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In northern biomes, growth is nitrogen (N) limited, but bryophytes are abundant. These bryophytes often host N2-fixing microorganisms (diazotrophs) that play a crucial role in the N cycle of these ecosystems. Despite their importance, how the bryophyte-associated N2-fixation varies across species and seasons (summer, autumn, winter, and spring) remains poorly understood. We measured N2-fixation rates for 10 bryophyte species in situ throughout the entire year in the Arctic with additional incubations to verify the method. We measured positive N2-fixation during most of the year, except for the coldest period (February). The species growing in the wettest conditions (Sphagnum spp.) had the highest N2-fixation rates in summer, while bryophytes in drier conditions peaked in N2-fixation rates in spring and autumn. The seasonal variation in N2-fixation activity was pronounced, but similar patterns were found among different species. This study reveals that bryophyte-associated N2-fixation in northern ecosystems is larger than previously assumed, as it occurs over a more extended part of the year than previously inferred. Furthermore, the importance of bryophyte-associated diazotrophs cannot be quantified without considering both the diversity of bryophytes and their variation in N2-fixing seasonal activity patterns. Both future changes in climatic conditions and biodiversity of bryophytes can thus have large implications for the N cycle in arctic regions.

Questions: Alpine grasslands are affected by a range of global change drivers, including land-use change, climate warming and pollution. How these drivers interact and affect plant functional communities is poorly understood. We used plant functional traits to test the single and interactive effects of warming, nitrogen addition and grazing on alpine grassland communities and assessed the importance of intraspecific trait variation.

Location: Alpine and sub-alpine grasslands in western Norway.

Methods: For three years, we applied global change treatments to test the effects of warming with nitrogen addition, and warming with grazing at an alpine and sub-alpine plant community. We measured six plant functional traits related to plant size and leaf economics, including intraspecific trait variation.

Results: Our results show that warming and nitrogen addition shifted size-related traits in plant communities towards taller plants with larger leaves, and more strongly in the alpine than in the sub-alpine plant community. Warming also affected leaf economic traits, promoting faster traits in the alpine and slower traits in the sub-alpine plant community. Grazing shifted communities to faster leaves (grazing tolerant) in the sub-alpine community and slower leaves (grazing avoidance) in the alpine community. There were no interactive effects between the global change drivers. The relative contributions of species turnover and intraspecific trait variation to overall trait variation differed between origins of the two plant communities.

Conclusions: We show that these global change drivers shift alpine and sub-alpine plant communities in different directions, likely due to differences in resource availability. Our results support the need for site-specific management strategies in these systems.

Aims: Rising ambient air temperatures may have adverse effects on alpine plant communities. To avoid extinction and to mitigate the demographic impacts of climate warming, migration and the ability to adapt become increasingly important. Experimental studies that simulate warming help test the extent and direction of functional adaptation. We ask: (1) To what extent does experimental warming drive intraspecific trait shifts in alpine species? (2) Do these trait responses vary across a precipitation gradient? (3) Do responses vary between forbs and graminoids?.

Location: Alpine grasslands along a precipitation gradient in south-western Norway.

Methods: At three alpine sites spanning 1315–3601 mm of annual precipitation, we measured 10 plant functional traits across 17 species of graminoids, perennial forbs and dwarf shrubs. We compared the traits of plants in open-top warming chambers with those under ambient temperature conditions. Effect sizes were estimated using Cohen's d and analysed with respect to precipitation regimes and growth form.

Results: Plant height generally increased in response to warming across all sites for both growth forms. For other traits, warming effects were context-dependent and varied among species. At the medium precipitation site, plants showed shifts toward more conservative resource-use strategies in response to warming, characterised by increased leaf area and leaf dry matter content, along with reduced species leaf area and leaf nitrogen concentration. The enrichment in heavier nitrogen and carbon isotopes with warming is consistent with the expectation that warming selects for individuals with high water-use efficiency. Forbs generally exhibited stronger but more variable responses to warming than graminoids.

Conclusion: Warming induces trait shifts in alpine species, but responses depend on local conditions and growth form. Predicting alpine community responses to climate change requires trait-based approaches and research designs that allow assessing and exploring patterns in both taxonomic and environmental context dependencies.

Plant functional trait-based approaches are powerful tools to assess the consequences of global environmental changes for plant ecophysiology, population and community ecology, ecosystem functioning, and landscape ecology. Here, we present data capturing these ecological dimensions from grazing, nitrogen addition, and warming experiments conducted along a 821 m a.s.l. elevation gradient and from a climate warming experiment conducted across a 3,200 mm precipitation gradient in boreal and alpine grasslands in Vestland County, western Norway. From these systems we collected 28,762 plant and leaf functional trait measurements from 76 vascular plant species, 88 leaf assimilation-temperature responses, 577 leaf handheld hyperspectral readings, 2.26 billion leaf temperature measurements, 3,696 ecosystem CO2 flux measurements, and 10.69 ha of multispectral (10-band) and RGB cm-resolution imagery from 4,648 individual images obtained from airborne sensors. These data augment existing longer-term data on local climate, soils, plant populations, plant community composition, and ecosystem functioning from within the same experiments and study systems and from similar systems in other mountain regions globally.

In boreal and arctic ecosystems, the seasons experience pronounced temporal variation, leading to high variability in environmental conditions. Plants are challenged by a short (aboveground) growing season during the summer and an extended period of cold temperatures, low light or snow cover during the long winter. Furthermore, nutrients are scarce and highly contested. Plants have evolved a range of mechanisms to survive and grow even in these harsh conditions, and both vascular plants and bryophytes may be able to use the “shoulder seasons” of spring and autumn. Yet, much of the research has focused on the processes during the summer season and less on seasons beyond; the full extent of annual seasonal variation in plantactivity remains unexplored.

In this thesis, I explore three aspects of plant seasonal activity in a low arctic ecosystem in Northern Sweden. I traced root potential nitrogen (N) uptake in vascular plants by using isotopic ¹⁵N labelling over multiple points throughout the year in the field. Furthermore, I explored two aspects of bryophyte activity in a range of species throughout a year: N₂-fixation, measured with the acetylene reduction assay, as well as photosynthesis.

My results show that vascular plants can acquire N at any time during the year in equal proportions, and potentially even more efficiently in winter. Bryophytes are less active in the middle of the winter, but activity in both photosynthesis and N₂-fixation peaks in the shoulder seasons for a majority of species. Interestingly, some species show activity even in the early and late winter. In contrast activity during the summer is much more limited, potentially because of drought, for many bryophyte species.

Overall, winter—a season often omitted from studies in boreal and arctic ecosystems—is as important for plant activity as summer, if not more so given its longer duration. To fully understand plant activity in these high-latitude ecosystems, the winter season has to be considered. The results from the vascular plants show that there is a temporal mismatch in the acquisition of nutrients and carbon, while bryophytes display a continued ability to acquire both nutrients and carbon. This has implications for understanding plant growth and survival in these ecosystems along with carbon and N dynamics. The concepts of growing season and “winter” should shift towards a more nuanced seasonal aspect where activity during the winter season is integrated and considered.

Aims: Plant growth in the Arctic is often nutrient limited due to temperature constraints on decomposition and low atmospheric input of nitrogen (N). Local hotspots of nutrient enrichment found in up to 4000-year-old archaeological deposits can be used to explore the recycling and long-term retention of nutrients in arctic ecosystems.

Methods: We investigated old Inuit and Norse deposits (known as middens) and adjacent tundra ecosystems along a wet-dry fjord gradient in western Greenland to explore the isotopic fingerprinting of plant and soil carbon and nitrogen (C-13/C-12 and(15)N/N-14) derived from human presence.

Results: At all locations we observed a significant isotopic fingerprint in soil and plant N related to human deposits. This demonstrates a century-long legacy of past human habitation on plant and soil characteristics and indicates a surprisingly high N retention in these ecosystems. This is consistent with the significantly higher plant biomass in areas with archaeological deposits.

Conclusion: Vegetation composition and N in plants and soils displayed marked differences along the wet-dry fjord gradient. Furthermore, the profound nutrient enrichment and organic matter accumulation in archaeological deposits compared to surrounding tundra demonstrates a century-long legacy of past habitation on plant and soil characteristics as well as efficient N cycling with surprisingly limited N loss.