The response of Arctic vegetation and soils following an unusually severe tundra fire

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Philosophical Transactions of the Royal Society B Biological Sciences 368 (2013): 20120490, doi:10.1098/rstb.2012.0490.Fire causes dramatic short-term changes in vegetation and ecosystem function, and may promote rapid vegetation change by creating recruitment opportunities. Climate warming likely will increase the frequency of wildfire in the Arctic, where it is not common now. In 2007, the unusually severe Anaktuvuk River fire burned 1039 km2 of tundra on Alaska's North Slope. Four years later, we harvested plant biomass and soils across a gradient of burn severity, to assess recovery. In burned areas, above-ground net primary productivity of vascular plants equalled that in unburned areas, though total live biomass was less. Graminoid biomass had recovered to unburned levels, but shrubs had not. Virtually all vascular plant biomass had resprouted from surviving underground parts; no non-native species were seen. However, bryophytes were mostly disturbance-adapted species, and non-vascular biomass had recovered less than vascular plant biomass. Soil nitrogen availability did not differ between burned and unburned sites. Graminoids showed allocation changes consistent with nitrogen stress. These patterns are similar to those seen following other, smaller tundra fires. Soil nitrogen limitation and the persistence of resprouters will likely lead to recovery of mixed shrub–sedge tussock tundra, unless permafrost thaws, as climate warms, more extensively than has yet occurred.This work was supported by NSF (no. OPP-0632264) and NSF (no. OPP-1107892) to M. S. Bret-Harte, NSF (no. OPP-0856853) to G. R. Shaver and NSF (no. OPP-6737545) to M. C. Mack

Circum-Arctic distribution of chemical anti-herbivore compounds suggests biome-wide trade-off in defence strategies in Arctic shrubs

Spatial variation in plant chemical defence towards herbivores can help us understand variation in herbivore top-down control of shrubs in the Arctic and possibly also shrub responses to global warming. Less defended, non-resinous shrubs could be more influenced by herbivores than more defended, resinous shrubs. However, sparse field measurements limit our current understanding of how much of the circum-Arctic variation in defence compounds is explained by taxa or defence functional groups (resinous/non-resinous). We measured circum-Arctic chemical defence and leaf digestibility in resinous (Betula glandulosa, B. nana ssp. exilis) and non-resinous (B. nana ssp. nana, B. pumila) shrub birches to see how they vary among and within taxa and functional groups. Using liquid chromatography-mass spectrometry (LC-MS) metabolomic analyses and in vitro leaf digestibility via incubation in cattle rumen fluid, we analysed defence composition and leaf digestibility in 128 samples from 44 tundra locations. We found biogeographical patterns in anti-herbivore defence where mean leaf triterpene concentrations and twig resin gland density were greater in resinous taxa and mean concentrations of condensing tannins were greater in non-resinous taxa. This indicates a biome-wide trade-off between triterpene- or tannin-dominated defences. However, we also found variations in chemical defence composition and resin gland density both within and among functional groups (resinous/non-resinous) and taxa, suggesting these categorisations only partly predict chemical herbivore defence. Complex tannins were the only defence compounds negatively related to in vitro digestibility, identifying this previously neglected tannin group as having a potential key role in birch anti-herbivore defence. We conclude that circum-Arctic variation in birch anti-herbivore defence can be partly derived from biogeographical distributions of birch taxa, although our detailed mapping of plant defence provides more information on this variation and can be used for better predictions of herbivore effects on Arctic vegetation.Peer reviewe

Circum-Arctic distribution of chemical anti-herbivore compounds suggests biome-wide trade-off in defence strategies in Arctic shrubs

Spatial variation in plant chemical defence towards herbivores can help us understand variation in herbivore top-down control of shrubs in the Arctic and possibly also shrub responses to global warming. Less defended, non-resinous shrubs could be more influenced by herbivores than more defended, resinous shrubs. However, sparse field measurements limit our current understanding of how much of the circum-Arctic variation in defence compounds is explained by taxa or defence functional groups (resinous/non-resinous). We measured circum-Arctic chemical defence and leaf digestibility in resinous (Betula glandulosa, B. nana ssp. exilis) and non-resinous (B. nana ssp. nana, B. pumila) shrub birches to see how they vary among and within taxa and functional groups. Using liquid chromatography-mass spectrometry (LC-MS) metabolomic analyses and in vitro leaf digestibility via incubation in cattle rumen fluid, we analysed defence composition and leaf digestibility in 128 samples from 44 tundra locations. We found biogeographical patterns in anti-herbivore defence where mean leaf triterpene concentrations and twig resin gland density were greater in resinous taxa and mean concentrations of condensing tannins were greater in non-resinous taxa. This indicates a biome-wide trade-off between triterpene- or tannin-dominated defences. However, we also found variations in chemical defence composition and resin gland density both within and among functional groups (resinous/non-resinous) and taxa, suggesting these categorisations only partly predict chemical herbivore defence. Complex tannins were the only defence compounds negatively related to in vitro digestibility, identifying this previously neglected tannin group as having a potential key role in birch anti-herbivore defence. We conclude that circum-Arctic variation in birch anti-herbivore defence can be partly derived from biogeographical distributions of birch taxa, although our detailed mapping of plant defence provides more information on this variation and can be used for better predictions of herbivore effects on Arctic vegetation

Circum-Arctic distribution of chemical anti-herbivore compounds suggests biome-wide trade-off in defence strategies in Arctic shrubs

Spatial variation in plant chemical defence towards herbivores can help us understand variation in herbivore top?down control of shrubs in the Arctic and possibly also shrub responses to global warming. Less defended, non-resinous shrubs could be more influenced by herbivores than more defended, resinous shrubs. However, sparse field measurements limit our current understanding of how much of the circum-Arctic variation in defence compounds is explained by taxa or defence functional groups (resinous/non-resinous). We measured circum-Arctic chemical defence and leaf digestibility in resinous (Betula glandulosa, B. nana ssp. exilis) and non-resinous (B. nana ssp. nana, B. pumila) shrub birches to see how they vary among and within taxa and functional groups. Using liquid chromatography?mass spectrometry (LC?MS) metabolomic analyses and in vitro leaf digestibility via incubation in cattle rumen fluid, we analysed defence composition and leaf digestibility in 128 samples from 44 tundra locations. We found biogeographical patterns in anti-herbivore defence where mean leaf triterpene concentrations and twig resin gland density were greater in resinous taxa and mean concentrations of condensing tannins were greater in non-resinous taxa. This indicates a biome-wide trade-off between triterpene- or tannin-dominated defences. However, we also found variations in chemical defence composition and resin gland density both within and among functional groups (resinous/non-resinous) and taxa, suggesting these categorisations only partly predict chemical herbivore defence. Complex tannins were the only defence compounds negatively related to in vitro digestibility, identifying this previously neglected tannin group as having a potential key role in birch anti-herbivore defence. We conclude that circum-Arctic variation in birch anti-herbivore defence can be partly derived from biogeographical distributions of birch taxa, although our detailed mapping of plant defence provides more information on this variation and can be used for better predictions of herbivore effects on Arctic vegetation

Measurement of the inclusive isolated-photon cross section in pp collisions at √s = 13 TeV using 36 fb−1 of ATLAS data

The differential cross section for isolated-photon production in pp collisions is measured at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 36.1 fb. The differential cross section is presented as a function of the photon transverse energy in different regions of photon pseudorapidity. The differential cross section as a function of the absolute value of the photon pseudorapidity is also presented in different regions of photon transverse energy. Next-to-leading-order QCD calculations from Jetphox and Sherpa as well as next-to-next-to-leading-order QCD calculations from Nnlojet are compared with the measurement, using several parameterisations of the proton parton distribution functions. The predictions provide a good description of the data within the experimental and theoretical uncertainties. [Figure not available: see fulltext.

Seedling and seedbank data

Environmental characterization, seedling count data, seedbank size and germination dat

Seedling recruitment, genetic diversity, and secondary growth of deciduous shrubs in Arctic tundra disturbed by retrogressive thaw slump thermokarst on Alaska's North Slope

Thesis (Ph.D.) University of Alaska Fairbanks, 2020Since the 1970s, Arctic temperatures have risen by 2.7 °C, more than twice that of lower latitudes. Productivity of tundra vegetation is historically nutrient-limited, largely due to low rates of decomposition in soils underlain by permafrost, where cold temperatures limit nutrient uptake by plants. However, climate warming is implicated in the recent expansion of tall (≥ 0.5 m) deciduous woody shrubs across the Arctic. Among the largest tundra plants, deciduous shrubs exert strong controls on hydrology, heat balance, nutrient cycling, and food webs. These shrubs may be key players in carbon storage and re-stabilization of thaw-deformed permafrost landscapes (thermokarst), however, shrub-climate feedbacks are complex and their magnitude remains uncertain. Warming associated with recent thermokarst activity includes large (≥ 1 ha) de-vegetated depressions on hillslopes caused by mass soil thaw, known as retrogressive thaw slumps (RTS). RTS have increased on Alaska's North Slope by two-thirds since the 1980s. Within a few decades, some RTS near Toolik Lake support tall willow (Salix spp.) and dwarf birch (Betula nana) colonies. This study quantified three aspects of plant response in RTS of different ages (chronosequences) at two North Slope lakes: 1) recruitment (seedlings m⁻² and percent germination of soil seedbanks), 2) clonal (asexual) growth of dominant vegetation (willow), and 3) secondary growth (annual rings) of dwarf birch and willow. I hypothesized that conditions in RTS support greater recruitment, genetic diversity, and growth than conditions in undisturbed moist acidic tussock tundra, and that the climate signal (June mean temperature) is amplified in RTS shrub ring widths. The study found higher seedling density and seedbank viability associated with warm, nutrient-rich bare soil in recent RTS. Willow species richness was higher in RTS than in undisturbed tundra, but all willows showed high heterozygosity and low clonal spread regardless of disturbance. Ramets (branches) within clones were more widely spaced in RTS, suggesting that RTS can fragment and disperse asexual propagules. Shrub rings in RTS were wider than in undisturbed tundra, but climate sensitivity to warmer temperatures was not amplified in the growth rings of most RTS shrubs. Most RTS shrubs had wider rings associated with greater September precipitation in the previous year, while shrubs growing outside of RTS did not, which suggests protective effects of early snow accumulations in RTS depressions. These results demonstrate that some North Slope RTS support greater seedling recruitment and shrub growth than undisturbed tundra and may enhance tundra shrub growth.the Arctic Institute of North America (DCH), the National Science Foundation (DEB 1556481, DEB 1637459 and PLR 1623461 to MSBH), the University of Alaska Fairbanks Center for Global Change/Alaska Climate Center Student Research Grant (DCH), the University of Alaska Fairbanks Dissertation Completion Grant (DCH), the University of Alaska Fairbanks Institute of Arctic Biology Director’s Office (DCH), the University of Alaska Fairbanks Institute of Arctic Biology Graduate Research Fellowship (DCH), the University of Alaska Fairbanks Office of the Vice Chancellor for Research (DCH), and the University of Alaska Fairbanks Randy Howenstein Memorial Field Research Fund (DCH).Chapter 1. General introduction -- Chapter 2. Microsite conditions in retrogressive thaw slumps may facilitate increased seedling recruitment in the Alaskan Low Arctic -- Chapter 3. Recruitment dynamics and population structure of willows in tundra disturbed by retrogressive thaw slump thermokarst on Alaska's North Slope -- Chapter 4. Secondary growth responses of deciduous shrubs in retrogressive thaw slump thermokarsts in the Alaskan Low Arctic -- Chapter 5. General Conclusion

Data from: Microsite conditions in retrogressive thaw slumps may facilitate increased seedling recruitment in the Alaskan Low Arctic

In Low Arctic tundra, thermal erosion of ice-rich permafrost soils (thermokarst) has increased in frequency since the 1980s. Retrogressive thaw slumps (RTS) are thermokarst disturbances forming large open depressions on hillslopes through soil wasting and vegetation displacement. Tall (> 0.5 m) deciduous shrubs have been observed in RTS a decade after disturbance. RTS may provide conditions suitable for seedling recruitment, which may contribute to arctic shrub expansion. We quantified in situ seedling abundance, and size and viability of soil seedbanks in greenhouse trials for two RTS chronosequences near lakes on Alaska’s North Slope. We hypothesized recent RTS provide microsites for greater recruitment than mature RTS or undisturbed tundra. We also hypothesized soil seedbanks demonstrate quantity-quality trade-offs: younger seedbanks contain smaller numbers of mostly viable seed that decrease in viability as seed accumulates over time. We found five times as many seedlings in younger RTS as in older RTS, including birch and willow, and no seedlings in undisturbed tundra. Higher seedling counts were associated with bare soil, warmer soils, higher soil available nitrogen, and less plant cover. Seedbank viability was unrelated to size. Older seedbanks were larger at one chronosequence, with no difference in percent germination. At the other chronosequence, germination was lower from older seedbanks but seedbank size was not different. Seedbank germination was positively associated with in situ seedling abundance at one RTS chronosequence, suggesting post-disturbance revegetation from seedbanks. Thermal erosion may be important for recruitment in tundra by providing bare microsites that are warmer, more nutrient rich, and less vegetated than in undisturbed conditions. Differences between two chronosequences in seedbank size, viability, and species composition suggest disturbance interacts with local conditions to form seedbanks. RTS may act as seedling nurseries to benefit many arctic species as climate changes, particularly those that do not produce persistent seed