Reindeer herding has a long history in northern Norway, Sweden and Finland, where it has contributed to shaping the Fennoscandian mountain landscape. Through extensive grazing, semi-domestic reindeer influence vegetation structure and composition, and can partly mitigate climate change effects on vegetation. However, northern pastoralism is increasingly challenged by cumulative pressures stemming from land-use changes, climate warming and predator pressure. These pressures act at different spatial and temporal scales, complicating our understanding of how multiple, interacting pressures affect reindeer grazing behaviour. Since reindeer grazing is both an ecological driver of tundra plant communities and is shaped by many factors, disentangling the relative importance of these relationships is therefore critical to better anticipate future ecosystem change in the mountain tundra.

In this thesis, I investigated how climate and land-use changes, in interaction with other factors, influence reindeer grazing patterns in northern Fennoscandia, and may subsequently affect the tundra vegetation. I specifically assessed the spatial exposure to multiple pressures, quantified reindeer grazing behaviour in space and time, and examined how shifts in grazing patterns may cascade through tundra plant communities in the Swedish mountain tundra. To do so, I combined spatial analyses of cumulative pressures, long-term climate data, GPS tracking with accelerometer-based behavioural data, and vegetation surveys.

Over the whole Fennoscandian herding region, I show that the vast majority of the grazing lands is exposed to one or multiple land-use pressures, often co-occurring with predator presence (Paper I). In that same study, I estimated a regional warming of 1.5–2°C over the past sixty years. Such warming in summer implies shifts in grazing patterns, that was further analysed in Paper II. This study showed that reindeer grazing was strongly limited spatially and temporally by warm summer temperatures in the mountain tundra. Such constraining effect of heat is becoming more common with warming summers, likely diminishing herbivory pressure (Paper II). At a local scale, grazing patterns were also shaped by abiotic conditions. Soil wetness emerged as a key predictor of where reindeer grazed, with wetter sites being significantly less grazed, enhancing distinct plant communities (Paper III). Additionally, human presence in the mountains was generally associated with a reduced reindeer occurrence and grazing activity (Paper IV). Although, if it offered protection from predators, reindeer would tolerate human disturbance, yet usually at the cost of grazing less.

Taken together, these results show that cumulative pressures constrain reindeer grazing both spatially and temporally, leading to a fragmented use of summer pastures. Areas that are consistently under-grazed or avoided, particularly near human infrastructures and in warm and wet habitats, are likely to experience a weakened top-down control on vegetation. By demonstrating how anthropogenic activities, environment and predators jointly may alter reindeer behaviour and its ecological functions, this work emphasizes the need to consider cumulative and interacting pressures when predicting future ecosystem change in northern mountain landscapes.

Climate change is predicted to alter species interactions by exposing ecosystems to increasingly frequent and intense warm spells. In the mountain tundra, grazing by large herbivores, particularly reindeer, can limit shrub expansion and preserve Arctic plant diversity. However, the impact of rising temperatures on herbivores themselves remains understudied. 

Here, we combine long-term weather data with spatially-explicit behavioural data from 31 free-ranging reindeer from three Swedish herding districts equipped with GPS, temperature sensors, and tri-axial accelerometers over two consecutive summers to investigate how warming affects grazing. We hypothesise that both heat stress and insect harassment reduce grazing under warm conditions. 

First, we show that reindeer significantly reduce grazing beyond a body surface temperature (T_R) of 20.3°C, likely due to insect harassment. As reindeer speed sharply declines beyond 24°C T_R, our results suggest an onset of physiological heat stress, indicating that warm spells limit grazing through insect harassment, but also overheating. Second, warming also triggers a shift in habitat-use, as reindeer relocate their grazing activity outside their primary grazing land for less favourable high-elevation habitats, further reducing foraging efficiency. These behavioural and spatial shifts result in a net loss of foraging, with no evidence of compensatory grazing. Third, we find that warm spells - defined as 24-hour period with a maximum air temperature above 13°C - have become more frequent over the last 30 years, now occurring during over half the summer.

Overall, this study highlights how thermal discomfort can disrupt and relocate the foraging patterns of reindeer, a keystone herbivore in the tundra. Such reduced herbivory pressure could have severe cascading consequences by accelerating shrubification and contributing to local biodiversity loss. Hence, climate warming does not only alter abiotic conditions, but can also disrupt biotic processes that underpin the resilience of cold ecosystems.

Questions: The relative importance of abiotic versus biotic factors on structuring plant communities is debated, especially in the Arctic tundra where the harsh environment is limiting together with the effects of grazing by reindeer. To understand the relative and interactive effect of abiotic (bottom-up) and biotic (top-down) factors on vegetation in the Swedish mountain tundra, we ask how do bottom-up factors and their interaction affect reindeer grazing activity and vegetation composition?. Location: Summer pastures of Gran reindeer herding district, in Vindelfjällen mountain tundra (northern Sweden). Methods: We surveyed the composition of vascular plants, bryophytes, and lichens across 34 sites (17 north-facing and 17 south-facing) along a grazing duration gradient based on data collected from accelerometers collared on reindeer. Data on the bottom-up factors slope, soil wetness, soil depth, primary productivity and the top-down factor grazing duration were extracted for each of our sampled plots (n = 102). The additive and interactive relation between all factors and vegetation composition and species richness was analyzed using generalized linear models. Results: Reindeer grazed for a longer time in drier than wetter sites, indicating an important interaction between grazing and soil wetness. Bottom-up factors prevailed as the dominant driver of local vegetation patterns, while grazing duration had weak effects on the vegetation. Wetter sites with longer grazing duration had more graminoid species, whereas drier sites with shorter grazing duration had more shrub and lichen species. Conclusions: The study shows that species richness of vascular plants, lichens and bryophytes is related to soil wetness but is also influenced by reindeer grazing intensity. Based on these results, we stress the importance of further investigating the interaction between grazing and soil wetness in order to foresee changes in the tundra vegetation, especially as plant communities might change under altered grazing regimes and future hydrological conditions as an effect of predicted climate change.

Expansion of boreal species into tundra ecosystems is a consequence of climate change and human exploitation that threatens local species through increased predation, competition, and pathogen transmission. Under these circumstances, efficient control of expanding boreal species may be necessary, but the efficiency of such action depends on understanding the ecological influences of expansion. The red fox (Vulpes vulpes) is expanding into the tundra across the Arctic. In Scandinavia, red foxes threaten local tundra species and communities including the endangered Arctic fox (V. lagopus). The ecological dynamics in the tundra are influenced by small rodent cycles (classified into different phases based on seasonal abundance fluctuations), which can affect red fox expansion, distribution, and abundance. We used a 17-year (2004–2020) dataset from the tundra in Sweden, consisting of raw snow track data, to test how cyclic prey influenced red fox distribution and abundance, and subsequently red fox control. The winter abundance of red fox was influenced by small rodent phase, with higher abundance during high prey availability (i.e., increased number of prey numbers) with no support for a time lag between red fox and small rodent abundance. This suggests that high prey availability attracts red foxes to the tundra and that higher immigration from the boreal zone can be expected in response to increased prey abundances. There was no relationship between red fox control and small rodent availability, but control was influenced by red fox abundance during the previous year, which highlights an opportunistic control strategy. We recommend an adaptive management strategy where authorities include small rodent dynamics in the planning and execution of red fox control.

Spatially synchronous fluctuations of animal populations have profound ecological consequences, especially in northern latitudes. Spatially coupled fluctuations are often seen in small rodent populations, albeit with local and regional variations. While both resource limitation and predation influence rodent dynamics, their relative importance for generating spatial variation is less clear, particularly during winter. In this study, we quantify spatial variation in winter abundance of the Norwegian lemming (Lemmus lemmus) across three ecologically connected mountain areas in northern Sweden and evaluate whether the relative strength of bottom-up and top-down regulation influences such variation. Our data included observations of predated and nonpredated winter nests as well as environmental characteristics of nest locations and nest predation. While the direction of annual changes in lemming nest abundance was perfectly synchronized among the three areas, there were differences in nest abundance, potentially caused by contrasting amplitudes of temporal fluctuations in lemming winter populations. Mustelid predation was positively associated with decreasing lemming populations but did not differ in occurrence among the three areas. Lemming nests were predominantly observed in meadows, whereas areas prone to flooding and close to the tree line were underrepresented. Mustelid predation was most common close to the tree line, but not associated with geomorphological characteristics related to snow depth. We suggest that the observed differences in lemming winter abundances were caused by variations in the relative strength of bottom-up and top-down regulation in the three mountain areas. We encourage further studies evaluating how the relative strength of different processes influence local population regulation, and how such processes influence spatial variation in animal population dynamics at different spatial scales.

Traditional grazing areas in Europe have declined substantially over the last century. Specifically, in northern Fennoscandia, the grazing land is disturbed by cumulative land-use pressures. Here we analysed the configuration of the grazing land for reindeer and sheep in northern Fennoscandia in relation to the concurrent land-use pressures from tourism, road and railway networks, forestry, industrial and wind energy facilities, together with predator presence and climate change. Our results show that 85% of the region is affected by at least one land-use pressure and 60% is affected by multiple land-use pressures, co-occurring with predator presence and rising temperatures. As such, a majority of the grazing land is exposed to cumulative pressures in northern Fennoscandia. We stress that, if the expansion of cumulative pressures leads to grazing abandonment of disturbed areas and grazing intensification in other areas, it could irreversibly change northern vegetation and the Fennoscandian mountain landscape.

European populations of geese, swans and cranes have increased considerably since the 1970s raising conflicts between conservation and farming interests. Crop damage caused by geese, swans and cranes across the national scale needs a trans-boundary approach that captures the site-specific characteristics of crop damage at a more refined spatial scale, to deal with the high spatio-temporal variation inherent in the system and to avoid conflict displacement. In the present study we use long-term crop damage data (2000-2015) in Sweden to evaluate seasonal and annual patterns of crop damage. We show that crop damage increased over years but followed a fairly consistent seasonal pattern during the later parts of the study period. We show how these seasonal patterns differ across the country such that trans-boundary regions with similar patterns of crop damage, relating to different nuisance species and damaged crops, can be identified. These findings about spatio-temporal variation of damage can be used to find appropriate scales of management units (e.g. areas with similar conditions), and to adapt damage mitigation strategies to temporal and spatial-specific conditions, e.g. guidance of when and where certain crop may be suitable as sacrificial crops.

Studies of ecological processes should focus on a relevant spatial scale, as crude spatial resolution will fail to detect small scale variation which is of potentially critical importance. Remote sensing methods based on multispectral satellite images are used to assess primary productivity and aerial photos to map vegetation structure. Both methods are based on the principle that photosynthetically active vegetation has a characteristic spectral signature. Yet they are applied differently due to technical differences. Satellite images are suitable for calculations of vegetation indices, for example Normalized Difference Vegetation Index (NDVI). Colour infrared aerial photography was developed for visual interpretation and never regarded for calculation of indices since the spectrum recorded and post processing differ from satellite images. With digital cameras and improved techniques for generating colour infrared orthophotos, the implications of these differences are uncertain and should be explored. We tested if plant productivity can be assessed using colour infrared aerial orthophotos (0.5 m resolution) by applying the standard NDVI equation. With 112 vegetation samples as ground truth, we evaluated an index that we denote rel‐NDVI_ortho in two areas of the Fennoscandian mountain tundra. We compared the results with conventional SPOT5 satellite‐based NDVI (10 m resolution). rel‐NDVI_ortho was related to plant productivity (Northern area: P = <0.001, R² = 0.73; Southern area: P = <0.001, R² = 0.39), performed similar to SPOT5 satellite NDVI (Northern area: P = <0.001, R² = 0.76; Southern area: P = <0.001, R² = 0.40) and the two methods were highly correlated (cor = 0.95 and cor = 0.84). Despite different plant composition, the results were consistent between areas. Our results suggest that vegetation indices based on colour infrared aerial orthophotos can be a valuable tool in the remote sensing toolbox, offering a high‐spatial resolution proxy for plant productivity with less signal degradation due to atmospheric interference and clouds, compared to satellite images. Further research should aim to investigate if the method is applicable to other ecosystems.

Global warming is predicted to change ecosystem functioning and structure in Arctic ecosystems by strengthening top‐down species interactions, i.e. predation pressure on small herbivores and interference between predators. Yet, previous research is biased towards the summer season. Due to greater abiotic constraints, Arctic ecosystem characteristics might be more pronounced in winter. Here we test the hypothesis that top‐down species interactions prevail over bottom‐up effects in Scandinavian mountain tundra (Northern Sweden) where effects of climate warming have been observed and top‐down interactions are expected to strengthen. But we test this ‘a priori’ hypothesis in winter and throughout the 3–4 yr rodent cycle, which imposes additional pulsed resource constraints. We used snowtracking data recorded in 12 winters (2004–2015) to analyse the spatial patterns of a tundra predator guild (arctic fox Vulpes lagopus, red fox Vulpes vulpes, wolverine Gulo gulo) and small prey (ptarmigan, Lagopus spp). The a priori top‐down hypothesis was then tested through structural equation modelling, for each phase of the rodent cycle. There was weak support for this hypothesis, with top‐down effects only discerned on arctic fox (weakly, by wolverine) and ptarmigan (by arctic fox) at intermediate and high rodent availability respectively. Overall, bottom‐up constraints appeared more influential on the winter community structure. Cold specialist predators (arctic fox and wolverine) showed variable landscape associations, while the boreal predator (red fox) appeared strongly dependent on productive habitats and ptarmigan abundance. Thus, we suggest that the unpredictability of food resources determines the winter ecology of the cold specialist predators, while the boreal predator relies on resource‐rich habitats. The constraints imposed by winters and temporary resource lows should therefore counteract productivity‐driven ecosystem change and have a stabilising effect on community structure. Hence, the interplay between summer and winter conditions should determine the rate of Arctic ecosystem change in the context of global warming.

1.     The long history of reindeer herding in the Fennoscandian mountains has shaped ecosystems and can help mitigate climate change effects on vegetation. Yet, outdoor tourism is discussed as a major pressure resulting in a changed grazing behaviour, potentially leading to shifts in the tundra plant communities. 

2.     We tagged reindeer with GPS and accelerometers in three reindeer herding districts in northern Sweden, and analysed their occurrence and grazing activity in relation to people, habitat and predators. 

3.     Overall, we found that reindeer presence and grazing activity was negatively associated with human presence, but where there was a positive association between reindeer and human presence, grazing activity remained mostly decreased. 

4.     In all districts, we detected interactive effects between predators and humans, suggesting a “human-shield” effect at play, i.e. reindeer use the proximity to specific types of human infrastructure as a refuge from predation.  

5.     Our study shows that human presence significantly influenced where reindeer chose to be and where they chose to graze, highlighting the need to recognize the human influence on herbivores, even when assessing vegetation change in the tundra. Our findings hence encourage to move beyond a single-driver perspective and to consider multiple, interacting pressures to better anticipate future ecological shifts under climate change.