Carbon isotopic signature of CO2 emitted by plant compartments and soil in two temperate deciduous forests

? Context

The carbon isotope composition of the CO₂ efflux (δ¹³C_E) from ecosystem components is widely used to investigate carbon cycles and budgets at different ecosystem scales. δ¹³C_E, was considered constant but is now known to vary along seasons. The seasonal variations have rarely been compared among different ecosystem components.

? Aims

We aimed to characterise simultaneously the seasonal dynamics of δ¹³C_E in different compartments of two temperate broadleaved forest ecosystems.

? Methods

Using manual chambers and isotope ratio mass spectrometry, we recorded simultaneously δ¹³C_E and δ¹³C of organic matter in sun leaves, current-year twigs, trunk bases and soil in an oak and a beech forest during 1 year.

? Results

In the two forests, δ¹³C_E displayed a larger variability in the tree components than in the soil. During the leafy period, a pronounced vertical zonation of δ¹³C_E was observed between the top (sun leaves and twigs with higher values) and bottom (trunk and soil with lower values) of the ecosystem. No correlation was found between δ¹³C_E and δ¹³C of organic matter. Causes for these seasonal variations and the vertical zonation in isotope signature are discussed.

? Conclusion

Our study shows clear differences in values as well as seasonal dynamics of δ¹³C_E among different components in the two ecosystems. The temporal and local variation of δ¹³C_E cannot be inferred from organic matter signature or CO₂ emission rates.     

Relationship between tree morphology and growth stress in mature European beech stands

Aims

In European Beech (Fagus sylvatica L.) large growth stresses lead to severe log end splitting that devaluate beech timber. Our study aimed at detecting relationships between growth stress and some morphology parameters in trees.

Methods

Growth stress indicators were recorded for 440 mature trees in nine stands from five European countries, together with morphology parameters.

Results

Most trees displayed an uneven distribution of growth stress around the trunk. Moreover, growth stress intensity varied largely between individual trees. Geometry of the trunk was a poor predictor of growth stress intensity. Crown asymmetry resulted in a larger stress dissymmetry within trees. Trunk inclination was not correlated to max tension stress, contrary to what is usually found in younger trees. In the case of small inclination, growth stress was close to expected from biomechanics of restoring verticality. Trees exhibiting a larger inclination probably evolved a different mechanical solution: a rather large crown, lower tree slenderness and a sufficient asymmetry in growth stress as to prevent a higher inclination due to growth.

Conclusion

A large slenderness is the best accurate predictor of a large growth stress, although variations in the ratio height/diameter at breast height explained only 10 % of the variability of growth stress. A large crown surface was the best predictor of a low level of growth stress. A large spacing between trees seems a good solution to lower the risk of growth stress in mature beech.     

Patterns of longitudinal and tangential maturation stresses in Eucalyptus nitens plantation trees

Context

Tree orientation is controlled by asymmetric mechanical stresses set during wood maturation. The magnitude of maturation stress differs between longitudinal and tangential directions, and between normal and tension woods.

Aims

We aimed at evaluating patterns of maturation stress on eucalypt plantation trees and their relation with growth, with a focus on tangential stress evaluation.

Methods

Released maturation strains along longitudinal and tangential directions were measured around the circumference of 29 Eucalyptus nitens trees, including both straight and leaning trees.

Results

Most trees produced asymmetric patterns of longitudinal maturation strain, but more than half of the maturation strain variability occurred between trees. Many trees produced high longitudinal tensile stress all around their circumference. High longitudinal tensile stress was not systematically associated with the presence of gelatinous layer. The average magnitude of released longitudinal maturation strain was found negatively correlated to the growth rate. A methodology is proposed to ensure reliable evaluation of released maturation strain in both longitudinal and tangential directions. Tangential strain evaluated with this method was lower than previously reported.

Conclusion

The stress was always tensile along the longitudinal direction and compressive along the tangential direction, and their respective magnitude was positively correlated. This correlation does not result from a Poisson effect but may be related to the mechanism of maturation stress generation.     

Intensifying weathering and land use in Iron Age Central Africa

About 3000 years ago, a major vegetation change occurred in Central Africa, when rainforest trees were abruptly replaced by savannas. Up to this point, the consensus of the scientific community has been that the forest disturbance was caused by climate change. We show here that chemical weathering in Central Africa, reconstructed from geochemical analyses of a marine sediment core, intensified abruptly at the same period, departing substantially from the long-term weathering fluctuations related to the Late Quaternary climate. Evidence that this weathering event was also contemporaneous with the migration of Bantu-speaking farmers across Central Africa suggests that human land-use intensification at that time had already made a major impact on the rainforest.     

The Paleozoic origin of enzymatic lignin decomposition reconstructed from 31 fungal genomes

Wood is a major pool of organic carbon that is highly resistant to decay, owing largely to the presence of lignin. The only organisms capable of substantial lignin decay are white rot fungi in the Agaricomycetes, which also contains non-lignin-degrading brown rot and ectomycorrhizal species. Comparative analyses of 31 fungal genomes (12 generated for this study) suggest that lignin-degrading peroxidases expanded in the lineage leading to the ancestor of the Agaricomycetes, which is reconstructed as a white rot species, and then contracted in parallel lineages leading to brown rot and mycorrhizal species. Molecular clock analyses suggest that the origin of lignin degradation might have coincided with the sharp decrease in the rate of organic carbon burial around the end of the Carboniferous period.     

Late accretion on the earliest planetesimals revealed by the highly siderophile elements

Late accretion of primitive chondritic material to Earth, the Moon, and Mars, after core formation had ceased, can account for the absolute and relative abundances of highly siderophile elements (HSEs) in their silicate mantles. Here we show that smaller planetesimals also possess elevated HSE abundances in chondritic proportions. This demonstrates that late addition of chondritic material was a common feature of all differentiated planets and planetesimals, irrespective of when they accreted; occurring ≤5 to ≥150 million years after the formation of the solar system. Parent-body size played a role in producing variations in absolute HSE abundances among these bodies; however, the oxidation state of the body exerted the major control by influencing the extent to which late-accreted material was mixed into the silicate mantle rather than removed to the core.     

Surgical management of first caudal nerve root foraminal compression secondary to intervertebral disc disease in a Cocker Spaniel

A nine-year-old spayed female Cocker Spaniel was investigated for an eight week history of licking and rubbing at the tail base, dullness, and signs of pain on manipulation of the tail. Left-sided intraforaminal compression of the first caudal nerve root due to intervertebral disc disease was diagnosed by radiographic, computed tomography, and magnetic resonance imaging examinations. The dog was nonresponsive to conservative medical therapy. A decompressive left-sided first-second caudal (Cd1-Cd2) foraminotomy was performed. Postoperative computed tomography confirmed surgical decompression of the involved nerve root. At the one month follow-up examination there was marked improvement in clinical signs. At two months, clinical signs were completely resolved and there was not any evidence of recurrence twelve months after surgery. Intervertebral disc disease should be considered as a differential diagnosis in dogs with discomfort at the tail base or signs of pain on manipulation of the tail. Surgical decompression may be indicated for management of these cases. This is the first report of diagnosis and surgical management of caudal intervertebral disc disease by foraminotomy in the dog. Surgical decompression by foraminotomy may therefore be a treatment option for this condition.     

Treatment of chenevotte, a co-product of industrial hemp fiber, by water or hydrochloric acid: impact on polymer mobility in the lignified cell walls

The impact of water- and of mild acid impregnation with hydrochloric acid (HCl) on the viscoelastic and breaking properties of woody hemp core (chenevotte) was studied from the polymer to the macroscale level. We have shown that transition temperature (T _g) values of lignin and hemicelluloses were not affected by the acid treatment, whereas the relaxation mechanism of hemicelluloses, expressed by the apparent activation energy, was modified. The opposite impacts were observed in the water extraction. Changes in the sample strength were also noted, as the chenevotte treated with mild HCl became more brittle, as opposed to the samples extracted with water. It is suggested here that the moderate HCl treatment impacts chenevotte micro- and macro-properties through acid depolymerization of polysaccharides and extraction of low molecular mass entities, whereas water immersion contributes solely to the later mechanism.