Short Rotation Woody Crops Research Articles

Productivity of hybrid sweetgum (Liquidambar formosana x styraciflua) plantations compared to native sweetgum (L. Styraciflua) in the Western Gulf Region of south-central U.S.

Sweetgum (Liquidambar styraciflua) is an excellent short-rotation woody crop species for producing hardwood pulp and biofuel feedstock on marginal sites. A recent development which may improve growth is the production of hybrid sweetgum (L. formosana x styraciflua) clones. We compared the productivity and attributes, e.g., wood specific gravity and stand growth, of operational plantations of hybrid sweetgum clones to improved native sweetgum half-sib families in 36 stands in Oklahoma, Texas, and Louisiana, U.S. The least square means (LSMean) of wood specific gravity was significantly greater for hybrids clones than the half-sibs (0.49 vs. 0.46 g/cm3), and the hybrid clones had less taper which necessitated developing genotype-specific volume and biomass equations. Despite having less fertilizer inputs on average, the hybrids had greater standing green stem biomass than the half-sibs at an age of 8.5 years (LSMean of 73.3 vs. 51.9 Mg/ha). The hybrids as a group also exhibited greater green biomass growth during the 2022 growing season when compared to the half-sibs at 8.5 years (LSMean of 15.8 vs. 11.4 Mg/ha/y). Therefore, the hybrids provide growth and wood quality advantages that may provide an opportunity to increase productivity and profitability of hardwood SRWC plantings.

Willow, Poplar, and Black Locust Debarked Wood as Feedstock for Energy and Other Purposes

Solid biomass can be used for energy generation and the production of various renewable bioproducts. The aim of this study was to determine the yield and characteristics of wood obtained as debarking residue from 14 genotypes of short-rotation woody crops (SRWCs). These included five Populus genotypes, one Robinia genotype, and eight Salix genotypes, harvested in both annual and quadrennial cycles. The results showed that the highest dry wood yield (12.42 Mg ha−1 y−1 DM) and yield energy value (244.34 GJ ha−1 y−1) were obtained from willow (cultivar Żubr) harvested in a quadrennial cycle. The best effect among the poplar genotypes was achieved for the Hybryda275, and it was particularly marked in the quadrennial harvest cycle. The poorest results were determined for black locust. The Robinia characteristics included the significantly lowest moisture content (31.6%), which was a positive attribute from the energy point of view, but, on the other hand, it had some adverse characteristics—the highest levels of sulfur (0.033% DM), nitrogen (0.38% DM), and ash (0.69% DM). More beneficial properties in this respect were determined for willow and poplar wood. Moreover, willow and poplar wood contained more cellulose—51.8 and 50.0% DM, respectively—compared with black locust. Extending the SRWC shoot harvest cycle from annual to quadrennial resulted in an increase in cellulose, lignin, and carbon, higher heating value, and a decrease in nitrogen, sulfur, ash, and moisture content. Therefore, extending the harvest cycle improved the parameters of SRWC wood as an energy feedstock.

Early Dynamics of Carbon Accumulation as Influenced by Spacing of a Populus deltoides Planting

The fast-growing tree, eastern cottonwood (Populus deltoides), currently is being planted to catalyze native forest restoration on degraded agricultural sites in the southeastern United States. Many of these restoration sites are appropriate for short rotation woody crop (SRWC) culture that addresses climate mitigation objectives, but information needed to optimize climate mitigation objectives through such plantings is limited. Therefore, we established a 10-year experiment on degraded agricultural land located in the Mississippi Alluvial Valley, USA, aiming to quantify the dynamics of aboveground carbon (AGC) accumulation in a cottonwood planting of four replicated spacing levels (3.7 × 3.7 m, 2.7 × 1.8 m, 2.1 × 0.8 m, and (0.8 + 1.8) × 0.8 m) aligned with SRWC systems targeting various ecosystem services. Annual sampling revealed a substantial range in increments of AGC and year 10 carbon stocks among stands of different densities. Mean annual increments for AGC (MAIAGC) were similar for the two tightest spacing levels, peaking higher than for the other two spacings at about 7.5 Mg ha−1 y−1 in year 7. Year 10 AGC ranged between 22.3 Mg ha−1 for stands spaced 3.7 × 3.7 m and 70.1 Mg ha−1 for stands of the two tightest spacings, leading us to conclude that a spacing between 2.1 × 0.8 m and 2.7 × 1.8 m would maximize aboveground carbon stocks through year 10 on sites of similar agricultural degradation. Increments and accumulation of AGC on the degraded site trended lower than values reported from more productive sites but illustrate that quick and substantial transformation of the carbon stock status of degraded agricultural sites can be achieved with the application of SRWCs to restore forests for climate mitigation and other compatible ecosystem services.

Proportion of Nonindustrial Private Forest Lands That Potentially Can Be Leased to Bioenergy Enterprises to Grow Short Rotation Woody Crops

Abstract Determining how much nonindustrial private forest (NIPF) land could potentially be allocated to growing short rotation woody crops (SRWCs) is critical for estimating the future supply of woody biomass for biofuels and overall bioenergy production. This study estimated the proportion of the land NIPF landowners would be willing to allocate to SRWC cultivation. In view of the bounded nature of the dependent binary variable [0,1] and model performance, a fractional logit regression model was used to estimate model parameters. The estimated proportion of land allocated to SRWCs was positively influenced by the rental rate per acre, owning land for investment or fee-access hunting purposes, and the landowner’s age. In contrast, size of the ownership, owning land for the production of agricultural products, and residing on the land negatively influenced the proportion of land allocated to SRWCs. Lease contract duration, location of the largest tract owned, owning land for personal recreation or ecosystem services, and landowner familiarity with the concept of alternative fuels production did not influence the proportion of land allocated to SRWCs. These results will contribute to a better understanding of SRWC biomass supply from NIPF land and provide important benchmark information for conducting feasibility analyses of biofuel and biopower manufacturing in Mississippi and other states with abundant woody feedstocks. Study Implications: To achieve targeted levels of bioenergy production and carbon dioxide (CO2) emission reductions and diversify NIPF landowner income, participation of NIPF landowners in woody biomass feedstock production will be critical, because they own a majority of forestland in the region. Moreover, they have diverse reasons for owning land which has extensive implications not only in terms of incentive types or monetary compensation levels they might require for cultivating SRWCs but also policies related to the development of the bioenergy sector in Mississippi, the location of individual bioenergy processing facilities, and feedstock types that will be developed. Thus, forest managers and biomass procurement foresters might need to decide whether to focus their efforts on maximizing the land area allocated to growing SRWCs or considering a mix of SRWCs at lower lease rates per acre and material obtained from thinnings, which might be more amenable to landowners who value environmental amenities.

Green Energy Prospects of Electricity Generated from Short-Rotation Woody Crops—Quantifying the EROIg of Bioelectricity

The Intergovernmental Panel on Climate Change’s sixth assessment report (AR6) allocates 15% to 43% of global primary energy to biomass in 2050 across multiple mitigation scenarios. The report also emphasizes the importance of electrification. For increased reliance on electricity and on biomass, bioelectricity is expected to play a major role. It is therefore vital to know whether the energy generation potential of biomass electricity can support the removal of its environmental impact, particularly as generation at large scale is expected to rely almost solely on energy crops. This paper evaluates the potential of short-rotation woody crops in generating green electricity. This is performed using the “Green Energy Return on Investment (EROIg)” methodology, which indicates the net energy generated after investing in ecosystem maintenance energy (ESME). This study found that the EROIg of bioelectricity is marginally larger than unity when converted to its primary equivalent form (EROIg-PE). Three design options were proposed to improve bioenergy’s EROIg. Among these options, pelletizing wood chips has the largest advantage with an EROIg of 1.11 and an EROIg-PE of 3.17. We conclude with a discussion of the indirect advantages of growing energy crops, and discuss how this technique can be used alongside others to help generate cleaner energy.

Interactions with microbial consortia have variable effects in organic carbon and production of exometabolites among genotypes of Populus trichocarpa.

Poplar is a short-rotation woody crop frequently studied for its significance as a sustainable bioenergy source. The successful establishment of a poplar plantation partially depends on its rhizosphere-a dynamic zone governed by complex interactions between plant roots and a plethora of commensal, mutualistic, symbiotic, or pathogenic microbes that shape plant fitness. In an exploratory endeavor, we investigated the effects of a consortium consisting of ectomycorrhizal fungi and a beneficial Pseudomonas sp. strain GM41 on plant growth (including height, stem girth, leaf, and root growth) and as well as growth rate over time, across four Populus trichocarpa genotypes. Additionally, we compared the level of total organic carbon and plant exometabolite profiles across different poplar genotypes in the presence of the microbial consortium. These data revealed no significant difference in plant growth parameters between the treatments and the control across four different poplar genotypes at 7 weeks post-inoculation. However, total organic carbon and exometabolite profiles were significantly different between the genotypes and the treatments. These findings suggest that this microbial consortium has the potential to trigger early signaling responses in poplar, influencing its metabolism in ways crucial for later developmental processes and stress tolerance.

Physiological harmfulness of xylophagous insects in poplar and aspen stands in the Left-Bank Forest-Steppe

Introduction
 Poplars and aspens (Populus sp.) are common in forest stands, shelterbelts, rural landscaping, and in special plantations as short-rotation woody crops; they are promising tree species for biofuel and bioenergy production. Wood-boring insects browse galleries in a stem or branches that interrupt sap circulation and promote infestation by plant pathogens. Some of these insects can additionally weaken trees using foliage, bark, or phloem for maturation feeding. All these effects comprise the physiological harmfulness of wood-boring insects, whereas their technical harmfulness depends on the size and depth of the galleries, their location at the stem, and the timber value of a given tree species.
 The approach of wood-boring insect harmfulness assessment was suggested by E. Mozolevskaya and it is implemented for oak, pine, spruce, birch, and elm. It was shown that the harmfulness of the same insect species varies significantly depending on environmental conditions and population density.
 The aim of the research was to evaluate the physiological harmfulness of xylophagous insects for Populus sp. in the Left-Bank Forest-Steppe considering the ability of these insects to colonize and damage trees under certain health, transmit the pathogens, as well as host range and occurrence.
 Materials and Methods
 The research was carried out in 2019–2022 in Poltava, Sumy, and Kharkiv regions at the archive plantation of poplar clones, forest stands, and forest shelterbelts. The collections and research of Yu. Skrylnik since 2006 in the specified territories were also included in the analysis. In the analysis, only those insect species were used, for which the development in the stems and branches of Populus sp. was proved by our research and/or supported by publications. The range of host plants and the occurrence of insects were estimated. Their physiological harmfulness was calculated as the sum of scores for their physiological activity (ability to colonize trees under certain health condition), ability to damage the trees during maturation feeding, and ability to vector the pathogens.
 Results and Conclusions
 In the study, 72 insect species were analyzed, particularly 40 Cerambycidae, 19 Buprestidae, 7 Curculionidae, 2 Sesiidae, and 3 Cossidae. The proportion of abundant species was the highest among Scolytinae: Xyleborini. Species with a high physiological activity predominate among monophagous insects, particularly, Agrilus suvorovi, Cryptorhynchus lapathi, Xyleborus cryptographus, Paranthrene tabaniformis, Acossus terebra. Only five species (S. populnea, S. octopunctata, S. perforata, S. scalaris, S. carcharias) can cause significant damage to trees during maturation feeding by foliage or bark of branches. The average score of physiological harmfulness is the highest among abundant xylophagous species, it is almost twice as low in common species, and almost two times less in rare and single species. High physiological harmfulness (10 and more) for poplars and aspens is estimated for nineteen xylophagous species, particularly, 13 for S. populnea and S. carcharias, and 12 for Cryptorhynchus lapathi, Trypodendron signatum, Tremex fuscicornis, and Xyleborini species.
 3 Tab., 4 Fig., 35 Ref.

Toward the Adoption of New Farming Systems among Farmers: A Case study of Short Rotation Woody Crops in North Carolina

This study explores the human dimensions of the broad-based adoption of Short Rotation Woody Crops (SRWCs) among farmers in North Carolina. We used an actor diagramming and tracing approach to explore factors influencing farmers’ adoption of SRWCs. Results suggest four factors strongly influence the adoption process: 1) market availability, 2) education awareness, 3) funding, and 4) social networking. Based on these results, we recommend that Extension professionals use the following education modules to prompt the adoption of SRWCs practices and potentially adopt other new farming practices: 1) ecological sustainability, 2) financial considerations, 3) harvesting, and 4) community building.

Full-length transcriptome sequencing of the short-rotation woody crop Salix integra reveals a time series response to Pb stress

The short-rotation woody crop Salix integra is a low-cost, renewable feedstock that can be used in the medical and energy industry sectors. Lead (Pb) is a highly toxic, nondegradable heavy metal that accumulates in the soil, can reduce the biomass productivity of plants and threatens human health. Some genotypes of S. integra show strong tolerance to and high accumulation potential for Pb. Here, a highly tolerant S. integra clone was subjected to 0.3 mM Pb in a hydroponic system for 14 d, and roots were sampled at 9 time points. Cytotoxic substance contents and antioxidant enzyme activities were affected by Pb. Transcriptome data revealed 11039 differentially expressed transcripts (DETs); the expression levels of representative DETs were verified by RTqPCR. Weighted gene coexpression network analysis (WGCNA) and enrichment analyses showed that the response of S. integra to Pb stress could be divided into two main stages. In the early stage (mainly within 24 h), DETs were involved in the "sulfur assimilation and GSH synthesis" process and "ABC transporters" pathway. In the late stage (mainly after 3 d), the differential expression of DETs that regulate the synthesis of multiple secondary metabolites in the "phenylpropanoid biosynthesis" pathway was the main factor. Additionally, the continuous differential expression of DETs involved in the "plant hormone signal transduction" pathway suggested a complex response pattern. The results deeply revealed the Pb response pattern in S. integra driven by the coregulation of multiple pathways at different stages instead of the single Pb response pathway found in other species. This study provides new ideas for studying the Pb tolerance mechanisms of energy crops.

Preliminary Results on the Effects of Initial Stand Density and Supplemental Irrigation on Biomass of Eucalyptus camaldulensis Dehn. Grown as a Short Rotation Woody Crop under Semi-Arid Conditions

Most short rotation coppice (SRC) plantations are grown in temperate, subtropical, and tropical areas where land availability is limited. Very little is known about the potential biomass yield of Eucalyptus camaldulensis grown as SRC under semi-arid conditions with supplemental irrigation. Our objective was to maximize biomass production of E. camaldulensis under semi-arid conditions as a feedstock for biofuels by optimizing initial stem density (ISD) and irrigation amount. We tested the effects of four densities of 2500, 10,000, 40,000, and 160,000 stems per hectare, and supplemental irrigation of 100% or 200% of potential evapotranspiration on tree growth and biomass production. Our results showed that under semi-arid conditions and supplemental irrigation, trees grew rapidly and accumulated biomass at a rate equal to or exceeding that in tropical regions. As ISD increased, individual trees grew slightly taller, became much narrower, and had fewer stems. We concluded that competition for resources such as light and nutrients increased with ISD, resulting in significantly lower biomass accumulation by individual trees. However, the significantly greater number of individuals with increasing ISD was responsible for the higher biomass production per hectare, allowing us to achieve exceptionally high annual yields of eucalypt biomass under semi-arid conditions after three annual coppicing cycles.

Short rotation woody crops and forest biomass sawdust mixture pellet quality

Global pellet production has been growing steadily, and it was higher in 2021 by approximately 6.8% compared with 2020, with EU production increasing by 9%. In consequence, the demand for sawdust is growing while its availability is increasingly limited, and the prices are increasing. This is why woody biomass from short-rotation woody crops (SRWC) may have to be used to satisfy the demand for this raw material. Therefore, the aim of this study was to determine the thermophysical properties and elemental composition of pellets produced from forest wood (pine, birch) sawdust and biomass from SRWC (black locust, poplar and willow) and from their mixtures. Moreover, the study also seeks to demonstrate which types of pellets met the parameters laid down in selected standards which categorize pellets into various classes. A total of 23 types of pellets were produced – two from forest-derived biomass, three from SRWC biomass and 18 from mixtures of forest-derived and SRWC biomass. The lower heating value of all pellet types met the strict criteria for class A1 pellets (≥16.5 MJ kg–1). Furthermore, only three pellet types: those obtained from Scots pine and mixed pellets from 75% of Scots pine biomass and 25% willow or poplar biomass, met the strict criteria for class A1 and Grade 1 with respect to all the analyzed parameters. The quality of pellets from biomass mixtures decreased with increasing portions of SRWC biomass relative to pure sawdust from forest wood, especially due to the increasing content of ash, nitrogen, sulfur and chlorine.

A functional trait framework for integrating nitrogen‐fixing cover crops into short‐rotation woody crop systems

AbstractDeveloping approaches to simultaneously maximize short‐rotation woody crop (SRWC) productivity while minimizing footprints associated with intensive management is imperative to profitable and sustainable bioenergy production systems. Intercropping nitrogen (N)‐fixing cover crops in SRWC systems is an overlooked approach to sustainably intensify SRWC production by increasing N availability using less environmentally costly inputs. Here, we discuss how functional traits (e.g., seasonal activity, lifespan, leaf habit, soil exploration) of cover crops and SRWCs may interact through space and time influencing access to light, water, and nutrients to provide a framework for successful integration of cover crops into SRWCs. Next, we summarize the literature on intercropping forest plantations with N‐fixing cover crops to identity research gaps and outline future research needs and opportunities. And then, using empirical N demand and productivity data from SRWCs and cover crop N inputs from the literature, we illustrate how SRWC leaf habit (conifer evergreens and deciduous hardwoods) would influence successful integration of cover crops and potential N fixation. We estimate that integrating cover crops into SRWCs could supply 27% and 72% of the N demand across a 10‐year rotation for an evergreen and a deciduous hardwood, respectively. These figures suggest theseintegratedSRWC systems may approach a virtual minimal external N input when other biogeochemical cycles are considered. The guiding principles presented here are grounded in ecological theory and provide a framework for sustainable intensification of forest production.

Thermophysical Properties and Elemental Composition of Black Locust, Poplar and Willow Biomass

Biomass is currently the main renewable energy source (RES) in the EU, particularly in Poland. Solid biomass for energy purposes is primarily sourced from forests, the wood processing industry, and agriculture. A significant source of this energy feedstock could also be short-rotation woody crops (SRWCs), including black locust, poplar, and willow. Since numerous factors determine the SRWC biomass quality, the current study aimed at assessing biomass thermophysical properties and elemental composition depending on the plant species, soil enrichment procedure, and the plant harvest rotation over a consecutive 12-year period of cultivation. The characteristics under study, including the moisture content, ash content, volatile matter, fixed carbon, higher heating value (HHV), and the carbon, hydrogen, nitrogen, sulfur, and chlorine contents, were significantly differentiated by all the main factors, i.e., the SRWC species, the soil enrichment procedure, the harvest rotation, and the interactions between these factors. The SRWC species accounted for the highest percentage of the variation in the biomass moisture content, ash content, HHV, and nitrogen content, while the harvest rotation made the largest contribution to the variation in carbon, hydrogen, and chlorine contents. The black locust biomass was characterized by the significantly lowest moisture content (an average of 38.89%) and the highest sulfur content (an average of 0.033% DM), nitrogen content (an average of 0.91% DM), and chlorine content (an average of 0.032% DM). However, poplar was characterized by the highest HHV (an average of 19.84 GJ Mg−1 DM) and the highest moisture content (56.52% DM), carbon content (56.52% DM), and ash content (an average of 1.67% DM). Willow was characterized by the lowest ash content (an average of 1.67% DM), a medium moisture content, and the lowest nitrogen content (an average of 0.38% DM) and chlorine content (an average of 0.19% DM).

Whole-tree dormant season nitrogen pools for different species receiving combinations of fertilization and irrigation after one short rotation

Our understanding of the accretion and distribution of tree tissue nitrogen (N) pools across a variety of species and genotypes suitable for short-rotation woody crop (SRWC) production in response to water and N availability remains limited. We measured dormant-season, rotation-length, whole-tree N pools for five tree genotypes from four species (two eastern cottonwood, Populus deltoides Bartr., genotypes; American sycamore, Platanus occidentalis L.; American sweetgum, Liquidambar styraciflua L.; and loblolly pine, Pinus taeda L.) receiving irrigation (I), fertilization (F), their combination (IF), or no resources manipulation (C). Our results demonstrate that foliar nitrogen concentration [N] responded to fertilization but was constrained within genotype-specific ranges and varied temporally. Tree genotypes differed in their composite and component tissue N content (NC), and these differences mostly reflected tissue mass (i.e., larger components and trees resulted in higher NC). Resource amendments (I, F, IF) resulted in up to 3.8-fold increases in NC compared with C, which were most pronounced for sycamore and sweetgum with F and IF treatments, respectively. By the end of the rotation, forest stands accumulated 73 to 452 kg N ha−1 in tree tissues with 40 – 78 % distributed aboveground and 22 – 60 % distributed belowground. A critical difference between genotypes was that all hardwoods exhibited larger belowground N distributions than the evergreen conifer. Our results stress the importance of belowground N pools and highlight differences among genotypes. Our study underscores valuable information about N pools across genotypes suitable for SRWC production, which can be leveraged to inform fertilization plans and devise sustainable nutrient management as production expands across marginal lands.

Integrating Short Rotation Woody Crops into Conventional Agricultural Practices in the Southeastern United States: A Review

One of the United Nations Sustainable Development Goal’s (SDGs) aims is to enhance access to clean energy. In addition, other SDGs are directly related to the restoration of degraded soils to improve on-farm productivity and land management. Integrating Short Rotation Woody Crops (SRWC) for bioenergy into conventional agriculture provides opportunities for sustainable domestic energy production, rural economic development/diversification, and restoration of soil health and biodiversity. Extensive research efforts have been carried out on the development of SRWC for bioenergy, biofuels, and bioproducts. Recently, broader objectives that include multiple ecosystem services, such as carbon sequestration, and land mine reclamation are being explored. Yet, limited research is available on the benefits of establishing SRWC on degraded agricultural lands in the southeastern U.S. thereby contributing to environmental goals. This paper presents a literature review to (1) synthesize the patterns and trends in SWRC bioenergy production; (2) highlight the benefits of integrating short rotation woody crops into row crop agriculture; and (3) identify emerging technologies for efficiently managing the integrated system, while identifying research gaps. Our findings show that integrating SRWC into agricultural systems can potentially improve the climate of agricultural landscapes and enhance regional and national carbon stocks in terrestrial systems.

Nth-plant scenario for forest resources and short rotation woody crops: Biorefineries and depots in the contiguous US

• Nationwide depots & biorefineries to meet target costs, quality & quantity from woody resources. • Forest residues: trees, tops & limbs. Short rotation crops: poplar, willow, pine, & eucalyptus. • Maximum accessible forest residues at $79.07/dt & $85.51/dt was 103 and 106 M dt at any ash target. • Short rotation woody crops increased total to 153 & 195 M dt at 1% & 1.75% ash & $85.51/dt cost targets. • Woody resources could address 55% of EPA’s goals, 195 M dt/year at $85.51/dt cost & 1.75% ash target. Estimating the US potential of woody material is of vital importance to ensure cost-effective supply logistics and develop a sustainable bioenergy and bioproducts industry. We analyzed a mature conversion technology for woody resources for the contiguous US that takes advantage of economies of scale: the nth-plant. We developed a database to quantify the total accessible woody biomass within a distributed network of preprocessing depots and biorefineries considering both quality specifications for conversion and a target cost to compete with fossil fuels. We considered two categories of woody biomass: 1) forest residues from trees, tops and limbs produced from conventional thinning and timber harvesting operations as well as non-timber tree removal; and 2) short rotation woody crops such as poplar, willow, pine, and eucalyptus. A mixed integer linear programming model was developed to analyze scenarios with woody feedstock blends at variable biomass ash contents and cost targets at the biorefinery. When considering a target cost of $85.51/dry ton (2016$) at the biorefinery, the maximum accessible biomass from forest residues in 2040 remained constant at 106 million dry tons regardless of ash targets. Including short rotation woody crops as part of the blend increased the total accessible biomass to 153 and 195 million dry tons at ash targets of 1% and 1.75%, respectively. We concluded from our analysis that woody resources could address about 55% of EPA’s (Environmental Protection Agency) target of 16 billion gallons of cellulosic biofuel.

Environmental and Plant-Derived Controls on the Seasonality and Partitioning of Soil Respiration in an American Sycamore (Platanus occidentalis) Bioenergy Plantation Grown at Different Planting Densities

Bioenergy is one of the most considered alternatives to fossil fuels. Short-rotation woody crops (SRWCs) as bioenergy sources are capable of alleviating energy constraints and sequestering atmospheric CO2. However, studies investigating soil carbon (C) dynamics at SWRC plantations are scarce. We studied American sycamore (Platanus occidentalis) as a model tree species for SRWC at different planting densities ((1) 0.5 × 2.0 m (10,000 trees·ha−1 or tph), (2) 1.0 × 2.0 m (5000 tph), and (3) 2.0 × 2.0 m (2500 tph)) to examine seasonal variation in total soil respiration (Rtotal), partitioned into heterotrophic (Rh) and autotrophic (Ra) respiration, and we evaluated climatic and biological controls on soil respiration. Rtotal and Rh exhibited larger seasonal variation than Ra (p < 0.05). During the nongrowing seasons, the average Rtotal was 0.60 ± 0.21 g·C·m−2·day−1 in winter and 1.41 ± 0.73 g·C·m−2·day−1 in fall. During the growing season, Rtotal was 2–7 times higher in spring (3.49 ± 1.44 g·C·m−2·day−1) and summer (4.01 ± 1.17 g·C·m−2·day−1) than winter. Average Rtotal was 2.30 ± 0.63 g·C·m−2·day−1 in 2500 tph, 2.43 ± 0.64 g·C·m−2·day−1 in 5000 tph, and 2.41 ± 0.75 g·C·m−2·day−1 in 10,000 tph treatments. Average Rh was 1.72 ± 0.40 g·C·m−2·day−1 in 2500 tph, 1.57 ± 0.39 g·C·m−2·day−1 in 5000 tph, and 1.93 ± 0.64 g·C·m−2·day−1 in 10,000 tph, whereas Ra had the lowest rates, with 0.59 ± 0.53 g·C·m−2·day−1 in 2500 tph, 0.86 ± 0.51 g·C·m−2·d−1 in 5000 tph, and 0.48 ± 0.34 g·C·m−2·day−1 in 10,000 tph treatments. Rh had a greater contribution to Rtotal (63%–80%) compared to Ra (20%–37%). Soil temperature was highly correlated to Rtotal (R2 = 0.92) and Rh (R2 = 0.77), while the correlation to Ra was weak (R2 = 0.21). Rtotal, Rh, and Ra significantly declined with soil water content extremes (e.g., <20% or >50%). Total root biomass in winter (469 ± 127 g·C·m−2) was smaller than in summer (616 ± 161 g·C·m−2), and the relationship of total root biomass to Rtotal, Rh, and Ra was only significant during the growing seasons (R2 = 0.12 to 0.50). The litterfall in 5000 tph (121 ± 16 g DW·m−2) did not differ (p > 0.05) from the 2500 tph (108 ± 16 g DW·m−2) or 10,000 tph (132 ± 16 g DW·m−2) treatments. In no circumstances were Rtotal, Rh, and Ra significantly correlated with litterfall amount across planting densities and seasons (p > 0.05). Overall, our results show that Rtotal in American sycamore SRWC is dominated by the heterotrophic component (Rh), is strongly correlated to soil environmental conditions, and can be minimized by planting at a certain tree density (5000 tph).

Comparative analysis of different alternatives for sustainable short rotation woody crops in Central Italy.

Short rotation woody crops (SRWCs) can be a sustainable solution for producing biomass for bioenergy and, at the same time, mitigating CO2 emissions. Although the contribution SRWCs can give to the transition to a low-carbon energy economy, farmers are hesitant to introduce them, as they have a low added value and there are uncertain economic prospects for cultivating SRWCs. Therefore, this study aims to analyze the economic sustainability of poplar woodchip production in Central Italy. Thus, the work compares three plantations, characterized by different durations (one year, two years, and five years) and harvesting systems, to identify the solution with the optimal duration. The results show that the quinquennial harvesting system is the most advantageous according to all the indicators, whereas the biennial performs the worst.

Productivity of Short-Rotation Poplar Crops: A Case Study in the NE of Romania

In Romania, the productivity of the new clones of hybrid poplar has not been tested in recent years. This case study aims to fill a gap on the productivity map of the new poplar clones, estimating, by biomass measurements, the productivity of two clones (AF2 and AF8) with different planting densities (from 1333 trees·ha−1 to 2667 trees·ha−1). The short-rotation woody crops (SRWCs) were established in homogeneous conditions, in North-East (NE) Romania and the northern part of Suceava County. Using a specifically developed method, biomass production was estimated using destructive methods, with 190 poplar trees being harvested, measured, and weighed to compute the accumulated biomass for each growing season The biomass production of the crops with 1667 trees·ha−1 planting density highlighted significant differences in productivity in favour of the AF2 clone after five growing seasons. The crops shared similar annual growth patterns, and the stem biomass represents approximately 73–80% of the total biomass of the trees. The second research question concerning planting density influence on productivity showed fluctuations of biomass accumulations at different planting densities (1333 trees ha−1, 1667 trees ha−1 and 2667 trees ha−1) for a 5-year rotation. The outcomes emphasized the influence of the annual weather conditions—primarily the rainfall in May–June—on poplar growth, showing that productivity also depends on the genotype, density and biotic disturbances.

Growth and Development of Short-Rotation Woody Crops for Rural and Urban Applications

Woody biomass from short-rotation woody crops (SRWCs) plays a substantial role in feedstock production for alternative energy sources throughout the world, thus helping to mitigate climate change driven by excessive use of fossil fuels [...]