is often lethal to vascular plants, did not have any negative effects on the studied lichens. The results indicate that complete snow and ice melt followed by refreezing can be detrimental to terricolous lichen ecosystems. Reduced lichen biomass will have a negative effect both on reindeer winter survival and the indigenous peoples who herd reindeer.     

Long-term responses in arctic ungulate dynamics to changes in climatic and trophic processes

 Following predictions from climatic general circulation models, the effects of perturbations in global climate are expected to be most pronounced in the Northern Hemisphere. Elaborating on a recently developed plant–herbivore–climate model, we explore statistically how different winter climate regimes and density-dependent processes during the past century have affected population dynamics of two arctic ungulate species. Our analyses were performed on the dynamics of six muskox and six caribou populations. In muskoxen, variation in winter climate, mediated through the North Atlantic Oscillation (NAO), explained up to 24% of the variation in interannual abundance, whereas in caribou up to 16% was explained by the NAO. Muskoxen responded negatively following warm and snowy winters, whereas caribou responded negatively to dry winters. Direct and delayed density dependence was recorded in most populations and explained up to 32% and 90% of variations in abundance of muskoxen and caribou, respectively. Received: November 19, 2001 / Accepted: May 28, 2002