Experimental study of the transport properties of rough self-affine fractures

An experimental study of the transport properties of fluid-saturated joints composed of two complementary rough fracture surfaces, translated with respect to each other and brought in contact, is reported. Quantitative roughness measurements on different fractured granite samples show that the surfaces have a self-affine geometry from which the dependence of the mean aperture on the relative displacement of fracture surfaces kept in contact can be predicted. Variations of the hydraulic and electrical conductances of the joint are measured as functions of its mean aperture. A simple parallel plane model accounts for the global trend of the measurements, but significant deviations are observed when a relative lateral displacement of the surfaces is introduced. A theoretical analysis of their origin shows that they are due both to the randomness of the aperture field and to a nonzero local slope of the surface near the injection hole; the corresponding conductivity fluctuation amplitudes have power law and linear variations with the lateral displacement, and are enhanced by the radial injection geometry

Le drageonnage pour la régénéreration d'espèces médicinales en Afrique tropicale : cas du Spathodea campanulata en Ouganda

En Afrique, depuis plus d'un demi siècle, les formations arborées, denses ou claires, sont fortement sollicitées pour leur valeur commerciale ou médicinale et par la recherche perpétuelle de nouveaux espaces pour l'agriculture et l'élevage. De nombreuses espèces ligneuses en Afrique équatoriale et tropicale disparaissent des systèmes agro-pastoraux et forestiers, conséquence d'une forte exploitation et d'une régénération naturelle peu efficace. En Ouganda, et particulièrement dans les zones habitées du sud-ouest du pays, la situation s'aggrave très rapidement et il devient nécessaire de régénérer ces écosystèmes si l'on veut que les conditions socio-économiques n'empirent pas. La maîtrise de la multiplication végétative à faible coût par induction du drageonnage semblerait apporter une réelle alternative à la régénération des essences recherchées par les populations locales. Les observations sont faites en Ouganda, sur plusieurs Spathodea campanulata, dont le système racinaire a été endommagé. Après avoir observé les réactions physiologiques induites par ce stress au niveau des racines, on constate que le sectionnement complet d'une racine engendre toujours la néoformation d'un ou plusieurs drageons exclusivement sur la partie de la racine déconnectée de l'arbre mère. La partie connectée se stabilise ou produit de nouvelles racines et les blessures superficielles ou profondes sur des racines à l'air libre ou sous terre n'engendrent pas d'organogenèse (ni radicelle, ni drageon). On observe dans ce cas la formation d'un cal cicatriciel. Le développement durable passe nécessairement par la régénération à faible coût des ressources ligneuses dont les populations souvent démunies de l'essentiel ont besoin pour lutter contre les maladies et augmenter leurs revenus par la commercialisation de produits forestiers non ligneux. Ces observations confirment qu'il est possible d'initier l'apparition de drageons lors d'inductions artificielles. Il est souhaitable à l'avenir d'entreprendre des études de cas d'espèces différentes. (Résumé d'auteur

Roughness of sandstone fracture surfaces: Profilometry and shadow length investigations

The geometrical properties of fractured sandstone surfaces were studied by measuring the length distribution of the shadows appearing under grazing illumination. Three distinct domains of variation were found: at short length scales a cut-off of self-affinity is observed due to the inter-granular rupture of sandstones, at long length scales, the number of shadows falls off very rapidly because of the non-zero illumination angle and of the finite roughness amplitude. Finally, in the intermediate domain, the shadow length distribution displays a power law decrease with an exponent related to the roughness exponent measured by mechanical profilometry. Moreover, this method is found to be more sensitive to deviations from self-affinity than usual methods

Predictors of organoleptic quality of boiled and dried pulp of safou (dacryodes edulis) and the shelf life of its fresh fruits

Introduction. The high intra-specific variation in safou traits and the perishable nature of the fruit hampers commercialization. Knowledge of the parameters that determine the acceptability of safou products and the shelf life of fresh fruits is critical for marketing and useful as a first step toward cultivar development. Materials and methods. Safou fruits of different pulp colors, skin colors and stages of ripening were collected from clones. Physicochemical and organoleptic analyses were performed on fresh and processed safou pulp, respectively, to elucidate parameters that could be used to determine the acceptability of boiled and dried safou pulp, and the shelf life of fresh fruits. Results and discussions. Our study demonstrated that the pH and color of raw pulp could be used to determine the acceptability of boiled and dried safou. Consumers preferred products which were not acidic (sour), with a nice aroma. There was a negative correlation between the pH of raw pulp and the acceptability of boiled and dried safou pulp. Likewise, there was a positive correlation between the color of raw pulp and the acceptability of boiled and dried safou pulp. These parameters were used to predict the taste of finished products. Polygalacturonase activity was higher in fully ripe and unripe fruits than in semi-ripe fruits. Polygalacturonase activity had a strong positive correlation with percentage loss of fully ripe fruit during storage and, as such, can also be used to predict the postharvest loss of safou. Conclusion. The pH and color of raw (uncooked) pulp of safou can be used to determine the taste and acceptability of boiled and dried pulp. Likewise, the activity of polygalacturonase in raw pulp can be used to predict its postharvest shelf life. (Résumé d'auteur

Productivity and management of agroforestry parklands in the Sudan zone of Burkina Faso, west Africa

This study focuses on the bio-economic justification of maintaining trees in parklands of a frontier village of southern Burkina Faso, West Africa. It also attempts to understand how parklands are established and managed by farmers. Methods included comparative vegetation inventories of fields and adjacent uncultivated lands, farmer interviews, sorghum yield trials between karite (Vitellaria paradoxa) trees, and a two-year karite fruit production trial. Parklands result from drastic changes in density and species richness of the original woody vegetation at clearing time. Slightly declining density, increasing dominance of a few species and increasing average tree diameter characterize developing parklands. Practice and duration of fallow, gender of field manager, method of field cultivation, soil mapping unit, original karite density, and farmer perception of the effect of trees on crop production affected parkland tree density. Crop grain and biomass production were higher at the edge of medium-sized karite canopies than either under the canopy or at mid-distance between trees. The zone at the edge of canopies had higher soil fertility (organic matter and potassium) and soil moisture, and lower sunlight intensity than in the middle of the field. Karite trees depressed grain yields under their crowns by an average of 15%. The decreased grain yields under crowns are probably due to competition for light or excessive moisture. Karite trees were found to increase overall sorghum production at densities between 12 and 31 trees/ha. Therefore, farmers do not improve cereal production by reducing tree densities below these levels. Karite trees produced an average of 2 kg of dry kernels. There was a ten-fold difference between average and highest nut-producing trees. Additionally, nut production was four times higher in 1994 than in 1993. Mass selection would thus be valuable for increasing karite nut production. The intercropping of medium-sized and large karite trees in Thiougou was economically superior to a treeless annual cropping system

Scattered shade trees improve low-input smallholder Arabica coffee productivity in the Northern Lake Kivu region of Rwanda

In suboptimal conditions for coffee growth, the use of shade tree is usually considered beneficial to production. This study aimed to evaluate this benefit in the poorly documented East African smallholder Arabica coffee systems in optimal climatic but suboptimal management conditions. In a 4 year observational trial in 50 coffee farms, the association of shade trees and coffee generated an average 55 % cherry production increase. Neither delay in berry maturation nor buffer in alternate bearing patterns were observed, probably due to the low productivity of unfertilized coffee plants. Quality wise, the presence of shade trees did not result in an increase of larger green beans, but it reduced the proportion of altered and lighter cherries in 2009, a low production year. A shade species effect was detected through the positive influence of two non-leguminous shade trees, Persea americana and Ficus thonningii on production. The effect was correlated to greater canopy openness and increase in K soil content. In general, soil mineral content was not influenced by the presence of trees, but the legume species Inga oerstediana appeared responsible for a slight increase of total C and N soil content. It is concluded that in the small holder context of the Northern Lake Kivu region, the association of mature trees is beneficial to coffee production and can contribute to the improvement of producer's income. (Résumé d'auteur

Le drageonnage pour la regénération d’espèces médicinales en Afrique tropicale : cas du Spathodea campanulata en Ouganda

In Africa since more than one half century, deciduous forests and open woodlands are seriously harvested for their commercial value and/or for medicinal purposes, and also for the continual research of new lands for agriculture or livestock. Many woody species are disappearing in agricultural and forest systems of Equatorial and Tropical Africa, due to harvesting practices and low natural regeneration. In Uganda, and particularly in the south-western habited zones of the country, the situation is getting worse very rapidly and it is urgent to regenerate these ecosystems before the socio-economical condition deteriorates too much. Vegetative propagation techniques at low cost, such as the root sucker induction method, can provide alternatives for a sustainable regeneration of species required by local populations. Observations were made in Uganda on Spathodea campanulata, for which root systems have been damaged by human activities. We observed physiological reactions on stressed roots. The total severance of a root always induced neo-formation of one or several root suckers on the distal part of the root section (root disconnected from the mother tree). Proximal parts of the roots stabilized or created new roots. Superficial or deep injuries on earthed or unearthed roots did not produce any roots or root suckers. In such cases, we observed the formation of callosity covering the injury. Sustainable development depends necessary on the regeneration at low cost of the woody resources. These poor populations, who don’t even have the essential, need these resources in order to combat disease and increase the incomes by the commercialisation of non woody forest products. This study confirms that artificial induction may lead to root sucker development and thus control the propagation of the species. Through similar case studies and species-based observations, such vegetative propagation methods can be replicated and transferred to threatened woody species in Tropical Africa