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The Arctic is warming and significant changes to the landscape, including increased vegetative cover ("greening"), are expected in the near future. These landscape changes may alter nitrogen (N) availability in terrestrial, stream, and coastal ecosystems, where production is often N limited, but the exact changes in nutrient cycling are uncertain. Here, we analyzed the relationship between vegetation greenness (i.e., NDVI) and dissolved inorganic (DIN) and organic (DON) concentrations in streams draining 14 headwater catchments (mean 3.6 km2, range 0.4–11 km2) across three samplings in the Zackenberg area, Northeast Greenland. We found large variation in DIN and DON concentrations across the sampled streams. We further show that this variation is correlated to water temperature and catchment NDVI, such that increased vegetation greenness and temperature correlated with lower DIN, and increased greenness also correlated with higher DON concentrations in streams. The results suggest that increased terrestrial vegetation due to rising air temperature could substantially alter dissolved N concentrations and form in streams, with potentially cascading impacts on coastal areas. Plain Language Summary: This study links terrestrial vegetation cover to stream water chemistry in High Arctic headwater catchments. We found significant spatial and seasonal variation in dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON) availability in streams. Nitrate (NO3− ${{\text{NO}}_{3}}^{-}$) concentration in stream water decreases and DON increases with increasing spectral median normalized difference vegetation index (NDVI) in the stream catchment (i.e., proxy for terrestrial vegetation). This shift in the form of available nitrogen (N) has significant implications as terrestrial vegetation is changing quickly in High Arctic areas, which may then alter the amount and the dominant form of N exported from catchments to coastal areas. Key Points: High terrestrial vegetation greenness (NDVI) correlates negatively with dissolved inorganic nitrogen concentrations in High Arctic catchmentsDissolved inorganic nitrogen decreases and organic nitrogen increases with increased terrestrial vegetation greennessFuture climate changes in High Arctic areas may alter the amount and form of nitrogen exported from land to sea [ABSTRACT FROM AUTHOR]

Contrary to the general "greening of the Arctic", the Siberian Indigirka Lowlands show strong "browning" (a decrease in the Normalized Difference Vegetation Index or "NDVI") in various recent satellite records. Since greening and browning are generally indicative of increases and losses in photosynthetically active biomass, this browning trend may have implications for the carbon balance and vegetation of this Arctic tundra region. To explore potential mechanisms responsible for this trend break from general Arctic greening, we studied timeseries of Landsat summer maximum NDVI, weather data, and high‐resolution maps of vegetation compositional change, topography, geomorphology and hydrology. We find that a significant proportion of browning (lower summer NDVI) is explained by moisture dynamics, with high snow depths and resulting floods as well as summer drought coinciding with low NDVI. Relations between seasonal weather variables and NDVI are spatially heterogeneous, with floodplains, drained thaw lake basins and Yedoma ridges showing different patterns of association with weather variables. Low summer NDVI after high snowfall was particularly evident in floodplains, likely explained by early summer floods. Local small‐scale vegetation changes explained only small amounts of variance in browning rates in Landsat NDVI. Local expansion of Sphagnum vegetation in particular may have contributed to recent browning of our study site, but higher resolution NDVI timeseries are necessary to accurately constrain the role of small‐scale vegetation shifts. Overall, associations identified in this study suggest that future increases in Arctic precipitation variability and extremes may limit tundra greening, but to different extents even across comparatively small topographical contrasts. Plain Language Summary: Across Arctic regions, satellite images show that tundra ecosystems have been "greening". This suggests that plant growth, and thereby uptake of carbon from the atmosphere, is increasing in the Arctic. The Indigirka Lowlands in Siberia provide a stark contrast; in this lowland tundra region, satellite images show "browning", which suggests a decline in plant growth. We explored why this might be the case, in order to better assess whether browning should be expected across the wider Arctic in a changing climate. We compare local browning rates, calculated as changes in spectral indices from Landsat satellite images, to annual weather data, detailed maps of local vegetation changes and maps of terrain properties such as elevation and hydrology. We find that browning is associated with high snow depths and resulting floods, but also with dry summer conditions. Browning was not strongly related to local, small‐scale changes in vegetation (the most common of which was expansion of peat moss). We also find that the degree of association with potential drivers of browning (e.g., flooding, dry summers) differs across landforms. Floodplains show particularly strong browning following high snowfall, likely explained by early summer floods. This suggests that expected increases in year‐to‐year variability and extremes in Arctic precipitation may limit future greening. Key Points: Our study site in the Indigirka Lowlands shows strong (−0.0039 NDVI units yr−1) and ubiquitous (76% of area) recent browning in Landsat NDVIHigh snowfall and low summer rainfall were associated with lower summer NDVI, with differential impacts across topographical gradientsLocal small‐scale shifts in vegetation types and thermokarst activity show weak association with Landsat NDVI trends [ABSTRACT FROM AUTHOR]

Evergreen dwarf shrubs respond swiftly to warming in the cool and dry High Arctic, but their response in the warmer Low Arctic, where they are expected to be outcompeted by taller species under future warming, remains to be clarified. Here, 12,528 annual growth increments, covering 122 years (1893–2014), were measured of 764 branches from 25 individuals of the evergreen dwarf shrub Cassiope tetragona from a Low‐Arctic erect dwarf‐shrub tundra site in western Greenland. In addition, branch initiation and mortality frequency time series were developed. The influence of seasonal climate and correspondence with fluctuations in regional normalized difference vegetation index (NDVI), a satellite‐proxy for vegetation productivity, were studied. Summer temperatures were the main driver of growth, while winter temperatures were the main non‐summer‐climate driver. During past and recent warm episodes, shrub growth diverged from summer temperatures. In recent decades, early summer precipitation has become the main growth‐limiting factor for some individuals, likely through micro‐topography‐determined soil moisture availability, and more than half of the shrubs studied became irresponsive to summer temperatures. There was correspondence between climatic drivers, C. tetragona growth and branch initiation frequency, and satellite‐observed vegetation productivity, suggesting the area's shrub‐dominated tundra vegetation is limited by similar climatic factors. Winter warming events were likely the predominant cause of branch mortality, while branching increased after years with poor growth and cooler‐than‐average summers. These findings show that the erect dwarf‐shrub tundra in the Low Arctic has already and will likely become decreasingly temperature‐ and increasingly moisture‐limited and that winter warming supports shrub growth, but increased extreme winter warming event frequency may increase branch mortality and vegetation damage. Such counter‐acting mechanisms could offer an explanation for the vegetation stability observed over large parts of the Arctic. [ABSTRACT FROM AUTHOR]

In early studies, northern vegetation response to global warming recognised both increases in biomass/cover and shrinking of species' distributional ranges. Subsequent field measurements focussed on vegetation cover and biomass increases ("greening"), and more recently decreases ("browning"). However, satellite observations show that more than 50% of arctic vegetation has not changed significantly despite rapid warming. While absence of change in remote sensing data does not necessarily mean no ecological change on the ground, the significant proportion of the Arctic that appears to be stable in the face of considerable climate change points to a greater need to understand Arctic ecosystem stability. In this paper, we performed an extensive review of the available literature to seek balances or imbalances between research focussing on "greening", "browning" and "stability/no change". We find that greening studies dominate the literature though two relatively small areas of the Arctic are disproportionately represented for this main change process. Critically, there are too few studies anywhere investigating stability. We highlight the need to understand the mechanisms driving Arctic ecosystem stability, and the potential longer-term consequences of remaining stable in a rapidly changing climate. [ABSTRACT FROM AUTHOR]

Over the past several decades, various trends in vegetation productivity, from increases to decreases, have been observed throughout Arctic-Boreal ecosystems. While some of this variation can be explained by recent climate warming and increased disturbance, very little is known about the impacts of permafrost thaw on productivity across diverse vegetation communities. Active layer thickness data from 135 permafrost monitoring sites along a 10° latitudinal transect of the Northwest Territories, Canada, paired with a Landsat time series of normalized difference vegetation index from 1984 to 2019, were used to quantify the impacts of changing permafrost conditions on vegetation productivity. We found that active layer thickness contributed to the observed variation in vegetation productivity in recent decades in the northwestern Arctic-Boreal, with the highest rates of greening occurring at sites where the near-surface permafrost recently had thawed. However, the greening associated with permafrost thaw was not sustained after prolonged periods of thaw and appeared to diminish after the thaw front extended outside the plants' rooting zone. Highest rates of greening were found at the mid-transect sites, between 62.4° N and 65.2° N, suggesting that more southernly sites may have already surpassed the period of beneficial permafrost thaw, while more northern sites may have yet to reach a level of thaw that supports enhanced vegetation productivity. These results indicate that the response of vegetation productivity to permafrost thaw is highly dependent on the extent of active layer thickening and that increases in productivity may not continue in the coming decades., (© 2023 His Majesty the King in Right of Canada and The Authors. Global Change Biology published by John Wiley & Sons Ltd. Reproduced with the permission of the Minister of Natural Resources Canada.)

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One expected response to climate warming in the Arctic is an increase in the abundance and extent of shrubs in tundra areas. Repeat photography shows that there has been an increase in shrub cover over the past 50 years in northern Alaska. Using 202 pairs of old and new oblique aerial photographs, we have found that across this region spanning 620 km east to west and 350 km north to south, alder, willow, and dwarf birch have been increasing, with the change most easily detected on hill slopes and valley bottoms. Plot and remote sensing studies from the same region using the normalized difference vegetation index are consistent with the photographic results and indicate that the smaller shrubs between valleys are also increasing. In Canada, Scandinavia, and parts of Russia, there is both plot and remote sensing evidence for shrub expansion. Combined with the Alaskan results, the evidence suggests that a pan-Arctic vegetation transition is underway. If continued, this transition will alter the fundamental architecture and function of this ecosystem with important ramifications for the climate, the biota, and humans. [ABSTRACT FROM AUTHOR]

Rapid climate warming has widely been considered as the main driver of recent increases in Arctic tundra productivity. Field observations and remote sensing both show that tundra "greening" has been widespread, but heterogeneity in regional and landscape-scale trends suggest that additional controls are mediating the response of tundra vegetation to warming. In this study, we examined the relationship between changes in vegetation productivity in the western Canadian Arctic and biophysical variables by analyzing trends in the Enhanced Vegetation Index (EVI) obtained from nonparametric regression of annual Landsat surface reflectance composites. We used Random Forests classification and regression tree modelling to predict the trajectory and magnitude of greening from 1984 to 2016 and identify biophysical controls. More than two-thirds of our study area showed statistically significant increases in vegetation productivity, but observed changes were heterogeneous, occurring most rapidly within areas of the Southern Arctic that were: (1) dominated by dwarf and upright shrub cover types, (2) moderately sloping, and (3) located at lower elevation. These findings suggest that the response of tundra vegetation to warming is mediated by regional- and landscape-scale variation in microclimate, topography and soil moisture, and physiological differences among plant functional groups. Our work highlights the potential of the joint analysis of annual remotely sensed vegetation indices and broad-scale biophysical data to understand spatial variation in tundra vegetation change. • A 33-year time-series of Landsat EVI was analyzed using machine learning. • Over 68% of the western Canadian Arctic greened significantly from 1984 to 2016. • Greening was most rapid in the Southern Arctic region in shrub dominated terrain. • Tundra responses to warming are mediated by microclimate, topography, and moisture. [ABSTRACT FROM AUTHOR]

The Arctic tundra is warming rapidly, yet the exact mechanisms linking warming and observed ecological changes are often unclear. Understanding mechanisms of change requires long‐term monitoring of multiple ecological parameters. Here, we present the findings of a collaboration between government scientists, local people, park rangers, and academic researchers that provide insights into changes in plant composition, phenology, and growth over 18 yr on Qikiqtaruk‐Herschel Island, Canada. Qikiqtaruk is an important focal research site located at the latitudinal tall shrub line in the western Arctic. This unique ecological monitoring program indicates the following findings: (1) nine days per decade advance of spring phenology, (2) a doubling of average plant canopy height per decade, but no directional change in shrub radial growth, and (3) a doubling of shrub and graminoid abundance and a decrease by one‐half in bare ground cover per decade. Ecological changes are concurrent with satellite‐observed greening and, when integrated, suggest that indirect warming from increased growing season length and active layer depths, rather than warming summer air temperatures alone, could be important drivers of the observed tundra vegetation change. Our results highlight the vital role that long‐term and multi‐parameter ecological monitoring plays in both the detection and attribution of global change. [ABSTRACT FROM AUTHOR]