喀斯特次生林幼树更新空间分布格局及种间关联性

本研究以贵州省喀斯特典型区域紫云苗族布依族自治县撂荒30余年后自然恢复形成的次生林为对象,设置140 m×120 m固定样地,系统调查样地内幼树更新,并采用空间点格局分析方法分析幼树更新优势种群在不同空间尺度下的分布格局和种间关联性。结果表明: 调查样地中幼树共计1291株,包括39个树种,其中光皮桦、化香、马尾松、枫香和山杨5个树种的幼树个体数量总和达83.7%,重要值总和达77.8%,为幼树更新的优势树种。光皮桦、化香和枫香3个幼树优势种群的空间分布格局在0～60 m空间尺度上均呈现较强的聚集分布;马尾松和山杨2个幼树优势种群在小尺度上呈现聚集分布,大尺度上则随机分布。幼树优势种群空间关联性多呈现正关联,仅马尾松与枫香和山杨在小尺度呈现正关联,大尺度呈现不相关。调查样地5个幼树优势种群空间分布格局及种间关联性差别较大,可能与树种的生物学特性、生境及空间资源的利用密切相关。目前,林分多以先锋树种为主,群落结构不稳定;以马尾松和光皮桦为优势种群的松-桦混交林可能成为下一阶段演替方向,建议通过森林经营措施加快植被恢复进程。     

Nitrous oxide emissions during establishment of eight alternative cellulosic bioenergy cropping systems in the North Central United States

Greenhouse gas (GHG) emissions from soils are a key sustainability metric of cropping systems. During crop establishment, disruptive land‐use change is known to be a critical, but under reported period, for determining GHG emissions. We measured soil N₂O emissions and potential environmental drivers of these fluxes from a three‐year establishment‐phase bioenergy cropping systems experiment replicated in southcentral Wisconsin (ARL) and southwestern Michigan (KBS). Cropping systems treatments were annual monocultures (continuous corn, corn–soybean–canola rotation), perennial monocultures (switchgrass, miscanthus, and poplar), and perennial polycultures (native grass mixture, early successional community, and restored prairie) all grown using best management practices specific to the system. Cumulative three‐year N₂O emissions from annuals were 142% higher than from perennials, with fertilized perennials 190% higher than unfertilized perennials. Emissions ranged from 3.1 to 19.1 kg N₂O‐N ha^?1 yr^?1 for the annuals with continuous corn > corn–soybean–canola rotation and 1.1 to 6.3 kg N₂O‐N ha^?1 yr^?1 for perennials. Nitrous oxide peak fluxes typically were associated with precipitation events that closely followed fertilization. Bayesian modeling of N₂O fluxes based on measured environmental factors explained 33% of variability across all systems. Models trained on single systems performed well in most monocultures (e.g., R² = 0.52 for poplar) but notably worse in polycultures (e.g., R² = 0.17 for early successional, R² = 0.06 for restored prairie), indicating that simulation models that include N₂O emissions should be parameterized specific to particular plant communities. Our results indicate that perennial bioenergy crops in their establishment phase emit less N₂O than annual crops, especially when not fertilized. These findings should be considered further alongside yield and other metrics contributing to important ecosystem services.     

Soil carbon sequestration and land use change associated with biofuel production: empirical evidence

Soil organic carbon (SOC) change can be a major impact of land use change (LUC) associated with biofuel feedstock production. By collecting and analyzing data from worldwide field observations of major LUCs from cropland, grassland, and forest to lands producing biofuel crops (i.e. corn, switchgrass, Miscanthus, poplar, and willow), we were able to estimate SOC response ratios and sequestration rates and evaluate the effects of soil depth and time scale on SOC change. Both the amount and rate of SOC change were highly dependent on the specific land transition. Irrespective of soil depth or time horizon, cropland conversions resulted in an overall SOC gain of 6–14% relative to initial SOC level, while conversion from grassland or forest to corn (without residue removal) or poplar caused significant carbon loss (9–35%). No significant SOC changes were observed in land converted from grasslands or forests to switchgrass, Miscanthus, or willow. The SOC response ratios were similar in both 0–30 and 0–100 cm soil depths in most cases, suggesting SOC changes in deep soil and that use of top soil only for SOC accounting in biofuel life cycle analysis (LCA) might underestimate total SOC changes. Soil carbon sequestration rates varied greatly among studies and land transition types. Generally, the rates of SOC change tended to be the greatest during the 10 years following land conversion and had declined to approach 0 within about 20 years for most LUCs. Observed trends in SOC change were generally consistent with previous reports. Soil depth and duration of study significantly influence SOC change rates and so should be considered in carbon emission accounting in biofuel LCA. High uncertainty remains for many perennial systems and forest transitions, additional field trials, and modeling efforts are needed to draw conclusions about the site‐ and system‐specific rates and direction of change.     

Effects of heat and drought stress on post‐illumination bursts of volatile organic compounds in isoprene‐emitting and non‐emitting poplar

Over the last decades, post‐illumination bursts (PIBs) of isoprene, acetaldehyde and green leaf volatiles (GLVs) following rapid light‐to‐dark transitions have been reported for a variety of different plant species. However, the mechanisms triggering their release still remain unclear. Here we measured PIBs of isoprene‐emitting (IE) and isoprene non‐emitting (NE) grey poplar plants grown under different climate scenarios (ambient control and three scenarios with elevated CO₂ concentrations: elevated control, periodic heat and temperature stress, chronic heat and temperature stress, followed by recovery periods). PIBs of isoprene were unaffected by elevated CO₂ and heat and drought stress in IE, while they were absent in NE plants. On the other hand, PIBs of acetaldehyde and also GLVs were strongly reduced in stress‐affected plants of all genotypes. After recovery from stress, distinct differences in PIB emissions in both genotypes confirmed different precursor pools for acetaldehyde and GLV emissions. Changes in PIBs of GLVs, almost absent in stressed plants and enhanced after recovery, could be mainly attributed to changes in lipoxygenase activity. Our results indicate that acetaldehyde PIBs, which recovered only partly, derive from a new mechanism in which acetaldehyde is produced from methylerythritol phosphate pathway intermediates, driven by deoxyxylulose phosphate synthase activity.     

Selection and optimization of a suitable pretreatment method for miscanthus and poplar raw material

Miscanthus and poplar are very promising second‐generation feedstocks due to the high growth rates and low nutrient demand. The aim of the study was to develop a systematic approach for choosing suitable pretreatment methods evaluated with the modified severity factor (log ). Optimal pretreatment results in a high delignification grade, low cellulose solubilization and increased accessibility for enzymatic hydrolysis while revealing minimal log values. In order to do so, several reaction approaches were compared. Acid‐catalyzed organosolv processing carried out for miscanthus and poplar revealed the highest delignification grade leading to a relatively high glucose yield after enzymatic saccharification. In both cases, a design of experiments approach was used to study the influence of relevant parameters. Modeling the data resulted in the identification of optimum pretreatment conditions for miscanthus with concentrations of 0.16% H₂SO₄ and 50% EtOH at 185°C for a retention time of 60 min. Experimental validation of these conditions revealed an even higher delignification degree (88%) and glucose yield (85%) than predicted. 0.19% H₂SO₄ and 50% EtOH were determined as optimum concentrations, 182°C and 48 min identified as optimum pretreatment conditions for poplar; the delignification degree was 84% and the resulting glucose yield 70%.     

Sex pheromone component produced by microbial associates of the forest pest Megaplatypus mutatus

Megaplatypus mutatus (Chapuis) (Coleoptera: Platypodidae) is an ambrosia beetle native to South America that causes economic loss and was recently introduced to Italy, where it attacks and damages live poplar trees. Sulcatol and sulcatone are male‐produced pheromone components involved in the mating process of M. mutatus. Their relative proportions are highly variable among insects, although the temporal pattern shows that initially only sulcatol is present, and sulcatone increases with time, until they are finally both depleted. Sulcatol and sulcatone may be produced de novo by the beetles, they may be produced by fungi, or both pathways may contribute to their production. Sulcatol is stored in the males’ hindgut but sulcatone is only present in emissions, so there is an oxidation process to transform the alcohol to the ketone before or during pheromone release. It is our hypothesis that fungi associated with M. mutatus are responsible for this process. In this work, we studied a possible contribution of associated microorganisms in the conversion of sulcatol into sulcatone and its consequent role in the temporal release pattern of these sex pheromone components observed in male insects. Moreover, we inhibited the postulated enzymes involved in this pheromone conversion process – 3‐hydroxy‐3‐methyl‐glutatyl‐CoA reductase (HMGR) and P450 enzymes of a fungal strain – and added an antibiotic and a fungicide to the homogenate during sulcatol‐sulcatone conversion. Among the fungal species, particular interest was given to Graphium basitruncatum (Matsush.) Seifert & Okada (Microascales), as it is present in male but not in female exoskeletons and in insect gallery samples, suggesting a possible different role in pherome production, as the male is the pheromone‐producing sex. Several isolated strains were able to convert sulcatol to sulcatone, whereas the fungus G. basitruncatum showed the highest production of this ketone. Additionally, inhibition of P450 enzymes and HMGR from G. basitruncatum on this alcohol‐ketone conversion demonstrated that HMGR is involved in sulcatone generation using sulcatol as precursor, and that P450 enzymes are not. Finally, sulcatone production diminished significantly in homogenized tissues of male and female M. mutatus following addition of an antibiotic and a fungicide. The results suggest that fungi associated with M. mutatus are involved in pheromone production.     

Direct comparison of four methods to construct xylem vulnerability curves: Differences among techniques are linked to vessel network characteristics

During periods of dehydration, water transport through xylem conduits can become blocked by embolism formation. Xylem embolism compromises water supply to leaves and may lead to losses in productivity or plant death. Vulnerability curves (VCs) characterize plant losses in conductivity as xylem pressures decrease. VCs are widely used to characterize and predict plant water use at different levels of water availability. Several methodologies for constructing VCs exist and sometimes produce different results for the same plant material. We directly compared four VC construction methods on stems of black cottonwood (Populus trichocarpa), a model tree species: dehydration, centrifuge, X‐ray–computed microtomography (microCT), and optical. MicroCT VC was the most resistant, dehydration and centrifuge VCs were intermediate, and optical VC was the most vulnerable. Differences among VCs were not associated with how cavitation was induced but were related to how losses in conductivity were evaluated: measured hydraulically (dehydration and centrifuge) versus evaluated from visual information (microCT and optical). Understanding how and why methods differ in estimating vulnerability to xylem embolism is important for advancing knowledge in plant ecophysiology, interpreting literature data, and using accurate VCs in water flux models for predicting plant responses to drought.     

Effect of auxin transport inhibitors and ethylene on the wood anatomy of poplar

The influence of the auxin transport inhibitors naphthylphthalamic acid (NPA) and methyl-2-chloro-9-hydroxyflurene-9-carboxylate (CF), as well as the gaseous hormone ethylene on cambial differentiation of poplar was determined. NPA treatment induced clustering of vessels and increased vessel length. CF caused a synchronized differentiation of cambial cells into either vessel elements or fibres. The vessels in CF-treated wood were significantly smaller and fibre area was increased compared with controls. Under the influence of ethylene, the cambium produced more parenchyma, shorter fibres and shorter vessels than in controls. Since poplar is the model tree for molecular biology of wood formation, the modulation of the cambial differentiation of poplar towards specific cell types opens an avenue to study genes important for the development of vessels or fibres.