Publications by Boris Rewald
Annals of Botany, 2024
Background and Aims Mixed forest plantations are increasingly recognised for their role in mitiga... more Background and Aims Mixed forest plantations are increasingly recognised for their role in mitigating the impacts of climate change and enhancing ecosystem resilience. Yet, there remains a significant gap in understanding the early-stage dynamics of species trait diversity and interspecies interactions, particularly in pure deciduous mixtures. This study aims to explore the timing and mechanisms by which trait diversity of deciduous species and competitive interactions influence yield, carbon allocation, and space occupation in mixed forests, both above- and belowground. Methods A forest inventory was conducted in planted monocultures, 2-species, and 4-species mixtures of European Acer, Tilia, Carpinus, and Quercus, representing a spectrum from acquisitive to conservative tree species. Competition effects were assessed with linear mixed-effects models at the level of biomass and space acquisition, including leaf, canopy, stem, and fine root traits. Key Results Early aboveground growth effects were observed six years post-planting, with significant biomass accumulation after eight years, strongly influenced by species composition. Mixtures, especially with acquisitive species, exhibited aboveground overyielding, 1.5- to 1.9-times higher than monocultures. Fine roots showed substantial overyielding in high diversity stands. Biomass allocation was species-specific and varied markedly by tree size, the level of diversity, and between acquisitive Acer and the more conservative species. No root segregation was found. Conclusions Our findings underscore the critical role of species trait diversity in enhancing productivity in mixed deciduous forest plantations. Allometric changes highlight the need to differentiate between (active) acclimations and (passive) tree size-related changes, but illustrate major consequences of competitive interactions for the functional relation between leaves, stem, and roots. This study points towards the significant contributions of both above- and belowground components to overall productivity of planted mixed-species forests.
Irrigation management and biochar × mineral fertilization for improving rice yield and profitability
Agronomy Journal , 2025
Inefficient fertilizer management and increasing water scarcity are affecting rice yields and thu... more Inefficient fertilizer management and increasing water scarcity are affecting rice yields and thus food security in Central African lowlands. The objective of the study was to determine the effect of biochar and mineral fertilizer application and irrigation regime on paddy rice growth, yield and profitability. A randomized complete block design with a split‐split plot arrangement and three replications was used at three field sites and for two consecutive seasons. The main plots consisted of two irrigation regimes, permanent flooding and alternate wetting and drying. Rice (Oryza sativa L.) cultivars TAI and AR2017105 were assigned to subplots. Six fertilization treatments were applied in sub‐subplots: control, rice husk biochar or manure‐charged biochar, each with and without mineral fertilizer. Alternate wetting and drying did not penalize yield. The combined use of biochar and mineral fertilizer increased paddy rice yield compared to fertilizer alone (+16%) and the control (+85%). The sole application of biochar, with or without manure, did increase yield in the first (+14%) but not in the second season compared to the unfertilized control. In both cropping seasons, the combined use of biochar and mineral fertilizer significantly increased the gross margin of paddy rice farming compared to the sole application of fertilizer. The combined application of biochar and mineral fertilizer under alternate wetting and drying can thus be recommended as a key climate‐smart agricultural practice to increase food security and the agronomic profitability of rice cropping in Central African lowlands and comparable rice growing areas worldwide.
Agrosystems, Geosciences & Environment, 2025
Timely crop monitoring and yield prediction are essential in guiding management decision making. ... more Timely crop monitoring and yield prediction are essential in guiding management decision making. The aim of the study was to estimate the agronomic traits of paddy rice (Oryza sativa L.) using unmanned aerial vehicle (UAV)‐multispectral imaging. A randomized complete block design field experiment with a split–split plot arrangement was set up in the Ruzizi plain, Democratic Republic of Congo (DRC). Spectral imaging data were collected at rice tillering and panicle initiation stages. Predictive analysis of rice agronomic traits was performed using linear and decision tree‐based machine learning techniques. Paddy rice trait predictions were critically sensitive to the timing of image acquisition but not largely affected by the model. The most accurate predictions were made at rice panicle initiation stage, with R² values of 0.62, 0.65, and 0.75 for yield, aboveground biomass, and plant nitrogen (N) uptake, respectively. The visible atmospherically resistant index (VARI), modified chlorophyll absorption in reflective index, and ratio vegetation index, along with near infrared and green bands, played a critical role in predicting paddy rice N uptake and yield. The same spectral features associated with crop height and canopy data were essential for predicting paddy rice aboveground biomass. UAV‐multispectral data were able to assess agricultural intensification strategies at field/landscape scale irrespective of soil types, watering regimes, and cultivars. Special consideration should be attributed to VARI, as it enables economical prediction of paddy rice traits. The UAV technologies are therefore reliable tools for monitoring rice production and can be applied in agricultural extension in the DRC.
Plant and Soil , 2025
Background and aims Hyperspectral imaging is becoming a key, high-throughput technique in plant r... more Background and aims Hyperspectral imaging is becoming a key, high-throughput technique in plant research. However, its application to roots has not yet received sufficient attention. The aims of this study are to identify spectral features that distinguish fine roots from soil, non-woody roots of different species, and dead from living roots, and to identify appropriate analytical techniques. Methods Roots of Alopecurus pratensis (meadow foxtail) and Urtica dioica (nettle) and the rhizosphere were imaged in rhizoboxes in the wavelength range 400–1700 nm, covering both visible near- (VISNIR) and shortwave infrared (SWIR) regions. Principal Component Analysis, K-means clustering, and Generalised Linear Model, Partial Least Squares Discriminant Analysis, and Distributed Random Forest models were used to classify groups. Wavebands critical for classification were identified. Results Our results demonstrate the intricate nature of spectra clustering, highlighting the challenges in the VISNIR range and the promise of SWIR data for enhanced separability. While species differentiation is challenging, the determination of the living conditions of the roots is possible within the SWIR range. The analysis reveals the significance of specific spectral regions, notably those associated with water content and senescence, in distinguishing between living and dead roots. Water content regions (mainly 1245 nm and 1450 nm) were most important in discriminating between roots and soil. Conclusions This study highlights the potential of spectral analysis, particularly in the SWIR region, for distinguishing roots by species and vitality. Further efforts are needed to develop robust methods for mixed data sets containing roots of different species and degrees of vitality.
Forest Temperature Buffering in Pure and Mixed Stands: A High-Resolution Temporal Analysis with Generalized Additive Models
Forest Ecology and Management , 2025
Forests foster buffered microclimates, but causal mechanisms have rarely been studied on longer t... more Forests foster buffered microclimates, but causal mechanisms have rarely been studied on longer timescales and in differently diverse stands. Here, we explore temperature regulation by a young experimental forest in Austria, focusing on four common colline broadleaf species (Acer platanoides L., Tilia cordata Mill., Quercus robur L., Carpinus betulus L.) in monocultures, 2- and 4-species mixed stands. Air temperature was monitored in 28 forest plots for two years and compared to open-field controls. Using generalized additive models (GAMs), we investigated direct temperature offsets and lags between open field and sub-canopy temperatures, considering diurnal and seasonal changes, and causal factors such as global mean radiation, relative air humidity, wind, and leaf area index (LAI). Forests generally had a cooling effect during the summer and a warming effect in winter, where the cooling magnitude varied with species composition and environmental conditions. Specifically, Acer platanoides and Carpinus betulus demonstrated the highest cooling capacities, and Quercus robur the lowest. Mixed species stands exhibited higher temperature buffering effects relative to monospecific stands, suggesting that species diversity in forests can increase the ability to regulate microclimates. Solar radiation, relative air humidity, wind speed, and LAI all significantly influenced offsets. These findings are crucial for urban forestry and environmental planning, suggesting that careful selection of tree species can optimize temperature regulation, thereby improving human thermal comfort and ecosystem processes alike.
To log or not to log: Salvaging bark-beetle affected spruce stands results in direct losses of leaf litter C, stable topsoil C stocks, and shifts in enzyme stoichiometry
Forest Ecology and Management , 2025
Disturbances by bark beetles and subsequent salvage logging affect forest carbon (C) stocks. Alth... more Disturbances by bark beetles and subsequent salvage logging affect forest carbon (C) stocks. Although deadwood retention is recognised to maintain soil organic C (SOC), the effects of standing deadwood vs. salvage logging on soil properties are poorly understood. This study examines C stocks and soil biochemistry at 21 low-elevation Norway spruce forests 1–3 years after disturbance in the Czech Republic. One third of the stands served as undisturbed control, one third remained dead standing, and one third was salvage logged. The litter C stocks of salvaged plots were immediately reduced to 2.8±0.8 Mg C ha-1, whereas the litter and topsoil C stocks of dead standing plots decrease from 26.8 and 43.8 Mg C ha-1 to 4.4 and 24.9 Mg C ha-1, respectively, over the first three years. Consequently, the topsoil C stocks were ~24% lower at the dead than those of the salvaged plots. NH4+-N contents increased two- to three-fold following dieback but decreased to the level of control within the third year. Extracellular enzyme stoichiometry indicated lower organic topsoil C and P microbial limitations at salvaged plots. Our results highlight the rapid dynamics of SOC pools following spruce forest dieback and a marked redistribution of SOC towards topsoil layers when salvaged. Thus, the incorporation of harvest residues and/or changes in soil microbial processes prevented a significant decrease in SOC stocks due to logging. Although deadwood retained higher ecosystem C stocks at the unlogged plots, detailed data on SOC dynamics are required for managing forests to maximise C stocks.
Drought and salt are key abiotic stressors in temperate cities. Biogenic volatile organic compoun... more Drought and salt are key abiotic stressors in temperate cities. Biogenic volatile organic compound (BVOC) emissions, often dominated by trees, influence the urban troposphere and mediate plant-plant and plant-insect interactions. While knowledge on constitutive BVOC emissions is increasing, modulation of blends by stress has yet received little attention. We thus investigated BVOC blends and herbivore-related subsets ('bouquets') of Quercus robur, Fagus sylvatica, Betula pendula and Carpinus betulus seedlings under control, and after 2-weeks of drought and salt stress using PTR-Tof-MS. 22 BVOCs were related to metabolic pathways, and changes among blends and bouquets were assessed. Drought led to a slight increase of isoprene and monoterpene emissions from Q. robur and F. sylvatica, respectively, while total non-isoprene emissions generally declined under water stress. Both drought and salinity led to distinct, species-specific changes in the emission rates of single BVOCs, and resulted in markedly different herbivore-related bouquets in F. sylvatica and C. betulus. Oxygenated VOCs and green leaf volatiles in particular increased in these two salt-sensitive species, while pathway-specific effects were less clear under drought. Findings indicate significant consequences of common urban stressors on BVOC emission spectra, including tropospheric ozone formation and severely hampered plant communication cues under stress.
Urban Forestry & Urban Greening , 2023
Drought and salt are key abiotic stressors in temperate cities. Biogenic volatile organic compoun... more Drought and salt are key abiotic stressors in temperate cities. Biogenic volatile organic compound (BVOC) emissions, often dominated by trees, influence the urban troposphere and mediate plant-plant and plant-insect interactions. While knowledge on constitutive BVOC emissions is increasing, modulation of blends by stress has yet received little attention. We thus investigated BVOC blends and herbivore-related subsets ('bouquets') of Quercus robur, Fagus sylvatica, Betula pendula and Carpinus betulus seedlings under control, and after 2-weeks of drought and salt stress using PTR-Tof-MS. 22 BVOCs were related to metabolic pathways, and changes among blends and bouquets were assessed. Drought led to a slight increase of isoprene and monoterpene emissions from Q. robur and F. sylvatica, respectively, while total non-isoprene emissions generally declined under water stress. Both drought and salinity led to distinct, species-specific changes in the emission rates of single BVOCs, and resulted in markedly different herbivore-related bouquets in F. sylvatica and C. betulus. Oxygenated VOCs and green leaf volatiles in particular increased in these two salt-sensitive species, while pathway-specific effects were less clear under drought. Findings indicate significant consequences of common urban stressors on BVOC emission spectra, including tropospheric ozone formation and severely hampered plant communication cues under stress.
Environmental Pollution, 2023
Particulate matter (PM) pollution poses a significant threat to human health. Greenery, particula... more Particulate matter (PM) pollution poses a significant threat to human health. Greenery, particularly trees, can act as effective filters for PM, reducing associated health risks. Previous studies have indicated that tree traits play a crucial role in determining the amount of PM accumulated on leaves, although findings have often been sitespecific. To comprehensively investigate the key factors influencing PM binding to leaves across diverse tree species and geographical locations, we conducted an extensive analysis using data extracted from 57 publications. The data covers 11 countries and 190 tree species from 1996 to 2021. We categorized tree species into functional groups: evergreen conifers, deciduous conifers, deciduous broadleaves, and evergreen broadleaves based on leaf habit and phylogeny. Evergreen conifers exhibited the highest PM accumulation on leaves, and in general, evergreen leaves accumulated more PM compared to deciduous leaves across all PM size classes. Specific leaf traits, such as epicuticular wax, played a significant role. The highest PM loads on leaves were observed in peri-urban areas along the rural-peri-urban-urban gradient. However, the availability of global data was skewed, with most data originating from urban and peri-urban areas, primarily from China and Poland. Among different climate zones, substantial data were only available for warm temperate and cold steppe climate zones. Understanding the problem of PM pollution and the role of greenery in urban environments is crucial for monitoring and controlling PM pollution. Our systematic review of the literature highlights the variation on PM loading among different vegetation types with varying leaf characteristics. Notably, epicuticular wax emerged as a marker trait that exhibited variability across PM size fractions and different vegetation types. In conclusion, this review emphasizes the importance of greenery in mitigation PM pollution. Our findings underscore the significance of tree traits in PM binding. However, lack of data stresses the need for further research and data collection initiatives.
Plant Methods, 2023
Background Manual analysis of (mini-)rhizotron (MR) images is tedious. Several methods have been ... more Background Manual analysis of (mini-)rhizotron (MR) images is tedious. Several methods have been proposed for semantic root segmentation based on homogeneous, single-source MR datasets. Recent advances in deep learning (DL) have enabled automated feature extraction, but comparisons of segmentation accuracy, false positives and transferability are virtually lacking. Here we compare six state-of-the-art methods and propose two improved DL models for semantic root segmentation using a large MR dataset with and without augmented data. We determine the performance of the methods on a homogeneous maize dataset, and a mixed dataset of > 8 species (mixtures), 6 soil types and 4 imaging systems. The generalisation potential of the derived DL models is determined on a distinct, unseen dataset. Results The best performance was achieved by the U-Net models; the more complex the encoder the better the accuracy and generalisation of the model. The heterogeneous mixed MR dataset was a particularly challenging for the non-U-Net techniques. Data augmentation enhanced model performance. We demonstrated the improved performance of deep meta-architectures and feature extractors, and a reduction in the number of false positives. Conclusions Although correction factors are still required to match human labelled root lengths, neural network architectures greatly reduce the time required to compute the root length. The more complex architectures illustrate how future improvements in root segmentation within MR images can be achieved, particularly reaching higher segmentation accuracies and model generalisation when analysing real-world datasets with artefacts-limiting the need for model retraining.
Forest Ecology and Management, 2023
Adapted management methodologies are required to alleviate increasing and/or more frequent water ... more Adapted management methodologies are required to alleviate increasing and/or more frequent water stress under climate change, enabling successful tree seedling establishment after planting. Hydrogel amendments have been suggested to increase soil water holding capacity, tree seedling growth and survival. However, the effect of a non-uniform addition of hydrogels to tree planting pits as compared to homogenously admixed hydrogels in agricultural soils, as well as effects of hydrogels on woody root system establishment, have been poorly studied. Here we studied the effect of two commercial hydrogels (Polyter and Stockosorb) on the above-and belowground growth of Picea abies seedlings during a ~ 5-weeks-long drought period. Rhizoboxes with a non-uniform distribution of nutrients and hydrogels were used to monitor root system development in a stratified soil. Biomass allocation, root morphology and root elongation rates were related to evapotranspiration rates. Proximity effects of hydrogels on root traits were determined. Total plant biomass was significantly greater if plants were grown in Polyter-amended soil (+16, 17 %) compared to the Stockosorb and control treatments, respectively. Dead needle mass was significantly greater in Stockosorb than in Polyter (+719 %) and control (+274 %). Root elongation was faster in both hydrogel treatments compared to the control during the first part of the drought period, but subsequently ceased to the order Polyter > control > Stockosorb. Root morphological changes in relation to hydrogels were limited. The better performance of Polyter-amended seedlings was related to a slow water release from the hydrogel, while Stockosorb released the additional soil water content prematurely. The latter might have imposed a "false sense of security", and the subsequent lack of acclimation responses early in the drought resulted in an even more severe drought stress and a higher needle loss of Stockosorb-amended seedlings as compared to control. However, both hydrogels did not result in preferential growth towards or away from the hydrogel, suggesting that roots did neither sense the hydrogel-bound water nor that secondary compounds hampered root growth. A deeper root system placement and thus potential advantages for drought events at later growth stages were not encountered, asPolyter also had a strong positive effect on root growth in the fertile top soil. As hydrogel amendments may influence root system establishment and seedling vitality differently during drought, further studies are urgently needed before large scale applications to mitigate climate change effects on forest regeneration / restoration can take place.
Frontiers in Plant Science, 2023
Vessel traits are key in understanding trees’ hydraulic efficiency, and related characteristics l... more Vessel traits are key in understanding trees’ hydraulic efficiency, and related characteristics like growth performance and drought tolerance. While most plant hydraulic studies have focused on aboveground organs, our understanding of root hydraulic functioning and trait coordination across organs remains limited. Furthermore, studies from seasonally dry (sub-)tropical ecosystems and mountain forests are virtually lacking and uncertainties remain regarding potentially different hydraulic strategies of plants differing in leaf habit. Here, we compared wood anatomical traits and specific hydraulic conductivities between coarse roots and small branches of five drought-deciduous and eight evergreen angiosperm tree species in a seasonally dry subtropical Afromontane forest in Ethiopia. We hypothesized that largest vessels and highest hydraulic conductivities are found in roots, with greater vessel tapering between roots and equally-sized branches in evergreen angiosperms due to their drought-tolerating strategy. We further hypothesized that the hydraulic efficiencies of root and branches cannot be predicted from wood density, but that wood densities across organs are generally related. Root-to-branch ratios of conduit diameters varied between 0.8 and 2.8, indicating considerable differences in tapering from coarse roots to small branches. While deciduous trees showed larger branch xylem vessels compared to evergreen angiosperms, root-to-branch ratios were highly variable within both leaf habit types, and evergreen species did not show a more pronounced degree of tapering. Empirically determined hydraulic conductivity and corresponding root-to-branch ratios were similar between both leaf habit types. Wood density of angiosperm roots was negatively related to hydraulic efficiency and vessel dimensions; weaker relationships were found in branches. Wood density of small branches was neither related to stem nor coarse root wood densities. We conclude that in seasonally dry subtropical forests, similar-sized coarse roots hold larger xylem vessels than small branches, but the degree of tapering from roots to branches is highly variable. Our results indicate that leaf habit does not necessarily influence the relationship between coarse root and branch hydraulic traits. However, larger conduits in branches and a low carbon investment in less dense wood may be a prerequisite for high growth rates of drought-deciduous trees during their shortened growing season. The correlation of stem and root wood densities with root hydraulic traits but not branch wood points toward large trade-offs in branch xylem towards mechanical properties.
Forests, 2022
Abstract: In the Ethiopian highlands, clearance of Afromontane dry forest and conversion to crop ... more Abstract: In the Ethiopian highlands, clearance of Afromontane dry forest and conversion to crop and grazing land lead to land degradation and loss of soil organic matter (SOM). Eucalyptus is often grown on degraded soils, and this results in the partial recovery of soil carbon stocks. The aim of this work was to assess the biological sources of SOM in this land-use sequence. In top-soils (0–10 cm) of four land-use systems, namely remnant natural forest, eucalyptus plantation, cropland, and grazing land, in the Ethiopian highlands, the origin of SOM was investigated. For this, a sequential extraction method was used, involving a solvent extraction, base hydrolysis, and a subsequent CuO oxidation. In these extracts, biomarkers (molecular proxies) were identified to characterize the SOM of the soil Citation: Assefa, D.; Mentler, A.; Sandén, H.; Rewald, B.; Godbold, D.L. The Biological Origins of Soil Organic Matter in Different Land-Uses in the Highlands of Ethiopia. Forests 2022, 13, 560. https://doi.org/10.3390/f13040560 Academic Editor: Choonsig Kim Received: 23 February 2022 Accepted: 27 March 2022 Published: 31 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). of the four land-uses. Putative lipid monomers of leaf, root, and microbial degradation products suggest that root inputs and microbial inputs dominate in SOM of all the land-uses, except grazing land. The ratios of syringyls, vanillyls, and cinnamyls showed that non-woody angiosperm plants were the predominant source for lignin in eucalyptus, cropland, and grazing land soil. In the soils of the natural forest, lignin originates from both woody angiosperms and woody gymnosperms. Our study shows the importance of root and microbial inputs in the formation of SOM, but also that, in the natural forest, legacies of previous forest cover are p
Frontiers in Plant Science, 2021
Current conservation strategies are targeted at preserving species, without explicitly aiming at ... more Current conservation strategies are targeted at preserving species, without explicitly aiming at the maintenance of ecosystem functions. In a physically highly connected world, the unintentional relocation of terrestrial, marine, and microbial life is therefore unavoidable and has been an integral part of human evolution for thousands of years. Here, we challenge the default perception often shared among conservation ecologists that preserving native species at all costs and reducing the number of exotic species and their abundance is the only way to conservation and restoration success. While this strategy is valuable in cases where exotic species disrupt ecological function, there are examples where exotic species have similar functional traits to the threatened or extinct native species and can in fact help maintain the overall or target function of an ecosystem. In the race to cope with global environmental change, we argue that ecosystem function and ecosystem services need to be viewed not only through a taxonomic lens, but increasingly also through a functional, trait-based one.
Frontiers in Plant Science , 2022
Vegetable grafting is increasingly recognized as an effective and sustainable plant production al... more Vegetable grafting is increasingly recognized as an effective and sustainable plant production alternative. Grafted plants usually show increased uptake of water and minerals compared with self-rooted plants, mostly thought a consequence of the vigorous rootstocks selected. However, while studies frequently addressed the effects of rootstocks on the performance of scions, knowledge on the influences of scions on biomass allocation, morphology, and metabolic activity of roots is rare. In particular, the plasticity of root traits affecting resource acquisition and its efficiency remains poorly understood. Two different rootstock species, Cucurbita maxima × Cucurbita moschata and Lagenaria siceraria, were grafted in combination with melon (Cucumis melo) and watermelon (Citrullus lanatus). Self-grafted rootstocks were used as control. Plant biomass and root traits were determined after destructive harvesting 30 and/or 60 days after grafting. Traits included biomass allocation, leaf and root morphology, potential activities of four extracellular enzymes on root tips and basal root segments, and root respiration. Successfully grafted scions increase the ratio of root to whole plant dry matter (RMF), and increased ratios of root length to whole plant dry matter (RLR) and to plant leaf area (RL : LA). In contrast, morphological root traits such as diameter, tissue density, and specific root length remain surprisingly stable, and thus scion-induced changes of those traits may only play a minor role for the beneficial effects of grafting in Cucurbitaceae. Incompatibility in melon/L. siceraria grafts, however, was likely responsible for the reduced root growth in combination with clear changes in root morphological traits. Reduced root respiration rates seem to be the effects of a non-compatible rootstock-scion combination rather than an active, C-efficiency increasing acclimation. In contrast, heterografts with melon and watermelon frequently resulted in root-stock-specific, often enhanced potential enzymatic activities of acid phosphatase, b-glucosidase, leucine-amino-peptidase, and N-acetylglucosaminidase both at root tips and basal parts of lateral roots-presenting a potential and complementary mechanism of grafted plants to enhance nutrient foraging. The studied melon and watermelon scions may thus increase the nutrient foraging capacity of grafted plants by fostering the Frontiers in Plant Science frontiersin.org 01
Deforestation and land use strongly effect soil organic carbon and nitrogen stock in Northwest Ethiopia
Soil is the largest terrestrial organic carbon pool and can act as a source or sink for atmospher... more Soil is the largest terrestrial organic carbon pool and can act as a source or sink for atmospheric CO2. Although reliable soil carbon (SOC) stock measurements of major ecosystems are essential for predicting the influence of advancing climate change, comprehensive data on SOC stocks is still scarce for most ecosystems in subtropical areas. In this study, SOC and N stocks of different land use systems were investigated along a climatic gradient in Northwest Ethiopia. The land use systems ranged from dry subtropical Afromontane forest, as the baseline, to cropland as the most degraded system. In addition, we investigated the changes of SOC stocks after interventions to recover vegetation cover; these were eucalyptus plantations and an exclosure to prevent grazing. Total SOC varied between land use systems and ranged from 3.1 kg C m(-2) in croplands to 23.9 kg C m(-2) in natural forest, and average N stock ranged from 0.4 kg N m(-2) in croplands to 2.1 kg N m(-2) in natural forest. In forests, there were a clear vertical gradient in SOC and N stock down the soil profile, and 60% of the total SOC and N stocks were found in the upper 10 cm soil depth. Using the Sr/Ca and Ba/Ca ratios and the vertical distribution of the C/N ratio of the soil, the losses of SOC were shown to be due to loss of the of the upper soil layer. Afforestation of degraded croplands and grazing lands with eucalyptus increased SOC stocks to nearly 70% of the natural forest levels within 30 years. Exclosure, which removed grazing pressure and allowed regeneration of native vegetation, increased SOC in the top soil only.
Fine Root Morphology, Biochemistry and Litter Quality Indices of Fast- and Slow-growing Woody Species in Ethiopian Highland Forest
Ecosystems, 2017
Fine root turnover of trees is a major C input to soil. However, the quality of litter input is i... more Fine root turnover of trees is a major C input to soil. However, the quality of litter input is influenced by root morphological traits and tissue chemical composition. In this study, fine roots of ten tropical woody species were collected from an Afromontane forest in the northern highlands of Ethiopia. The fine roots were analysed for root morphological traits and tissue chemistry measured as proxy carbon fractionations. Based on stem increment the 10 species were divided into faster and slower growing species. Faster-growing species exhibited higher specific root length (1362 cm g-1) than slower-growing species (923 cm g-1). Similarly specific root area was higher in faster-growing species (223 cm2 g-1) than in slower-growing species (167 cm2 g-1). Among the carbon fractions, the acid-insoluble fraction (AIF) was the highest (44-51%). The carbon content, AIF, and the lignocellulose index were higher for slower-growing species. Root tissue density was lower in faster-growing species (0.33 g cm-3) than slower-growing species (0.40 g cm-3 26 ) and showed a strong positive correlation with carbon content (r2 =0.84;) and the AIF (r pearson=0.93). The morphological parameters of fine roots between faster- and slower-growing species reflect the ecological strategy they employ. Slower-growing species have a higher tissue density which may reflect a greater longevity.
Plant Cell & Environment, 2017
Faba bean (Vicia faba L.) is an important source of protein but breeding for increased yield stab... more Faba bean (Vicia faba L.) is an important source of protein but breeding for increased yield stability and stress tolerance is hampered by the scarcity of phenotyping information. Because comparisons of cultivars adapted to different agro-climatic zones improve our understanding of stress tolerance mechanisms, the root architecture and morphology of 16 pan-European faba bean cultivars were studied at maturity. Different machine learning (ML) approaches were tested in their usefulness to analyse trait variations between cultivars. A supervised, i.e. hypothesis-driven, ML approach revealed that cultivars from Portugal feature greater and coarser but less frequent lateral roots at the top of the taproot, potentially enhancing water uptake from deeper soil horizons. Unsupervised clustering revealed that trait differences between Northern and Southern cultivars are not predominant but that two cultivar groups, independently from major and minor types, differ largely in overall root system size. Methodological guidelines on how to use powerful machine learning methods such as random forest models for enhancing the phenotypical exploration of plants are given.
Plasticity of biomass allometry and root traits of two tomato cultivars under deficit irrigation × chemically induced drought hardening by Paclobutrazol
Irrigation Science, 2017
Tomato yield is seriously affected by water stress. Paclobutrazol, a fungicide and plant growth r... more Tomato yield is seriously affected by water stress. Paclobutrazol, a fungicide and plant growth retardant, has previously shown the potential to improve drought tolerance of crops. However, knowledge on the impact of Paclobutrazol on root system traits is scarce. Seeds of two tomato cultivars were primed with three different rates of Paclobutrazol. After an establishment phase of 60 days, greenhouse-grown plants were partially subjected to deficit irrigation (60% ET). Subsequently, biomass and surface area allometry between plant organs and within the root system were determined. The morphology and architecture of the root system was studied in detail. Changes in root system traits under deficit irrigation and after Paclobutrazol treatments were largely based on basal roots’ plasticity. The proportion of basal roots significantly increased with increasing Paclobutrazol concentration; deficit irrigation resulted in both cultivars in increased branching of basal roots. Cultivars differed in their plastic response of root system biomass, enhancing tap root and lateral root growth in one cultivar but basal root proportions in the other. Paclobutrazol-priming of seeds facilitated basal root versus tap root system growth in tomato. It is hypothesized that basal roots, positioned in the topsoil, are most advantageous to maximize water uptake under (deficit) irrigation.
Frontiers in Plant Science, 2013
The drivers underlying the development of deep root systems, whether genetic or environmental, ar... more The drivers underlying the development of deep root systems, whether genetic or environmental, are poorly understood but evidence has accumulated that deep rooting could be a more widespread and important trait among plants than commonly anticipated from their share of root biomass. Even though a distinct classification of "deep roots" is missing to date, deep roots provide important functions for individual plants such as nutrient and water uptake but can also shape plant communities by hydraulic lift (HL). Subterranean fauna and microbial communities are highly influenced by resources provided in the deep rhizosphere and deep roots can influence soil pedogenesis and carbon storage.Despite recent technological advances, the study of deep roots and their rhizosphere remains inherently time-consuming, technically demanding and costly, which explains why deep roots have yet to be given the attention they deserve. While state-of-the-art technologies are promising for laboratory studies involving relatively small soil volumes, they remain of limited use for the in situ observation of deep roots. Thus, basic techniques such as destructive sampling or observations at transparent interfaces with the soil (e.g., root windows) which have been known and used for decades to observe roots near the soil surface, must be adapted to the specific requirements of deep root observation. In this review, we successively address major physical, biogeochemical and ecological functions of deep roots to emphasize the significance of deep roots and to illustrate the yet limited knowledge. In the second part we describe the main methodological options to observe and measure deep roots, providing researchers interested in the field of deep root/rhizosphere studies with a comprehensive overview. Addressed methodologies are: excavations, trenches and soil coring approaches, minirhizotrons (MR), access shafts, caves and mines, and indirect approaches such as tracer-based techniques.
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Publications by Boris Rewald