Aspen Species Research Articles

Chromosome-level genome assembly of the Asian aspen Populus davidiana Dode

The genome of Populus davidiana, a keystone aspen species, has been sequenced to improve our understanding of the evolutionary and functional genomics of the Populus genus. The Hi-C scaffolding genome assembly resulted in a 408.1 Mb genome with 19 pseudochromosomes. The BUSCO assessment revealed that 98.3% of the genome matched the embryophytes dataset. A total of 31,862 protein-coding sequences were predicted, of which 31,619 were functionally annotated. The assembled genome was composed of 44.9% transposable elements. These findings provide new knowledge about the characteristics of the P. davidiana genome and will facilitate comparative genomics and evolutionary research on the genus Populus.

Genomic insights into speciation history and local adaptation of an alpine aspen in the Qinghai–Tibet Plateau and adjacent highlands

AbstractNatural selection serves as an important agent to drive and maintain interspecific divergence. Populus rotundifolia Griff. is an alpine aspen species that mainly occurs in the Qinghai–Tibet Plateau (QTP) and adjacent highlands, whereas its sister species, P. davidiana Dode, is distributed across southwest and central to northeast China in much lower altitude regions. In this study, we collected genome resequencing data of 53 P. rotundifolia and 42 P. davidiana individuals across their natural distribution regions. Our population genomic data suggest that the two species are well delimitated in the allopatric regions, but with hybrid zones in their adjacent region in the eastern QTP. Coalescent simulations suggest that P. rotundifolia diverged from P. davidiana in the middle Pleistocene with following continuous gene flow since divergence. In addition, we found numerous highly diverged genes with outlier signatures that are likely associated with high‐altitude adaptation of these alpine aspens. Our finding indicate that Quaternary climatic changes and natural selection have greatly contributed to the origin and distinction maintenance of P. rotundifolia in the QTP.

Herbivore Gender Effects on Volatile Induction in Aspen and on Olfactory Responses in Leaf Beetles

Hybrid aspen (Populus tremula × tremuloides Michx.) is a fast-growing tree species used for short-rotation forestry in northern latitudes. Aspen species have a rich herbivore fauna, including defoliating leaf beetles that induce emissions of volatile organic compounds (VOCs) when feeding on aspen leaves. We investigated the differential induction of VOCs by male and female Phratora laticollis leaf beetles feeding on hybrid aspen and the differences in the orientation of beetles in response to gender-specific induced VOCs. The hypotheses for the study were (1) the VOCs in the headspace of plants infested with beetles of the two genders individually and in mixed aggregates would vary subtly, and (2) foraging adult beetles would be able to detect differences in VOC blends and use them to fine-tune their orientation choices. In Y-tube bioassays, both females and males preferred VOCs from leaves damaged by one gender (females or males) over undamaged leaves. However, if leaves were damaged by a two-gender population, neither females nor males indicated a preference over volatiles of undamaged leaves. Leaves damaged by both beetle genders simultaneously had significantly increased green leaf volatile (GLV), benzenoid and homoterpene emissions compared to undamaged leaves. Emissions of these compounds possibly indicate higher herbivore pressure and a higher risk of attack by parasitoids and predators and could thus be the cause of the lack of beetle preference. Our findings provide new basic information on gender-based host plant selection by herbivores and may be helpful in the development of sustainable biogenic VOC-based herbivore-control methods for intensive short-rotation hybrid aspen production.

Evidence for widespread selection in shaping the genomic landscape during speciation of Populus.

Increasing our understanding of how evolutionary processes drive the genomic landscape of variation is fundamental to a better understanding of the genomic consequences of speciation. However, genome-wide patterns of within- and between- species variation have not been fully investigated in most forest tree species despite their global ecological and economic importance. Here, we use whole-genome resequencing data from four Populus species spanning the speciation continuum to reconstruct their demographic histories and investigate patterns of diversity and divergence within and between species. Using Populus trichocarpa as an outgroup species, we further infer the genealogical relationships and estimate the extent of ancient introgression among the three aspen species (Populus tremula, Populus davidiana and Populus tremuloides) throughout the genome. Our results show substantial variation in these patterns along the genomes with this variation being strongly predicted by local recombination rates and the density of functional elements. This implies that the interaction between recurrent selection and intrinsic genomic features has dramatically sculpted the genomic landscape over long periods of time. In addition, our findings provide evidence that, apart from background selection, recent positive selection and long-term balancing selection have also been crucial components in shaping patterns of genome-wide variation during the speciation process.

Features of the Stand and Deadwood in Postfire Aspen and Birch Forests in Northern Urals

By now, most of the forests in the European part of Russia have been transformed by drastic fires and clear cutting. Significant areas of primary dark coniferous forests in the Northern Urals have been replaced by either birch or aspen stands. These communities pass different stages of secondary succession. They remain understudied despite their wide occurrence. We consider the composition and structure of the tree story and deadwood in two postfire communities (80- and 120-years old). Both fires were initiated from the slash-and-burn cultivation, as has been the practice up to the mid-20th century. The composition of the stands in these communities indicates the gradual replacement of early succession tree species—aspen and willow—by late succession spruce and fir. Weibull function was applied to fit the distribution of diameters of the trees. We show that these communities do not differ by most indicators of stand and deadwood structures and characterize the same successional stage.

Functional and evolutionary genomic inferences in Populus through genome and population sequencing of American and European aspen

The Populus genus is one of the major plant model systems, but genomic resources have thus far primarily been available for poplar species, and primarily Populus trichocarpa (Torr. & Gray), which was the first tree with a whole-genome assembly. To further advance evolutionary and functional genomic analyses in Populus, we produced genome assemblies and population genetics resources of two aspen species, Populus tremula L. and Populus tremuloides Michx. The two aspen species have distributions spanning the Northern Hemisphere, where they are keystone species supporting a wide variety of dependent communities and produce a diverse array of secondary metabolites. Our analyses show that the two aspens share a similar genome structure and a highly conserved gene content with P. trichocarpa but display substantially higher levels of heterozygosity. Based on population resequencing data, we observed widespread positive and negative selection acting on both coding and noncoding regions. Furthermore, patterns of genetic diversity and molecular evolution in aspen are influenced by a number of features, such as expression level, coexpression network connectivity, and regulatory variation. To maximize the community utility of these resources, we have integrated all presented data within the PopGenIE web resource (PopGenIE.org).

Integrating natural and engineered remediation strategies for water quality management within a low-impact development (LID) approach.

The objective of this paper is to demonstrate an interdisciplinary strategy combining both engineering- and biology-based approaches for stormwater and wastewater treatment. The work involves a novel and environmentally friendly surface material that can withstand urban load over its design service life, allows preliminary treatment through filtration, and diverts water to the subsurface to conduct secondary treatment below the surface through phytoremediation via the extensive rooting systems of trees. The present study highlights an interdisciplinary low-impact development (LID) approach developed for a polluted industrial wastewater site, for a cleaner and greener environment. The LID system involves (i) rhizofiltration and phytoremediation methods for removing heavy metals and organic pollutants using a hybrid poplar and aspen species; (ii) porous infrastructure produced using industrial waste, referred to as geopolymer pavers; and (iii) use of Silva cells as a tree-friendly and load support system. The design of the pavers over the Silva cells is innovative as it can deal with rainwater runoff and urban transportation loads simultaneously. The proposed system has the ability to extract heavy metals that are common in urban runoff or domestic and industrial effluents thus preserving the ecosystem naturally. The test site is only 15m2, but designed for a water-retention capacity of 2m3 (roughly 1:100year design volume draining a 10 × 10m parking lot), and remediation levels for Cu and Zn are expected to reach 180mg/kg dry weight and 1200mg/kg dry weight, respectively.

The complete mitochondrial genome of the lesser aspen webworm moth Meroptera pravella (Insecta: Lepidoptera: Pyralidae)

The lesser aspen webworm moth, Meroptera pravella, is a small pyralid that uses quaking aspen (Populus tremuloides) and related tree species as larval hosts. Whole-genome Illumina sequencing allowed the assembly of a complete circular mitochondrial genome of 15,260 bp consisting of 80.7% AT nucleotides, 22 tRNAs, 13 protein-coding genes, 2 rRNAs and a control region. Mitogenome structure maintains complete synteny with other sequenced pyralid mitogenomes. Parsimony and maximum-likelihood phylogenetic reconstruction places M. pravella within monophyletic subfamily Phycitinae and monophyletic family Pyralidae. The Pyralidae, with monophyletic sister family Crambidae, constitute monophyletic superfamily Pyraloidea, which is consistent with conventional taxonomy.

Shungite Nanopowder as a Modificator of Mechanical Properties and Water Absorption for Three-Ply Particle Board

The article presents the methods and the results of the experimental investigation of the additive influence of shungite nanopowder when mixed in the glue for three-ply particle board. The hypothesis of shungite nanopowder influence on physical-mechanical properties, water absorption and thickness swelling of wood particle board was formulated. The results of experimental investigations proved that the optimum shungite nanopowder quantity in glue solution makes up about 10 % of absolutely dry resin mass. Wooden particles of aspen and coniferous species, as well as glue solution based on carbamide-formaldehyde resin, were used in boards manufacturing. The samples were tested in order to determine physical-mechanical properties. It was found out that the 10 % shungite nanopowder additive increases the breaking point of three-ply particle board under bending strength by 18.3-25.7 %, the breaking point of three-ply particle board under tension perpendicular to the face of board by 7.5-11.7 %. As the result of experimental investigation it was found out that the 10 % shungite nanopowder when mixed in the glue decreases water thickness swelling of three-ply particle board up to 14.2 % and water absorption by 10.6-20.1 %. The shungite nanopowder powder contained particles of 50...100 nm in size and specific surface of 120 m2/g. In the course of the experiment three-ply particle boards were used with the thickness of 15.6 mm.

To each its own: differential response of specialist and generalist herbivores to plant defence in willows.

Plant-insect food webs tend to be dominated by interactions resulting from diffuse co-evolution between plants and multiple lineages of herbivores rather than by reciprocal co-evolution and co-cladogenesis. Plants therefore require defence strategies effective against a broad range of herbivore species. In one extreme, plants could develop a single universal defence effective against all herbivorous insects, or tailor-made strategies for each herbivore species. The evolution and ecology of plant defence has to be studied with entire insect assemblages, rather than small subsets of pairwise interactions. The present study examines whether specialists and generalists in three coexisting insect lineages, forming the leaf-chewing guild, respond uniformly to plant phylogeny, secondary metabolites, nutrient content and mechanical antiherbivore defences of their hosts, thus permitting universal plant defence strategies against specialized and generalist folivorous insects from various taxa. The extensive data on folivorous assemblages comprising three insect orders and 193 species are linked with plant phylogeny, secondary chemistry (salicylates, flavonoids and tannins), leaf morphological traits [specific leaf area (SLA) and trichome coverage], nutrient (C:N) content and growth form of eight willow (Salix) and one aspen (Populus) species growing in sympatry. Generalists responded to overall host plant chemistry and trichomes, whilst specialists responded to host plant phylogeny and secondary metabolites that are unique to willows and that are capable of being utilized as an antipredator protection. We did not find any significant impact of other plant traits, that is SLA, C:N ratio, flavonoids, tannins and growth form, on the composition of leaf-chewing communities. Our results show that the response to plant traits is differential among specialists and generalists. This finding constrains the ability of plants to develop defensive traits universally effective against herbivores and may lead to diversification of plant defensive mechanisms into several complementary syndromes, required for effective protection against generalists and specialists from multiple insect taxa comprising most leaf-chewing assemblages. These results point to the necessity of broad studies of plant-herbivore interactions, across multiple insect taxa and guilds.

Modeling forest ecosystem responses to elevated carbon dioxide and ozone using artificial neural networks

Rising atmospheric levels of carbon dioxide and ozone will impact productivity and carbon sequestration in forest ecosystems. The scale of this process and the potential economic consequences provide an incentive for the development of models to predict the types and rates of ecosystem responses and feedbacks that result from and influence of climate change. In this paper, we use phenotypic and molecular data derived from the Aspen Free Air CO2 Enrichment site (Aspen-FACE) to evaluate modeling approaches for ecosystem responses to changing conditions. At FACE, it was observed that different aspen clones exhibit clone-specific responses to elevated atmospheric levels of carbon dioxide and ozone. To identify the molecular basis for these observations, we used artificial neural networks (ANN) to examine above and below-ground community phenotype responses to elevated carbon dioxide, elevated ozone and gene expression profiles. The aspen community models generated using this approach identified specific genes and subnetworks of genes associated with variable sensitivities for aspen clones. The ANN model also predicts specific co-regulated gene clusters associated with differential sensitivity to elevated carbon dioxide and ozone in aspen species. The results suggest ANN is an effective approach to predict relevant gene expression changes resulting from environmental perturbation and provides useful information for the rational design of future biological experiments.

Wood Quality and Growth Characterization across Intra- and Inter-Specific Hybrid Aspen Clones

Trembling aspen (Populus tremuloides Michx.) is one of the most abundant poplar species in North America; it is native, displays substantial breadth in distribution inhabiting several geographical and climatic ecoregions, is notable for its rapid growth, and is ecologically and economically important. As the demand for raw material continues to increase rapidly, there is a pressing need to improve both tree quality and growth rates via breeding efforts. Hybridization is considered one of the most promising options to simultaneously accelerate these tree characteristics, as it takes advantage of heterosis. Two aspen species showing particular promise for hybridization with trembling aspen are European aspen (P. tremula) and Chinese aspen (P. davidiana) because their native climates are similar to that of P. tremuloides and are also very easy to hybridize. In 2003, aspen clones were planted in Athabasca, Alberta from the following species crosses: open pollinated (OP) P. tremuloides (NN), OP P. davidiana (CC), P. tremula × P. tremula (EE), P. tremula × P. tremuloides (EN), and P. tremuloides × P. davidiana (CN). In November 2010, growth measurements and core samples were taken from seven-year field grown clones. Comparisons of the mean growth and cell wall traits were made between crosses using generalized linear model least squares means tests for stem volume, fiber length, fiber width, coarseness, wood density, microfibril angle, total cell wall carbohydrate and lignin content, and lignin composition. The results clearly indicated that the inter-specific crosses offer a means to breed for more desirable wood characteristics than the intra-specific Populus spp. crosses.

Photosynthetic characteristics and growth performance of closely related aspen taxa: On the systematic relatedness of the Eurasian Populus tremula and the North American P. tremuloides

The European and American aspen species Populus tremula and P. tremuloides are closely related taxa with very large distribution ranges and high economic importance. Genetic and morphological data are not fully congruent with respect to the question of the systematic relatedness of these sister taxa, pointing either at separate species on the two continents or a single aggregate species with circumarctic distribution. In a replicated growth trial with 1-year-old saplings, we compared about 30 morphological (leaf size, leaf area, leaf numbers, leaf growth, leaf phenology and the ratio of leaves lost to leaves produced) and physiological traits (Amax, quantum yield, carboxylation efficiency, maximum rates of carboxylation and electron transport, leaf dark respiration, leaf conductance, leaf water potential and WUE) with the aim to obtain evidence in support of or against the one-species hypothesis and to identify key determinants of growth in the two aspen taxa.American aspen showed a by 20% higher canopy carbon gain than European aspen which was nearly entirely caused by a larger mean leaf area of P. tremuloides, while differences in mean assimilation rate and the length of the leafy period were of minor importance. Species-specific differences in the onset of leaf abscission in early autumn were identified as main determinants of the size of mean plant leaf area and thus of canopy carbon gain. American aspen with a larger leaf area and C gain possessed by far fewer leaves and side branches than European aspen, but had considerably larger leaves. These morphological and phenological contrasts support the view that the two aspen collectives are indeed different species, what has been questioned recently. However, the large majority of physiological traits is not significantly different between the two species and they are much less variable among the individuals of a species than the morphological traits indicating a conservative inheritance of physiological traits.

Metabolite Signature during Short-Day Induced Growth Cessation in Populus

The photoperiod is an important environmental signal for plants, and influences a wide range of physiological processes. For woody species in northern latitudes, cessation of growth is induced by short photoperiods. In many plant species, short photoperiods stop elongational growth after a few weeks. It is known that plant daylength detection is mediated by Phytochrome A (PHYA) in the woody hybrid aspen species. However, the mechanism of dormancy involving primary metabolism remains unclear. We studied changes in metabolite profiles in hybrid aspen leaves (young, middle, and mature leaves) during short-day-induced growth cessation, using a combination of gas chromatography–time-of-flight mass spectrometry, and multivariate projection methods. Our results indicate that the metabolite profiles in mature source leaves rapidly change when the photoperiod changes. In contrast, the differences in young sink leaves grown under long and short-day conditions are less distinct. We found short daylength induced growth cessation in aspen was associated with rapid changes in the distribution and levels of diverse primary metabolites. In addition, we conducted metabolite profiling of leaves of PHYA overexpressor (PHYAOX) and those of the control to find the discriminative metabolites between PHYAOX and the control under the short-day conditions. The metabolite changes observed in PHYAOX leaves, together with those in the source leaves, identified possible candidates for the metabolite signature (e.g., 2-oxo-glutarate, spermidine, putrescine, 4-amino-butyrate, and tryptophan) during short-day-induced growth cessation in aspen leaves.

Near-ambient Ozone Concentrations Reduce the Vigor ofBetulaandPopulusSpecies in Finland

In this review the main growth responses of Finnish birch (Betula pendula, B. pubescens) and aspen species (Populus tremula and P. tremuloides x P. tremula) are correlated with ozone exposure, indicated as the AOT40 value. Data are derived from 23 different laboratory, open-top chamber, and free-air fumigation experiments. Our results indicate that these tree species are sensitive to increasing ozone concentrations, though high intraspecific variation exists. The roots are the most vulnerable targets in both genera. These growth reductions, determined from trees grown under optimal nutrient and water supply, were generally accompanied by increased visible foliar injuries, carbon allocation toward defensive compounds, reduced carbohydrate contents of leaves, impaired photosynthesis processes, disturbances in stomatal function, and earlier autumn senescence. Because both genera have shown complex ozone defense and response mechanisms, which are modified by variable environmental conditions, a mechanistically based approach is necessary for accurate ozone risk assessment.

Resistance of European and hybrid aspen wood against two brown-rot fungi

The decay resistance of European (Populus tremula L.) and hybrid (P. tremula x tremuloides) aspen wood against brown-rot fungi was investigated after three different treatments, i.e. conventional drying, press drying and heat treatment. For both aspen species, the mass loss after exposure to Gloeophyllum trabeum was higher than after exposure to Coniophora puteana, regardless of the wood treatment. Conventionally and press dried aspen wood had similar mass losses. However, heat treatment appears to be an effective method to improve the decay resistance of aspen wood, reducing the mass loss by about 30% compared with conventional and press drying. As a function of mass loss, wood exposed to G. trabeum had lower moisture content than wood exposed to C. puteana. This is thought to be due to differences in the degradation pattern between the two brown-rot fungi. On the other hand, European aspen appears to be slightly more resistant to decay than hybrid aspen.

Effect of drying technology on aspen wood properties

This article reports the impacts of three different drying treatments on selected physical and mechanical properties of European (Populus tremula L.) and hybrid (P. tremula x tremuloides) aspen wood. The material originates from 5 European aspen stands and 7 hybrid aspen stands in southern and central Finland. After processing the logs at a saw mill, sawn timber samples were dried using 1) conventional warm air drying, 2) press drying, or 3) heat treatment into Thermo-S grade by the Finnish Thermowood® method. Finally, small clearwood specimens were manufactured from different within-stem positions for the measurements of physical and mechanical properties. Both press dried and heat treated specimens absorbed water at significantly slower pace than the conventionally dried specimens. In normal climate, the conventionally dried, press dried and heat treated specimens conditioned at equilibrium moisture contents of 12.2, 8.7, and 8.9 per cent, respectively. It appears that the butt logs between 2–6 metres contain the lightest and, thus, weakest wood in aspen stems. Radial compression strength was at its highest in heat treated specimens, whereas conventionally and press dried specimens did not differ from each other. Press dried specimens had the highest longitudinal compression strength, also heat treated specimens showed higher values than the conventionally dried ones. Radial Brinell hardness of press dried specimens was higher than that of conventionally dried or heat treated specimens. Both modulus of elasticity and modulus of rupture were at their highest in press dried specimens. Irrespective of the drying treatment, the tangential shear strength of European aspen specimens was approximately 5% higher than that of hybrid aspen. Heat treated specimens indicated significantly lower tangential shear strength values than the conventionally dried ones. In case of both aspen species, the longitudinal tensile strengths of heat treated specimens were significantly lower than those of conventionally and press dried specimens. Heat treated specimens had the highest variability among the results. The inherent flaws in aspen wood material, e.g., wetwood and density

Modelling the change in aspen species composition in boreal mixedwoods

Summary The dynamics of aspen ( Populus tremuloides Michx.) species composition (SC, defi ned as the ratio of aspen basal area to total stand basal area) in boreal mixedwoods were modelled in this study using the difference equation method, based on datasets collected from repeatedly measured permanent sample plots (PSPs) across Alberta. The aspen SC measured at time 1 was taken as a key predictor variable to project SC values at future ages. Since site quality was found to impact the aspen SC, site index (SI) was incorporated into the model. To correct the autocorrelation and heteroskedasticity problems associated with PSPs data, the non-linear mixed-model technique was applied to accommodate the variance – covariance structure of the error terms and to estimate model parameters. The fi nal model was evaluated on an independent dataset collected from a different region in Alberta, based on a number of statistical measures including a goodness of prediction index (GOPI) from forward and backward projections, and examinations of residual and studentized residual plots. The low prediction bias and high GOPI value obtained from forward and backward projections on the validation data suggest that the model fi tted the data well and can be reliably applied to predict changes in aspen SC in boreal mixedwoods across Alberta. The model showed a decline in the SC after year 30, but the decline was steeper in sites of low SI. The model can be used for modelling the transitions of forest compositions in boreal mixedwood forests.

Aspen Knots, a Rich Source of Flavonoids

Hydrophilic extractives in heartwood, sapwood, and knots of three aspen species were analyzed by gas chromatography, gas chromatography‐mass spectrometry, and high‐performance size‐exclusion chromatography. The amounts of flavonoids were considerably higher in the knots relative to the stem wood. Flavonoids identified were dihydrokaempferol, naringenin, kaempferol, catechin, and taxifolin. In addition, glycosides of dihydrokaempferol, naringenin, and kaempferol were identified by mass spectrometry and shown to be glucosides by enzymatic hydrolysis by β‐D‐glucosidase. Dihydrokaempferol and its glucoside dominated in all knot samples although there were variations in the amounts and composition of the extractives. The total amounts of flavonoids varied between 11 and 43 mg/g in Populus tremula, 12 and 62 mg/g in Populus tremuloides and 47 and 82 mg/g in Populus grandidentata. The aspen knots were found to be a rich source of bioactive flavonoids. This work is part of the activities at the Åbo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Programme (2000–2005) selected by the Academy of Finland and is a part of the project “Bioextra” funded by the National Technology Agency of Finland (Tekes).

Growth-independent mortality of Lithuanian forest tree species

Growth-independent tree mortality (mortality of trees from the upper layer of the stand canopy or mortality besides self-thinning) was investigated in north-temperate forests of central Europe (Lithuania). Data from long-term tree observations on 420 experimental plots and data from 900 permanent forest monitoring plots were used. Investigations covered stands of 20–120 years of age with a variety of the following tree species: Scots pine (Pinus sylvestris L.), Norway spruce [Picea abies (L.) Karst.], birch (Betula pendula Roth. and B. pubescens Ehrh.), aspen (Populus tremula L.), common ash (Fraxinus excelsior L.), pedunculate oak (Quercus robur L.) and alder (Alnus glutinosa L. and Alnus incana L.). Periodic measurements were carried out every 5–10 years in experimental plots and annually in forest monitoring plots. The average annual growth-independent mortality rate of all tree species in Lithuania was 0.55% (on experimental plots) and 0.62% (on forest monitoring plots). The lower mortality rate was typical for Scots pine, pedunculate oak and common ash trees (tree species with a higher maturity age), while the higher rate was typical for birch, Norway spruce, and aspen (species with a lower maturity age). The mortality of species with the lower mortality rate increased significantly with increasing age. Especially high mortality (2–3%) was recorded in older stands (birch > 60–70 years, Norway spruce > 70–80 years, and aspen > 80–90 years). More than 60% of dead trees had symptoms of wind damage. Stem insects and root diseases were identified as mortality causes for 14.3% and 16.5% of dead trees, respectively. Significant fluctuations in tree mortality rate over time were caused by extreme meteorological conditions (wind, drought) and insect invasions. Under such conditions tree mortality increased by up to 10 times.