Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Frankincense in peril
Bongers, Frans ; Groenendijk, Peter ; Bekele, Tesfaye ; Birhane, Emiru ; Damtew, Abebe ; Decuyper, Mathieu ; Eshete, Abeje ; Gezahgne, Alemu ; Girma, Atkilt ; Khamis, Mohamed A. ; Lemenih, Mulugeta ; Mengistu, Tefera ; Ogbazghi, Woldeselassie ; Sass-Klaassen, Ute ; Tadesse, Wubalem ; Teshome, Mindaye ; Tolera, Motuma ; Sterck, Frank J. ; Zuidema, Pieter A. - \ 2019
Nature Sustainability 2 (2019). - ISSN 2398-9629 - p. 602 - 610.
The harvest of plant parts and exudates from wild populations contributes to the income, food security and livelihoods of many millions of people worldwide. Frankincense, an aromatic resin sourced from natural populations of Boswellia trees and shrubs, has been cherished by world societies for centuries. Boswellia populations are threatened by over-exploitation and ecosystem degradation, jeopardizing future resin production. Here, we reveal evidence of population collapse of B. papyrifera—now the main source of frankincense—throughout its geographic range. Using inventories of 23 populations consisting of 21,786 trees, growth-ring data from 202 trees and demographic models on the basis of 7,246 trees, we find that over 75% of studied populations lack small trees, natural regeneration has been absent for decades, and projected frankincense production will be halved in 20 yr. These changes are caused by increased human population pressure on Boswellia woodlands through cattle grazing, frequent burns and reckless tapping. A literature review showed that other Boswellia species experience similar threats. Populations can be restored by establishing cattle exclosures and fire-breaks, and by planting trees and tapping trees more carefully. Concerted conservation and restoration efforts are urgently needed to secure the long-term availability of this iconic product.
Hyper-temporal SPOT-NDVI dataset parameterization captures species distributions
Girma, Atkilt ; Bie, C.A.J.M. de; Skidmore, Andrew K. ; Venus, Valentijn ; Bongers, Frans - \ 2016
International Journal of Geographical Information Science 30 (2016)1. - ISSN 1365-8816 - p. 89 - 107.
Boswellia papyrifera - HANTS - hyper-temporal - MAXENT - SPOT-NDVI

Hyper-temporal SPOT NDVI images contain useful information about the environment in which a species occurs, including information such as the beginning, end, peak, and curvature of photosynthetically active vegetation (PAV) greenness signatures. This raises the question: can parameterization of hyper-temporal SPOT NDVI images be useful to predict species distribution? A set of SPOT-NDVI images for the whole of Ethiopia covering nine years was classified using the unsupervised ISODATA clustering algorithm to group similar NDVI pixel values. The HANTS (Harmonic ANalysis of Time Series) algorithm, that fits series of smoothing cosine waves, was then applied to the time series for each of the NDVI classes to generate seven output Fourier components. These components, together with the topographic parameters slope and elevation, were used as predictors in a species distribution model using MAXENT. Presence-only data of one test species, Boswellia papyrifera, were modelled. This species is diminishing at an alarming rate and requires conservation. The performance of the model was evaluated by the area under curve (AUC) of the receiver-operating characteristics value. The output distribution map was tested for its agreement with the NDVI-clustering approach and conventional B. papyrifera distribution map using Kappa. The relative contributions of the first four predictors to the MAXENT in sequence were: 2nd harmonic phase, elevation, amplitude of the 1st harmonics, and amplitude of the 2nd harmonics. The average AUC test result for the 100 runs was 0.98 with a standard deviation of 0.002. The probability distribution map clearly shows high correlation with the B. papyrifera occurrence data. In addition, the distribution map was found to be in agreement with the NDVI-clustered and conventional map with improved details. Classifying hyper-temporal NDVI images and extracting their parameters through the use of the HANTS algorithm captures the PAV greenness behaviour (parameters) of the environment of the species studied. These parameters have proved successful in predicting the distribution of B. papyrifera.

Understanding Boswellia papyrifera tree secondary metabolites through bark spectral analysis
Girma, A. ; Skidmore, A.K. ; Bie, C.A.J.M. de; Bongers, F. - \ 2015
ISPRS Journal of Photogrammetry and Remote Sensing 105 (2015). - ISSN 0924-2716 - p. 30 - 37.
Decision makers are concerned whether to tap or rest Boswellia Papyrifera trees. Tapping for the production of frankincense is known to deplete carbon reserves from the tree leading to production of less viable seeds, tree carbon starvation and ultimately tree mortality. Decision makers use traditional experience without considering the amount of metabolites stored or depleted from the stem-bark of the tree. This research was designed to come up with a non-destructive B. papyrifera tree metabolite estimation technique relevant for management using spectroscopy. The concentration of biochemicals (metabolites) found in the tree bark was estimated through spectral analysis. Initially, a random sample of 33 trees was selected, the spectra of bark measured with an Analytical Spectral Device (ASD) spectrometer. Bark samples were air dried and ground. Then, 10 g of sample was soaked in Petroleum ether to extract crude metabolites. Further chemical analysis was conducted to quantify and isolate pure metabolite compounds such as incensole acetate and boswellic acid. The crude metabolites, which relate to frankincense produce, were compared to plant properties (such as diameter and crown area) and reflectance spectra of the bark. Moreover, the extract was compared to the ASD spectra using partial least square regression technique (PLSR) and continuum removed spectral analysis. The continuum removed spectral analysis were performed, on two wavelength regions (1275–1663 and 1836–2217) identified through PLSR, using absorption features such as band depth, area, position, asymmetry and the width to characterize and find relationship with the bark extracts. The results show that tree properties such as diameter at breast height (DBH) and the crown area of untapped and healthy trees were strongly correlated to the amount of stored crude metabolites. In addition, the PLSR technique applied to the first derivative transformation of the reflectance spectrum was found to estimate the concentration of the metabolites reliably at higher coefficient of determination. The most influential maximum slope positions of the spectrum obtained through PLSR analysis of the petroleum ether extract (crude metabolites) and the pure compounds (incensole acetate and boswellic acid) were found to coincide and concentrate in the region between 1383–1406 nm and 1861–1896 nm. However, analysis on these two individual specific region absorption features relationship with the bark extract, using the continuum removed approach, was not as robust as the PLSR analysis. This reveals that the ability to estimate metabolites in the stem-bark of B. papyrifera using spectral analysis opens a new approach on how to manage B. papyrifera tree. Development of such technique provides quick and reliable information for decision makers to decide on when to tap or for how long to rest the trees. However, to implement the technique, further research need to be conducted to determine how B. papyrifera tree metabolites vary due to tapping. In addition, research to determine the lowest metabolites quantity threshold, useful for management of the tree, needs to be conducted.
The impact of contracts on organic honey producers' incomes in southwestern Ethiopia
Girma, J. ; Gardebroek, C. - \ 2015
Forest Policy and Economics 50 (2015). - ISSN 1389-9341 - p. 259 - 268.
propensity score - countries - schemes - program - africa - bias
In southwestern Ethiopia honey is a non-timber forest product that provides income for many smallholders. Some of these beekeepers supply their honey under contract to a company that markets their organic honey internationally allowing them to access premium markets. Since both production and marketing depend crucially on the forest, both smallholders and the company have an interest in preserving the forest. An important question is whether smallholders also benefit economically from supplying under contract. The objective of this study is to examine the contribution of participation in contract supply of organic honey to beekeepers' income levels in the Sheka zone in southwestern Ethiopia. Results indicate that contract supply improved quality of honey delivered, the prices beekeepers received, and total honey income per household. The findings illustrate the potential of contract supply of forest product for sustainable management of forests.
Detection and attribution of observed impacts
Cramer, W. ; Yohe, G.W. ; Auffhammer, M. ; Huggel, C. ; Molau, U. ; Dias, M.A.F.S. ; Leemans, R. - \ 2014
In: Climate Climate Change 2014 : Impacts, Adaptation, and Vulnerability / Field, C.B., Barros, V.R., Dokken, D.J., Mach, K.J., Mastrandrea, M.D., Bilir, T.E., Chatterjee, M., Ebi, K.L., Estrada, Y.O., Genova, R.C., Girma, B., Kissel, E.S., Levy, A.N., MacCracken, S., Mastrandrea, P.R., White, L.L., Cambridge, United Kingdom and New York, NY, USA : Cambridge University Press - ISBN 9781107058071 - p. 979 - 1038.
This chapter synthesizes the scientific literature on the detection and attribution of observed changes in natural and human systems in response to observed recent climate change. For policy makers and the public, detection and attribution of observed impacts will be a key element to determine the necessity and degree of mitigation and adaptation efforts. For most natural and essentially all human systems, climate is only one of many drivers that cause change—other factors such as technological innovation, social and demographic changes, and environmental degradation frequently play an important role as well. Careful accounting of the importance of these and other confounding factors is therefore an important part of the analysis.

At any given location, observed recent climate change has happened as a result of a combination of natural, longer term fluctuations and anthropogenic alteration of forcings. To inform about the sensitivity of natural and human systems to ongoing climate change, the chapter assesses the degree to which detected changes in such systems can be attributed to all aspects of recent climate change. For the development of adaptation policies, it is less important whether the observed changes have been caused by anthropogenic climate change or by natural climate fluctuations. Where possible, the relative importance of anthropogenic drivers of climate change is assessed as well.
Photosynthetic bark: use of chlorophyll absorption continuum index to estimate Boswellia papyrifera bark chlorophyll content
Girma, A. ; Skidmore, A.K. ; Bie, C.A.J.M. de; Bongers, F. ; Schlerf, M. - \ 2013
International Journal of applied Earth Observation and Geoinformation 23 (2013)August. - ISSN 0303-2434 - p. 71 - 80.
least-squares regression - red edge position - hyperspectral measurements - spectral reflectance - vegetation indexes - leaves - stems - frankincense - performance - prospect
Quantification of chlorophyll content provides useful insight into the physiological performance of plants. Several leaf chlorophyll estimation techniques, using hyperspectral instruments, are available. However, to our knowledge, a non-destructive bark chlorophyll estimation technique is not available. We set out to assess Boswellia papyrifera tree bark chlorophyll content and to provide an appropriate bark chlorophyll estimation technique using hyperspectral remote sensing techniques. In contrast to the leaves, the bark of B. papyrifera has several outer layers masking the inner photosynthetic bark layer. Thus, our interest includes understanding how much light energy is transmitted to the photosynthetic inner bark and to what extent the inner photosynthetic bark chlorophyll activity could be remotely sensed during both the wet and the dry season. In this study, chlorophyll estimation using the chlorophyll absorption continuum index (CACI) yielded a higher R2 (0.87) than others indices and methods, such as the use of single band, simple ratios, normalized differences, and conventional red edge position (REP) based estimation techniques. The chlorophyll absorption continuum index approach considers the increase or widening in area of the chlorophyll absorption region, attributed to high concentrations of chlorophyll causing spectral shifts in both the yellow and the red edge. During the wet season B. papyrifera trees contain more bark layers than during the dry season. Having less bark layers during the dry season (leaf off condition) is an advantage for the plants as then their inner photosynthetic bark is more exposed to light, enabling them to trap light energy. It is concluded that B. papyrifera bark chlorophyll content can be reliably estimated using the chlorophyll absorption continuum index analysis. Further research on the use of bark signatures is recommended, in order to discriminate the deciduous B. papyrifera from other species during the dry season.
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