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Climate change is rapidly transforming high latitude ecosystems, yet comparative growth responses between trees and shrubs at the northern treeline remain understudied. The Arctic and Sub-Arctic, including northern Scandinavia, has warmed nearly four times faster than the global average since 1979, leading to reduced snow cover and earlier onset of the growing season. These shifts highlight the need to understand how different life forms respond to ongoing warming. This study compares the climatic sensitivity of Pinus sylvestris L. trees and Juniperus communis L. shrubs in northern Norway, Øvre Pasvik region, positioned at the Europe’s northernmost treeline. Specifically, we aimed to assess whether the temperature sensitivity of two co-existing woody plant forms at the northern treeline are coherent and stable over time. We developed 385- and 423-year-long tree-ring width chronologies for Pinus trees and Juniperus shrubs, respectively. For both species we analysed their relationships with instrumental climate data: air temperature, precipitation and Standardized Precipitation-Evapotranspiration Index (SPEI, scale =3) for the period 1901–2022. We revealed that Pinus trees maintained strong and stable correlation with July temperatures (r = 0.56), while Juniperus shrubs exhibited weaker July temperature signal (r = 0.35) that declined and became largely non-significant after the 1970s. While increased drought stress is often suggested as a cause for temperature signal loss in high latitudes, this does not appear to be the case in Øvre Pasvik, as correlations between Juniperus shrubs and SPEI are generally weak or negative for the recent period. Additionally, growth of Pinus trees was positively related to May precipitation, while Juniperus shrubs exhibited negative relationship with May precipitation. Our findings highlight reliability of Pinus trees chronology as a temperature proxy at the northernmost treeline. We also underscore the necessity of high-resolution microtomy and microscopy techniques for accurate ring detection in Juniperus shrubs as standard method minimizing the risk of misdating or discarding ecologically valuable material.

An important factor for controlling the chemical signature of surface water is the interactions with soil particles and groundwater. In permafrost landscapes, ground ice restricts groundwater flow, which implies a limited influence of processes such as weathering on the chemical signature of the runoff. The aim of this study was to examine how freeze-thaw processes, hydrology and water age interact to shape the chemical and stable hydrogen and oxygen isotopic signature of surface water in a catchment in West Greenland. Measuring runoff in remote catchments is challenging, and therefore we used a validated hydrological model to estimate daily runoff over multiple years. We also applied a particle tracking simulation to determine groundwater ages and used data on stable isotopic and chemical composition from various water types-including surface water, groundwater, lake water and precipitation-spanning from early snowmelt to the end of the thawed season. Our results show that groundwater age generally is less than one year and rarely exceeds four years, total runoff is dominated by groundwater, and overland flow is restricted to the snowmelt period and after heavy rain events. Monitoring of thaw rates in the active layer indicates a rapid thawing in connection with running water, and meltwater from ground ice quickly becomes an important fraction of the runoff. Taken together, our data suggest that even in continuous permafrost landscapes with thin active layers and an absence of truly old and mobile groundwater, soil processes exert a strong influence on the chemical and stable isotopic signature of runoff, i.e., similar to what has been observed in other climatic settings.

The high temperature sensitivity of pine trees in northern Fennoscandia has led to some of the most reliable tree-ring climate reconstructions in the world for the past millennia. However, wood anatomical anomalies that likely reflect temperature-induced reductions in cell wall lignification, the so-called Blue Rings (BRs), have not yet been systematically investigated in trees and shrubs in northern Europe. Here, we present frontier research on the occurrence of BRs in Pinus sylvestris trees and Juniperus communis (L) s.l. shrubs from the upper treeline in northern Norway (69°N) in relation to instrumental temperature data covering the last ca. 150 years. The highest number of BRs was found in 1902, with 96% of Pinus trees and 68% of Juniperus shrubs showing BRs. These corresponded on average to a 42% vs. 27% proportion of the growth ring in 1902 which was less-lignified in Pinus trees and Juniperus shrubs, respectively. Another peak in BRs recorded for 1877 was more pronounced in Pinus trees (88%) than in Juniperus shrubs (36%), with a lower proportion of less lignified rings. We found the lowest monthly sums of growing degree days in June 1902 and August 1877, resulting in more uniform non-lignified BRs in 1902 than in 1877. Prolonged early growing season cooling shortened the growing season in 1902 and resulted in much thinner cell walls in trees and shrubs than in 1877, which was characterized by extended cooling at the end of the growing season. Also, after 1902 BR, Pinus trees exclusively showed no recovery in the mean cell wall thickness in the following year. Our study provides the first evidence for different impacts of early versus late growing season cooling on cell wall lignification in trees and shrubs at the northern treeline. Using the anatomy of BRs, we demonstrated the potential to refine summer cooling event reconstructions at an intra-annual resolution in northern Fennoscandia and beyond.

We perform a first permafrost higher education curriculum survey in Norden. Permafrost is part of the education within both bio- and geosciences and engineering, and the variation in educational activities reflect this. Five permafrost-specific geoscience and engineering permafrost courses exist, whereas there are 23 bachelor and 25 master courses with a permafrost content ranging from 1% to 50 %. The is large potential and clear needs for closer permafrost teaching collaboration. This could focus on permafrost course development, teaching methods, sharing practical experiences including fieldwork and further developing the educational offer. Such collaboration could establish: 1) An online, joint Nordic specific course on permafrost, sharing the special permafrost competences existing across the universities using digital teaching tools, 2) Nordic collaboration on developing joint, both general but also specific, PhD courses on permafrost, 3) Lifelong education in permafrost, and 4) Internships a part of active permafrost education to better meet the future employers and society’s needs. The Nordic region might also gain largely from establishing an overview-providing interdisciplinary joint Nordic course aiming to characterize the region and its diversity broadly including both natural and social sciences, and naturally covering different topics including permafrost and seasonally frozen ground. The mapping done for this paper will function as a first overall roadmap catalogue providing an overview of all offered courses on permafrost. The overall outcome of our survey shows large potential for increased and deeper inter-university collaboration for further developing joint permafrost higher education both in the form of courses and other educational activities between institutions across Norden, and potentially with ambitions for joint permafrost degrees between several institutions. Based on the presented results and the mapped different future plans for permafrost education across Norden, we discuss the implications of our results, specifically concerning the potential for increased collaboration in Nordic permafrost education. These focus on permafrost course development, teaching methods, sharing practical experiences including fieldwork and further developing the educational offer. In more detail increased collaboration could establish: 1) An online, joint Nordic-specific course on permafrost, sharing the special permafrost competences existing across the universities using digital teaching tools, 2) Nordic collaboration on developing joint PhD courses on permafrost, 3) Lifelong education in permafrost, and 4) Internships as part of active permafrost education to better meet the needs of future employers and society. The Nordic region might also gain largely from establishing an interdisciplinary joint Nordic course, aiming to characterize the region and its diversity broadly and including both natural and social sciences, and naturally covering different topics including permafrost and seasonally frozen ground.

In Arctic landscapes, the active layer forms a near-surface aquifer on top of the permafrost where water and nutrients are available for plants or subject to downslope transport. Warmer summer air temperatures can increase the thickness of the active layer and alter the partitioning of water into evapotranspiration and discharge by increasing the potential evapotranspiration, the depth to the water table, and changing the flow paths but the interacting processes are poorly understood. In this study, a numerical model for surface- and subsurface cryo-hydrology is calibrated based on field observations from a discontinue permafrost area in West Greenland considered sensitive to future climate changes. The validated model is used to simulate the effect of three summers with contrasting temperature regimes to quantify the variations in the active layer thickness, the resulting changes in the water balance, and the implications on solute transport. We find that an increase of summer air temperature by 1.6°C, under similar precipitation can increase the active layer thickness by 0.25 m, increase evapotranspiration by 5%, and reduce the total discharge compared to a colder summer by 9%. Differences in soil moisture and evapotranspiration between upslope and downslope were amplified in a warm summer. These hydrological differences impact solute transport which is 1.6 times faster in a cold summer. Surprisingly, we note that future warmer summer with increase in permafrost thaw may not necessary lead to an increase in discharge along a hill slope with underlying permafrost.

• Precipitation has increased over recent decades, especially incold seasons, and is associated with an increase in rainfallin all seasons and a decrease in snowfall in summer, withspatially varying trends in winter. This century, precipitationevents presently regarded as extremes are expected to becomeroutine. Snow mass has decreased across northern NorthAmerica, but in Eurasia the trend has been negligible andsnow depth has increased in parts of Eurasia.

• Permafrost thaw is likely to drive changes in the waterbalance in Arctic areas, but the relevant subsurface processesare difficult to observe directly at the catchment scale.However, observed changes in streamflow dynamics andwater chemistry indicate that permafrost thaw is influencinghydrological connectivity by creating deeper and longerwaterflow pathways through catchments across the Arctic.

• Increasing trends in annual river discharge to the Arctic Oceanfrom both continents have continued, providing compellingevidence of intensification of the Arctic water cycle. Asignificant increase in base streamflow during the cold seasonis observed across most regions of the pan-Arctic drainagebasin. The magnitude of maximum river discharge has notchanged significantly; however, the timing of snowmelt freshethas become earlier almost everywhere across the pan-Arctic.

• Lake area is declining across the discontinuous permafrostzone. In the continuous permafrost zone, however, the numberof sites with decreasing lake area is similar to the numberwith increasing lake area. Stronger lake area declines in thediscontinuous permafrost zone is consistent with permafrostthaw being further advanced there than in the continuouspermafrost zone.

• Ice-cover duration on rivers has declined significantly in coldregions over the past several decades due to later freeze-upand earlier breakup. The observed decline in river ice is likelyto continue in the future due to the projected increase in airtemperature. Maximum river-ice thickness has decreasedsignificantly on most pan-Arctic rivers over the last 50 to60 years, with the greatest decrease observed before 2000.

• Lakes are rapidly losing ice across the Northern Hemisphere,with later ice-on dates, earlier ice-off dates, and in some years,some lakes not freezing at all.

• Freshwater delivery from Arctic land ice is roughly equivalentto that from North American rivers. Eurasian river dischargeis roughly three times higher. However, the increase in Arcticriver discharge was 1.6 times smaller than the increase infreshwater flux from Arctic land ice. Most of the increasedland-ice freshwater discharge originated from Greenland andArctic Canada. A further increase in freshwater flux fromland ice reduction is likely to continue with the projectedfuture increase in Arctic warming.

• Changes in the terrestrial hydrological system have importantimpacts on ecosystems and Arctic livelihoods. Declining snowcover, permafrost, lake area, and lake ice have implications forecosystems, as well as for hunting, fishing, reindeer herding,transportation, and drinking water availability. Impactsalso include feedbacks to the climate and ocean circulationthrough increased freshwater fluxes to the Arctic Ocean andchanges in lake area and ice cover.