Vegetation responses in Alaskan arctic tundra after 8 years of a summer warming and winter snow manipulation experiment

We used snow fences and small (1 m²) open‐topped fiberglass chambers (OTCs) to study the effects of changes in winter snow cover and summer air temperatures on arctic tundra. In 1994, two 60 m long, 2.8 m high snow fences, one in moist and the other in dry tundra, were erected at Toolik Lake, Alaska. OTCs paired with unwarmed plots, were placed along each experimental snow gradient and in control areas adjacent to the snowdrifts. After 8 years, the vegetation of the two sites, including that in control plots, had changed significantly. At both sites, the cover of shrubs, live vegetation, and litter, together with canopy height, had all increased, while lichen cover and diversity had decreased. At the moist site, bryophytes decreased in cover, while an increase in graminoids was almost entirely because of the response of the sedge Eriophorum vaginatum. These community changes were consistent with results found in studies of responses to warming and increased nutrient availability in the Arctic. However, during the time period of the experiment, summer temperature did not increase, but summer precipitation increased by 28%. The snow addition treatment affected species abundance, canopy height, and diversity, whereas the summer warming treatment had few measurable effects on vegetation. The interannual temperature fluctuation was considerably larger than the temperature increases within OTCs (<2°C), however. Snow addition also had a greater effect on microclimate by insulating vegetation from winter wind and temperature extremes, modifying winter soil temperatures, and increasing spring run‐off. Most increases in shrub cover and canopy height occurred in the medium snow‐depth zone (0.5–2 m) of the moist site, and the medium to deep snow‐depth zone (2–3 m) of the dry site. At the moist tundra site, deciduous shrubs, particularly Betula nana, increased in cover, while evergreen shrubs decreased. These differential responses were likely because of the larger production to biomass ratio in deciduous shrubs, combined with their more flexible growth response under changing environmental conditions. At the dry site, where deciduous shrubs were a minor part of the vegetation, evergreen shrubs increased in both cover and canopy height. These changes in abundance of functional groups are expected to affect most ecological processes, particularly the rate of litter decomposition, nutrient cycling, and both soil carbon and nitrogen pools. Also, changes in canopy structure, associated with increases in shrub abundance, are expected to alter the summer energy balance by increasing net radiation and evapotranspiration, thus altering soil moisture regimes.     

Greater deciduous shrub abundance extends tundra peak season and increases modeled net CO2 uptake

Satellite studies of the terrestrial Arctic report increased summer greening and longer overall growing and peak seasons since the 1980s, which increases productivity and the period of carbon uptake. These trends are attributed to increasing air temperatures and reduced snow cover duration in spring and fall. Concurrently, deciduous shrubs are becoming increasingly abundant in tundra landscapes, which may also impact canopy phenology and productivity. Our aim was to determine the influence of greater deciduous shrub abundance on tundra canopy phenology and subsequent impacts on net ecosystem carbon exchange (NEE) during the growing and peak seasons in the arctic foothills region of Alaska. We compared deciduous shrub‐dominated and evergreen/graminoid‐dominated community‐level canopy phenology throughout the growing season using the normalized difference vegetation index (NDVI). We used a tundra plant‐community‐specific leaf area index (LAI) model to estimate LAI throughout the green season and a tundra‐specific NEE model to estimate the impact of greater deciduous shrub abundance and associated shifts in both leaf area and canopy phenology on tundra carbon flux. We found that deciduous shrub canopies reached the onset of peak greenness 13 days earlier and the onset of senescence 3 days earlier compared to evergreen/graminoid canopies, resulting in a 10‐day extension of the peak season. The combined effect of the longer peak season and greater leaf area of deciduous shrub canopies almost tripled the modeled net carbon uptake of deciduous shrub communities compared to evergreen/graminoid communities, while the longer peak season alone resulted in 84% greater carbon uptake in deciduous shrub communities. These results suggest that greater deciduous shrub abundance increases carbon uptake not only due to greater leaf area, but also due to an extension of the period of peak greenness, which extends the period of maximum carbon uptake.     

Environmental Limits of Tall Shrubs in Alaska’s Arctic National Parks

We sampled shrub canopy volume (height times area) and environmental factors (soil wetness, soil depth of thaw, soil pH, mean July air temperature, and typical date of spring snow loss) on 471 plots across five National Park Service units in northern Alaska. Our goal was to determine the environments where tall shrubs thrive and use this information to predict the location of future shrub expansion. The study area covers over 80,000 km² and has mostly tundra vegetation. Large canopy volumes were uncommon, with volumes over 0.5 m³/m² present on just 8% of plots. Shrub canopy volumes were highest where mean July temperatures were above 10.5°C and on weakly acid to neutral soils (pH of 6 to 7) with deep summer thaw (>80 cm) and good drainage. On many sites, flooding helped maintain favorable soil conditions for shrub growth. Canopy volumes were highest where the typical snow loss date was near 20 May; these represent sites that are neither strongly wind-scoured in the winter nor late to melt from deep snowdrifts. Individual species varied widely in the canopy volumes they attained and their response to the environmental factors. Betula sp. shrubs were the most common and quite tolerant of soil acidity, cold July temperatures, and shallow thaw depths, but they did not form high-volume canopies under these conditions. Alnus viridis formed the largest canopies and was tolerant of soil acidity down to about pH 5, but required more summer warmth (over 12°C) than the other species. The Salix species varied widely from S. pulchra, tolerant of wet and moderately acid soils, to S. alaxensis, requiring well-drained soils with near neutral pH. Nearly half of the land area in ARCN has mean July temperatures of 10.5 to 12.5°C, where 2°C of warming would bring temperatures into the range needed for all of the potential tall shrub species to form large canopies. However, limitations in the other environmental factors would probably prevent the formation of large shrub canopies on at least half of the land area with newly favorable temperatures after 2°C of warming.     

Variable sensitivity of plant communities in Iceland to experimental warming

Facing an increased threat of rapid climate change in cold‐climate regions, it is important to understand the sensitivity of plant communities both in terms of degree and direction of community change. We studied responses to 3–5 years of moderate experimental warming by open‐top chambers in two widespread but contrasting tundra communities in Iceland. In a species‐poor and nutrient‐deficient moss heath, dominated by Racomitrium lanuginosum, mean daily air temperatures at surface were 1–2°C higher in the warmed plots than the controls whereas soil temperatures tended to be lower in the warmed plots throughout the season. In a species‐rich dwarf shrub heath on relatively rich soils at a cooler site, dominated by Betula nana and R. lanuginosum, temperature changes were in the same direction although more moderate. In the moss heath, there were no detectable community changes while significant changes were detected in the dwarf shrub heath: the abundance of deciduous and evergreen dwarf shrubs significantly increased (>50%), bryophytes decreased (18%) and canopy height increased (100%). Contrary to some other studies of tundra communities, we detected no changes in species richness or other diversity measures in either community and the abundance of lichens did not change. It is concluded that the sensitivity of Icelandic tundra communities to climate warming varies greatly depending on initial conditions in terms of species diversity, dominant species, soil and climatic conditions as well as land‐use history.     

Decomposition of Senesced Leaf Litter is Faster in Tall Compared to Low Birch Shrub Tundra

Many Low Arctic tundra regions are currently undergoing a vegetation shift towards increasing growth and groundcover of tall deciduous shrubs due to recent climate warming. Vegetation change directly affects ecosystem carbon balance, but it can also affect soil biogeochemical cycling through physical and biological feedback mechanisms. Recent studies indicate that enhanced snow accumulation around relatively tall shrubs has negligible physical effect on litter decomposition rates. However, these investigations were no more than 3 years, and therefore may be insufficient to detect differences in inherently slow biogeochemical processes. Here, we report a 5-year study near Daring Lake, Canada, comparing Betula neoalaskana foliar litter decay rates within unmanipulated and snowfenced low-stature birch (height: ~?0.3 m) plots to test the physical effect of experimentally deepened snow, and within tall birch (height: ~?0.8 m) plots to test the combined physical and biological effects, that is, deepened snow plus strong birch dominance. Having corrected for carbon gain by the colonizing decomposers, actual litter carbon loss increased by approximately 25% in the tall birch relative to both low birch sites. Decay of lignin-like acid unhydrolizable litter residues also accelerated in the tall birch site, and a similar but lower magnitude response in the snowfenced low birch site indicated that physical effects of deepened snow were at least partially responsible. In contrast, deepened snow alone did not affect litter carbon loss. Our findings suggest that a combination of greater litter inputs, altered soil microbial community, enhanced soil nutrient pools, and warmer winter soils together promote relatively fast decomposition of recalcitrant litter carbon in tall birch shrub environments.     

Seasonal and Long-Term Changes to Active-Layer Temperatures after Tall Shrubland Expansion and Succession in Arctic Tundra

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.     

Browsing‐mediated shrub canopy changes drive composition and species richness in forest‐tundra ecosystems

Changes in climate and in browsing pressure are expected to alter the abundance of tundra shrubs thereby influencing the composition and species richness of plant communities. We investigated the associations between browsing, tundra shrub canopies and their understory vegetation by utilizing a long‐term (10–13 seasons) experiment controlling reindeer and ptarmigan herbivory in the subarctic forest tundra ecotone in northwestern Fennoscandia. In this area, there has also been a consistent increase in the yearly thermal sum and precipitation during the study period. The cover of shrubs increased 2.8–7.8 fold in exclosures and these contrasted with browsed control areas creating a sharp gradient of canopy cover of tundra shrubs across a variety of vegetation types. Browsing exclusions caused significant shifts in more productive vegetation types, whereas little or no shift occurred in low‐productive tundra communities. The increased deciduous shrub cover was associated with significant losses of understory plant species and shifts in functional composition, the latter being clearest in the most productive plant community types. The total cover of understory vegetation decreased along with increasing shrub cover, while the cover of litter showed the opposite response. The cover of cryptogams decreased along with increasing shrub cover, while the cover of forbs was favoured by a shrub cover. Increasing shrub cover decreased species richness of understory vegetation, which was mainly due to the decrease in the cryptogam species. The effects were consistent across different types of forest tundra vegetation indicating that shrub increase may have broad impacts on arctic vegetation diversity. Deciduous shrub cover is strongly regulated by reindeer browsing pressure and altered browsing pressure may result in a profound shrub expansion over the next one or two decades. Results suggest that the impact of an increase in shrubs on tundra plant richness is strong and browsing pressure effectively counteracts the effects of climate warming‐driven shrub expansion and hence maintains species richness.     

The Cooling Capacity of Mosses: Controls on Water and Energy Fluxes in a Siberian Tundra Site

Arctic tundra vegetation composition is expected to undergo rapid changes during the coming decades because of changes in climate. Higher air temperatures generally favor growth of deciduous shrubs, often at the cost of moss growth. Mosses are considered to be very important to critical tundra ecosystem processes involved in water and energy exchange, but very little empirical data are available. Here, we studied the effect of experimental moss removal on both understory evapotranspiration and ground heat flux in plots with either a thin or a dense low shrub canopy in a tundra site with continuous permafrost in Northeast Siberia. Understory evapotranspiration increased with removal of the green moss layer, suggesting that most of the understory evapotranspiration originated from the organic soil layer underlying the green moss layer. Ground heat flux partitioning also increased with green moss removal indicating the strong insulating effect of moss. No significant effect of shrub canopy density on understory evapotranspiration was measured, but ground heat flux partitioning was reduced by a denser shrub canopy. In summary, our results show that mosses may exert strong controls on understory water and heat fluxes. Changes in moss or shrub cover may have important consequences for summer permafrost thaw and concomitant soil carbon release in Arctic tundra ecosystems.     

Arctic arthropod assemblages in habitats of differing shrub dominance

Recent climate warming in the Arctic has caused advancement in the timing of snowmelt and expansion of shrubs into open tundra. Such an altered climate may directly and indirectly (via effects on vegetation) affect arctic arthropod abundance, diversity and assemblage taxonomic composition. To allow better predictions about how climate changes may affect these organisms, we compared arthropod assemblages between open and shrub‐dominated tundra at three field sites in northern Alaska that encompass a range of shrub communities. Over ten weeks of sampling in 2011, pitfall traps captured significantly more arthropods in shrub plots than open tundra plots at two of the three sites. Furthermore, taxonomic richness and diversity were significantly greater in shrub plots than open tundra plots, although this pattern was site‐specific as well. Patterns of abundance within the five most abundant arthropod orders differed, with spiders (Order: Araneae) more abundant in open tundra habitats and true bugs (Order: Hemiptera), flies (Order: Diptera), and wasps and bees (Order: Hymenoptera) more abundant in shrub‐dominated habitats. Few strong relationships were found between vegetation and environmental variables and arthropod abundance; however, lichen cover seemed to be important for the overall abundance of arthropods. Some arthropod orders showed significant relationships with other vegetation variables, including maximum shrub height (Coleoptera) and foliar canopy cover (Diptera). As climate warming continues over the coming decades, and with further shrub expansion likely to occur, changes in arthropod abundance, richness, and diversity associated with shrub‐dominated habitat may have important ecological effects on arctic food webs since arthropods play important ecological roles in the tundra, including in decomposition and trophic interactions.     

Greater shrub dominance alters breeding habitat and food resources for migratory songbirds in Alaskan arctic tundra

Climate warming is affecting the Arctic in multiple ways, including via increased dominance of deciduous shrubs. Although many studies have focused on how this vegetation shift is altering nutrient cycling and energy balance, few have explicitly considered effects on tundra fauna, such as the millions of migratory songbirds that breed in northern regions every year. To understand how increasing deciduous shrub dominance may alter breeding songbird habitat, we quantified vegetation and arthropod community characteristics in both graminoid and shrub dominated tundra. We combined measurements of preferred nest site characteristics for Lapland longspurs (Calcarius lapponicus) and Gambel's White‐crowned sparrows (Zonotrichia leucophrys gambelii) with modeled predictions for the distribution of plant community types in the Alaskan arctic foothills region for the year 2050. Lapland longspur nests were found in sedge‐dominated tussock tundra where shrub height does not exceed 20 cm, whereas White‐crowned sparrows nested only under shrubs between 20 cm and 1 m in height, with no preference for shrub species. Shrub canopies had higher canopy‐dwelling arthropod availability (i.e. small flies and spiders) but lower ground‐dwelling arthropod availability (i.e. large spiders and beetles). Since flies are the birds' preferred prey, increasing shrubs may result in a net enhancement in preferred prey availability. Acknowledging the coarse resolution of existing tundra vegetation models, we predict that by 2050 there will be a northward shift in current White‐crowned sparrow habitat range and a 20–60% increase in their preferred habitat extent, while Lapland longspur habitat extent will be equivalently reduced. Our findings can be used to make first approximations of future habitat change for species with similar nesting requirements. However, we contend that as exemplified by this study's findings, existing tundra modeling tools cannot yet simulate the fine‐scale habitat characteristics that are critical to accurately predicting future habitat extent for many wildlife species.     

Direct and indirect control by snow cover over decomposition in alpine tundra along a snowmelt gradient

We assessed direct and indirect effects of snow cover on litter decomposition and litter nitrogen release in alpine tundra. Direct effects are driven by the direct influence of snow cover on edaphoclimatic conditions, whereas indirect effects result from the filtering effect of snow cover on species’ abundance and traits. We compared the in situ decomposition of leaf litter from four dominant plant species (two graminoids, two shrubs) at early and late snowmelt locations using a two-year litter-bag experiment. A seasonal experiment was also performed to estimate the relative importance of winter and summer decomposition. We found that growth form (graminoids vs. shrubs) are the main determinants of decomposition rate. Direct effect of snow cover exerted only a secondary influence. Whatever the species, early snowmelt locations showed consistently reduced decomposition rates and delayed final stages of N mineralization. This lower decomposition rate was associated with freezing soil temperatures during winter. The results suggest that a reduced snow cover may have a weak and immediate direct effect on litter decomposition rates and N availability in alpine tundra. A much larger impact on nutrient cycling is likely to be mediated by longer term changes in the relative abundance of lignin-rich dwarf shrubs.     

Impacts of the removal of vascular plants on physiological and biochemical characteristics of Syntrichia caninervis during winter season in a temperate desert

Aims Patchily distributed biological soil crusts and shrubs is one of the main vegetation cover types in Gurbantünggüt desert. The existence of shrubs in desert areas serves not only as a shelter for small animals, but also a good living condition for cryptogams and some herbs. Syntrichia caninervis, a dominant moss species in Gurbantünggüt desert, is patchily distributed under shrub canopy and open spaces between vascular plants. To our knowledge, the impacts of the removal of shrub canopy on physiological and biochemical characteristics of S. caninervis during the winter is still unknown.
Methods We simulated grazing of animals on Ephedra distachya at various rates (shrubs left intact, 50% shrubs removed, and shrubs removed completely) by cutting different percent of above-ground shoots of E. distachya. The shoot water content, chlorophyll fluorescence, proline content, soluble sugar content, soluble protein content, malonyldialdehyde (MDA) content, peroxidase (POD) and superoxide dismutase (SOD) activity were compared.
Important findings The results show that, as for proline and soluble sugar contents of tested S. caninervis, there were no significant differences among three treatments of shrub canopy. However, the MDA content, and the activities of POD, SOD, CAT were significantly higher than nature shrubs in snow cover periods. During snow-melting periods, soil moisture and temperature did not served as limiting factors on the growth of S. caninervis. The soluble sugar content, and the activities of POD, CAT with shrub removal were significantly higher than shrubs left intact. This may indicate that the decline of the function of maintaining constant soil temperature due to shrub canopy increased the damage of temperature on moss plants. In addition, as for physiological and biochemical characteristics, there were no significant differences between two treatments of 50% shrub canopy and shrub canopy left intact. In snow covered periods and snow melting periods, the MDA content, the activities of POD, SOD and CAT of S. caninervis under shrub canopy removed completely were significantly higher than that of 50% shrub canopy and shrub canopy left intact, except for soluble protein content. The duration of chlorophyll fluorescence activity of S. caninervis growing in habitats with shrubs removed completely was significantly shorter than that of S. caninervis growing in habitats of 50% shrub canopy and shrub canopy left intact. This result indicated that the removal of shrubs may increase the UV-B radiation and weaken the “moisture island effects” developed by the existence of shrub canopy.     

Phenology and species determine growing‐season albedo increase at the altitudinal limit of shrub growth in the sub‐Arctic

Arctic warming is resulting in reduced snow cover and increased shrub growth, both of which have been associated with altered land surface–atmospheric feedback processes involving sensible heat flux, ground heat flux and biogeochemical cycling. Using field measurements, we show that two common Arctic shrub species (Betula glandulosa and Salix pulchra), which are largely responsible for shrub encroachment in tundra, differed markedly in albedo and that albedo of both species increased as growing season progressed when measured at their altitudinal limit. A moveable apparatus was used to repeatedly measure albedo at six precise spots during the summer of 2012, and resampled in 2013. Contrary to the generally accepted view of shrub‐covered areas having low albedo in tundra, full‐canopy prostrate B. glandulosa had almost the highest albedo of all surfaces measured during the peak of the growing season. The higher midsummer albedo is also evident in localized MODIS albedo aggregated from 2000 to 2013, which displays a similar increase in growing‐season albedo. Using our field measurements, we show the ensemble summer increase in tundra albedo counteracts the generalized effect of earlier spring snow melt on surface energy balance by approximately 40%. This summer increase in albedo, when viewed in absolute values, is as large as the difference between the forest and tundra transition. These results indicate that near future (<50 years) changes in growing‐season albedo related to Arctic vegetation change are unlikely to be particularly large and might constitute a negative feedback to climate warming in certain circumstances. Future efforts to calculate energy budgets and a sensible heating feedback in the Arctic will require more detailed information about the relative abundance of different ground cover types, particularly shrub species and their respective growth forms and phenology.     

灌丛移除对荒漠齿肋赤藓越冬过程中生理生化特性的影响

灌丛和生物土壤结皮镶嵌分布是古尔班通古特沙漠一种重要的地表覆被类型, 灌丛的存在不仅为小型动物提供了庇护场所, 也为隐花植物及部分草本植物创造了良好的生存条件。然而在初冬时期移除灌丛是否会影响这些隐花植物的越冬及生长还不得而知。该研究通过模拟放牧及鼠害, 移除50%双穗麻黄(Ephedra distachya)灌丛、移除全部双穗麻黄灌丛及自然对照, 测定齿肋赤藓(Syntrichia caninervis)植株脯氨酸、可溶性糖和可溶性蛋白含量, 以及丙二醛(MDA)含量和超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CAT) 3种抗氧化酶活性, 并测定原位光化学效率, 探讨初冬灌丛丧失对荒漠藓类植物越冬的影响。结果表明, 在积雪期3种处理下齿肋赤藓的游离脯氨酸和可溶性糖含量之间差异并不显著, 但去灌丛后其MDA含量、POD、SOD和CAT 3种抗氧化酶活性均显著高于自然灌丛下齿肋赤藓的相关生理指标。即使是在融雪期, 去灌丛下齿肋赤藓的可溶性糖含量及POD和CAT活性仍显著高于自然灌丛, 而可溶性蛋白明显较低, 这可能是由于灌丛的丧失造成的温度波动加剧了冬季低温对藓类植物的伤害。灌丛的部分移除(50%)对齿肋赤藓的生理生化特性影响不显著, 就积雪融化期叶绿素荧光活性持续时间而言, 与自然灌丛和移除50%灌丛相比, 完全移除灌丛的齿肋赤藓植株叶绿素荧光活性持续时间显著缩短。这可能是由于灌丛移除导致其UV-B辐射增加及“湿岛效应”消失所致, UV-B辐射的增加加剧了对植物的伤害, 而春季融雪期保水能力的下降也是其叶绿素荧光活性时间缩短的重要原因。     

The Effects of Snow,Soil Microenvironment,and Soil Organic Matter Quality on N Availability in Three Alaskan Arctic Plant Communities

Climate warming in The Arctic may lead to a shift from graminoid to shrub dominance, which may, in turn, alter the structure and function of the ecosystem through shrub influences on the abiotic and/or biotic controls over biogeochemical cycles of carbon (C) and nitrogen (N). In Arctic tundra, near Toolik Lake, Alaska, we quantified net N-mineralization rates under ambient and manipulated snow treatments at three different plant communities that varied in abundance of deciduous shrubs. Our objective was twofold: (1) to test whether the amount of snow that can accumulate around Arctic deciduous shrubs maintains winter soil temperatures high enough to stimulate microbial activity and increase soil N levels (effect of soil microclimate) and (2) to compare the relative effects of snow versus shrubs on N availability via effects on the main drivers of N-mineralization: SOM quality versus microclimate. Winter snow addition had a positive effect on summer, but not winter, N-mineralization rates. Soil organic matter quality had a nine times larger effect on N-mineralization than did soil microclimate in the summer season and only SOM quality had a detectable effect on N-mineralization in the winter. Here we conclude that on a short time scale, shrub interactions with snow may play a role in increasing plant available N, primarily through effects on the summer soil microenvironment. In addition, differences in SOM quality can drive larger differences in net N-mineralization than changes in soil microclimate of the magnitude of what we saw across our three sites.     

Multiple effects of shrubs on annual plant communities in arid lands of South Australia

Abstract The presence of shrubs in arid lands creates spatial heterogeneity that affects the distribution and performance of annual plants; several possible mechanisms have been implicated. A preliminary survey in a chenopod shrubland in South Australia showed differences in the distribution of annual plants under canopies of Atriplex vesicaria and Maireana sedifolia (the two dominant shrub species) and open spaces. A series of experiments were conducted to test the potential contribution to these patterns of nutrient enrichment under shrubs, differential seed accumulation, stress reduction by the canopy, competition by shrub roots, and protection against grazing. The germinable soil seed‐bank under A. vesicaria and M. sedifolia was different from that in open spaces, but these differences can only explain a fraction of the differences observed in the growing annual plant community in different microsites. The soil under A. vesicaria had higher total nitrogen content than soil in open spaces, whereas soil under M. sedifolia had lower available phosphorus than open spaces. Although annual plant densities under A. vesicaria were higher than in open spaces, experimental removal of shrubs increased their density, suggesting that shrub canopies inhibit annual plants in this system. Surprisingly, trenching of open areas close to shrubs (severing lateral shrub roots) decreased annual plant density. We suggest that water moves laterally through shrub roots, in a process akin to a hydraulic lift, increasing water availability for the annual plants. Exclusion of vertebrate grazers had a stronger effect on annual plant biomass in open spaces than under M. sedifolia, suggesting that this shrub provides shelter against herbivory. Overall our results show that shrubs can have simultaneously facilitative and inhibitory effects on the annual plant community through different mechanisms, but more importantly that different shrub species have different effects. This is a potential mechanism allowing for species coexistence of annual plants.     

Tall shrub and tree expansion in Siberian tundra ecotones since the 1960s

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.     

Long‐term deepened snow promotes tundra evergreen shrub growth and summertime ecosystem net CO2 gain but reduces soil carbon and nutrient pools

Arctic climate warming will be primarily during winter, resulting in increased snowfall in many regions. Previous tundra research on the impacts of deepened snow has generally been of short duration. Here, we report relatively long‐term (7–9 years) effects of experimentally deepened snow on plant community structure, net ecosystem CO₂ exchange (NEE), and soil biogeochemistry in Canadian Low Arctic mesic shrub tundra. The snowfence treatment enhanced snow depth from 0.3 to ~1 m, increasing winter soil temperatures by ~3°C, but with no effect on summer soil temperature, moisture, or thaw depth. Nevertheless, shoot biomass of the evergreen shrub Rhododendron subarcticum was near‐doubled by the snowfences, leading to a 52% increase in aboveground vascular plant biomass. Additionally, summertime NEE rates, measured in collars containing similar plant biomass across treatments, were consistently reduced ~30% in the snowfenced plots due to decreased ecosystem respiration rather than increased gross photosynthesis. Phosphate in the organic soil layer (0–10 cm depth) and nitrate in the mineral soil layer (15–25 cm depth) were substantially reduced within the snowfences (47–70 and 43%–73% reductions, respectively, across sampling times). Finally, the snowfences tended (p = .08) to reduce mineral soil layer C% by 40%, but with considerable within‐ and among plot variation due to cryoturbation across the landscape. These results indicate that enhanced snow accumulation is likely to further increase dominance of R. subarcticum in its favored locations, and reduce summertime respiration and soil biogeochemical pools. Since evergreens are relatively slow growing and of low stature, their increased dominance may constrain vegetation‐related feedbacks to climate change. We found no evidence that deepened snow promoted deciduous shrub growth in mesic tundra, and conclude that the relatively strong R. subarcticum response to snow accumulation may explain the extensive spatial variability in observed circumpolar patterns of evergreen and deciduous shrub growth over the past 30 years.     

Snow-induced changes in dwarf birch chemistry increase moth larval growth rate and level of herbivory

Changes in snow cover might influence arctic ecosystems to the same extent as increased temperatures. Although the duration of snow cover is generally expected to decrease in the future as a result of global warming, the amounts of snow might increase in arctic areas where much of the elevated precipitation will fall as snow. We examined the effects of an increased snow cover, as a result of a snow fence treatment, on soil nitrogen mineralization, plant phenology, plant chemistry (nitrogen and potential defense compounds), the level of invertebrate herbivory, and performance of invertebrate herbivores in an arctic ecosystem, using dwarf birch (Betula nana) and the autumnal moth (Epirrita autumnata) as study organisms. An enhanced and prolonged snow cover increased the level of herbivory on dwarf birch leaves. Larvae feeding on plants that had experienced enhanced snow cover grew faster and pupated earlier than larvae fed with plant material from control plots, indicating that plants from enhanced snow-lie plots produce higher-quality food to herbivores. The increased larval growth rate was strongly correlated with higher leaf nitrogen concentration in plants subjected to snow manipulation, and also to certain phenolic acids. Snow manipulation did not change net nitrogen mineralization rates in the soil or total carbon concentration in leaves, but it altered the within-season fluctuating pattern of leaf phenolic compounds. This study demonstrates a positive relationship between increased snow cover and level of herbivory on deciduous shrubs, thus proposing a negative feedback on the climate-induced dwarf shrub expansion in arctic areas.     

The influence of subterranean termites on the hydrological characteristics of a Chihuahuan desert ecosystem

Summary Rainfall simulation at an average intensity of 124 mm·h^-1 was used to compare infiltration and run off on arid areas where subterranean termites had been eliminated four years prior to the initiation of the study (termite free) with adjacent areas populated by subterranean termites (termites present). Infiltration rates on termite free plots with less than 5% perennial plant cover were significantly lower 51.3±6.8 mm·h^-1 than rates on comparable termites present plots 88.4±5.6 mm·h^-1. On plots centered on Larrea tridentata shrubs, there were no differences in infiltration rates with or without termites. Plots with shrub cover had the highest infiltration rates 101±6 mm·h^-1. Highest run-off volumes were recorded from termite free <5% grass cover plots and the lowest from plots with shrubs. There were no differences in suspended sediment concentrations from termites present and termite free plots. Average bed load concentration was more than three times greater from termite free, <5% cover plots than from termites present, <5% cover plots.The reduction in infiltration, high run-off volumes and high bedloads from termite free areas without shrub cover is related to increased soil bulk density resulting from the collapse of subterranean galleries of the termites that provide avenues of bulk flow into the soil. Subterranean termites affect the hydrology of Chihuahuan desert systems by enhancing water infiltration and retention of top soil. The presence of a shrub canopy and litter layer cancels any effect of subterranean termites on hydrological parameters. Since approximately 2/3 of the area is not under shrub canopies, subterranean termites are considered to be essential for the maintenance of the soil water characteristics that support the present vegetation.