Impacts of multiple extreme winter warming events on sub‐Arctic heathland: phenology,reproduction, growth,and CO2 flux responses

Extreme weather events can have strong negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme, short‐lived, winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (for instance, 2–10 °C for 2–14 days) but upon return to normal winter climate exposes the ecosystem to much colder temperatures due to the loss of insulating snow. Single events have been shown to reduce plant reproduction and increase shoot mortality, but impacts of multiple events are little understood as are the broader impacts on community structure, growth, carbon balance, and nutrient cycling. To address these issues, we simulated week‐long extreme winter warming events – using infrared heating lamps and soil warming cables – for 3 consecutive years in a sub‐Arctic heathland dominated by the dwarf shrubs Empetrum hermaphroditum, Vaccinium vitis‐idaea (both evergreen) and Vaccinium myrtillus (deciduous). During the growing seasons after the second and third winter event, spring bud burst was delayed by up to a week for E. hermaphroditum and V. myrtillus, and berry production reduced by 11–75% and 52–95% for E. hermaphroditum and V. myrtillus, respectively. Greater shoot mortality occurred in E. hermaphroditum (up to 52%), V. vitis‐idaea (51%), and V. myrtillus (80%). Root growth was reduced by more than 25% but soil nutrient availability remained unaffected. Gross primary productivity was reduced by more than 50% in the summer following the third simulation. Overall, the extent of damage was considerable, and critically plant responses were opposite in direction to the increased growth seen in long‐term summer warming simulations and the ‘greening’ seen for some arctic regions. Given the Arctic is warming more in winter than summer, and extreme events are predicted to become more frequent, this generates large uncertainty in our current understanding of arctic ecosystem responses to climate change.     

Impact of rainfall manipulations and biotic controls on soil respiration in Mediterranean and desert ecosystems along an aridity gradient

Spatially heterogeneous ecosystems form a majority of land types in the vast drylands of the globe. To evaluate climate‐change effects on CO₂ fluxes in such ecosystems, it is critical to understand the relative responses of each ecosystem component (microsite). We investigated soil respiration (R_s) at four sites along an aridity gradient (90–780 mm mean annual precipitation, MAP) during almost 2 years. In addition, R_s was measured in rainfall manipulations plots at the two central sites where ～30% droughting and ～30% water supplementation treatments were used over 5 years. Annual R_s was higher by 23% under shrub canopies compared with herbaceous gaps between shrubs, but R_s at both microsites responded similarly to rainfall reduction. Decreasing precipitation and soil water content along the aridity gradient and across rainfall manipulations resulted in a progressive decline in R_s at both microsites, i.e. the drier the conditions, the larger was the effect of reduction in water availability on R_s. Annual R_s on the ecosystem scale decreased at a slope of 256/MAP g C m^?2 yr^?1 mm^?1 (r²=0.97). The reduction in R_s amounted to 77% along the aridity gradient and to 16% across rainfall manipulations. Soil organic carbon (SOC) decreased with declining precipitation, and variation in SOC stocks explained 77% of the variation in annual R_s across sites, rainfall manipulations and microsites. This study shows that rainfall manipulations over several years are a useful tool for experimentally predicting climate‐change effects on CO₂ fluxes for time scales (such as approximated by aridity gradients) that are beyond common research periods. Rainfall reduction decreases rates of R_s not only by lowering biological activity, but also by drastically reducing shrub cover. We postulate that future climate change in heterogeneous ecosystems, such as Mediterranean and deserts shrublands will have a major impact on R_s by feedbacks through changes in vegetation structure.     

基于异速生长理论的准噶尔盆地荒漠灌丛形态研究

为了揭示荒漠灌丛形态的发生发展机制并认识其在荒漠生态系统中的功能, 从形态和结构决定功能的原理出发, 对生长在准噶尔荒漠东南部的岛状灌丛进行了形态学调查。依据Malthusian方程微分形式, 根据异速生长理论, 建立了冠幅与株高生长、灌丛表面积与体积生长的数学关系式, 利用植被调查数据进行了验证, 并最终得出不同灌丛在不同株高时的情景示意图。结果表明: 1)将荒漠灌丛形态假设成半三轴椭球体是合理的; 2)虽然灌丛形态发展趋势可以是扁平、近半球和竖直3种类型, 但是形态建成后, 一般维持在扁平和近半球两种类型; 3) 18类荒漠灌丛的体积和表面积的数量关系具有一定的一致性, 可能与同处于相同环境条件下的水分利用效率相近有关。     

Marker Density and Read Depth for Genotyping Populations Using Genotyping-by-Sequencing

Genotyping-by-sequencing (GBS) approaches provide low-cost, high-density genotype information. However, GBS has unique technical considerations, including a substantial amount of missing data and a nonuniform distribution of sequence reads. The goal of this study was to characterize technical variation using this method and to develop methods to optimize read depth to obtain desired marker coverage. To empirically assess the distribution of fragments produced using GBS, ∼8.69 Gb of GBS data were generated on the Zea mays reference inbred B73, utilizing ApeKI for genome reduction and single-end reads between 75 and 81 bp in length. We observed wide variation in sequence coverage across sites. Approximately 76% of potentially observable cut site-adjacent sequence fragments had no sequencing reads whereas a portion had substantially greater read depth than expected, up to 2369 times the expected mean. The methods described in this article facilitate determination of sequencing depth in the context of empirically defined read depth to achieve desired marker density for genetic mapping studies.     

Extensive land cover change across Arctic–Boreal Northwestern North America from disturbance and climate forcing

A multitude of disturbance agents, such as wildfires, land use, and climate‐driven expansion of woody shrubs, is transforming the distribution of plant functional types across Arctic–Boreal ecosystems, which has significant implications for interactions and feedbacks between terrestrial ecosystems and climate in the northern high‐latitude. However, because the spatial resolution of existing land cover datasets is too coarse, large‐scale land cover changes in the Arctic–Boreal region (ABR) have been poorly characterized. Here, we use 31 years (1984–2014) of moderate spatial resolution (30 m) satellite imagery over a region spanning 4.7 × 10⁶ km² in Alaska and northwestern Canada to characterize regional‐scale ABR land cover changes. We find that 13.6 ± 1.3% of the domain has changed, primarily via two major modes of transformation: (a) simultaneous disturbance‐driven decreases in Evergreen Forest area (?14.7 ± 3.0% relative to 1984) and increases in Deciduous Forest area (+14.8 ± 5.2%) in the Boreal biome; and (b) climate‐driven expansion of Herbaceous and Shrub vegetation (+7.4 ± 2.0%) in the Arctic biome. By using time series of 30 m imagery, we characterize dynamics in forest and shrub cover occurring at relatively short spatial scales (hundreds of meters) due to fires, harvest, and climate‐induced growth that are not observable in coarse spatial resolution (e.g., 500 m or greater pixel size) imagery. Wildfires caused most of Evergreen Forest Loss and Evergreen Forest Gain and substantial areas of Deciduous Forest Gain. Extensive shifts in the distribution of plant functional types at multiple spatial scales are consistent with observations of increased atmospheric CO₂ seasonality and ecosystem productivity at northern high‐latitudes and signal continental‐scale shifts in the structure and function of northern high‐latitude ecosystems in response to climate change.     

Enhanced shrub growth in the Arctic increases habitat connectivity for browsing herbivores

Habitat connectivity is a key factor influencing species range dynamics. Rapid warming in the Arctic is leading to widespread heterogeneous shrub expansion, but impacts of these habitat changes on range dynamics for large herbivores are not well understood. We use the climate–shrub–moose system of northern Alaska as a case study to examine how shrub habitat will respond to predicted future warming, and how these changes may impact habitat connectivity and the distribution of moose (Alces alces). We used a 19 year moose location dataset, a 568 km transect of field shrub sampling, and forecasted warming scenarios with regional downscaling to map current and projected shrub habitat for moose on the North Slope of Alaska. The tall‐shrub habitat for moose exhibited a dendritic spatial configuration correlated with river corridor networks and mean July temperature. Warming scenarios predict that moose habitat will more than double by 2099. Forecasted warming is predicted to increase the spatial cohesion of the habitat network that diminishes effects of fragmentation, which improves overall habitat quality and likely expands the range of moose. These findings demonstrate how climate change may increase habitat connectivity and alter the distributions of shrub herbivores in the Arctic, including creation of novel communities and ecosystems.     

Molecular Characterization of Begomoviruses Infecting Sauropus androgynus in Thailand

Begomoviruses were detected in leaf samples of Sauropus androgynus (L.) Merr. plants showing leaf curling with or without yellowing symptoms in Kamphaeng Saen, Nakhon Pathom, Thailand in 2009 and 2010. From eight plants with symptoms, 17 complete begomoviral DNA‐As were amplified by polymerase chain reaction and sequenced. No DNA‐B was detected in any of the plants. All the DNA‐As had the characteristic begomovirus genome organization of six open reading frames, two in the virion‐sense orientation and four in the complementary orientation. Sequence comparison of these virus isolates indicated that one isolate belongs to Tomato leaf curl New Delhi virus, 12 isolates belong to Ageratum yellow vein virus and four isolates belong to a novel species with the tentative name Sauropus leaf curl virus. Five of the eight samples were found to be co‐infected by isolates of two different begomovirus species. Recombination analysis indicated that all but one of the isolates were probably the product of one or more recombination events. The results indicated that S. androgynus plants act as natural hosts as well as potential nurseries for genetic recombination between begomovirus species and strains.     

Estimation of population allele frequencies from next‐generation sequencing data: pool‐versus individual‐based genotyping

Molecular markers produced by next‐generation sequencing (NGS) technologies are revolutionizing genetic research. However, the costs of analysing large numbers of individual genomes remain prohibitive for most population genetics studies. Here, we present results based on mathematical derivations showing that, under many realistic experimental designs, NGS of DNA pools from diploid individuals allows to estimate the allele frequencies at single nucleotide polymorphisms (SNPs) with at least the same accuracy as individual‐based analyses, for considerably lower library construction and sequencing efforts. These findings remain true when taking into account the possibility of substantially unequal contributions of each individual to the final pool of sequence reads. We propose the intuitive notion of effective pool size to account for unequal pooling and derive a Bayesian hierarchical model to estimate this parameter directly from the data. We provide a user‐friendly application assessing the accuracy of allele frequency estimation from both pool‐ and individual‐based NGS population data under various sampling, sequencing depth and experimental error designs. We illustrate our findings with theoretical examples and real data sets corresponding to SNP loci obtained using restriction site–associated DNA (RAD) sequencing in pool‐ and individual‐based experiments carried out on the same population of the pine processionary moth (Thaumetopoea pityocampa). NGS of DNA pools might not be optimal for all types of studies but provides a cost‐effective approach for estimating allele frequencies for very large numbers of SNPs. It thus allows comparison of genome‐wide patterns of genetic variation for large numbers of individuals in multiple populations.     

A greener Greenland? Climatic potential and long-term constraints on future expansions of trees and shrubs

Warming-induced expansion of trees and shrubs into tundra vegetation will strongly impact Arctic ecosystems. Today, a small subset of the boreal woody flora found during certain Plio-Pleistocene warm periods inhabits Greenland. Whether the twenty-first century warming will induce a re-colonization of a rich woody flora depends on the roles of climate and migration limitations in shaping species ranges. Using potential treeline and climatic niche modelling, we project shifts in areas climatically suitable for tree growth and 56 Greenlandic, North American and European tree and shrub species from the Last Glacial Maximum through the present and into the future. In combination with observed tree plantings, our modelling highlights that a majority of the non-native species find climatically suitable conditions in certain parts of Greenland today, even in areas harbouring no native trees. Analyses of analogous climates indicate that these conditions are widespread outside Greenland, thus increasing the likelihood of woody invasions. Nonetheless, we find a substantial migration lag for Greenland''s current and future woody flora. In conclusion, the projected climatic scope for future expansions is strongly limited by dispersal, soil development and other disequilibrium dynamics, with plantings and unintentional seed dispersal by humans having potentially large impacts on spread rates.