Influence of reindeer herbivory on Arctic ecosystem structure and functioning across spatial and temporal scales

Top-down and bottom-up forces interact to regulate ecosystem structure and functioning across Earth’s biomes. In the Arctic, warming is outpacing the global average and driving extensive changes including increases in permafrost thaw and vegetation productivity. These ecosystem responses are mediated by herbivores, particularly reindeer, which are the only large herbivores presently widespread in tundra environments. Herbivory in the Arctic affects vegetation productivity, plant community composition, and soil thermal regimes, and thus has implications for the carbon cycle and energy balance potentially relevant to global-scale climate feedbacks. However, our understanding of these Earth system consequences is hindered by uncertainty as to whether herbivory remains a major control on ecosystem processes over larger, regional scales in the Arctic and how the effects of herbivory vary in space and time.
This thesis aims to detect and characterise the influence of reindeer herbivory from the regional to the plot scale and depending on the time since and duration of herbivory events. The research focussed on the Yamal Peninsula, where Indigenous Nenets mobile pastoralists manage the world’s largest domesticated reindeer herds, and included analysis of remote sensing products and field-based data collection. The objectives of the thesis are addressed through three studies presented as separate research articles.
The first article, ‘Top-down regulation by a reindeer herding system limits climate-driven Arctic vegetation change at a regional scale,’ used spatial comparisons to determine whether differences in vegetation across a large swath of the Yamal Peninsula were linked to variations in reindeer herbivory pressure. By creating new maps of herd sizes and decades-old migration routes and integrating these data with remote sensing products, this work found that higher reindeer density was linked to lower coverage of tall woody vegetation and less sensitivity of productivity to climate within this vegetation type. These findings were consistent across six separate lines of evidence, leading to the conclusion that reindeer have had a significant, albeit modest, effect at the regional scale that has limited the cover of tall woody vegetation and its growth and expansion with warming.
The second article, ‘Heterogeneity in immediate and lagged effects of reindeer herbivory over spatially comprehensive fine scales,’ included landscape-scale analysis that involved conducting repeat unmanned aerial vehicle (UAV) surveys next to a campsite of Indigenous Nenets managing a large herd. The surveys occurred immediately before the herd arrived at the campsite, immediately after they had left the site to migrate onwards, and one month after the herd’s activity to map vegetation changes, continuously over space at very fine, centimetre-scale resolution. The processing of the UAV imagery employed structure-from-motion (SfM) photogrammetry in a novel workflow that enabled detection of significant canopy height changes between surveys exceeding a spatially variable limit of detection based on photogrammetric and rasterisation uncertainty. The significant changes showed primarily decreases in vegetation height immediately after the herbivory event and increases one month later, but with high spatial heterogeneity, which nevertheless showed spatial structure. Evaluation of this spatial heterogeneity by mapping habitat types, deriving vegetation height, and incorporating reindeer GPS collar data revealed that areas with high reindeer activity showed evidence of reindeer browsing around the edges of erect shrubs and trampling of low-lying vegetation as well as lagged growth in low-lying vegetation one month later. Areas with lower herbivory pressure showed strikingly little change, despite belonging to similar habitat types, demonstrating the effects of a reindeer herbivory event on vegetation change within a growing season.
Lastly, the third article, ‘Offsetting impacts of different size classes of herbivores on CO2 uptake in a tundra ecosystem,’ employed a more traditional plot-based ecological approach to compare CO2 fluxes within a seven-year-old exclosure experiment excluding three different size classes of herbivores within three different habitat types. Variations in fluxes were driven primarily by differences in photosynthesis, rather than respiration. Consistently, controls accessible to all herbivores were the weakest CO2 sinks, but plots open to small herbivores and small- and medium-sized herbivores were progressively stronger CO2 sinks than plots where all herbivores were excluded. After ruling out differences in litter cover and plant community composition, the most likely explanation for these dynamics included a combination of defoliation from reindeer and nutrient fertilisation by smaller herbivores. The effects of reindeer were most pronounced in willow meadows, whereas smaller herbivores had the largest influence within forb tundra. These findings highlight the importance of considering the different functions of herbivores and how they interact with different environmental contexts in the Arctic to understand the aggregate, non-additive consequences for ecosystem processes.
Overall, this thesis demonstrates new methodologies for assessing the impacts of herbivores with spatially comprehensive measurements and offers novel insights into the effects of reindeer in tundra ecosystems. The impacts of reindeer were apparent and consistent across spatial scales, including at the regional scale, suggesting that they have significance for the Earth system. The lines of evidence showed that reindeer herbivory can limit the growth of shrubs in the Arctic, mediated by topography and landcover, with implications for ecosystem processes, namely carbon cycling. This work can inform future studies of Arctic herbivory and facilitate the inclusion of top-down regulation in ecosystem models, which is necessary to accurately predict the responses of tundra ecosystems to climate change and the resulting feedbacks.

Keywords: structure-from-motion
climate change
Rangifer tarandus
Arctic ecology
remote sensing
UAV
flux chamber
herbivory
Yamal
Advisor: Macias-Fauria, M