Publications by authors named "Hans Beeckman"

38 Publications

Genome skimming reveals novel plastid markers for the molecular identification of illegally logged African timber species.

PLoS One 2021 11;16(6):e0251655. Epub 2021 Jun 11.

Meise Botanic Garden, Meise, Belgium.

Tropical forests represent vast carbon stocks and continue to be key carbon sinks and buffer climate changes. The international policy constructed several mechanisms aiming at conservation and sustainable use of these forests. Illegal logging is an important threat of forests, especially in the tropics. Several laws and regulations have been set up to combat illegal timber trade. Despite significant enforcement efforts of these regulations, illegal logging continues to be a serious problem and impacts for the functioning of the forest ecosystem and global biodiversity in the tropics. Microscopic analysis of wood samples and the use of conventional plant DNA barcodes often do not allow to distinguish closely-related species. The use of novel molecular technologies could make an important contribution for the identification of tree species. In this study, we used high-throughput sequencing technologies and bioinformatics tools to obtain the complete de-novo chloroplast genome of 62 commercial African timber species using the genome skimming method. Then, we performed a comparative genomic analysis that revealed new candidate genetic regions for the discrimination of closely-related species. We concluded that genome skimming is a promising method for the development of plant genetic markers to combat illegal logging activities supporting CITES, FLEGT and the EU Timber Regulation.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0251655PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8195358PMC
June 2021

Resistance of African tropical forests to an extreme climate anomaly.

Proc Natl Acad Sci U S A 2021 May;118(21)

Forestry Research Institute of Ghana (FORIG), Kumasi, Ghana.

The responses of tropical forests to environmental change are critical uncertainties in predicting the future impacts of climate change. The positive phase of the 2015-2016 El Niño Southern Oscillation resulted in unprecedented heat and low precipitation in the tropics with substantial impacts on the global carbon cycle. The role of African tropical forests is uncertain as their responses to short-term drought and temperature anomalies have yet to be determined using on-the-ground measurements. African tropical forests may be particularly sensitive because they exist in relatively dry conditions compared with Amazonian or Asian forests, or they may be more resistant because of an abundance of drought-adapted species. Here, we report responses of structurally intact old-growth lowland tropical forests inventoried within the African Tropical Rainforest Observatory Network (AfriTRON). We use 100 long-term inventory plots from six countries each measured at least twice prior to and once following the 2015-2016 El Niño event. These plots experienced the highest temperatures and driest conditions on record. The record temperature did not significantly reduce carbon gains from tree growth or significantly increase carbon losses from tree mortality, but the record drought did significantly decrease net carbon uptake. Overall, the long-term biomass increase of these forests was reduced due to the El Niño event, but these plots remained a live biomass carbon sink (0.51 ± 0.40 Mg C ha y) despite extreme environmental conditions. Our analyses, while limited to African tropical forests, suggest they may be more resistant to climatic extremes than Amazonian and Asian forests.
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http://dx.doi.org/10.1073/pnas.2003169118DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8166131PMC
May 2021

When xylarium and herbarium meet: linking Tervuren xylarium wood samples with their herbarium specimens at Meise Botanic Garden.

Biodivers Data J 2021 31;9:e62329. Epub 2021 Mar 31.

Meise Botanic Garden, Meise, Belgium Meise Botanic Garden Meise Belgium.

Background: The current data paper aims to interlink the African plant collections of the Meise Botanic Garden Herbarium (BR) and the Royal Museum for Central Africa Xylarium (Tw). Complementing both collections strengthens the reference value of each institutional collection, as more complete metadata are made available and it enables increased quality control for the identification of wood specimens. Furthermore, the renewed connection enables the linking of available wood trait data with data on phenology, leaf morphology or even molecular information for many tree species, allowing assessments of performance of individual trees. In addition to studies at the interspecific level, comparisons at the intraspecific level become possible, which could lead to important new insights into resilience to and impact of global change, as well as biodiversity conservation or forest management of Central African forest ecosystems.

New Information: By interlinking the Tervuren Xylarium Wood database with the recently digitised herbarium of Meise Botanic Garden, we were able to establish a link between 6,621 xylarium and 9,641 herbarium records for 6,953 plant specimens. Both institutional databases were complemented with reliable specimen metadata. The Tervuren xylarium now profits from taxonomic revisions made by botanists at Meise Botanic Garden and a list of phenotypical features for woody African species can be extended with wood anatomical descriptors. New metadata from the Tw xylarium records were used to add the country of collection to 50 linked BR herbarium specimens for which this information was missing. Furthermore, metadata available from the labels on digitised BR herbarium specimens was used to update Tw xylarium records with the date of collection (817 records), collection locality (698 records), coordinates (482 records) and altitude (817 records). In conclusion, we created a reference database with reliable botanic identities which can be used in a range of studies, such as modelling analyses, community assessments or trait analyses, all framed in a spatiotemporal context. Furthermore, the linked collections hold historical reference data and specimens that can be studied in the context of global changes.
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http://dx.doi.org/10.3897/BDJ.9.e62329DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8026533PMC
March 2021

Long-term thermal sensitivity of Earth's tropical forests.

Science 2020 05 21;368(6493):869-874. Epub 2020 May 21.

Wageningen Environmental Research, Wageningen, Netherlands.

The sensitivity of tropical forest carbon to climate is a key uncertainty in predicting global climate change. Although short-term drying and warming are known to affect forests, it is unknown if such effects translate into long-term responses. Here, we analyze 590 permanent plots measured across the tropics to derive the equilibrium climate controls on forest carbon. Maximum temperature is the most important predictor of aboveground biomass (-9.1 megagrams of carbon per hectare per degree Celsius), primarily by reducing woody productivity, and has a greater impact per °C in the hottest forests (>32.2°C). Our results nevertheless reveal greater thermal resilience than observations of short-term variation imply. To realize the long-term climate adaptation potential of tropical forests requires both protecting them and stabilizing Earth's climate.
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http://dx.doi.org/10.1126/science.aaw7578DOI Listing
May 2020

Century-long apparent decrease in intrinsic water-use efficiency with no evidence of progressive nutrient limitation in African tropical forests.

Glob Chang Biol 2020 08 29;26(8):4449-4461. Epub 2020 May 29.

Isotope Bioscience Laboratory - ISOFYS, Department of Green Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Gent, Belgium.

Forests exhibit leaf- and ecosystem-level responses to environmental changes. Specifically, rising carbon dioxide (CO ) levels over the past century are expected to have increased the intrinsic water-use efficiency (iWUE) of tropical trees while the ecosystem is gradually pushed into progressive nutrient limitation. Due to the long-term character of these changes, however, observational datasets to validate both paradigms are limited in space and time. In this study, we used a unique herbarium record to go back nearly a century and show that despite the rise in CO concentrations, iWUE has decreased in central African tropical trees in the Congo Basin. Although we find evidence that points to leaf-level adaptation to increasing CO -that is, increasing photosynthesis-related nutrients and decreasing maximum stomatal conductance, a decrease in leaf δ C clearly indicates a decreasing iWUE over time. Additionally, the stoichiometric carbon to nitrogen and nitrogen to phosphorus ratios in the leaves show no sign of progressive nutrient limitation as they have remained constant since 1938, which suggests that nutrients have not increasingly limited productivity in this biome. Altogether, the data suggest that other environmental factors, such as increasing temperature, might have negatively affected net photosynthesis and consequently downregulated the iWUE. Results from this study reveal that the second largest tropical forest on Earth has responded differently to recent environmental changes than expected, highlighting the need for further on-ground monitoring in the Congo Basin.
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http://dx.doi.org/10.1111/gcb.15145DOI Listing
August 2020

Asynchronous carbon sink saturation in African and Amazonian tropical forests.

Nature 2020 03 4;579(7797):80-87. Epub 2020 Mar 4.

Center for International Forestry Research (CIFOR), Bogor, Indonesia.

Structurally intact tropical forests sequestered about half of the global terrestrial carbon uptake over the 1990s and early 2000s, removing about 15 per cent of anthropogenic carbon dioxide emissions. Climate-driven vegetation models typically predict that this tropical forest 'carbon sink' will continue for decades. Here we assess trends in the carbon sink using 244 structurally intact African tropical forests spanning 11 countries, compare them with 321 published plots from Amazonia and investigate the underlying drivers of the trends. The carbon sink in live aboveground biomass in intact African tropical forests has been stable for the three decades to 2015, at 0.66 tonnes of carbon per hectare per year (95 per cent confidence interval 0.53-0.79), in contrast to the long-term decline in Amazonian forests. Therefore the carbon sink responses of Earth's two largest expanses of tropical forest have diverged. The difference is largely driven by carbon losses from tree mortality, with no detectable multi-decadal trend in Africa and a long-term increase in Amazonia. Both continents show increasing tree growth, consistent with the expected net effect of rising atmospheric carbon dioxide and air temperature. Despite the past stability of the African carbon sink, our most intensively monitored plots suggest a post-2010 increase in carbon losses, delayed compared to Amazonia, indicating asynchronous carbon sink saturation on the two continents. A statistical model including carbon dioxide, temperature, drought and forest dynamics accounts for the observed trends and indicates a long-term future decline in the African sink, whereas the Amazonian sink continues to weaken rapidly. Overall, the uptake of carbon into Earth's intact tropical forests peaked in the 1990s. Given that the global terrestrial carbon sink is increasing in size, independent observations indicating greater recent carbon uptake into the Northern Hemisphere landmass reinforce our conclusion that the intact tropical forest carbon sink has already peaked. This saturation and ongoing decline of the tropical forest carbon sink has consequences for policies intended to stabilize Earth's climate.
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http://dx.doi.org/10.1038/s41586-020-2035-0DOI Listing
March 2020

A large-scale species level dated angiosperm phylogeny for evolutionary and ecological analyses.

Biodivers Data J 2020 21;8:e39677. Epub 2020 Jan 21.

Universite Libre de Bruxelles, Brussels, Belgium Universite Libre de Bruxelles Brussels Belgium.

Phylogenies are a central and indispensable tool for evolutionary and ecological research. Even though most angiosperm families are well investigated from a phylogenetic point of view, there are far less possibilities to carry out large-scale meta-analyses at order level or higher. Here, we reconstructed a large-scale dated phylogeny including nearly 1/8th of all angiosperm species, based on two plastid barcoding genes, (incl. ) and Novel sequences were generated for several species, while the rest of the data were mined from GenBank. The resulting tree was dated using 56 angiosperm fossils as calibration points. The resulting megaphylogeny is one of the largest dated phylogenetic tree of angiosperms yet, consisting of 36,101 sampled species, representing 8,399 genera, 426 families and all orders. This novel framework will be useful for investigating different broad scale research questions in ecological and evolutionary biology.
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http://dx.doi.org/10.3897/BDJ.8.e39677DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6987248PMC
January 2020

TRY plant trait database - enhanced coverage and open access.

Glob Chang Biol 2020 01 31;26(1):119-188. Epub 2019 Dec 31.

Arizona State University, Tempe, AZ, USA.

Plant traits-the morphological, anatomical, physiological, biochemical and phenological characteristics of plants-determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait-based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits-almost complete coverage for 'plant growth form'. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait-environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives.
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http://dx.doi.org/10.1111/gcb.14904DOI Listing
January 2020

Advanced X-ray CT scanning can boost tree ring research for earth system sciences.

Ann Bot 2019 11;124(5):837-847

UGent-Woodlab, Laboratory of Wood Technology, Department of Environment, Ghent University, Gent, Belgium.

Background And Aims: Tree rings, as archives of the past and biosensors of the present, offer unique opportunities to study influences of the fluctuating environment over decades to centuries. As such, tree-ring-based wood traits are capital input for global vegetation models. To contribute to earth system sciences, however, sufficient spatial coverage is required of detailed individual-based measurements, necessitating large amounts of data. X-ray computed tomography (CT) scanning is one of the few techniques that can deliver such data sets.

Methods: Increment cores of four different temperate tree species were scanned with a state-of-the-art X-ray CT system at resolutions ranging from 60 μm down to 4.5 μm, with an additional scan at a resolution of 0.8 μm of a splinter-sized sample using a second X-ray CT system to highlight the potential of cell-level scanning. Calibration-free densitometry, based on full scanner simulation of a third X-ray CT system, is illustrated on increment cores of a tropical tree species.

Key Results: We show how multiscale scanning offers unprecedented potential for mapping tree rings and wood traits without sample manipulation and with limited operator intervention. Custom-designed sample holders enable simultaneous scanning of multiple increment cores at resolutions sufficient for tree ring analysis and densitometry as well as single core scanning enabling quantitative wood anatomy, thereby approaching the conventional thin section approach. Standardized X-ray CT volumes are, furthermore, ideal input imagery for automated pipelines with neural-based learning for tree ring detection and measurements of wood traits.

Conclusions: Advanced X-ray CT scanning for high-throughput processing of increment cores is within reach, generating pith-to-bark ring width series, density profiles and wood trait data. This would allow contribution to large-scale monitoring and modelling efforts with sufficient global coverage.
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http://dx.doi.org/10.1093/aob/mcz126DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6868372PMC
November 2019

Asynchronous leaf and cambial phenology in a tree species of the Congo Basin requires space-time conversion of wood traits.

Ann Bot 2019 09;124(2):245-253

UGCT-UGent-Woodlab, Ghent University, Laboratory of Wood Technology, Department of Environment, Gent, Belgium.

Background And Aims: Wood traits are increasingly being used to document tree performance. In the Congo Basin, however, weaker seasonality causes asynchrony of wood traits between trees. Here, we monitor growth and phenology data to date the formation of traits.

Methods: For two seasons, leaf and cambial phenology were monitored on four Terminalia superba trees (Mayombe) using cameras, cambial pinning and dendrometers. Subsequently, vessel lumen and parenchyma fractions as well as high-resolution isotopes (δ13C/δ18O) were quantified on the formed rings. All traits were dated and related to weather data.

Key Results: We observed between-tree differences in green-up of 45 d, with trees flushing before and after the rainy season. The lag between green-up and onset of xylem formation was 59 ± 21 d. The xylem growing season lasted 159 ± 17 d with between-tree differences of up to 53 d. Synchronized vessel, parenchyma and δ13C profiles were related to each other. Only parenchyma fraction and δ13C were correlated to weather variables, whereas the δ18O pattern showed no trend.

Conclusions: Asynchrony of leaf and cambial phenology complicates correct interpretation of environmental information recorded in wood. An integrated approach including high-resolution measurements of growth, stable isotopes and anatomical features allows exact dating of the formation of traits. This methodology offers a means to explore the asynchrony of growth in a rainforest and contribute to understanding this aspect of forest resilience.
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http://dx.doi.org/10.1093/aob/mcz069DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6758582PMC
September 2019

Large-sized rare tree species contribute disproportionately to functional diversity in resource acquisition in African tropical forest.

Ecol Evol 2019 Apr 2;9(8):4349-4361. Epub 2019 Apr 2.

Department of Green Chemistry and Technology Ghent University Gent Belgium.

Increasing evidence is available for a positive effect of biodiversity on ecosystem productivity and standing biomass, also in highly diverse systems as tropical forests. Biodiversity conservation could therefore be a critical aspect of climate mitigation policies. There is, however, limited understanding of the role of individual species for this relationship, which could aid in focusing conservation efforts and forest management planning. This study characterizes the functional specialization and redundancy for 95% of all tree species (basal area weighted percentage) in a diverse tropical forest in the central Congo Basin and relates this to species' abundance, contribution to aboveground carbon, and maximum size. Functional characterization is based on a set of traits related to resource acquisition (wood density, specific leaf area, leaf carbon, nitrogen and phosphorus content, and leaf stable carbon isotope composition). We show that within both mixed and monodominant tropical forest ecosystems, the highest functional specialization and lowest functional redundancy are solely found in rare tree species and significantly more in rare species holding large-sized individuals. Rare species cover the entire range of low and high functional redundancy, contributing both unique and redundant functions. Loss of species supporting functional redundancy could be buffered by other species in the community, including more abundant species. This is not the case for species supporting high functional specialization and low functional redundancy, which would need specific conservation attention. In terms of tropical forest management planning, we argue that specific conservation of large-sized trees is imperative for long-term maintenance of ecosystem functioning.
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http://dx.doi.org/10.1002/ece3.4836DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6476792PMC
April 2019

The persistence of carbon in the African forest understory.

Nat Plants 2019 02 21;5(2):133-140. Epub 2019 Jan 21.

Plant Systematic and Ecology Laboratory, Higher Teachers' Training College, University of Yaounde, Yaounde, Cameroon.

Quantifying carbon dynamics in forests is critical for understanding their role in long-term climate regulation. Yet little is known about tree longevity in tropical forests, a factor that is vital for estimating carbon persistence. Here we calculate mean carbon age (the period that carbon is fixed in trees) in different strata of African tropical forests using (1) growth-ring records with a unique timestamp accurately demarcating 66 years of growth in one site and (2) measurements of diameter increments from the African Tropical Rainforest Observation Network (23 sites). We find that in spite of their much smaller size, in understory trees mean carbon age (74 years) is greater than in sub-canopy (54 years) and canopy (57 years) trees and similar to carbon age in emergent trees (66 years). The remarkable carbon longevity in the understory results from slow and aperiodic growth as an adaptation to limited resource availability. Our analysis also reveals that while the understory represents a small share (11%) of the carbon stock, it contributes disproportionally to the forest carbon sink (20%). We conclude that accounting for the diversity of carbon age and carbon sequestration among different forest strata is critical for effective conservation management and for accurate modelling of carbon cycling.
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http://dx.doi.org/10.1038/s41477-018-0316-5DOI Listing
February 2019

Reconciling biodiversity and carbon stock conservation in an Afrotropical forest landscape.

Sci Adv 2018 03 28;4(3):eaar6603. Epub 2018 Mar 28.

Evolutionary Ecology Group, University of Antwerp, 2020 Antwerp, Belgium.

Protecting aboveground carbon stocks in tropical forests is essential for mitigating global climate change and is assumed to simultaneously conserve biodiversity. Although the relationship between tree diversity and carbon stocks is generally positive, the relationship remains unclear for consumers or decomposers. We assessed this relationship for multiple trophic levels across the tree of life (10 organismal groups, 3 kingdoms) in lowland rainforests of the Congo Basin. Comparisons across regrowth and old-growth forests evinced the expected positive relationship for trees, but not for other organismal groups. Moreover, differences in species composition between forests increased with difference in carbon stock. These variable associations across the tree of life contradict the implicit assumption that maximum co-benefits to biodiversity are associated with conservation of forests with the highest carbon storage. Initiatives targeting climate change mitigation and biodiversity conservation should include both old-growth and regenerating forests to optimally benefit biodiversity and carbon storage.
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http://dx.doi.org/10.1126/sciadv.aar6603DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5903881PMC
March 2018

Present-day central African forest is a legacy of the 19th century human history.

Elife 2017 01 17;6. Epub 2017 Jan 17.

TERRA Research Centre, Central African Forests, University of Liège - Gembloux Agro-Bio Tech, Gembloux, Belgium.

The populations of light-demanding trees that dominate the canopy of central African forests are now aging. Here, we show that the lack of regeneration of these populations began ca. 165 ya (around 1850) after major anthropogenic disturbances ceased. Since 1885, less itinerancy and disturbance in the forest has occurred because the colonial administrations concentrated people and villages along the primary communication axes. Local populations formerly gardened the forest by creating scattered openings, which were sufficiently large for the establishment of light-demanding trees. Currently, common logging operations do not create suitable openings for the regeneration of these species, whereas deforestation degrades landscapes. Using an interdisciplinary approach, which included paleoecological, archaeological, historical, and dendrological data, we highlight the long-term history of human activities across central African forests and assess the contribution of these activities to present-day forest structure and composition. The conclusions of this sobering analysis present challenges to current silvicultural practices and to those of the future.
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http://dx.doi.org/10.7554/eLife.20343DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5241113PMC
January 2017

Functional community structure of African monodominant forest influenced by local environmental filtering.

Ecol Evol 2017 01 20;7(1):295-304. Epub 2016 Dec 20.

Department of Applied Analytical and Physical Chemistry Isotope Bioscience Laboratory - ISOFYS Ghent University Gent Belgium; Instituto Multidisciplinario de Biología Vegetal Universidad Nacional de Córdoba & CONICET Cordoba Argentina.

Monodominant patches of forest dominated by are commonly found in central African tropical forests, alongside forests with high species diversity. Although these forests are generally found sparsely distributed along rivers, their occurrence is not thought to be (clearly) driven by edaphic conditions but rather by trait combinations of that aid in achieving monodominance. Functional community structure between these monodominant and mixed forests has, however, not yet been compared. Additionally, little is known about nondominant species in the monodominant forest community. These two topics are addressed in this study. We investigate the functional community structure of 10 one-hectare plots of monodominant and mixed forests in a central region of the Congo basin, in DR Congo. Thirteen leaf and wood traits are measured, covering 95% (basal area weighted) of all species present in the plots, including leaf nutrient contents, leaf isotopic compositions, specific leaf area, wood density, and vessel anatomy. The trait-based assessment of shows an ensemble of traits related to water use and transport that could be favorable for its location near forest rivers. Moreover, indications have been found for N and P limitations in the monodominant forest, possibly related to ectomycorrhizal associations formed with . Reduced leaf N and P contents are found at the community level for the monodominant forest and for different nondominant groups, as compared to those in the mixed forest. In summary, this work shows that environmental filtering does prevail in the monodominant forest, leading to lower functional diversity in this forest type, with the dominant species showing beneficial traits related to its common riverine locations and with reduced soil N and P availability found in this environment, both coregulating the tree community assembly.
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http://dx.doi.org/10.1002/ece3.2589DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5216677PMC
January 2017

Capacitive water release and internal leaf water relocation delay drought-induced cavitation in African Maesopsis eminii.

Tree Physiol 2017 04;37(4):481-490

Laboratory of Plant Ecology, Department of Applied Ecology and Environmental Biology, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, B-9000Ghent, Belgium.

The impact of drought on the hydraulic functioning of important African tree species, like Maesopsis eminii Engl., is poorly understood. To map the hydraulic response to drought-induced cavitation, sole reliance on the water potential at which 50% loss of xylem hydraulic conductivity (ψ50) occurs might be limiting and at times misleading as the value alone does not give a comprehensive overview of strategies evoked by M. eminii to cope with drought. This article therefore uses a methodological framework to study the different aspects of drought-induced cavitation and water relations in M. eminii. Hydraulic functioning of whole-branch segments was investigated during bench-top dehydration. Cumulative acoustic emissions and continuous weight measurements were used to quantify M. eminii's vulnerability to drought-induced cavitation and hydraulic capacitance. Wood structural traits, including wood density, vessel area, diameter and wall thickness, vessel grouping index, solitary vessel index and vessel wall reinforcement, were used to underpin observed physiological responses. On average, M. eminii's ψ50 (±SE) was -1.9 ± 0.1 MPa, portraying its xylem as drought vulnerable, just as one would expect for a common tropical pioneer. However, M. eminii additionally employed an interesting desiccation delay strategy, fuelled by internal relocation of leaf water, hydraulic capacitance and the presence of parenchyma around the xylem vessels. Our findings suggest that exclusive dependence on ψ50 would have misdirected our assessments of M. eminii's drought stress vulnerability. Hydraulic capacitance linked to anatomy and leaf-water relocation behaviour was equally important to better understand M. eminii's drought survival strategies. Because our study was conducted on branches of 3-year-old greenhouse-grown M. eminii seedlings, the findings cannot be simply extrapolated to adult M. eminii trees or their mature wood, because structural and physiological plant properties change with age. The techniques and methodological framework used in this study are, however, transferable to other species regardless of age.
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http://dx.doi.org/10.1093/treephys/tpw128DOI Listing
April 2017

A field-to-desktop toolchain for X-ray CT densitometry enables tree ring analysis.

Ann Bot 2016 06 23;117(7):1187-96. Epub 2016 Apr 23.

UGCT-Woodlab-UGent, Ghent University, Laboratory of Wood Technology, Department of Forest and Water Management, Coupure Links 653, B- 9000 Gent, Belgium.

Background And Aims: Disentangling tree growth requires more than ring width data only. Densitometry is considered a valuable proxy, yet laborious wood sample preparation and lack of dedicated software limit the widespread use of density profiling for tree ring analysis. An X-ray computed tomography-based toolchain of tree increment cores is presented, which results in profile data sets suitable for visual exploration as well as density-based pattern matching.

Methods: Two temperate (Quercus petraea, Fagus sylvatica) and one tropical species (Terminalia superba) were used for density profiling using an X-ray computed tomography facility with custom-made sample holders and dedicated processing software.

Key Results: Density-based pattern matching is developed and able to detect anomalies in ring series that can be corrected via interactive software.

Conclusions: A digital workflow allows generation of structure-corrected profiles of large sets of cores in a short time span that provide sufficient intra-annual density information for tree ring analysis. Furthermore, visual exploration of such data sets is of high value. The dated profiles can be used for high-resolution chronologies and also offer opportunities for fast screening of lesser studied tropical tree species.
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http://dx.doi.org/10.1093/aob/mcw063DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4904179PMC
June 2016

Climatic Signals in Tree Rings of Heritiera fomes Buch.-Ham. in the Sundarbans, Bangladesh.

PLoS One 2016 29;11(2):e0149788. Epub 2016 Feb 29.

Wood Biology Service, Royal Museum for Central Africa (RMCA), Leuvensesteenweg 13, 3080, Tervuren, Belgium.

Mangroves occur along the coastlines throughout the tropics and sub-tropics, supporting a wide variety of resources and services. In order to understand the responses of future climate change on this ecosystem, we need to know how mangrove species have responded to climate changes in the recent past. This study aims at exploring the climatic influences on the radial growth of Heritiera fomes from a local to global scale. A total of 40 stem discs were collected at breast height position from two different zones with contrasting salinity in the Sundarbans, Bangladesh. All specimens showed distinct tree rings and most of the trees (70%) could be visually and statistically crossdated. Successful crossdating enabled the development of two zone-specific chronologies. The mean radial increment was significantly higher at low salinity (eastern) zone compared to higher salinity (western) zone. The two zone-specific chronologies synchronized significantly, allowing for the construction of a regional chronology. The annual and monsoon precipitation mainly influence the tree growth of H. fomes. The growth response to local precipitation is similar in both zones except June and November in the western zone, while the significant influence is lacking. The large-scale climatic drivers such as sea surface temperature (SST) of equatorial Pacific and Indian Ocean as well as the El Niño-Southern Oscillation (ENSO) revealed no teleconnection with tree growth. The tree rings of this species are thus an indicator for monsoon precipitation variations in Bangladesh. The wider distribution of this species from the South to South East Asian coast presents an outstanding opportunity for developing a large-scale tree-ring network of mangroves.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0149788PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4771160PMC
July 2016

Wood Specific Gravity Variations and Biomass of Central African Tree Species: The Simple Choice of the Outer Wood.

PLoS One 2015 10;10(11):e0142146. Epub 2015 Nov 10.

Landscape Ecology and Plant Production Systems Unit, Université libre de Bruxelles, CP264-2, B-1050 Bruxelles, Belgium.

Context: Wood specific gravity is a key element in tropical forest ecology. It integrates many aspects of tree mechanical properties and functioning and is an important predictor of tree biomass. Wood specific gravity varies widely among and within species and also within individual trees. Notably, contrasted patterns of radial variation of wood specific gravity have been demonstrated and related to regeneration guilds (light demanding vs. shade-bearing). However, although being repeatedly invoked as a potential source of error when estimating the biomass of trees, both intraspecific and radial variations remain little studied. In this study we characterized detailed pith-to-bark wood specific gravity profiles among contrasted species prominently contributing to the biomass of the forest, i.e., the dominant species, and we quantified the consequences of such variations on the biomass.

Methods: Radial profiles of wood density at 8% moisture content were compiled for 14 dominant species in the Democratic Republic of Congo, adapting a unique 3D X-ray scanning technique at very high spatial resolution on core samples. Mean wood density estimates were validated by water displacement measurements. Wood density profiles were converted to wood specific gravity and linear mixed models were used to decompose the radial variance. Potential errors in biomass estimation were assessed by comparing the biomass estimated from the wood specific gravity measured from pith-to-bark profiles, from global repositories, and from partial information (outer wood or inner wood).

Results: Wood specific gravity profiles from pith-to-bark presented positive, neutral and negative trends. Positive trends mainly characterized light-demanding species, increasing up to 1.8 g.cm-3 per meter for Piptadeniastrum africanum, and negative trends characterized shade-bearing species, decreasing up to 1 g.cm-3 per meter for Strombosia pustulata. The linear mixed model showed the greater part of wood specific gravity variance was explained by species only (45%) followed by a redundant part between species and regeneration guilds (36%). Despite substantial variation in wood specific gravity profiles among species and regeneration guilds, we found that values from the outer wood were strongly correlated to values from the whole profile, without any significant bias. In addition, we found that wood specific gravity from the DRYAD global repository may strongly differ depending on the species (up to 40% for Dialium pachyphyllum).

Main Conclusion: Therefore, when estimating forest biomass in specific sites, we recommend the systematic collection of outer wood samples on dominant species. This should prevent the main errors in biomass estimations resulting from wood specific gravity and allow for the collection of new information to explore the intraspecific variation of mechanical properties of trees.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0142146PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4640573PMC
June 2016

Charcoal-inferred Holocene fire and vegetation history linked to drought periods in the Democratic Republic of Congo.

Glob Chang Biol 2015 Jun 6;21(6):2296-308. Epub 2015 Feb 6.

Department of Forest and Water Management, Laboratory of Wood Technology, Ghent University, Coupure Links 653, B-9000, Gent, Belgium; Royal Museum for Central Africa, Laboratory for Wood Biology, Leuvensesteenweg 13, B-3080, Tervuren, Belgium.

The impact of Holocene drought events on the presumably stable Central African rainforest remains largely unexplored, in particular the significance of fire. High-quality sedimentary archives are scarce, and palynological records mostly integrate over large regional scales subject to different fire regimes. Here, we demonstrate a direct temporal link between Holocene droughts, palaeofire and vegetation change within present-day Central African rainforest, using records of identified charcoal fragments extracted from soil in the southern Mayumbe forest (Democratic Republic of Congo). We find three distinct periods of local palaeofire occurrence: 7.8-6.8 ka BP, 2.3-1.5 ka BP, 0.8 ka BP - present. These periods are linked to well-known Holocene drought anomalies: the 8.2 ka BP event, the 3rd millennium BP rainforest crisis and the Mediaeval Climate Anomaly. During and after these Holocene droughts, the Central African rainforest landscape was characterized by a fragmented pattern with fire-prone open patches. Some fires occurred during the drought anomalies although most fires seem to lag behind them, which suggests that the open patches remained fire-prone after the actual climate anomalies. Charcoal identifications indicate that mature rainforest patches did persist through the Early to Mid-Holocene climatic transition, the subsequent Holocene thermal optimum and the third millennium BP rainforest crisis, until 0.8 ka BP. However, disturbance and fragmentation were probably more prominent near the boundary of the southern Mayumbe forest. Furthermore, the dominance of pioneer and woodland savanna taxa in younger charcoal assemblages indicates that rainforest regeneration was hampered by increasingly severe drought conditions after 0.8 ka BP. These results support the notion of a dynamic forest ecosystem at multicentury time scales across the Central African rainforest.
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http://dx.doi.org/10.1111/gcb.12844DOI Listing
June 2015

Effects of experimental sedimentation on the phenological dynamics and leaf traits of replanted mangroves at Gazi bay, Kenya.

Ecol Evol 2014 Aug 22;4(16):3187-200. Epub 2014 Jul 22.

Laboratory of Plant Biology and Nature Management (APNA), Vrije Universiteit Brussel B-1050, Brussels, Belgium.

Sedimentation results in the creation of new mudflats for mangroves to colonize among other benefits. However, large sediment input in mangrove areas may be detrimental to these forests. The dynamics of phenological events of three mangrove tree species (Avicennia marina, Ceriops tagal, and Rhizophora mucronata) were evaluated under experimental sediment burial simulating sedimentation levels of 15, 30, and 45 cm.While there was generally no shift in timing of phenological events with sedimentation, the three mangrove tree species each responded differently to the treatments.Partially buried A. marina trees produced more leaves than the controls during the wet season and less during the dry season. Ceriops tagal on the other hand had higher leaf loss and low replacement rates in the partially buried trees during the first 6 months of the experiment but adapted with time, resulting in either equal or higher leaf emergence rates than the controls.Rhizophora mucronata maintained leaf emergence and loss patterns as the unaffected controls but had a higher fecundity and productivity in the 15-cm sedimentation level.The results suggest that under incidences of large sedimentation events (which could be witnessed as a result of climate change impacts coupled with anthropogenic disturbances), mangrove trees may capitalize on "advantages" associated with terrestrial sediment brought into the biotope, thus maintaining the pattern of phenological events.
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http://dx.doi.org/10.1002/ece3.1154DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4222206PMC
August 2014

How to catch the patch? A dendrometer study of the radial increment through successive cambia in the mangrove Avicennia.

Ann Bot 2014 Mar 6;113(4):741-52. Epub 2014 Feb 6.

Laboratory of Plant Biology and Nature Management (APNA), Vrije Universiteit Brussel, B-1050 Brussels, Belgium.

Background And Aims: Successive vascular cambia are involved in the secondary growth of at least 200 woody species from >30 plant families. In the mangrove Avicennia these successive cambia are organized in patches, creating stems with non-concentric xylem tissue surrounded by internal phloem tissue. Little is known about radial growth and tree stem dynamics in trees with this type of anatomy. This study aims to (1) clarify the process of secondary growth of Avicennia trees by studying its patchiness; and (2) study the radial increment of Avicennia stems, both temporary and permanent, in relation to local climatic and environmental conditions. A test is made of the hypothesis that patchy radial growth and stem dynamics enable Avicennia trees to better survive conditions of extreme physiological drought. Methods Stem variations were monitored by automatic point dendrometers at four different positions around and along the stem of two Avicennia marina trees in the mangrove forest of Gazi Bay (Kenya) during 1 year.

Key Results: Patchiness was found in the radial growth and shrinkage and swelling patterns of Avicennia stems. It was, however, potentially rather than systematically present, i.e. stems reacted either concentrically or patchily to environment triggers, and it was fresh water availability and not tidal inundation that affected radial increment.

Conclusions: It is concluded that the ability to develop successive cambia in a patchy way enables Avicennia trees to adapt to changes in the prevailing environmental conditions, enhancing its survival in the highly dynamic mangrove environment. Limited water could be used in a more directive way, investing all the attainable resources in only some locations of the tree stem so that at least at these locations there is enough water to, for example, overcome vessel embolisms or create new cells. As these locations change with time, the overall functioning of the tree can be maintained.
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http://dx.doi.org/10.1093/aob/mcu001DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3936594PMC
March 2014

Conventional tree height-diameter relationships significantly overestimate aboveground carbon stocks in the Central Congo Basin.

Nat Commun 2013 ;4:2269

Laboratory of Plant Ecology, Department of Applied Ecology and Environmental Biology, Faculty of Bioscience Engineering, Ghent University, Ghent 9000, Belgium.

Policies to reduce emissions from deforestation and forest degradation largely depend on accurate estimates of tropical forest carbon stocks. Here we present the first field-based carbon stock data for the Central Congo Basin in Yangambi, Democratic Republic of Congo. We find an average aboveground carbon stock of 162 ± 20  Mg  C  ha(-1) for intact old-growth forest, which is significantly lower than stocks recorded in the outer regions of the Congo Basin. The best available tree height-diameter relationships derived for Central Africa do not render accurate canopy height estimates for our study area. Aboveground carbon stocks would be overestimated by 24% if these inaccurate relationships were used. The studied forests have a lower stature compared with forests in the outer regions of the basin, which confirms remotely sensed patterns. Additionally, we find an average soil carbon stock of 111 ± 24  Mg  C  ha(-1), slightly influenced by the current land-use change.
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http://dx.doi.org/10.1038/ncomms3269DOI Listing
February 2014

Above-ground biomass and structure of 260 African tropical forests.

Philos Trans R Soc Lond B Biol Sci 2013 22;368(1625):20120295. Epub 2013 Jul 22.

Department of Geography, University College London, UK.

We report above-ground biomass (AGB), basal area, stem density and wood mass density estimates from 260 sample plots (mean size: 1.2 ha) in intact closed-canopy tropical forests across 12 African countries. Mean AGB is 395.7 Mg dry mass ha⁻¹ (95% CI: 14.3), substantially higher than Amazonian values, with the Congo Basin and contiguous forest region attaining AGB values (429 Mg ha⁻¹) similar to those of Bornean forests, and significantly greater than East or West African forests. AGB therefore appears generally higher in palaeo- compared with neotropical forests. However, mean stem density is low (426 ± 11 stems ha⁻¹ greater than or equal to 100 mm diameter) compared with both Amazonian and Bornean forests (cf. approx. 600) and is the signature structural feature of African tropical forests. While spatial autocorrelation complicates analyses, AGB shows a positive relationship with rainfall in the driest nine months of the year, and an opposite association with the wettest three months of the year; a negative relationship with temperature; positive relationship with clay-rich soils; and negative relationships with C : N ratio (suggesting a positive soil phosphorus-AGB relationship), and soil fertility computed as the sum of base cations. The results indicate that AGB is mediated by both climate and soils, and suggest that the AGB of African closed-canopy tropical forests may be particularly sensitive to future precipitation and temperature changes.
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http://dx.doi.org/10.1098/rstb.2012.0295DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3720018PMC
March 2014

Olive tree-ring problematic dating: a comparative analysis on Santorini (Greece).

PLoS One 2013 28;8(1):e54730. Epub 2013 Jan 28.

WSL Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland.

Olive trees are a classic component of Mediterranean environments and some of them are known historically to be very old. In order to evaluate the possibility to use olive tree-rings for dendrochronology, we examined by various methods the reliability of olive tree-rings identification. Dendrochronological analyses of olive trees growing on the Aegean island Santorini (Greece) show that the determination of the number of tree-rings is impossible because of intra-annual wood density fluctuations, variability in tree-ring boundary structure, and restriction of its cambial activity to shifting sectors of the circumference, causing the tree-ring sequences along radii of the same cross section to differ.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0054730PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3557290PMC
August 2013

Cambial growth season of brevi-deciduous Brachystegia spiciformis trees from south central Africa restricted to less than four months.

PLoS One 2012 10;7(10):e47364. Epub 2012 Oct 10.

Laboratory for Tree-Ring Research, University of Arizona, Tucson, AZ, USA.

We investigate cambial growth periodicity in Brachystegia spiciformis, a dominant tree species in the seasonally dry miombo woodland of southern Africa. To better understand how the brevi-deciduous (experiencing a short, drought-induced leaf fall period) leaf phenology of this species can be linked to a distinct period of cambial activity, we applied a bi-weekly pinning to six trees in western Zambia over the course of one year. Our results show that the onset and end of cambial growth was synchronous between trees, but was not concurrent with the onset and end of the rainy season. The relatively short (three to four months maximum) cambial growth season corresponded to the core of the rainy season, when 75% of the annual precipitation fell, and to the period when the trees were at full photosynthetic capacity. Tree-ring studies of this species have found a significant relationship between annual tree growth and precipitation, but we did not observe such a correlation at intra-annual resolution in this study. Furthermore, a substantial rainfall event occurring after the end of the cambial growth season did not induce xylem initiation or false ring formation. Low sample replication should be taken into account when interpreting the results of this study, but our findings can be used to refine the carbon allocation component of process-based terrestrial ecosystem models and can thus contribute to a more detailed estimation of the role of the miombo woodland in the terrestrial carbon cycle. Furthermore, we provide a physiological foundation for the use of Brachystegia spiciformis tree-ring records in paleoclimate research.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0047364PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3468463PMC
February 2013

Fluctuations of cambial activity in relation to precipitation result in annual rings and intra-annual growth zones of xylem and phloem in teak (Tectona grandis) in Ivory Coast.

Ann Bot 2012 Sep 17;110(4):861-73. Epub 2012 Jul 17.

UGCT-Laboratory of Wood Technology, Department of Forest and Water Management, Ghent University, Ghent, Belgium.

Background And Aims: Teak forms xylem rings that potentially carry records of carbon sequestration and climate in the tropics. These records are only useful when the structural variations of tree rings and their periodicity of formation are known.

Methods: The seasonality of ring formation in mature teak trees was examined via correlative analysis of cambial activity, xylem and phloem formation, and climate throughout 1·5 years. Xylem and phloem differentiation were visualized by light microscopy and scanning electron microscopy.

Key Results: A 3 month dry season resulted in semi-deciduousness, cambial dormancy and formation of annual xylem growth rings (AXGRs). Intra-annual xylem and phloem growth was characterized by variable intensity. Morphometric features of cambium such as cambium thickness and differentiating xylem layers were positively correlated. Cambium thickness was strongly correlated with monthly rainfall (R(2) = 0·7535). In all sampled trees, xylem growth zones (XGZs) were formed within the AXGRs during the seasonal development of new foliage. When trees achieved full leaf, the xylem in the new XGZs appeared completely differentiated and functional for water transport. Two phloem growth rings were formed in one growing season.

Conclusions: The seasonal formation pattern and microstructure of teak xylem suggest that AXGRs and XGZs can be used as proxies for analyses of the tree history and climate at annual and intra-annual resolution.
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http://dx.doi.org/10.1093/aob/mcs145DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3423803PMC
September 2012

Successive cambia: a developmental oddity or an adaptive structure?

PLoS One 2011 Jan 31;6(1):e16558. Epub 2011 Jan 31.

Laboratory for Plant Biology and Nature Management, APNA, Vrije Universiteit Brussel, VUB, Brussels, Belgium.

Background: Secondary growth by successive cambia is a rare phenomenon in woody plant species. Only few plant species, within different phylogenetic clades, have secondary growth by more than one vascular cambium. Often, these successive cambia are organised concentrically. In the mangrove genus Avicennia however, the successive cambia seem to have a more complex organisation. This study aimed (i) at understanding the development of successive cambia by giving a three-dimensional description of the hydraulic architecture of Avicennia and (ii) at unveiling the possible adaptive nature of growth by successive cambia through a study of the ecological distribution of plant species with concentric internal phloem.

Results: Avicennia had a complex network of non-cylindrical wood patches, the complexity of which increased with more stressful ecological conditions. As internal phloem has been suggested to play a role in water storage and embolism repair, the spatial organisation of Avicennia wood could provide advantages in the ecologically stressful conditions species of this mangrove genus are growing in. Furthermore, we could observe that 84.9% of the woody shrub and tree species with concentric internal phloem occurred in either dry or saline environments strengthening the hypothesis that successive cambia provide the necessary advantages for survival in harsh environmental conditions.

Conclusions: Successive cambia are an ecologically important characteristic, which seems strongly related with water-limited environments.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0016558PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3031581PMC
January 2011

High-resolution proxies for wood density variations in Terminalia superba.

Ann Bot 2011 Feb 3;107(2):293-302. Epub 2010 Dec 3.

Ghent University, Department of Forest and Water Management, Faculty of Bioscience Engineering, Coupure Links 653, 9000 Ghent, Belgium.

Background And Aims: Density is a crucial variable in forest and wood science and is evaluated by a multitude of methods. Direct gravimetric methods are mostly destructive and time-consuming. Therefore, faster and semi- to non-destructive indirect methods have been developed.

Methods: Profiles of wood density variations with a resolution of approx. 50 µm were derived from one-dimensional resistance drillings, two-dimensional neutron scans, and three-dimensional neutron and X-ray scans. All methods were applied on Terminalia superba Engl. & Diels, an African pioneer species which sometimes exhibits a brown heart (limba noir).

Key Results: The use of X-ray tomography combined with a reference material permitted direct estimates of wood density. These X-ray-derived densities overestimated gravimetrically determined densities non-significantly and showed high correlation (linear regression, R(2) = 0·995). When comparing X-ray densities with the attenuation coefficients of neutron scans and the amplitude of drilling resistance, a significant linear relation was found with the neutron attenuation coefficient (R(2) = 0·986) yet a weak relation with drilling resistance (R(2) = 0·243). When density patterns are compared, all three methods are capable of revealing the same trends. Differences are mainly due to the orientation of tree rings and the different characteristics of the indirect methods.

Conclusions: High-resolution X-ray computed tomography is a promising technique for research on wood cores and will be explored further on other temperate and tropical species. Further study on limba noir is necessary to reveal the causes of density variations and to determine how resistance drillings can be further refined.
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http://dx.doi.org/10.1093/aob/mcq224DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3025726PMC
February 2011

Tyloses and phenolic deposits in xylem vessels impede water transport in low-lignin transgenic poplars: a study by cryo-fluorescence microscopy.

Plant Physiol 2010 Oct 16;154(2):887-98. Epub 2010 Jul 16.

Laboratory for Wood Biology and Xylarium, Royal Museum for Central Africa, Tervuren, Belgium.

Of 14 transgenic poplar genotypes (Populus tremula × Populus alba) with antisense 4-coumarate:coenzyme A ligase that were grown in the field for 2 years, five that had substantial lignin reductions also had greatly reduced xylem-specific conductivity compared with that of control trees and those transgenic events with small reductions in lignin. For the two events with the lowest xylem lignin contents (greater than 40% reduction), we used light microscopy methods and acid fuchsin dye ascent studies to clarify what caused their reduced transport efficiency. A novel protocol involving dye stabilization and cryo-fluorescence microscopy enabled us to visualize the dye at the cellular level and to identify water-conducting pathways in the xylem. Cryo-fixed branch segments were planed in the frozen state on a sliding cryo-microtome and observed with an epifluorescence microscope equipped with a cryo-stage. We could then distinguish clearly between phenolic-occluded vessels, conductive (stain-filled) vessels, and nonconductive (water- or gas-filled) vessels. Low-lignin trees contained areas of nonconductive, brown xylem with patches of collapsed cells and patches of noncollapsed cells filled with phenolics. In contrast, phenolics and nonconductive vessels were rarely observed in normal colored wood of the low-lignin events. The results of cryo-fluorescence light microscopy were supported by observations with a confocal microscope after freeze drying of cryo-planed samples. Moreover, after extraction of the phenolics, confocal microscopy revealed that many of the vessels in the nonconductive xylem were blocked with tyloses. We conclude that reduced transport efficiency of the transgenic low-lignin xylem was largely caused by blockages from tyloses and phenolic deposits within vessels rather than by xylem collapse.
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http://dx.doi.org/10.1104/pp.110.156224DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2949004PMC
October 2010