Patterns and trends of Northern Hemisphere snow mass from 1980 to 2018.

Warming surface temperatures have driven a substantial reduction in the extent and duration of Northern Hemisphere snow cover1-3. These changes in snow cover affect Earth's climate system via the surface energy budget, and influence freshwater resources across a large proportion of the Northern Hemisphere4-6. In contrast to snow extent, reliable quantitative knowledge on seasonal snow mass and its trend is lacking7-9. Here we use the new GlobSnow 3.0 dataset to show that the 1980-2018 annual maximum snow mass in the Northern Hemisphere was, on average, 3,062 ± 35 billion tonnes (gigatonnes). Our quantification is for March (the month that most closely corresponds to peak snow mass), covers non-alpine regions above 40° N and, crucially, includes a bias correction based on in-field snow observations. We compare our GlobSnow 3.0 estimates with three independent estimates of snow mass, each with and without the bias correction. Across the four datasets, the bias correction decreased the range from 2,433-3,380 gigatonnes (mean 2,867) to 2,846-3,062 gigatonnes (mean 2,938)-a reduction in uncertainty from 33% to 7.4%. On the basis of our bias-corrected GlobSnow 3.0 estimates, we find different continental trends over the 39-year satellite record. For example, snow mass decreased by 46 gigatonnes per decade across North America but had a negligible trend across Eurasia; both continents exhibit high regional variability. Our results enable a better estimation of the role of seasonal snow mass in Earth's energy, water and carbon budgets.

Publisher Correction: Patterns and trends of Northern Hemisphere snow mass from 1980 to 2018.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Solving Challenges of Assimilating Microwave Remote Sensing Signatures With a Physical Model to Estimate Snow Water Equivalent.

Global monitoring of seasonal snow water equivalent (SWE) has advanced significantly over the past decades. However, challenges remain when estimating SWE from passive and active microwave signatures, because a priori characterization of snow properties is required for SWE retrievals. Numerical experiments have shown that utilizing physical snow models to acquire snowpack characterization can potentially improve microwave-based SWE retrievals. This study aims to identify the challenges of assimilating active and passive microwave signatures with physical snow models, and to examine solutions to those challenges. Guided by observations from a point-based study, we designed a sensitivity experiment to quantify the effects of changes in the physically modeled SWE-and of corresponding changes to other snowpack properties-to the microwave-based SWE retrievals. The results indicate that assimilating microwave signatures with physical snow models face some critical challenges associated with the physical relationship between SWE and snow microstructure. We demonstrate these challenges can be overcome if the microwave algorithms account for these relationships.

Early snowmelt significantly enhances boreal springtime carbon uptake.

We determine the annual timing of spring recovery from space-borne microwave radiometer observations across northern hemisphere boreal evergreen forests for 1979-2014. We find a trend of advanced spring recovery of carbon uptake for this period, with a total average shift of 8.1 d (2.3 d/decade). We use this trend to estimate the corresponding changes in gross primary production (GPP) by applying in situ carbon flux observations. Micrometeorological CO2 measurements at four sites in northern Europe and North America indicate that such an advance in spring recovery would have increased the January-June GPP sum by 29 g⋅C⋅m-2 [8.4 g⋅C⋅m-2 (3.7%)/decade]. We find this sensitivity of the measured springtime GPP to the spring recovery to be in accordance with the corresponding sensitivity derived from simulations with a land ecosystem model coupled to a global circulation model. The model-predicted increase in springtime cumulative GPP was 0.035 Pg/decade [15.5 g⋅C⋅m-2 (6.8%)/decade] for Eurasian forests and 0.017 Pg/decade for forests in North America [9.8 g⋅C⋅m-2 (4.4%)/decade]. This change in the springtime sum of GPP related to the timing of spring snowmelt is quantified here for boreal evergreen forests.

SodSAR: A Tower-Based 1-10 GHz SAR System for Snow, Soil and Vegetation Studies.

We introduce SodSAR, a fully polarimetric tower-based wide frequency (1-10 GHz) range Synthetic Aperture Radar (SAR) aimed at snow, soil and vegetation studies. The instrument is located in the Arctic Space Centre of the Finnish Meteorological Institute in Sodankylä, Finland. The system is based on a Vector Network Analyzer (VNA)-operated scatterometer mounted on a rail allowing the formation of SAR images, including interferometric pairs separated by a temporal baseline. We present the description of the radar, the applied SAR focusing technique, the radar calibration and measurement stability analysis. Measured stability of the backscattering intensity over a three-month period was observed to be better than 0.5 dB, when measuring a target with a known radar cross section. Deviations of the estimated target range were in the order of a few cm over the same period, indicating also good stability of the measured phase. Interforometric SAR (InSAR) capabilities are also discussed, and as a example, the coherence of subsequent SAR acquisitions over the observed boreal forest stand are analyzed over increasing temporal baselines. The analysis shows good conservation of coherence in particular at L-band, while higher frequencies are susceptible to loss of coherence in particular for dense vegetation. The potential of the instrument for satellite calibration and validation activities is also discussed.

GlobSnow v3.0 Northern Hemisphere snow water equivalent dataset.

We describe the Northern Hemisphere terrestrial snow water equivalent (SWE) time series covering 1979-2018, containing daily, monthly and monthly bias-corrected SWE estimates. The GlobSnow v3.0 SWE dataset combines satellite-based passive microwave radiometer data (Nimbus-7 SMMR, DMSP SSM/I and DMSP SSMIS) with ground based synoptic snow depth observations using bayesian data assimilation, incorporating the HUT Snow Emission model. The original GlobSnow SWE retrieval methodology has been further developed and is presented in its current form in this publication. The described GlobSnow v3.0 monthly bias-corrected dataset was applied to provide continental scale estimates on the annual maximum snow mass and its trend during the period 1980 to 2018.

BpMADS4 has a central role in inflorescence initiation in silver birch (Betula pendula).

Acceleration of flowering would be beneficial for breeding trees with a long juvenile phase; conversely, inhibition of flowering would prevent the spread of transgenes from the genetically modified trees. We have previously isolated and characterized several MADS genes from silver birch (Betula pendula Roth). In this study, we investigated the more detailed function of one of them, BpMADS4, a member of the APETALA1/FRUITFULL group of MADS genes. The expression of BpMADS4 starts at very early stage of the male and female inflorescence development and the activity is high in the apex of the developing inflorescence. Later, some expression is detected in the bracts and in the flower initials. Ectopic expression of BpMADS4 accelerates flowering dramatically in normally flowering clones and also in the early-flowering birch clone, in which the earliest line flowered about 11 days after rooting, when the saplings were only 3 cm high. The birches transformed with the BpMADS4 antisense construct showed remarkable delay in flowering and the number of flowering individuals was reduced. Two of the transformed lines did not show any signs of flower development during our 2-year study, whereas all the control plants formed inflorescences within 107 days. Our results show that BpMADS4 has a critical role in the initiation of birch inflorescence development and that BpMADS4 seems to be involved in the transition from vegetative to reproductive development. Therefore, BpMADS4 provides a promising tool for the genetic enhancement of forest trees.

The expression and promoter specificity of the birch homologs for PISTILLATA/GLOBOSA and APETALA3/DEFICIENS.

B-function genes determine the identity of petals and stamens in the flowers of model plants such as Arabidopsis and Antirrhinum. Here, we show that a putative B-function gene BpMADS2, a birch homolog for PISTILLATA, is expressed in stamens and carpels of birch inflorescences. We also present a novel birch gene BpMADS8, a homolog for APETALA3/DEFICIENS, which is expressed in stamens. Promoter-GUS analysis revealed that BpMADS2 promoter is active in the receptacle of Arabidopsis flower buds while BpMADS8 promoter is highly specific in mature stamens. BpMADS2 promoter::BARNASE construct prevented floral organ development in Arabidopsis and tobacco. In birch, inflorescences with degenerated stamens and carpels were obtained. BpMADS8::BARNASE resulted in degeneration of stamens in Arabidopsis and birch causing male sterility. In tobacco, only sepals were developed instead of normal flowers. The results show that the BpMADS2::BARNASE construct can be used to specifically disrupt floral organ development in phylogenetically distant plant species. The stamen-specific promoter of BpMADS8 is a promising tool for biotechnological applications in inducing male sterility or targeting gene expression in the late stamen development.

Three MADS-box genes similar to APETALA1 and FRUITFULL from silver birch (Betula pendula).

Despite intensive research on genetic regulation of flower development there are still only a few studies on the early phases of this process in perennial plants like trees. The aim of this study has been to identify genes that regulate early stages of inflorescence development in silver birch (Betula pendula Roth) and to follow the expression of these genes during development of the unisexual birch inflorescences. Here we describe the cloning and characterization of 3 cDNAs representing MADS-box genes designated BpMADS3, BpMADS4 and BpMADS5, all belonging to the AP1/SQUA group of plant MADS-box genes. According to RNA blot analysis, all 3 genes are active during the development of both male and female inflorescences. However, differences in patterns of expression suggest that they play different roles. BpMADS3 is most similar in sequence to AP1 and SQUA, but it seems to have the highest expression at late developmental stages. BpMADS4 is most similar in sequence to the Arabidopsis gene FRUITFULL, but is expressed, in addition to developing inflorescences, in shoots and roots. BpMADS5 is also similar to FRUITFULL; its expression seems to be inflorescence-specific and continues during fruit development. Ectopic expression of either BpMADS3, BpMADS4 or BpMADS5 with the CaMV 35S promoter in tobacco results in extremely early flowering. All of these birch genes seem to act early during the transition to reproductive phase and might be involved in the determination of the identity of the inflorescence or flower meristem. They could apparently be used to accelerate flowering in various plant species.

Functional characterization of SEPALLATA3 and AGAMOUS orthologues in silver birch.

The development of flowers is regulated by a complex network of transcriptional activators and repressors, many of which belong to the MADS box gene family. In this study, we describe two MADS box genes of silver birch (Betula pendula Roth), BpMADS1 and BpMADS6, which are similar to SEPALLATA3 and AGAMOUS in Arabidopsis thaliana, respectively. In situ hybridization showed that BpMADS1 was expressed in the inflorescence meristem at a very early stage, but not later. Both genes were expressed in developing carpels, ovules and stamens but not in tepals or scales. Ectopic expression of BpMADS1 in Arabidopsis resulted in a reduced number of floral organs or whole whorls and in petaloid or carpelloid sepals, a phenotype reminiscent of that of fil mutants. 35S::BpMADS6 caused very early flowering in Arabidopsis. In tobacco, both 35S::BpMADS1 and 35S::BpMADS6 accelerated flowering and, in addition, 35S::BpMADS6 caused changes in sepals and petals. In some transgenic birch plants, 35S::BpMADS1 antisense resulted in the development of both male and female organs in the axil of a single bract and in a change of some inflorescences into vegetative shoots. In two plants, either 35S::BpMADS6 sense or antisense constructs resulted in an increase in the number of tepals and in complete lack of stamens in some male inflorescences. These results suggest that BpMADS1 participates both in inflorescence and in flower formation and BpMADS6 participates in flower formation and that they are functional homologues to SEPALLATA3 and AGAMOUS, respectively.

Changes in Lumbriculus variegatus metabolites under hypoxic exposure to benzo(a)pyrene, chlorpyrifos and pentachlorophenol: consequences on biotransformation.

The regulation of endogenous metabolites is still not fully understood in aquatic invertebrates exposed concurrently to toxicants and hypoxia. Despite the prevalence of hypoxia in the aquatic environment, toxicity estimations seldom account for multiple stressors thereby differing from natural conditions. In this study, we examined the influence of hypoxia (<30% O2) on contaminant uptake and the composition of intracellular metabolites in Lumbriculus variegatus exposed to benzo(a)pyrene (B(a)P, 3µgL(-1)), chlorpyrifos (CPF, 100µgL(-1)) or pentachlorophenol (PCP, 100µgL(-1)). Tissue extracts of worms were analyzed for 123 metabolites by gas chromatography-mass spectrometry and metabolite levels were then related to treatments and exposure time. Hypoxia markedly increased the accumulation of B(a)P and CPF, which underlines the significance of oxygen in chemical uptake. The oxygen effect on PCP uptake was less pronounced. Succinate and glycerol-3-phosphate increased significantly (p<0.0001) following hypoxic treatment, whereas sugars, cysteine, and cholesterol were effectively repressed. The buildup of succinate coupled with the corresponding decline in intracellular 2-oxo- and 2-hydroxy glutaric acid is indicative of an active hypoxia inducible factor mechanism. Glutamate, and TCA cycle intermediates (fumarate, and malate) were disturbed and evident in their marked suppression in worms exposed concurrently to hypoxia and PCP. Clearly, hypoxia was the dominant stressor for individuals exposed to B(a)P or CPF, but to a lesser extent upon PCP treatment. And since oxygen deprivation promotes the accumulation of different toxicants, there may be consequences on species composition of metabolites in natural conditions.

Response of Lumbriculus variegatus transcriptome and metabolites to model chemical contaminants.

Assessment of the underlying molecular events leading to xenobiotic toxicity is challenging especially when techniques are applied in isolation. We examined transcriptional and metabolic changes in Lumbriculus variegatus exposed to benzo(a)pyrene (B(a)P), cadmium (Cd) or pentachlorophenol (PCP) by DNA microarrays (7422 ESTs) and gas chromatography-mass spectrometry (GC-MS), respectively. In addition, the DNA damage response of worms exposed to B(a)P was assessed by a capillary electrophoresis laser induced fluorescence (CE-LIF) immunoassay. We found elevated expression of oxidative stress responsive genes, which correlated positively with the changes in antioxidant vitamin precursors including alpha-tocopherol and cholecalciferol. Other genes with strong differential expressions were mostly involved in actin related processes and proteolysis, despite an apparent delayed Cd response. Phosphates, sugars and fatty acids were effectively reduced and suggested that chemical treatments may have interfered with energy metabolism. The increased amount of B(a)P diol-epoxide (BPDE)-DNA adducts in exposed worms appeared to correlate with the variability in uridine, inosine and xanthine, which are key components of nucleoside metabolism. This suggests that DNA damage was imminent or peaked within 6h. The results conformed to transcriptional changes in B(a)P exposed worms and compliment other approaches to elucidate underlying molecular changes.

DNA sequence variation in BpMADS2 gene in two populations of Betula pendula.

The PISTILLATA (PI) homologue, BpMADS2, was isolated from silver birch (Betula pendula Roth) and used to study nucleotide polymorphism. Two regions (together about 2450 bp) comprising mainly untranslated sequences were sequenced from 10 individuals from each of two populations in Finland. The nucleotide polymorphism was low in the BpMADS2 locus, especially in the coding region. The synonymous site overall nucleotide diversity (pis) was 0.0043 and the nonsynonymous nucleotide diversity (pia) was only 0.000052. For the whole region, the pi values for the two populations were 0.0039 and 0.0045, and for the coding regions, the pi values were only 0 and 0.00066 (for the corresponding coding regions of Arabidopsis thaliana PI world-wide pi was 0.0021). Estimates of pi or theta did not differ significantly between the two populations, and the two populations were not diverged from each other. Two classes of BpMADS2 alleles were present in both populations, suggesting that this gene exhibits allelic dimorphism. In addition to the nucleotide site variation, two microsatellites were also associated within the haplotypes. This allelic dimorphism might be the result of postglacial re-colonization partly from northwestern, partly from southeastern/eastern refugia. The sequence comparison detected five recombination events in the regions studied. The large number of microsatellites in all of the three introns studied suggests that BpMADS2 is a hotspot for microsatellite formation.