KEY MESSAGE : We review the recent findings on the influence of drought on tree mortality, growth or ecosystem functioning in tropical rainforests. Drought plays a major role in shaping tropical ...rainforests and the response mechanisms are highly diverse and complex. The numerous gaps identified here require the international scientific community to combine efforts in order to conduct comprehensive studies in tropical rainforests on the three continents. These results are essential to simulate the future of these ecosystems under diverse climate scenarios and to predict the future of the global earth carbon balance. CONTEXT : Tropical rainforest ecosystems are characterized by high annual rainfall. Nevertheless, rainfall regularly fluctuates during the year and seasonal soil droughts do occur. Over the past decades, a number of extreme droughts have hit tropical rainforests, not only in Amazonia but also in Asia and Africa. The influence of drought events on tree mortality and growth or on ecosystem functioning (carbon and water fluxes) in tropical rainforest ecosystems has been studied intensively, but the response mechanisms are complex. AIMS : Herein, we review the recent findings related to the response of tropical forest ecosystems to seasonal and extreme droughts and the current knowledge about the future of these ecosystems. RESULTS : This review emphasizes the progress made over recent years and the importance of the studies conducted under extreme drought conditions or in through-fall exclusion experiments in understanding the response of these ecosystems. It also points to the great diversity and complexity of the response of tropical rainforest ecosystems to drought. CONCLUSION : The numerous gaps identified here require the international scientific community to combine efforts in order to conduct comprehensive studies in tropical forest regions. These results are essential to simulate the future of these ecosystems under diverse climate scenarios and to predict the future of the global earth carbon balance.
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Key message
The above- and belowground impacts due to
Acacia
invasions have been described in detail over the last 25 years. Future research should focus on the early detection and prevention of ...new
Acacia
introductions and on a cost-effective and sustainable management of the novel ecosystems resulting from invasions.
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Context
Invasive alien plants (IAPs) strongly alter ecosystems reducing biodiversity, modifying ecosystem services and increasing negative impacts at social and economic level. Among invasive taxa,
Acacia
is a highly problematic genus worldwide. In fact, almost 500 papers have been published on several aspects of
Acacia
invasions for the last 25 years.
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Aims
We aim at reviewing the current knowledge on the consequences of the invasion by Acacia genus in Mediterranean ecosystems. We also collect and propose different approaches for the management and recovery of invaded areas and suggest future perspectives on Acacia research.
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Methods
We compile, summarise and discuss recent findings on physicochemical, ecological, microbiological and socioeconomic aspects of invasion related to Australian acacias (Acacia dealbata, Acacia longifolia, Acacia mearnsii, Acacia saligna and Acacia melanoxylon) focusing on Mediterranean areas.
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Results
Acacia
invasion generally entails soil physicochemical alterations and changes in microbial function and structure. Consequences such as the decreased biodiversity, altered ecosystem structure, larger seed banks dominated by invasive species, new biotrophic relationships or alterations in water availability and fire regimes suggest that acacias are locally creating novel ecosystems.
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Conclusions
Forecasting invasions, modelling and managing ecosystems dominated by acacias are challenging tasks that should be addressed in the future, since climatic conditions and intensification in land uses are increasing the likelihood of
Acacia
invasions in Mediterranean areas. Unsuccessful management actions suggest that restoration should be meticulously monitored, but the magnitude of invasion or the inconsistency of economic investment indicate that eradication is often unfeasible. Alternatively, novel integrative and cost-effective solutions including the collaboration of society, politicians and stakeholders are necessary to prevent new introductions and achieve sustainable control of acacias. There is a growing interest in applied research on the valorisation or novel uses for acacias and their residues that result in economic benefits.
A three-dimensional numerical model was employed in simulating nonlinear transient moisture flow in wood and the wood’s hygro-mechanical and visco-elastic behaviour under such conditions. The model ...was developed using the finite element software Abaqus FEA
®
, while taking account of the fibre orientation of the wood. The purpose of the study was to assess the ability of the model to simulate the response of wood beams to bending and to the climate of northern Europe. Four-point bending tests of small and clear wood specimens exposed to a constant temperature and to systematic changes in relative humidity were conducted to calibrate the numerical model. A validation of the model was then performed on the basis of a four-point bending test of solid timber beams subjected to natural climatic conditions but sheltered from the direct effects of rain, wind and sunlight. The three-dimensional character of the model enabled a full analysis of the effects of changes in moisture content and in fibre orientation on stress developments in the wood. The results obtained showed a clear distinction between the effects of moisture on the stress developments caused by mechanical loads and the stress developments caused solely by changes in climate. The changes in moisture that occurred were found to have the strongest effect on the stress state that developed in areas in which the tangential direction of the material was aligned with the exchange surface of the beams. Such areas were found to be exposed to high-tension stress during drying and to stress reversal brought about by the uneven drying and shrinkage differences that developed between the outer surface and the inner sections of the beams.
•Thermal modification mainly affected the embedment strength parallel to the grain.•Density–embedment strength relationships still hold after the treatment.•Influence of moisture content on embedment ...properties followed similar trends after thermal modification.•Larger influence of load direction on embedment strength after thermal modification.•Smaller influence of calculation method on embedment strength after the treatment.
This study investigates the effect of thermal modification, ThermoWood Thermo-D treatment versus no treatment, on the embedment properties of Norway spruce timber (Picea abies L. Karst.) with dowel-type fasteners. The test specimens were reinforced to prevent splitting of the wood. The influence of density, load direction, test specimen configuration (full hole versus half hole), moisture content, gauge points and calculation method were also evaluated. Thermal modification primarily affected the embedment strength parallel to the grain, which was ∼ 25% higher after thermal modification, mainly due to the change in physical properties because of the treatment, i.e. the lower equilibrium moisture content. The influence of the investigated parameters on embedment properties of thermally modified spruce followed similar trends as for unmodified spruce. It was for example seen that the density–embedment strength relationships still hold after the treatment despite the decrease in density and the increase in embedment strength parallel to the grain. However, after thermal modification, the influence of load direction on embedment strength was ∼30% larger and the influence of calculation method (yield versus ultimate strength) on embedment strength perpendicular to the grain was ∼10% smaller.
Comprehensive approaches to predict performance of wood products are requested by international standards, and the first attempts have been made in the frame of European research projects. However, ...there is still an imminent need for a methodology to implement the durability and moisture performance of wood in an engineering design method and performance classification system. The aim of this study was therefore to establish an approach to predict service life of wood above ground taking into account the combined effect of wetting ability and durability data. A comprehensive data set was obtained from laboratory durability tests and still ongoing field trials in Norway, Germany and Sweden. In addition, four different wetting ability tests were performed with the same material. Based on a dose–response concept, decay rates for specimens exposed above ground were predicted implementing various indicating factors. A model was developed and optimised taking into account the resistance of wood against soft, white and brown rot as well as relevant types of water uptake and release. Decay rates from above-ground field tests at different test sites in Norway were predicted with the model. In a second step, the model was validated using data from laboratory and field tests performed in Germany and Sweden. The model was found to be fairly reliable, and it has the advantage to get implemented into existing engineering design guidelines. The approach at hand might furthermore be used for implementing wetting ability data into performance classification as requested by European standardisation bodies.
Bark from trees is considered a worthless raw material. However, this resource could be economically beneficial if utilized efficiently due to its rich chemical compounds. In this study, an ethanol ...toluene-soluble extractive, alpha-cellulose and lignin obtained from
Leucaena leucocephala
bark were characterized to determine their chemical functional groups. Based on FTIR spectral analysis, the results indicated that the bands of the functional groups of the extractive from the original bark remain unchanged; however, the absorbance intensity was found to be weaker in the group frequency and fingerprint regions. Removal of extractive, pectin, hemicellulose and lignin from the bark indirectly increased the strong absorbance intensity of cellulose. Broad peaks of OH stretching found in all spectra were assigned to the presence of phenolic OH and aliphatic structures for extractive and aromatic structures of lignin. It was revealed that aromatic functional groups were mainly found in the extractive, while water, carbonyl and ether were the dominant groups in cellulose, and methyl, methylene, carbonyl and carboxyl groups were enriched in lignin.
Graphic abstract
Lignin, an aromatic constituent of woody biomass, is a potential renewable aromatic feedstock for a sustainable future carbon economy. Pyrolysis-based technologies, such as fast pyrolysis and ...gasification, are promising methods for converting lignin into biochemicals, biomaterials, and biofuels. A better understanding of the molecular mechanisms involved in lignin pyrolysis/gasification would guide the development of the controlled pyrolysis and gasification systems to overcome issues with low product selectivity, an intrinsic drawback of current pyrolysis-based technologies. This review article summaries the state-of-the-art research into molecular mechanisms of lignin pyrolysis and gasification. This information should also be useful for understanding the influence of high temperature heat treatments on the properties of wood.
Key message
Wildfire danger and burnt areas should increase over the century in southern Europe, owing to climate warming. Fire-prone area expansion to the north and to Mediterranean mountains is a ...concern, while climate-induced burnt area increase might be limited by fuel availability in the most arid areas. Further studies are needed to both assess and reduce uncertainties on future trends.
Context
Wildfire is the main disturbance in forested ecosystems of southern Europe. Warmer and drier conditions projected in this region are expected to profoundly affect wildfire regimes.
Aims
In this review, we pursue a twofold objective: (i) report the trends in wildfire danger and activity projected under warming climate in southern Europe and (ii) discuss the limitations of these projections.
Methods
We reviewed 23 projection studies that examined future wildfire danger or wildfire activity at local, regional or continental scale.
Results
Under the scenarios with the highest greenhouse gas emissions, we found that projection studies estimate an increase in future fire danger and burnt areas varying, on average, from 2 to 4% and from 5 to 50% per decade, respectively. Further comparisons on the magnitude of increase remained challenging because of heterogeneous methodological choices between projection studies. We then described three main sources of uncertainty that may affect the reliability of wildfire projections: climate projections, climate-fire models and the influences of fuels, fire-vegetation feedbacks and human-related factors on the climate-fire relationships.
Conclusion
We suggest research directions to address some of these issues for the purpose of refining fire danger and fire activity projections in southern Europe.
Key message
An efficient use of thermal treatment of wood requires a depth understanding of the chemical modifications induced. This is a prerequisite to avoid problems of process control, and to ...provide high quality treated wood with accurately assessed properties to the market. Properties and structural anatomy of thermally modified woods are slightly different than un-processed woods from a same wood species. So it is necessary to create or adapt new analytical methods to control their quality.
Context
Heat treatment as a wood modification process is based on chemical degradation of wood polymer by heat transfer. It improves mainly the resistance of wood to decay and provides dimensional stability. These improvements, which come at the expense of a weakening of mechanical properties, have been extensively studied. Since a decade, researches focused mainly on the understanding of wood thermal degradation, on modelling, on quality prediction and quality control.
Aims
We aimed at reviewing the recent advances about (i) the analytical methods used to control thermal treatment; (ii) the effects on wood decay resistance and (iii) the advantages and drawbacks of a potential industrial use of wood heating.
Methods
We carried out a literature review of the main industrial methods used to evaluate the conferred wood properties, by thermal treatment. We used papers and reports published between 1970 and 2015, identified in the web of science data base..
Results
Approximately 100 papers mostly published after 2000 were retrieved. They concentrated on: (i) wood mass loss due to thermal degradation determination, (ii) spectroscopic analyses of wood properties, (iii) colour measurements, (iv) chemical composition, (v) non-destructive mechanical assessments and (vi) use of industrial data.
Conclusions
One of most interesting property of heat-treated wood remains its decay resistance. Durability test with modified wood in laboratory are expensive and time-consuming. This review displays data from different analytical methods, such as spectroscopy, thermogravimetry, chemical analyses or mechanical tests that have the potential to be valuable indicators to assess the durability of heat treated wood at industrial scale. However, each method has its limits and drawbacks, such as the required investment for the equipment, reliability and accuracy of the results and ease of use at industrial scale.
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