Poplar Production Research Articles

Planting spacing affects canopy structure, biomass production and stem roundness in poplar plantations

ABSTRACTTo improve the productivity and wood quality of poplar plantations, effects of four planting spacing on canopy characteristics, biomass production and stem roundness in poplar plantations were evaluated over 8 years. Planting spacing influenced canopy characteristics of the plantations, and further affected the understory vegetation and plantation productivity. Understory vegetation biomass and Shannon-Wiener index were negatively correlated with leaf area index, but both diversity indexes and aboveground biomass of understory vegetation were higher in stands with a wider spacing. Tree diameter growth increased with increasing planting spacing, while the increment in plantations of square configurations (5 × 5 m and 6 × 6 m) was higher than those with rectangular configurations (3 × 8 m, 4.5 × 8 m). The highest poplar biomass production was achieved in the plantation with 5 × 5 m spacing at age 8. Moreover, poplar trees showed a tendency with better stem roundness in a square configuration. The results suggest that planting spacing not only affect canopy characteristics, understory vegetation and tree growth but also wood quality, and square configurations (5 × 5 m and 6 × 6 m) could be a better option for poplar plywood timber production at similar sites.

Pest categorisation of Sphaerulina musiva.

Following a request from the European Commission, the EFSA Plant Health Panel performed a pest categorisation of Sphaerulina musiva, a well‐defined and distinguishable fungal species of the family Mycosphaerellaceae. Following a recent phylogenetic analysis of the genus Septoria and other closely related genera, a new name (S. musiva) was introduced for the species. The former species name Mycosphaerella populorum is used in the Council Directive 2000/29/EC. The pathogen is regulated in Annex IAI as a harmful organism whose introduction into the EU is banned. S. musiva is reported from North and South America and not known to occur in the EU. S. musiva causes Septoria leaf spots and cankers of poplar (Populus spp.). Of the poplars native to Europe, Populus nigra is reported as susceptible and Populus tremula as susceptible when planted in North America. The hybrid Populus x canadensis (arising from a cross of P. nigra and the North American Populus deltoides), widely grown in the EU, is also susceptible. The pest could enter the EU on plants for planting, cut branches, isolated bark and wood with and without bark. S. musiva could establish in the EU, as hosts are common and favourable climatic conditions are widespread, and could spread following establishment by natural dispersal and movement of infected plants for planting, cut branches, isolated bark and wood with or without bark. The pest introduction would have impacts in woodlands, plantations and nurseries. The pathogen is considered the most serious disease affecting hybrid poplar production in North America. Selection, breeding and planting of resistant species and clones are the main methods used to control the damage caused by the pathogen. There is some uncertainty on the geographical distribution of the pest in the Caucasus, the Crimean Peninsula and South America and on the level of susceptibility among Populus species native to Europe as well as Salix spp. The criteria assessed by the Panel for consideration as a potential quarantine pest are met. For regulated non‐quarantine pests, the criterion on the pest presence in the EU is not met.

Energy analysis of poplar production for bioenergy in Sweden

Poplar (Populus spp.) species are currently not widely grown in Sweden but offer interesting options for future large-scale biomass production for bioenergy. This study assesses the average annual net energy yield and the ratio between the gross energy yield and the total primary energy input for well-managed current commercial poplar production systems in Sweden, with and without fertilization with mineral nitrogen (N). The two systems perform similarly, mainly because the yield response to fertilization is estimated to be small. The average annual net energy yields with and without fertilization are estimated at 190 and 179 GJ ha−1 yr−1, respectively. The corresponding energy ratios are 32 and 34, i.e., the total primary energy inputs correspond to only ca. 3% of the gross energy yield, in both cases. Therefore, improving the net energy yield by increasing the harvest level may be considered the most important objective from an energy performance point of view. To do this, more research is needed in order to develop poplar clones that are better adapted to the Swedish climate and identify in which situations poplar responds well to fertilization. At the same time, by reducing the energy inputs associated with the most fuel-intensive activities (wood-chip transportation, chipping, harvesting, forwarding, and stump lifting), greenhouse gas emissions can be reduced. The results should be interpreted with caution since several parameters, especially the yield response to fertilization and fuel consumption rates, are associated with considerable uncertainties.

A Large-Scale Mutational Analysis of Two-Component Signaling Systems of Lonsdalea quercina Revealed that KdpD-KdpE Regulates Bacterial Virulence Against Host Poplar Trees.

Poplar, which is a dominant species in plant communities distributed in the northern hemisphere, is commonly used as a model plant in forestry studies. Poplar production can be inhibited by infections caused by bacteria, including Lonsdalea quercina subsp. populi, which is a gram-negative bacterium responsible for bark canker disease. However, the molecular basis of the pathogenesis remains uncharacterized. In this study, we annotated the two-component signal transduction systems (TCSs) encoded by the L. quercina subsp. populi N-5-1 genome and identified 18 putative histidine kinases and 24 response regulators. A large-scale mutational analysis revealed that 19 TCS genes regulated bacterial virulence against poplar trees. Additionally, the deletion of kdpE or overexpression of kdpD resulted in almost complete loss of bacterial virulence. We observed that kdpE and kdpD formed a bi-cistronic operon. KdpD exhibited autokinase activity and could bind to KdpE (Kd = 5.73 ± 0.64 μM). Furthermore, KdpE is an OmpR family response regulator. A chromatin immunoprecipitation sequencing analysis revealed that KdpE binds to an imperfect palindromic sequence within the promoters of 44 genes, including stress response genes Lqp0434, Lqp3037, and Lqp3270. A comprehensive analysis of TCS functions may help to characterize the regulation of poplar bark canker disease.

A nutrient-based sustainability assessment of purpose-grown poplar and switchgrass biomass production systems established on marginal lands in Canada

The sustainability of purpose-grown biomass production on marginal lands in Canada is uncertain. In this study, an assessment of biomass yield and sustainability was performed for two poplar clones (Poplus deltoides × P. nigra, DN-34—PDN, and P. nigra × P. maximowiczii, NM-6—PNM) and two switchgrass cultivars (Panicum virgatum ‘Cave-in-Rock’—SGC, and P. virgatum ‘Nebraska’—SGN) on three marginal lands in Guelph (ON), Kemptville (ON), and Nappan (NS) in Canada. The differences in stem biomass across sites were not significant; however, differences in stem biomass among plants were statistically significant between poplar and switchgrass (p < 0.0001) and between poplar clones (p < 0.0001). The 2-yr stem biomass yield in PNM (15.27 ± 1.28 t ha−1) was significantly higher than those in PDN (7.02 ± 0.54 t ha−1), SGC (2.57 ± 0.28 t ha−1), and SGN (1.45 ± 0.22 t ha−1). Two sustainability indices based on macronutrients (MBSI) and nitrogen (NBSI), were developed to assess sustainability. Both indices show that th...

Efficiency and sustainability indicators for papermaking from virgin pulp—An emergy-based case study

The pulp and paper sector is the fourth-largest industrial sector worldwide in terms of energy use, accounting for approximately 6% of the total industrial energy consumption and contributing to 2% of direct carbon dioxide (CO2) emissions produced by industries. The definition of the environmental profile of this industrial sector is crucial, due to the high market demand of paper and the increasing concern for the environmental costs of the whole papermaking process. A sustainability perspective should rely on a wider and holistic viewpoint, properly including all direct and indirect interactions with the environment. To this purpose, the Emergy (spelled with “m”) Accounting method (EMA) is very appropriate for the evaluation of the efficiency, effectiveness and sustainability of the papermaking process under different perspectives (resource quality, fossil energy and material consumption, environmental and human-driven support). Several studies concerning environmental impacts, eco-efficiency, and cleaner technologies in the pulp and paper sector have already been carried out, but none of them addressed resource quality and resource generation costs from a supply-side point of view. This study aims to fill this gap in the literature by highlighting the direct and indirect contribution in terms of natural capital and ecosystem services to the pulp and paper production process.By means of EMA performance indices, this paper aims to assess the environmental sustainability associated to the production of pulp and paper, so as to identify those process steps that entail the highest environmental costs and require improvements. Three forest management scenarios − based on Spruce/Pine, Eucalyptus and Poplar production for raw material supply − were evaluated to assess the sustainability and the efficiency of each species. Moreover, the marginal costs of achieving higher energy and material efficiency are investigated, with a special focus placed on the identification of the effects of energy input flows on additional demand for environmental services.The research results show that the largest supply-side environmental costs are generated by the industrial processing activities, due to high energy, water and chemicals consumption. Only a minor role is played by forestry activities that supply the raw feedstock, although forestry management practices certainly affect both the final productivity and the energy balance, through the amount and use efficiency of the farm inputs. Additionally, among the three forest systems under study, Spruce/Pine forest management displays the most sustainable option for paper production because, basing on the emergy indices, it presents the best sustainable contribution to both the economy and the environment of the investigated region. In conclusion, the application of EMA approach allowed a more comprehensive assessment of forestry and industrial operations, contributing to assist decision makers in implementing the best environmental management of papermaking process.

Impact of RAV1-engineering on poplar biomass production: a short-rotation coppice field trial

BackgroundEarly branching or syllepsis has been positively correlated with high biomass yields in short-rotation coppice (SRC) poplar plantations, which could represent an important lignocellulosic feedstock for the production of second-generation bioenergy. In prior work, we generated hybrid poplars overexpressing the chestnut gene RELATED TO ABI3/VP1 1 (CsRAV1), which featured c. 80% more sylleptic branches than non-modified trees in growth chambers. Given the high plasticity of syllepsis, we established a field trial to monitor the performance of these trees under outdoor conditions and a SRC management.ResultsWe examined two CsRAV1-overexpression poplar events for their ability to maintain syllepsis and their potential to enhance biomass production. Two poplar events with reduced expression of the CsRAV1 homologous poplar genes PtaRAV1 and PtaRAV2 were also included in the trial. Under our culture conditions, CsRAV1-overexpression poplars continued developing syllepsis over two cultivation cycles. Biomass production increased on completion of the first cycle for one of the overexpression events, showing unaltered structural, chemical, or combustion wood properties. On completion of the second cycle, aerial growth and biomass yields of both overexpression events were reduced as compared to the control.ConclusionsThese findings support the potential application of CsRAV1-overexpression to increase syllepsis in commercial elite trees without changing their wood quality. However, the syllepsis triggered by the introduction of this genetic modification appeared not to be sufficient to sustain and enhance biomass production.

Spatio-temporal dynamics of Sorghum halepense in poplar short-rotation coppice under several vegetation management systems

Weeds are one of the most important factors reducing productivity in poplar short-rotation coppices. Sorghum halepense is a very harmful perennial weed affecting irrigated crops preceding poplar plantations. The main goal of this study is to assess the potential hazards of residual S. halepense patches in newly established poplar crops and the effects of different vegetation control treatments. Our study objectives were: to describe the spatio-temporal dynamics of this perennial weed during the first rotation coppice of poplar in the presence of different vegetation management strategies and to assess the effect that S. halepense has on poplar growth. Experiments were conducted in an experimental field in central Spain and were repeated over time. S. halepense patches were artificially established in a random block design with four replications. Four types of vegetation management were performed: mechanical (field cultivator and rototiller) and chemical (glyphosate and fluazifop-p-butyl). Patches were monitored throughout all the experiments and poplar productivity parameters (height and biomass) were recorded. Monitoring indicated patch displacement towards crop rows, probably due to higher soil moisture levels in this irrigated area, and a reduction in patch density in the central inter-row area owing to different management strategies. However, displacement was barely detected lengthwise, as was originally expected, at least in mechanical management. Mardia’s test (circular statistics) indicated no significant differences in patch displacement between mechanical and chemical strategies. Regarding poplar growth, the mixed linear model indicated no consistent significant differences in terms of vegetation management, differences depending more on the proximity of S. halepense patches. This study has demonstrated that while S. halepense has very limited capacity to spread spatially in poplar short-rotation coppice, its proximity to crop rows may cause important damage in poplar productivity, especially during the first growing season. Therefore, proper management of this weed species is strongly recommended during this period.

Analysis of the energy efficiency of short rotation woody crops biomass as affected by different methods of soil enrichment

The aim of this study was to determine the energy input and energy efficiency of the production of willow, poplar and black locust chips in four-year harvest rotation. The highest energy input was made in poplar production when soil was enriched with lignin and by mineral fertilisation (33.02 GJ ha−1). For willow production it was 30.76 GJ ha−1 when lignin, mycorrhiza and mineral fertilisation were used. The energy input in the production of black locust was much lower. The largest energy gain was obtained in the production of poplar when soil was enriched with lignin and mineral fertilisation (673.7 GJ ha−1). A similar level of this parameter (669.7 GJ ha−1) was achieved in the production of willow when lignin, mycorrhiza and mineral fertilisation was used. In general, a higher energy gain was obtained in the production of willow and poplar than in the production of black locust. On the other hand, the best energy efficiency ratio was achieved for willow (28.9) in the option with lignin. The ratio for poplar production ranged from 19.7 to 25.9. On the other hand, the energy efficiency ratio for black locust ranged from 10.6 to 21.7.

Poplar growth and wood production on a grassland irrigated for decades with potato starch wastewater

Irrigation of grasslands with potato starch wastewater causes changes in soil quality parameters, often resulting in decreased crop yields and sometimes causing animal diseases. Reduced agricultural income leads to designation of such grasslands for afforestation aimed at production of bioenergy crops and improvement of soil quality. In this study, Populus alba L. and seven different poplar clones were planted in 2009 on the grassland irrigated in 1973–2008 with potato starch wastewater and in non-irrigated, experimental forest. The survival, growth and wood production potential of planted poplars after 2–3 growing seasons were analyzed. Morphophysiological parameters of leaves, nutrients in leaves and fine roots, sugars in fine roots, leaf rust resistance, frost hardiness and their effects on growth were also determined. Stem diameter and stem volume index of poplars growing on the grassland were higher than in the experimental forest. Only Populus deltoides Bartr. × P. maximowiczii Henry clone ‘Eridano’ (in short ERI) had a survival rate of 100 % and the highest values of stem diameter (77 mm) and stem volume index (0.034 m3) in comparison with other poplars in both sites after 3 growing seasons. Clone ERI was also characterized by a high leaf rust resistance and frost hardiness. The results suggest that clone ERI is suitable as a bioenergy crop on grasslands irrigated with potato starch wastewater. Afforestation of the grassland improved the fertility of the soil by increasing concentrations of soil organic matter and availability of mineral nutrients (N, P, K, S and Fe).

Effects of Veneer Treatment with Copper-Based Preservatives on the Decay Resistance and Mechanical Properties of Poplar LVL

To improve the durability of poplar laminated veneer lumber (LVL), the veneers were impregnated with alkaline copper quaternary (ACQ) and copper azole (CA) at four retention levels, and the effects of the treatments on the LVL properties were examined. The results demonstrated that the treatments substantially improved the decay resistance of LVLs, with the decreases in mass losses ranging from 42.6% to 87.3%. However, retention levels greater than the specified standard values for Use Category C4.1 might be required for sufficient protection of poplar LVL against decay fungi. The bending and shear properties of all treated LVLs met the requirements for high-grade products specified in the LVL standard GB/T 20241. No significant effects of the treatments were observed on the mechanical properties of LVLs, and the LVLs did not display any visible delamination after boil-dry treatment. These results imply that the treatments significantly improved the decay resistance of poplar LVL products and had no appreciable impact on their mechanical and physical properties.

Landscape patterns of bioenergy in a changing climate: implications for crop allocation and land-use competition.

Rural landscapes face changing climate, shifting development pressure, and loss of agricultural land. Perennial bioenergy crops grown on existing agricultural land may provide an opportunity to conserve rural landscapes while addressing increased demand for biofuels. However, increased bioenergy production and changing land use raise concerns for tradeoffs within the food-energy-environment trilemma. Heterogeneity of climate, soils, and land use complicate assessment of bioenergy potential in complex landscapes, creating challenges to evaluating future tradeoffs. The hypothesis addressed herein is that perennial bioenergy production can provide an opportunity to avoid agricultural land conversion to development. Using a process-based crop model, we assessed potential bioenergy crop growth through 2100 in a southern Appalachian Mountain region and asked: (1) how mean annual yield differed among three crops (switchgrass Panicum virgatum, giant miscanthus Miscanthus x giganteus, and hybrid poplar Populus x sp.) under current climate and climate change scenarios resulting from moderate and very high greenhouse gas emissions; (2) how maximum landscape yield, spatial allocation of crops, and bioenergy hotspots (areas with highest potential yield) varied among climate scenarios; and (3) how bioenergy hotspots overlapped with current crop production or lands with high development pressure. Under both climate change scenarios, mean annual yield of perennial grasses decreased (-4% to -39%), but yield of hybrid poplar increased (+8% to +20%) which suggests that a switch to woody crops would maximize bioenergy crop production. In total, maximum landscape yield increased by up to 90 000 Mg/yr (6%) in the 21st century due to increased poplar production. Bioenergy hotspots (> 18 Mg x ha(-1) x yr(-1)) consistently overlapped with high suburban/exurban development likelihood and existing row crop production. If bioenergy production is constrained to marginal (non-crop) lands, landscape yield decreased by 27%. The removal of lands with high development probability from crop production resulted in losses of up to 670 000 Mg/yr (40%). This study demonstrated that tradeoffs among bioenergy production, crop production, and exurban expansion in a mountainous changing rural landscape vary spatially with climate change over time. If markets develop, bioenergy crops could potentially counter losses of agricultural land to development.

Organosolv Pretreatment of Slurry Composting and Biofiltration of Liquid Fertilizer-Treated Yellow Poplar for Sugar Production

ABSTRACT The present study examines the influence of slurry composting and biofiltration liquid fertilizer (SCBLF) treatment on the biomass characteristics of yellow poplar, and the optimization of organosolv pretreatment for sugar production. After SCBLF treatment, total exchangeable cation contents of yellow poplar was increased from 3.1 g kg -1 to 4.4 g kg -1 , and as a result, biomass production of yellow poplar was also enhanced by 82.3%. Organosolv pretreatment was conducted with three independent variables: 1) reaction temperature: 133.2℃ to 166.8℃; 2) acid concentration: 0.2% to 1.8%; and 3) reaction time: 1.6 min to 18.4 min. Reaction temperature was the most significant variable in water insoluble solid (WIS) recovery rate. High overall sugar yield was attained from pretreatment conditions approximately 50% of WIS recovery rate, and the highest overall glucose yield (44.0%) was achieved from pretreatment at 140℃ with 1.5% acid concentration for 5 min. Consequently, 21.1% of glucose and 5.8% of xylose were produced from the organosolv pretreatment of SCBLF-treated 8-year-old yellow poplar.Keywords : slurry composting and biofiltration, yellow poplar, organosolv retreatment, biomass production, sugar production

Concentration- and flux-based ozone dose–response relationships for five poplar clones grown in North China

Concentration- and flux-based O3 dose–response relationships were developed for poplars in China. Stomatal conductance (gs) of five poplar clones was measured to parameterize a Jarvis-type multiplicative gs model. The maximum gs and other model parameters varied between clones. The strongest relationship between stomatal O3 flux and total biomass was obtained when phytotoxic ozone dose (POD) was integrated using an uptake rate threshold of 7 nmol m−2 s−1. The R2 value was similar between flux-based and concentration-based dose–response relationships. Ozone concentrations above 28–36 nmol mol−1 contributed to reducing the biomass production of poplar. Critical levels of AOT40 (accumulated O3 exposure over 40 nmol mol−1) and POD7 in relation to 5% reduction in total biomass for poplar were 12 μmol mol−1 h and 3.8 mmol m−2, respectively.

Lignin-enriched Fermentation Residues from Bioethanol Production of Fast-growing Poplar and Forage Sorghum

The current challenges in developing a cost-effective bioethanol industry include the production of not only high-volume, low cost biofuels but also high-value products with minimal downstream waste. The up-grading of side-stream lignins from bioethanol production plants to novel high-value products will improve the profitability of the bioethanol industry; to do that, a precise understanding of lignin is required. In the present study, lignin-enriched fermentation residues from bioethanol production (steam explosion pretreatment, saccharification, and fermentation) of fast-growing poplar and forage sorghum were characterized. In addition to the purity and composition, lignin structure (syringyl/guaiacyl (S/G) ratio, inter-unit linkages) was also analyzed with spectroscopy techniques such as Fourier transform infrared and two-dimensional nuclear magnetic resonance. Bioethanol processing and feedstock origins seemed to be the main factors determining the purity, composition, and structure of lignins. Residual lignins from poplar and forage sorghum contained significant amounts of sugar and protein impurities. Poplar lignin showed a very high S/G ratio associated with p-hydroxybenzoate. A lower S/G ratio together with H lignin units and p-hydroxycinnamates (p-coumarate and ferulate) was observed for forage sorghum lignin. The main inter-unit linkages present in both lignins were β-O-4´ aryl ether followed by resinols and phenylcoumarans.

Application of pre-emergence herbicides in poplar nursery production

In addition to pests and diseases, weeds are a major problem in poplar nursery production. The possibilities of herbicide application in juvenile poplar growth were researched, taking into account that weeds are one of the main limiting factors. The following pre-emergence herbicides were tested: acetochlor, S-metolachlor, metribuzin, oxifluorfen, and dimethenamid during two vegetation seasons at two locations, which differed by the soil physico-chemical characteristics. The study results show that the number of weeds on sample plots was significantly reduced by the tested herbicides when compared to control plots. The highest reduction in the number of weeds was achieved using the herbicides acetochlor and metribuzin. However, metribuzin showed a phytotoxic effect on sandy soil. Metribuzin application is recommended only on the soils with higher contents of organic matter, where the phytotoxic effect was absent. Acetochlor, S-metolachlor, oxifluorfen, and dimethenamid were not phytotoxic to poplars and can be used for weed suppression in the production of poplar plants.

Economic and cradle-to-gate life cycle assessment of poly-3-hydroxybutyrate production from plastic producing, genetically modified hybrid poplar leaves

Poly-3-hydroxybutyrate (PHB) is a renewable, biodegradable biopolymer that has shown great promise to offset the use of hydrocarbon-derived plastics. The genes encoding the bacterial PHB production pathway have been engineered into several higher order plant species providing an opportunity to produce PHB as a co-product on an industrial, agricultural scale. This study investigates the economic feasibility and estimates the life-cycle greenhouse gas (GHG) emissions produced during the PHB production from hybrid poplar leaves. An established, bench scale extraction procedure was extrapolated upon using SuperPro designer to estimate the product cost, raw materials and energy used during extraction of PHB from poplar leaves on an industrial scale. Assuming an economically feasible concentration of PHB could be produced in the leaf material, a cradle-to-gate life cycle assessment was performed under two of the most likely poplar production scenarios for the Northwest United States where poplar is commonly grown for biomass applications. The cost of PHB production was found to vary greatly with the PHB content in the leaves; from $33.28 per kg at 0.5% PHB to $1.72 per kg at 20% PHB content. Poplar production scenarios varied greatly in their emission of GHGs. An irrigated poplar production scenario is estimated to produce 248.8% more GHGs than production of the displaced polypropylene. An un-irrigated poplar production scenario produced 76.1% less GHGs. Both production scenarios produced significant amounts of volatile organic compounds (VOCs) associated with normal poplar growth that could prove detrimental to local air quality. PHB content of 15% in the poplar was required to bring the PHB production price to $2.26 per kg and make production competitive with petroleum-derived plastics.

Impacts of Supplyshed-Level Differences in Productivity and Land Costs on the Economics of Hybrid Poplar Production in Minnesota, USA

The joint effects of poplar biomass productivity and land costs on poplar production economics were compared for 12 Minnesota counties and two genetic groups, using a process-based model (3-PG) to estimate aboveground biomass productivity. The counties represent three levels of productivity which, due to spatial stratification, were analogous to three biomass supplysheds. An optimal rotation age (ORA) was calculated that minimizes the annualized, discounted per-dry megagrams biomass cost for each county, genetic group and land cover, and for two discount rates (5 and 10 %). The ORA for the lowest-cost county (Todd) with specialist genotypes and a 5 % discount rate is 14 years and the breakeven price at that age is US$71 dry Mg−1, while for the highest-cost county (McLeod), the generalist genotype and a 10 % discount rate, the ORA is 10 years and the breakeven price at that age is US$175 dry Mg−1. Planting after a previous poplar stand increased breakeven prices and increased the ORAs by 1 to 2 years relative to planting after a previous annual crop. An ANOVA analysis showed a significant genetic group effect and significant productivity class × land rent interactions. All other factors being equal, an increase in the discount rate from 5 to 10 % is expected to reduce ORAs by 2 to 3 years. High-productivity supplysheds can also be expected to have ORAs that are 2 to 3 years shorter than low-productivity ones. Land costs were not as closely correlated to productivity as we expected.

Shade tolerance plays an important role in biomass production of different poplar genotypes in a high-density plantation

Poplar (Populus sp.) canopy provides considerable shading in high-density plantations, imposing adverse impacts on crown characteristics relating to biomass production, such as leaf area index (LAI) and photosynthesis. However, it is yet unclear how the LAI, photosynthesis, and biomass production of poplars would respond to shading. In this study, the shading responses were investigated in three poplar genotypes with contrasting biomass performance in a high-density plantation (1667ha−1) established six years prior to the study. A three dimensional laser scanner and a portable photosynthesis system were used to measure the canopy structure and photosynthetic parameters. Principal component analysis (PCA) and the partial least squares method (PLS) were used to determine the importance of measured variables to LAI and biomass. Among all the factors studied, LAI was one of the most related factors to biomass. The genotype with greater shade tolerance ability had higher LAI. Moreover, the high biomass genotype showed a low light compensation point (LCP) and respiration rate (Rd) accompanied with a high ratio of photosynthesis to respiration (Amax/Rd) and a high maximum photosynthetic rate (Amax). This characteristic was more distinct in the middle (M) and lower (L) crown layers than in the upper (U) crown layer. Therefore, high LAI, Amax and Amax/Rd accompanied by low LSP and Rd are the key traits in obtain a high biomass genotype. Thus, improving shade tolerance in leaves of the middle and lower canopy layer to obtain a high LAI and biomass production can be an effective breeding method.

Effects of mixing clones on hybrid poplar productivity, photosynthesis and root development in northeastern Canadian plantations

Mixing tree cultivars or species in forest plantations can be efficient to reduce the risk of pest damages and could have a positive effect on yields if complementarity or facilitation between trees occurs. Four hybrid poplar clones (747215, Populus trichocarpa Torrey & A.Gray×P. balsamifera L.; 915004 and 915005, P. balsamifera×P. maximowiczii Henry; and 915319 P. maximowiczii×P. balsamifera) were planted in monoclonal and polyclonal plantations in three sites located in Quebec, Canada, to assess effects of clonal diversity on (i) aboveground biomass productivity, (ii) net photosynthesis and nutrient status of trees, and (iii) root spatial distribution. Stem growth was measured over five growing seasons, while root development, foliar nutrient concentrations and photosynthesis were measured during the fifth growing season. Results showed frequent but not general overyielding of trees in the polyclonal plots compared to monoclonal plots, five years after plantation establishment. Overall, stem volumes were 21% higher in the polyclonal (7.4m3ha−1) vs. monoclonal (6.1m3ha−1) plots. Effects of clone mixing on growth were greater in sites where soil nutrients were more limiting. However, foliar macronutrient concentrations (N, P, K, Ca and Mg) in trees growing in polyclonal plots were similar to those in monoclonal plots. Root development differed between the two plot layouts, with mean root:shoot ratio being greater in monoclonal (0.41:1) vs. the polyclonal (0.35:1) plots. Mixing clones increased biomass allocation aboveground, which we attributed to reduced competition between individuals of different clones and could explain overyielding in the polyclonal plots. The root fraction most distant from the stem (⩾60cm) was greater in monoclonal (67% of total root biomass) compared to polyclonal (47% of total root biomass) plots, suggesting greater belowground competition in the former, which forced roots to extend further from the stems. Effects of plot layout on net assimilation rate (Pn) depended on site, with trees in polyclonal plots having greater Pn in two of the three sites. Root total non-structural carbohydrates were greater in the polyclonal (216mgg−1) compared to the monoclonal (159mgg−1) plots. Mixing hybrid poplar clones often resulted in greater aboveground growth, lower root:shoot ratios, and different spatial root distributions, when compared to clones planted in monocultures.