Circumpolar expansion of tall shrubs and trees into Arctic tundra is widely thought to be occurring as a result of recent climate warming, but little quantitative evidence exists for northern ...Siberia, which encompasses the world's largest forest‐tundra ecotonal belt. We quantified changes in tall shrub and tree canopy cover in 11, widely distributed Siberian ecotonal landscapes by comparing very high‐resolution photography from the Cold War‐era ‘Gambit’ and ‘Corona’ satellite surveillance systems (1965–1969) with modern imagery. We also analyzed within‐landscape patterns of vegetation change to evaluate the susceptibility of different landscape components to tall shrub and tree increase. The total cover of tall shrubs and trees increased in nine of 11 ecotones. In northwest Siberia, alder (Alnus) shrubland cover increased 5.3–25.9% in five ecotones. In Taymyr and Yakutia, larch (Larix) cover increased 3.0–6.7% within three ecotones, but declined 16.8% at a fourth ecotone due to thaw of ice‐rich permafrost. In Chukotka, the total cover of alder and dwarf pine (Pinus) increased 6.1% within one ecotone and was little changed at a second ecotone. Within most landscapes, shrub and tree increase was linked to specific geomorphic settings, especially those with active disturbance regimes such as permafrost patterned‐ground, floodplains, and colluvial hillslopes. Mean summer temperatures increased at most ecotones since the mid‐1960s, but rates of shrub and tree canopy cover expansion were not strongly correlated with temperature trends and were better correlated with mean annual precipitation. We conclude that shrub and tree cover is increasing in tundra ecotones across most of northern Siberia, but rates of increase vary widely regionally and at the landscape scale. Our results indicate that extensive changes can occur within decades in moist, shrub‐dominated ecotones, as in northwest Siberia, while changes are likely to occur much more slowly in the highly continental, larch‐dominated ecotones of central and eastern Siberia.
High‐altitude treelines are temperature‐limited vegetation boundaries, but little quantitative evidence exists about the impact of climate change on treelines in untouched areas of Russia. Here, we ...estimated how forest‐tundra ecotones have changed during the last century along the Ural mountains. In the South, North, Sub‐Polar, and Polar Urals, we compared 450 historical and recent photographs and determined the ages of 11 100 trees along 16 altitudinal gradients. In these four regions, boundaries of open and closed forests (crown covers above 20% and 40%) expanded upwards by 4 to 8 m in altitude per decade. Results strongly suggest that snow was an important driver for these forest advances: (i) Winter precipitation has increased substantially throughout the Urals (~7 mm decade⁻¹), which corresponds to almost a doubling in the Polar Urals, while summer temperatures have only changed slightly (~0.05 °C decade⁻¹). (ii) There was a positive correlation between canopy cover, snow height and soil temperatures, suggesting that an increasing canopy cover promotes snow accumulation and, hence, a more favorable microclimate. (iii) Tree age analysis showed that forest expansion mainly began around the year 1900 on concave wind‐sheltered slopes with thick snow covers, while it started in the 1950s and 1970s on slopes with shallower snow covers. (iv) During the 20th century, dominant growth forms of trees have changed from multistemmed trees, resulting from harsh winter conditions, to single‐stemmed trees. While 87%, 31%, and 93% of stems appearing before 1950 were from multistemmed trees in the South, North and Polar Urals, more than 95% of the younger trees had a single stem. Currently, there is a high density of seedlings and saplings in the forest‐tundra ecotone, indicating that forest expansion is ongoing and that alpine tundra vegetation will disappear from most mountains of the South and North Urals where treeline is already close to the highest peaks.
Climate change is expected to cause extensive vegetation changes in the Arctic: deciduous shrubs are already expanding, in response to climate warming. The results from transect studies suggest that ...increasing shrub cover will impact significantly on the surface energy balance. However, little is known about the direct effects of shrub cover on permafrost thaw during summer. We experimentally quantified the influence of Betula nana cover on permafrost thaw in a moist tundra site in northeast Siberia with continuous permafrost. We measured the thaw depth of the soil, also called the active layer thickness (ALT), ground heat flux and net radiation in 10 m diameter plots with natural B. nana cover (control plots) and in plots in which B. nana was removed (removal plots). Removal of B. nana increased ALT by 9% on average late in the growing season, compared with control plots. Differences in ALT correlated well with differences in ground heat flux between the control plots and B. nana removal plots. In the undisturbed control plots, we found an inverse correlation between B. nana cover and late growing season ALT. These results suggest that the expected expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw. Increased shrub growth may thus partially offset further permafrost degradation by future temperature increases. Permafrost models need to include a dynamic vegetation component to accurately predict future permafrost thaw.
Recent climate warming and scenarios for further warming have led to expectations of rapid movement of ecological boundaries. Here we focus on the circumarctic forest–tundra ecotone (FTE), which ...represents an important bioclimatic zone with feedbacks from forest advance and corresponding tundra disappearance (up to 50% loss predicted this century) driving widespread ecological and climatic changes. We address FTE advance and climate history relations over the 20th century, using FTE response data from 151 sites across the circumarctic area and site‐specific climate data. Specifically, we investigate spatial uniformity of FTE advance, statistical associations with 20th century climate trends, and whether advance rates match climate change velocities (CCVs). Study sites diverged into four regions (Eastern Canada; Central and Western Canada and Alaska; Siberia; and Western Eurasia) based on their climate history, although all were characterized by similar qualitative patterns of behaviour (with about half of the sites showing advancing behaviour). The main associations between climate trend variables and behaviour indicate the importance of precipitation rather than temperature for both qualitative and quantitative behaviours, and the importance of non‐growing season as well as growing season months. Poleward latitudinal advance rates differed significantly among regions, being smallest in Eastern Canada (~10 m/year) and largest in Western Eurasia (~100 m/year). These rates were 1–2 orders of magnitude smaller than expected if vegetation distribution remained in equilibrium with climate. The many biotic and abiotic factors influencing FTE behaviour make poleward advance rates matching predicted 21st century CCVs (~103–104 m/year) unlikely. The lack of empirical evidence for swift forest relocation and the discrepancy between CCV and FTE response contradict equilibrium model‐based assumptions and warrant caution when assessing global‐change‐related biotic and abiotic implications, including land–atmosphere feedbacks and carbon sequestration.
Recent climate warming and scenarios for further warming have led to expectations of rapid movement of ecological boundaries, but northward advance of the subarctic forest–tundra ecotone (FTE) has been 1–2 orders of magnitude smaller than expected if vegetation distribution remained in equilibrium with climate. This mismatch makes poleward advance rates matching predicted 21st century climate change velocity (CCV) unlikely. The lack of empirical evidence for swift forest relocation and the discrepancy between CCV and FTE response contradict equilibrium model‐based assumptions and warrant caution when assessing global‐change‐related biotic and abiotic implications, including land–atmosphere feedbacks and carbon sequestration. Photo: Taymyr, Russia. Courtesy of Nikolai Kolupanov.
The tundra plays a pivotal role in the Arctic mercury
(Hg) cycle by storing atmospheric Hg deposition and shuttling it to the
Arctic Ocean. A recent study revealed that 70 % of the atmospheric Hg
...deposition to the tundra occurs through gaseous elemental mercury (GEM or Hg(0))
uptake by vegetation and soils. Processes controlling land–atmosphere
exchange of Hg(0) in the Arctic tundra are central, but remain
understudied. Here, we combine Hg stable isotope analysis of Hg(0) in the
atmosphere, interstitial snow air, and soil pore air, with Hg(0) flux
measurements in a tundra ecosystem at Toolik Field Station in northern
Alaska (USA). In the dark winter months, planetary boundary layer (PBL)
conditions and Hg(0) concentrations were generally stable throughout the day
and small Hg(0) net deposition occurred. In spring, halogen-induced
atmospheric mercury depletion events (AMDEs) occurred, with the fast
re-emission of Hg(0) after AMDEs resulting in net emission fluxes of Hg(0).
During the short snow-free growing season in summer, vegetation uptake of
atmospheric Hg(0) enhanced atmospheric Hg(0) net deposition to the Arctic
tundra. At night, when PBL conditions were stable, ecosystem uptake of
atmospheric Hg(0) led to a depletion of atmospheric Hg(0). The night-time
decline of atmospheric Hg(0) was concomitant with a depletion of lighter
Hg(0) isotopes in the atmospheric Hg pool. The enrichment factor,
ε202Hgvegetationuptake=-4.2 ‰ (±1.0 ‰) was consistent
with the preferential uptake of light Hg(0) isotopes by vegetation. Hg(0)
flux measurements indicated a partial re-emission of Hg(0) during daytime,
when solar radiation was strongest. Hg(0) concentrations in soil pore air
were depleted relative to atmospheric Hg(0) concentrations, concomitant with
an enrichment of lighter Hg(0) isotopes in the soil pore air, ε202Hgsoilair-atmosphere=-1.00 ‰
(±0.25 ‰) and E199Hgsoilair-atmosphere=0.07 ‰ (±0.04 ‰). These
first Hg stable isotope measurements of Hg(0) in soil pore air are
consistent with the fractionation previously observed during Hg(0) oxidation
by natural humic acids, suggesting abiotic oxidation as a cause for observed
soil Hg(0) uptake. The combination of Hg stable isotope fingerprints with
Hg(0) flux measurements and PBL stability assessment confirmed a dominant
role of Hg(0) uptake by vegetation in the terrestrial–atmosphere exchange of
Hg(0) in the Arctic tundra.
Question
Understanding the sensitivity and magnitude of plant community responses in tundra wetlands to herbivory and warming is pressing as these ecosystems are increasingly threatened by changes in ...grazing pressure and higher temperatures. Here, we ask to what extent different low‐Arctic coastal wetland plant communities are affected by short‐term goose grazing and warming, and whether these communities differ in their responses.
Location
Yukon–Kuskokwim Delta, Alaska.
Methods
We conducted an experiment where we simulated goose grazing by clipping the vegetation and summer warming by using open‐top chambers in three plant communities along a 6‐km coastal–inland gradient. We assessed plant community compositional changes following two years of treatments.
Results
Grazing had stronger effects than warming on both plant functional group and species composition. Overall, grazing decreased the abundance of grasses and sedges and increased the abundance of forbs, whereas warming only caused a decrease in forb abundance. However, plant communities and functional groups, both within and across communities, varied widely in their responses to treatments. Grazing decreased grass abundance (−25%) and increased forb abundance (+44%) in the two more coastal communities, and reduced sedge abundance (−22%) only in the most inland community. Warming only decreased forb abundance (−18%) in the most coastal community, which overall was the most responsive to treatments.
Conclusions
We show that short‐term goose grazing predominates over short‐term summer warming in eliciting compositional changes in three different low‐Arctic coastal wetland plant communities. Yet, responses varied among communities and the same functional groups could respond differently across them, highlighting the importance of investigating the effects of biotic and abiotic drivers in different contexts. By showing that tundra wetland plant communities can differ in their immediate sensitivity to goose grazing and, though to a lesser extent, warming, our findings have implications for the functioning of these rapidly changing high‐latitude ecosystems.
We show that two summers of experimental goose grazing and summer warming can elicit compositional changes in three distinct low‐Arctic coastal wetland plant communities of the Yukon–Kuskokwim Delta, Alaska. However, responses varied amongst communities and plant functional groups, highlighting the importance of investigating the effects of global change drivers in different contexts to obtain reliable short‐term predictions of tundra community‐specific changes.
Large migratory grazers commonly influence soil processes in tundra ecosystems. However, the extent to which grazing effects are limited to intensive grazing periods associated with migration has not ...previously been investigated. We analyzed seasonal patterns in soil nitrogen (N), microbial respiration and extracellular enzyme activities (EEAs) in a lightly grazed tundra and a heavily grazed tundra that has been subjected to intensive grazing during reindeer (Rangifer tarandus L.) migration for the past 50 years. We hypothesized that due to the fertilizing effect of the reindeer, microbial respiration and EEAs related to microbial C acquisition should be higher in heavily grazed areas compared to lightly grazed areas and that the effects of grazing should be strongest during reindeer migration. Reindeer migration caused a dramatic peak in soil N availability, but in contrast to our predictions, the effect of grazing was more or less constant over the growing season and the seasonal patterns of microbial activities and microbial N were strikingly uniform between the lightly and heavily grazed areas. Microbial respiration and the EEAs of β-glucosidase, acid-phosphatase, and leucine-aminopeptidase were higher, whereas that of N-acetylglucosamidase was lower in the heavily grazed area. Experimental fertilization had no effect on EEAs related to C acquisition at either level of grazing intensity. Our findings suggest that soil microbial activities were independent of grazing-induced temporal variation in soil N availability. Instead, the effect of grazing on soil microbial activities appeared to be mediated by substrate availability for soil microorganisms. Following a shift in the dominant vegetation in response to grazing from dwarf shrubs to graminoids, the effect of grazing on soil processes is no longer sensitive to temporal grazing patterns; rather, grazers exert a consistent positive effect on the soil microbial potential for soil C decomposition.
The expansion of shrubs into tundra areas is a key terrestrial change underway in the Arctic in response to elevated temperatures during the twentieth century. Repeat photography permits a glimpse ...into greening satellite pixels, and it shows that, since 1950, some shrub patches have increased rapidly (hereafter expanding), while others have increased little or not at all (hereafter stable). We characterized and compared adjacent expanding and stable shrub patches across Arctic Alaska by sampling a wide range of physical and chemical soil and vegetation properties, including shrub growth rings. Expanding patches of Alnus viridis ssp. fruticosa (Siberian alder) contained shrub stems with thicker growth rings than in stable patches. Alder growth in expanding patches also showed strong correlation with spring and summer warming, whereas alder growth in stable patches showed little correlation with temperature. Expanding patches had different vegetation composition, deeper thaw depth, higher mean annual ground temperature, higher mean growing season temperature, lower soil moisture, less carbon in mineral soil, and lower C:N values in soils and shrub leaves. Expanding patches—higher resource environments—were associated with floodplains, stream corridors, and outcrops. Stable patches—lower resource environments—were associated with poorly drained tussock tundra. Collectively, we interpret these differences as implying that preexisting soil conditions predispose parts of the landscape to a rapid response to climate change, and we therefore expect shrub expansion to continue penetrating the landscape via dendritic floodplains, streams, and scattered rock outcrops.
Warmer temperatures are accelerating the phenology of organisms around the world. Temperature sensitivity of phenology might be greater in colder, higher latitude sites than in warmer regions, in ...part because small changes in temperature constitute greater relative changes in thermal balance at colder sites. To test this hypothesis, we examined up to 20 years of phenology data for 47 tundra plant species at 18 high‐latitude sites along a climatic gradient. Across all species, the timing of leaf emergence and flowering was more sensitive to a given increase in summer temperature at colder than warmer high‐latitude locations. A similar pattern was seen over time for the flowering phenology of a widespread species, Cassiope tetragona. These are among the first results highlighting differential phenological responses of plants across a climatic gradient and suggest the possibility of convergence in flowering times and therefore an increase in gene flow across latitudes as the climate warms.
Warmer temperatures are accelerating the phenology of organisms around the world, and temperature sensitivity of phenology might be greater in colder, higher latitude sites than in warmer regions. We tested this hypothesis using phenology data for 47 tundra plant species at 18 high‐latitude sites along a climatic gradient. Across all species, the timing of leaf emergence and flowering was more sensitive to a given increase in summer temperature at colder than warmer high‐latitude locations. These are among the first results highlighting differential phenological responses of plants across a climatic gradient and suggest the possibility of convergence in flowering times and therefore an increase in gene flow across latitudes as the climate warms. (Photo credit: Anne D. Bjorkman)
Vegetation, active-layer soils, and snow cover regulate energy exchange between the atmosphere and permafrost. Therefore, interactions between changes to tundra vegetation and soil thermal regime ...will fundamentally affect permafrost in a warmer world. We recorded soil temperatures for approximately 1 year in a Siberian Low Arctic landscape with a known history of alder (Alnus) shrub expansion on disturbed microsites in patterned ground. We recorded near-surface soil temperatures and measured physical properties of soils and vegetation on sorted-circle microsites in four stages of shrubland development: (1) tundra lacking tall shrubs; (2) shrub colonization zones; (3) mature shrublands; and (4) paludified, long-established shrublands with thick soil organic layers. Summer soil temperatures declined with increasing shrub cover and soil organic thickness; shrub colonization suppressed cryoturbation, facilitating the development of continuous vegetation and a surface organic mat on circles. Compared to open tundra, mature shrubs cooled soils by up to 9 °C during summer, but warmed soils by greater than 10 °C in winter presumably because they developed highly insulative snowpacks. Paludified shrublands had the coldest summer active layers, but winter soil temperatures were much colder than mature shrublands and were similar to earlier stages. Our results indicate that although tall shrub establishment dramatically warms winter soils within decades, much of this warming is transient at paludification-prone sites because the buildup of wet peat favors cooling in winter and the stature and snow-trapping capacity of shrubs diminish over time. In the ecosystem we studied, shrub expansion has contrasting effects on active-layer temperatures both seasonally and over longer timescales due to successional processes.
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