Short-rotation Poplar Plantation Research Articles

Multi-tree cut-to-length harvesting of short-rotation poplar plantations

Small tree size represents the main challenge when designing a cost-effective harvesting system for European short-rotation plantations. This challenge is further complicated by the need to obtain 4-m logs for high-end products, which rules out the possibility of deploying whole-tree chipping. Both challenges can be met through mass or multi-tree handling (MTH), which must begin at the time of felling and continue uninterrupted along the whole supply chain. The objective was to: (1) gauge the productivity and the cost of CTL harvesting applied to these plantations; (2) assess log yield and capacity to match dimensional specifications; (3) determine if MTH applied to CTL technology offers a real benefit compared with conventional single-tree handling. The authors conducted a time study using a block design with a two-machine cut-to-length harvesting system (i.e. harvester and forwarder) in single- and multi-tree handling operations on the clear cutting of a hybrid poplar plantation in Poland, as well as we manually measured the produced volumes of the study. Higher productivity (+ 8%) was achieved under the multi-stem handling mode. The MTH system proved capable of containing harvesting costs below € 15 per green ton, while fulfilling set timber quality requirements in terms of value recovery and log quality specifications. A new, software-based, MTH system is recommended to be used in short-rotation poplar plantation for logs and biomass harvesting. When the coppicing season is over, the system can be deployed for the conventional thinning operations.

Water availability influences accumulation and allocation of nutrients and metals in short-rotation poplar plantation

We analyzed the effect of manipulated water availability on an accumulation of nutrients and metals, their stoichiometry, and allocation to roots or leaves in a short rotation coppice (SRC) poplar plantation. The aim of this study was also to clarify how these changes are related to the effects of drought on growth parameters. This study was conducted in Domanínek, Czech Republic in an SRC poplar clone J-105 (Populus nigra L. × P. Maximowiczii H.). This plantation was established as an uncoppiced (single stem) and later on converted into multi-stem (coppice).A rain-out shelter experiment (reduced throughfall) was established in the second year of coppice and the drought stress (DS) applied for 3 years. Water availability altered the accumulation and allocation of nutrients and metals in above and belowground biomass. Reduced water availability led, in particular, to the significantly lower accumulation of potassium (K) in both leaves and roots and a higher carbon (C) to potassium (K) ratio (C:K) in leaves. The significant decline of zinc (Zn) was also found in roots under reduced throughfall. Reduced water availability led to increased accumulation of cadmium (Cd) in leaves and decreased accumulation in roots. This resulted in significantly lower root:leaf ratio for Cd content. An opposite response was found for the allocation of copper (Cu). We also demonstrated that major changes in accumulation and allocation are associated with changes in growth. The results indicated that such knowledge may contribute to understanding the role of nutrient uptake and translocation in acclimation to DS and it may help in developing phytoextraction methods on contaminated soils.

Fluxes of the greenhouse gases (CO2, CH4 and N2O) above a short-rotation poplar plantation after conversion from agricultural land

The increasing demand for renewable energy may lead to the conversion of millions of hectares into bioenergy plantations with a possible substantial transitory carbon (C) loss. In this study we report on the greenhouse gas fluxes (CO2, CH4, and N2O) measured using eddy covariance of a short-rotation bioenergy poplar plantation converted from agricultural fields. During the first six months after the establishment of the plantation (June–December 2010) there were substantial CO2, CH4, and N2O emissions (a total of 5.36±0.52MgCO2eqha−1 in terms of CO2 equivalents). Nitrous oxide loss mostly occurred during a week-long peak emission after an unusually large rainfall. This week-long N2O emission represented 52% of the entire N2O loss during one and an half years of measurements. As most of the N2O loss occurred in just this week-long period, accurately capturing these emission events are critical to accurate estimates of the GHG balance of bioenergy. While initial establishment (June–December 2010) of the plantation resulted in a net CO2 loss into the atmosphere (2.76±0.16MgCO2eqha−1), in the second year (2011) there was substantial net CO2 uptake (−3.51±0.56MgCO2eqha−1). During the entire measurement period, CH4 was a source to the atmosphere (0.63±0.05MgCO2eqha−1 in 2010, and 0.49±0.05MgCO2eqha−1 in 2011), and was controlled by water table depth. Importantly, over the entire measurement period, the sum of the CH4 and N2O losses was much higher (3.51±0.52MgCO2eqha−1) than the net CO2 uptake (−0.76±0.58MgCO2eqha−1). As water availability was an important control on the GHG emission of the plantation, expected climate change and altered rainfall pattern could increase the negative environmental impacts of bioenergy.

Impacts of nitrogen fertilisation and coppicing on total and heterotrophic soil CO2 efflux in a short rotation poplar plantation

Short rotation forests can serve as sources of renewable energy and possibly for soil C storage. However, the high frequency of management practices and the fertilisation could reduce C storage into the soil, by increasing CO2 emissions and annulling the potential of C sequestration. The objectives of this work were to evaluate the impacts of coppicing and fertilisation on total soil CO2 efflux, soil heterotrophic processes and consequent changes of soil C storage in a short rotation poplar plantation. Field soil CO2 efflux, heterotrophic soil CO2 efflux and soil organic C were compared before and after coppicing. Temporal dynamics of fine root biomass and water-soluble carbon after coppicing were also analysed. Coppicing increased total soil CO2 efflux by more than 50%, while heterotrophic soil CO2 efflux remained unchanged. Nevertheless, an increase in total organic carbon was observed as a result of above and belowground litter inputs, as well as root re-growth and exudation. This trend was more evident in fertilised soils due to lower heterotrophic and autotrophic soil CO2 effluxes. Fertilisation can reduce the increase of CO2 emissions after coppicing. Although soil organic C storage increased, the accumulation of labile fractions may trigger microbial respiration in the following years.

Increased Litter Build Up and Soil Organic Matter Stabilization in a Poplar Plantation After 6 Years of Atmospheric CO2 Enrichment (FACE): Final Results of POP-EuroFACE Compared to Other Forest FACE Experiments

Free air CO2 enrichment (FACE) experiments in aggrading temperate forests and plantations have been initiated to test whether temperate forest ecosystems act as sinks for anthropogenic emissions of CO2. These FACE experiments have demonstrated increases in net primary production and carbon (C) storage in forest vegetation due to increased atmospheric CO2 concentrations. However, the fate of this extra biomass in the forest floor or mineral soil is less clear. After 6 years of FACE treatment in a short-rotation poplar plantation, we observed an additional sink of 32 g C m−2 y−1 in the forest floor. Mineral soil C content increased equally under ambient and increased CO2 treatment during the 6-year experiment. However, during the first half of the experiment the increase in soil C was suppressed under FACE due to a priming effect, that is, the additional labile C increased the mineralization of older SOM, whereas during the second half of the experiment the increase in soil C was larger under FACE. An additional sink of 54 g C m−2 y−1 in the top 10 cm of the mineral soil was created under FACE during the second half of the experiment. Although, this FACE effect was not significant due to a combination of soil spatial variability and the low number of replicates that are inherent to the present generation of forest stand FACE experiments. Physical fractionation by wet sieving revealed an increase in the C and nitrogen (N) content of macro-aggregates due to FACE. Further fractionation by density showed that FACE increased C and N contents of the light iPOM and mineral associated intra-macro-aggregate fractions. Isolation of micro-aggregates from macro-aggregates and subsequent fractionation by density revealed that FACE increased C and N contents of the light iPOM, C content of the fine iPOM and C and N contents of the mineral associated intra-micro-aggregate fractions. From this we infer that the amount of stabilized C and N increased under FACE treatment. We compared our data with published results of other forest FACE experiments and infer that the type of vegetation and soil base saturation, as a proxy for bioturbation, are important factors related to the size of the additional C sinks of the forest floor–soil system under FACE.

Elevated atmospheric CO 2 changes phosphorus fractions in soils under a short rotation poplar plantation (EuroFACE)

Future high levels of atmospheric carbon dioxide (CO 2) may increase biomass production of terrestrial plants and hence plant requirements for soil mineral nutrients to sustain a greater biomass production. Phosphorus (P), an element essential for plant growth, is found in soils both in inorganic and in organic forms. In this work, three genotypes of Populus were grown under ambient and elevated atmospheric CO 2 concentrations (FACE) for 5 years. An N fertilisation treatment was added in years 4 and 5 after planting. Using a fractionation scheme, total P was sequentially extracted using H 2O, NaOH, HCl and HNO 3, and P determined as both molybdate (Mo) reactive and total P. Molybdate-reactive P is defined as mainly inorganic but also some labile organic P which is determined by Vanado-molybdophosphoric acid colorimetric methods. Organic P was also measured to assess all plant available and weatherable P pools. We tested the hypotheses that higher P demand due to increased growth is met by a depletion of easily weatherable soil P pools, and that increased biomass inputs increases the amount of organic P in the soil. The concentration of organic P increased under FACE, but was associated with a decrease in total soil organic matter. The greatest increase in the soil P due to elevated CO 2 was found in the HCl-extractable P fraction in the non-fertilised treatment. In the NaOH-extractable fraction the Mo-reactive P increased under FACE, but total P did not differ between ambient and FACE. The increase in both the NaOH- and HCl-extractable fractions was smaller after N addition. The results showed that elevated atmospheric CO 2 has a positive effect on soil P availability rather than leading to depletion. We suggest that the increase in the NaOH- and HCl-extractable fractions is biologically driven by organic matter mineralization, weathering and mycorrhizal hyphal turnover.

Assessment of soil nitrogen and phosphorous availability under elevated CO2 and N-fertilization in a short rotation poplar plantation

Photosynthetic stimulation by elevated [CO2] is largely regulated by nitrogen and phosphorus availability in the soil. During a 6 year Free Air CO2 Enrichment (FACE) experiment with poplar trees in two short rotations, inorganic forms of soil nitrogen, extractable phosphorus, microbial and total nitrogen were assessed. Moreover, in situ and potential nitrogen mineralization, as well as enzymatic activities, were determined as measures of nutrient cycling. The aim of this study was to evaluate the effects of elevated [CO2] and fertilization on: (1) N mineralization and immobilization processes; (2) soil nutrient availability; and (3) soil enzyme activity, as an indication of microbial and plant nutrient acquisition activity. Independent of any treatment, total soil N increased by 23% in the plantation after 6 years due to afforestation. Nitrification was the main process influencing inorganic N availability in soil, while ammonification being null or even negative. Ammonium was mostly affected by microbial immobilization and positively related to total N and microbial biomass N. Elevated [CO2] negatively influenced nitrification under unfertilised treatment by 44% and consequently nitrate availability by 30% on average. Microbial N immobilization was stimulated by [CO2] enrichment and probably enhanced the transformation of large amounts of N into organic forms less accessible to plants. The significant enhancement of enzyme activities under elevated [CO2] reflected an increase in nutrient acquisition activity in the soil, as well as an increase of fungal population. Nitrogen fertilization did not influence N availability and cycling, but acted as a negative feed-back on phosphorus availability under elevated CO2.

Increased nitrogen-use efficiency of a short-rotation poplar plantation in elevated CO2 concentration

We estimated nitrogen (N) use by trees of three poplar species exposed for 3 years to free air CO(2) enrichment (FACE) and determined whether the CO(2) treatment affected the future N availability of the plantation. Trees were harvested at the end of the first 3-year rotation and N concentration and content of woody tissues determined. Nitrogen uptake of fine roots and litter was measured throughout the first crop rotation. The results were related to previously published variations in soil N content during the same period. We estimated retranslocation from green leaves and processes determining N mobilization and immobilization, such as mineralization and nitrification, and N immobilization in litter and microbial biomass. In all species, elevated CO(2) concentration ([CO(2)]) significantly increased nitrogen-use efficiency (NUE; net primary productivity per unit of annual N uptake), decreased N concentration in most plant tissues, but did not significantly change cumulative N uptake by trees over the rotation. Total soil N was depleted more in elevated [CO(2)] than in ambient [CO(2)], although not significantly for all soil layers. The effect of elevated [CO(2)] was usually similar for all species, although differences among species were sometimes significant. During the first 3-year rotation, productivity of the plantation remained high in the elevated [CO(2)] treatment. However, we observed a potential reduction in N availability in response to elevated [CO(2)].

Free atmospheric CO 2 enrichment (FACE) increased respiration and humification in the mineral soil of a poplar plantation

Free atmospheric CO 2 enrichment (FACE) studies conducted at the whole-tree and ecosystem scale indicate that there is a marked increase in primary production, mainly allocated into below-ground biomass. The enhanced carbon transfer to the root system may result in enhanced rhizodeposition and subsequent transfer to soil C pools. However, the impact of elevated CO 2 on soil C contents has yielded variable results. The fate and function of this extra C into the soil in response to elevated CO 2 are not clear. The POPFACE experiment was initiated early 1999 with the objective to determine the functional responses of a short-rotation poplar plantation to actual and future atmospheric CO 2 concentrations. During the first 2 years of the second rotation (2002–2003), the increase of total soil C% was larger under FACE than under ambient CO 2. Chemical fractionation revealed the presence of more labile soil C under FACE, which is in agreement with the larger input of plant litter and root exudates under FACE. In order to gain insight into the fate and function of this extra C into the soil and the dynamics of soil C, we incubated soil samples, measured respiration rates and used a simple soil C model to interpret the results. FACE increased the accumulated 28-day CO 2 production and the initial C slow pool content (metabolizable plant remains and partly decomposed SOM). FACE also increased the decomposition rates of the metabolizable C pools (C fast + C slow) in the top soil, while for the subsoil the opposite effect was observed. The modeled formation of humified SOM was also enhanced by FACE. Our results support the terrestrial feedback hypothesis, i.e. an increase of the long-term terrestrial C sink in response to increasing atmospheric CO 2 concentrations.

Effects of free atmospheric CO<sub>2</sub> enrichment (FACE), N fertilization and poplar genotype on the physical protection of carbon in the mineral soil of a polar plantation after five years

Abstract. Free air CO2 enrichment (FACE) experiments in aggrading forests and plantations have demonstrated significant increases in net primary production (NPP) and C storage in forest vegetation. The extra C uptake may also be stored in forest floor litter and in forest soil. After five years of FACE treatment at the EuroFACE short rotation poplar plantation, the increase of total soil C% was larger under elevated than under ambient CO2. However, the fate of this additional C allocated belowground remains unclear. The stability of soil organic matter is controlled by the chemical structure of the organic matter and the formation of micro-aggregates (within macro-aggregates) in which organic matter is stabilized and protected. FACE and N-fertilization treatment did not affect the micro- and macro-aggregate weight, C or N fractions obtained by wet sieving. However, Populus euramericana increased the small macro-aggregate and free micro-aggregate weight and C fractions. The obtained macro-aggregates were broken up in order to isolate recently formed micro-aggregates within macro-aggregates (iM-micro-aggregates). FACE increased the iM-micro-aggregate weight and C fractions, although not significantly. This study reveals that FACE did not affect the formation of aggregates. We did, however, observe a trend of increased stabilization and protection of soil C in micro-aggregates formed within macro-aggregates under FACE. Moreover, the largest effect on aggregate formation was due to differences in species, i.e. poplar genotype. P. euramericana increased the formation of free micro-aggregates which means that more newly incorporated soil C was stabilized and protected. The choice of species in a plantation, or the effect of global change on species diversity, may therefore affect the stabilization and protection of C in soils.

Impact of elevated CO 2 and nitrogen fertilization on foliar elemental composition in a short rotation poplar plantation

The experiment was carried out on a short rotation coppice culture of poplars (POP-EUROFACE, Central Italy), growing in a free air carbon dioxide enriched atmosphere (FACE). The specific objective of this work was to study whether elevated CO 2 and fertilization (two CO 2 treatments, elevated CO 2 and control, two N fertilization treatments, fertilized and unfertilized), as well as the interaction between treatments caused an unbalanced nutritional status of leaves in three poplar species ( P. x euramericana, P. nigra and P. alba). Finally, we discuss the ecological implications of a possible change in foliar nutrients concentration. CO 2 enrichment reduced foliar nitrogen and increased the concentration of magnesium; whereas nitrogen fertilization had opposite effects on leaf nitrogen and magnesium concentrations. Moreover, the interaction between elevated CO 2 and N fertilization amplified some element unbalances such as the K/N-ratio.

Free Atmospheric CO2 Enrichment (FACE) Increased Labile and Total Carbon in the Mineral Soil of a Short Rotation Poplar Plantation

The global net terrestrial carbon sink was estimated to range between 0.5 and 0.7 Pg C y−1 for the early 1990s. FACE (free atmospheric CO2 enrichment) studies conducted at the whole-tree and community scale indicate that there is a marked increase of primary production, mainly allocated into below-ground biomass. The enhanced carbon transfer to the root system may result in enhanced rhizodeposition and subsequent transfer to soil C pools. During the first rotation of the POP/EuroFACE experiment in a short-rotation Poplar plantation, total soil C content increased more under ambient CO2 treatment than under FACE, while under FACE more new C was incorporated than under ambient CO2. These unexpected and opposite effects may have been caused by a priming effect, where priming effect is defined as the stimulation of SOM decomposition caused by the addition of labile substrates. In order to gain insight into these processes affecting SOM decomposition, we obtained the labile, refractory and stable pools of soil C and N by chemical fractionation (acid hydrolysis) and measured rates of N-mineralization. Results of the first 2 years of the second rotation show a larger increase of total soil C% under FACE than under ambient CO2. In contrast to the first rotation, total C% is now increasing faster under FACE than under ambient CO2. Based on these observations we infer that the priming effect ceased during the second rotation. FACE treatment increased the labile C fraction at 0–10 cm depth, which is in agreement with the larger input of plant litter and root exudates under FACE. N-mineralization rates were not affected by FACE. We infer that the system switched from a state where extra labile C and sufficient N-availability (due to the former agricultural use of the soil) caused a priming effect (first rotation), to a state where extra C input is accumulating due to limited N-availability (second rotation). Our results on N-mineralization (second rotation) are in agreement with observations made at three forest FACE sites (Duke Forest, Oak Ridge, and Rhinelander), but our finding of increasing mineral soil C content contrasted with results at the Duke Forest where no significant increase in C content of the mineral soil occurred. However, the FACE induced increase in total C content occurred within the fraction with the shortest turnover time, i.e. the labile fraction. The refractory and stable fractions were not affected. The question remains whether the currently observed larger increase of total soil C and the increase of labile C under FACE will eventually result in long-term C storage in refractory and stable organic matter fractions.

Free-air CO2 enrichment (FACE) enhances biomass production in a short-rotation poplar plantation.

This paper investigates the possible contribution of Short Rotation Cultures (SRC) to carbon sequestration in both current and elevated atmospheric CO2 concentrations ([CO2]). A dense poplar plantation (1 x 1 m) was exposed to a [CO2] of 550 ppm in Central Italy using the free-air CO2 enrichment (FACE) technique. Three species of Populus were examined, namely P. alba L., P. nigra L. and P. x euramericana Dode (Guinier). Aboveground woody biomass of trees exposed to elevated [CO2] for three growing seasons increased by 15 to 27%, depending on species. As a result, light-use efficiency increased. Aboveground biomass allocation was unaffected, and belowground biomass also increased under elevated [CO2] conditions, by 22 to 38%. Populus nigra, with total biomass equal to 62.02 and 72.03 Mg ha-1 in ambient and elevated [CO2], respectively, was the most productive species, although its productivity was stimulated least by atmospheric CO2 enrichment. There was greater depletion of inorganic nitrogen from the soil after three growing seasons in elevated [CO2], but no effect of [CO2] on stem wood density, which differed significantly only among species.