Concept: Populus tremuloides
The genus Populus, which includes poplars, cottonwoods and aspen trees, represents a huge natural source of fibers with exceptional physical properties. In this study, the oil absorption properties of poplar seed hair fibers obtained from Populus nigra italica when tested with high-density motor oil and diesel fuel are reported. Poplar seed hair fibers are hollow hydrophobic microtubes with an external diameter between 3 and 12μm, an average length of 4±1mm and average tube wall thickness of 400±100nm. The solid skeleton of the hollow fibers consists of lignocellulosic material coated by a hydrophobic waxy coating. The exceptional chemical, physical and microstructural properties of poplar seed hair fibers enable super-absorbent behavior with high absorption capacity for heavy motor oil and diesel fuel. The absorption values of 182-211g heavy oil/g fiber and 55-60g heavy oil/g fiber for packing densities of 0.005g/cm(3) and 0.02g/cm(3), respectively, surpass all known natural absorbents. Thus, poplar seed hair fibers obtained from Populus nigra italica and other trees of the genus Populus are an extremely promising natural source for the production of oil super absorbents.
Ploidy levels sometimes vary among individuals or populations, particularly in plants. When such variation exists, accurate determination of cytotype can inform studies of ecology or trait variation and is required for population genetic analyses. Here we propose and evaluate a statistical approach for distinguishing low-level ploidy variants (e.g., diploids, triploids and tetraploids) based on genotyping-by-sequencing data. The method infers cytotypes based on observed heterozygosity and the ratio of DNA sequences containing different alleles at thousands of heterozygous SNPs (i.e., allelic ratios). Whereas the method does not require prior information on ploidy, a reference set of samples with known ploidy can be included in the analysis if it is available. We explore the power and limitations of this method using simulated data sets and GBS data from natural populations of aspen (Populus tremuloides) known to include both diploid and triploid individuals. The proposed method was able to reliably discriminate among diploids, triploids and tetraploids in simulated data sets, and this was true for different levels of genetic diversity, inbreeding and population structure. Power and accuracy were minimally affected by low coverage (i.e., 2X), but did sometimes suffer when simulated mixtures of diploids, autotetraploids and allotetraploids were analyzed. Cytotype assignments based on the proposed method closely matched those from previous microsatellite and flow cytometry data when applied to GBS data from aspen. An R package (gbs2ploidy) implementing the proposed method is available from CRAN. This article is protected by copyright. All rights reserved.
Range shifts are among the most ubiquitous ecological responses to anthropogenic climate change and have large consequences for ecosystems. Unfortunately, the ecophysiological forces that constrain range boundaries are poorly understood, making it difficult to mechanistically project range shifts. To explore the physiological mechanisms by which drought stress controls dry range boundaries in trees, we quantified elevational variation in drought tolerance and in drought avoidance-related functional traits of a widespread gymnosperm (ponderosa pine - Pinus ponderosa) and angiosperm (trembling aspen - Populus tremuloides) tree species in the southwestern USA. Specifically, we quantified tree-to-tree variation in growth, water stress (predawn and midday xylem tension), drought avoidance traits (branch conductivity, leaf/needle size, tree height, leaf area-to-sapwood area ratio), and drought tolerance traits (xylem resistance to embolism, hydraulic safety margin, wood density) at the range margins and range center of each species. Although water stress increased and growth declined strongly at lower range margins of both species, ponderosa pine and aspen showed contrasting patterns of clinal trait variation. Trembling aspen increased its drought tolerance at its dry range edge by growing stronger but more carbon dense branch and leaf tissues, implying an increased cost of growth at its range boundary. By contrast, ponderosa pine showed little elevational variation in drought-related traits but avoided drought stress at low elevations by limiting transpiration through stomatal closure, such that its dry range boundary is associated with limited carbon assimilation even in average climatic conditions. Thus, the same climatic factor (drought) may drive range boundaries through different physiological mechanisms - a result that has important implications for process-based modeling approaches to tree biogeography. Further, we show that comparing intraspecific patterns of trait variation across ranges, something rarely done in a range-limit context, helps elucidate a mechanistic understanding of range constraints.
- Proceedings of the National Academy of Sciences of the United States of America
- Published over 8 years ago
Forest ecosystems store approximately 45% of the carbon found in terrestrial ecosystems, but they are sensitive to climate-induced dieback. Forest die-off constitutes a large uncertainty in projections of climate impacts on terrestrial ecosystems, climate-ecosystem interactions, and carbon-cycle feedbacks. Current understanding of the physiological mechanisms mediating climate-induced forest mortality limits the ability to model or project these threshold events. We report here a direct and in situ study of the mechanisms underlying recent widespread and climate-induced trembling aspen (Populus tremuloides) forest mortality in western North America. We find substantial evidence of hydraulic failure of roots and branches linked to landscape patterns of canopy and root mortality in this species. On the contrary, we find no evidence that drought stress led to depletion of carbohydrate reserves. Our results illuminate proximate mechanisms underpinning recent aspen forest mortality and provide guidance for understanding and projecting forest die-offs under climate change.
Little is known about the ability of trees to recover hydraulic conductance (k) within a growing season by regrowth or refilling of embolized conduits. Recovery of k lost to drought or other causes would prevent chronic reductions in gas exchange and productivity. To test recovery ability we conducted a 2-year experiment (2014-15) on a cohort of aspen ramets (Populus tremuloides, Michx.). Whole-tree k was measured from mid-June through September from sapflow (Q) and pre-dawn and mid-day xylem pressure. We induced embolism in the treatment group with high air pressure delivered by a split pressure chamber sealed around the main trunk. Successful treatments reduced k and Q by 50% or more without causing rapid desiccation. The majority of trees recovered following treatment, rising to control levels of k and Q between 12 and 17 days. Failure to recover was correlated with drier climate conditions. The growing-season recovery of k was attributed to refilling of embolized vessels, based on the absence of diameter growth. Pre-dawn xylem pressures during recovery were similar to the threshold needed to passively collapse emboli. Successful recovery during the 2-year study was consistent with no reduction in cumulative Q or canopy area in treatment vs controls. However, non-recovering trees in 2014 exhibited lower basal area growth at the start of the 2015 growing season, suggesting a linkage between recovery ability and productivity. This study provides evidence for the potential of trees to recover xylem function by refilling during the growing season.
The fate of carbon © captured by forest trees during photosynthesis is influenced by the supply of other resources. Fixed C may be partitioned among biomolecules within the leaf and/or allocated throughout the tree to growth, storage and maintenance activities. Phosphorus (P) availability often limits tree productivity due to its high biological demand and strong interactions with soil minerals. As ectomycorrhizal (ECM) fungi play critical roles in enhancing phosphate (Pi) acquisition by their hosts, these symbioses will influence the fate of C within trees and forested ecosystems. Using Populus tremuloides Michx. (trembling aspen) in symbiosis with Laccaria bicolor (Marie) P.D. Orton or Paxillus involutus (Batsch) Fr., we assessed C acquisition, allocation and partitioning under Pi limitation, specifically focusing on primary and secondary C compounds. Both ECM fungi moderated the effects of low P on photosynthesis and C partitioning among carbohydrates and secondary metabolites by sustaining Pi uptake and translocation in P. tremuloides under Pi limitation. As leaf P declined, reductions in photosynthesis were accompanied by significant shifts in C partitioning from nonstructural carbohydrates (NSCs) to phenolic glycosides and tannins. Carbon partitioning in roots exhibited more complex patterns, with distinct increases in NSCs in nonmycorrhizal (NM) plants under Pi limitation that were not evident in plants colonized by either ECM symbiont. In general, aspen colonized by L. bicolor exhibited C partitioning patterns intermediate between those of NM and P. involutus aspen. The C cost of symbiosis was pronounced for plants supporting P. involutus, where ECM plants exhibited maintenance of photosynthesis yet reduced biomass in comparison with NM and L. bicolor aspen under Pi replete conditions. Our results indicate that the ECM symbiosis affects the disposition of C in forest trees in part by altering the acquisition of other limiting resources from soils, but also through ECM species-specific influences on host physiology. This modulation of C partitioning will have broad implications for forest ecosystem C capture, storage and cycling where nutrient resources may be limited.
Aspen groves along the Niobrara River in Nebraska have long been a biogeographic curiosity due to morphological differences from nearby remnant Populus tremuloides populations. Pleistocene hybridization between P. tremuloides and P. grandidentata has been proposed, but the nearest P. grandidentata populations are currently several hundred kilometers east. We tested the hybrid-origin hypothesis using genetic data and characterized putative hybrids phenotypically.
Carbon starvation as a mechanism of tree mortality is poorly understood. We exposed seedlings of aspen (Populus tremuloides) to complete darkness at 20 or 28 °C to identify minimum non-structural carbohydrate (NSC) concentrations at which trees die and to see if these levels vary between organs or with environmental conditions. We also first grew seedlings under different shade levels to determine if size affects survival time under darkness due to changes in initial NSC concentration and pool size and/or respiration rates. Darkness treatments caused a gradual dieback of tissues. Even after half the stem had died, substantial starch reserves were still present in the roots (1.3-3% dry weight), indicating limitations to carbohydrate remobilization and/or transport during starvation in the absence of water stress. Survival time decreased with increased temperature and with increasing initial shade level, which was associated with smaller biomass, higher respiration rates, and initially smaller NSC pool size. Dead tissues generally contained no starch, but sugar concentrations were substantially above zero and differed between organs (~2% in stems up to ~7.5% in leaves) and, at times, between temperature treatments and initial, pre-darkness shade treatments. Minimum root NSC concentrations were difficult to determine because dead roots quickly began to decompose, but we identify 5-6% sugar as a potential threshold for living roots. This variability may complicate efforts to identify critical NSC thresholds below which trees starve.
In the aspen-grassland ecotone of Riding Mountain, Manitoba, lightly browsed vigorous clones of trembling aspen (Populus tremuloides Michx.) occur in close proximity to heavily browsed dieback clones. This study examines whether intraspecific variation in the production of phenolic glycosides is correlated with this strong dichotomy in clonal vigor. Individual clones were sampled over four years at three sites located along a gradient of increasing soil moisture stress. At each site, eight aspen clones of similar size and age were sampled: four vigorous and four dieback clones (total of 24 individual clones). The severity of wapiti (elk) browsing was assessed as the ratio of browse-damaged to total branches per aspen ramet. Statistically significant differences in foliar concentrations of the phenolic glycosides salicortin and tremulacin were observed between vigorous and dieback clones: a mean of 14.8% dry mass for lightly browsed (vigorous) clones, versus just 7.0% for heavily browsed (dieback) clones. Mean concentrations of foliar phenolics were also significantly greater in more moisture-stressed sites. These results demonstrate that the strong dichotomy in clonal vigor (vigorous versus dieback clones) is associated with large differences in phenolic glycoside production. Vigorous clone ramets produce high amounts of phenolic glycosides and have low levels of herbivore browsing and low mortality rates, whereas dieback clone ramets have low amounts of phenolic glycosides and much higher herbivore browsing and mortality rates. This suggests that intraspecific variation in phenolic glycosides in trembling aspen is an important predisposing factor leading to ramet mortality, and by extension to the decline of aspen clones.
Insects, diseases, fire and drought and other disturbances associated with global climate change contribute to forest decline and mortality in many parts of the world. Forest decline and mortality related to drought or insect outbreaks have been observed in North American aspen forests. However, little research has been done to partition and estimate their relative contributions to growth declines. In this study, we combined tree-ring width and basal area increment series from 40 trembling aspen (Populus tremuloides Michx.) sites along a latitudinal gradient (from 52° to 58° N) in western Canada and attempted to investigate the effect of drought and insect outbreaks on growth decline, and simultaneously partition and quantify their relative contributions. Results indicated that the influence of drought on forest decline was stronger than insect outbreaks although both had significant effects. Furthermore, the influence of drought and insect outbreaks showed spatiotemporal variability. In addition, our data suggest that insect outbreaks could be triggered by warmer early spring temperature instead of drought, implicating that potentially increased insect outbreaks are expected with continued warming springs, which may further exacerbate growth decline and death in North America aspen mixed forests. This article is protected by copyright. All rights reserved.