Banded landscapes are comprised of alternating bands of vegetation and bare ground aligned along the contours in arid and semi-arid regions (50–750 mm rainfall), on very gentle and uniform slopes (0.2–2%). Vegetated bands can be perpendicular to the direction of the dominant wind, or more frequently of the slope. Under given climatic conditions, slope gradient is the controlling factor of the type of pattern (`spotted', `broadly' or `typically' banded). For a given slope gradient, mean annual rainfall determines the contrast between the vegetated and bare phase, as well as the band length and the interband width:band width ratio. A typical transect through such two-phase mosaic includes a dynamic succession of surface crusts which generates a run-off–run-on system. This favours the capture of limited water resources and thus a biomass production greater for banded patterns than spotted patterns or uniformly scattered vegetation. Moreover, vegetated bands act as natural bench structures that greatly limit soil erosion. As suggested by modelling, banded vegetation patterns can equally be derived from nearly bare areas or from dense vegetation patterns. Because banded vegetation patterns have often been considered as a form of degradation of previous continuous vegetation cover, many attempts have been made by foresters to restore this initial cover by reforestation of the bare interbands. However the numerous failures of reforestation of the bare interbands illustrate the key role of these mosaic components in the maintenance of the tiger bush ecosystem. Banded vegetation patterns are more resilient to climate change than to human disturbances. The loss of landscape patchiness due to the clearing of the bands either for fuelwood or for ephemeral cropping induces a rapid decline in soil fertility and water infiltration potential. Long-term monitoring shows that the interband width:band width ratio adapts to rainfall variations that are intrinsic of semi-arid zones. Moreover, climatic changes might also induce a change in the type of patterning, suggesting that banded vegetation patterns are highly self-sustainable. The ecological differences between the pioneer and the decaying edges of the vegetated bands, the temporal/spatial succession of soil crusts, as well as other indices such as the spatial distribution of soil organic matter and termite nests suggest the upslope migration of the bands. Nevertheless field evidence of such a shift remains scarce. Profound lessons can be learnt from the study of banded landscapes in terms of ecosystems functions. When designing water-harvesting systems, or restoring degraded arid and semi-arid land, banded vegetation patterns should be imitated to optimize biomass production and limit land degradation.

Soil and water components of banded vegetation patterns

Terra Preta soils of central Amazonia exhibit approximately three times more soil organic matter, nitrogen and phosphorus and 70 times more charcoal compared to adjacent infertile soils. The Terra Preta soils were generated by pre-Columbian native populations by chance or intentionally adding large amounts of charred residues (charcoal), organic wastes, excrements and bones. In this paper, it is argued that generating new Terra Preta sites (‘Terra Preta nova’) could be the basis for sustainable agriculture in the twenty-first century to produce food for billions of people, and could lead to attaining three Millennium Development Goals: (i) to combat desertification, (ii) to sequester atmospheric CO2 in the long term, and (iii) to maintain biodiversity hotspots such as tropical rainforests. Therefore, large-scale generation and utilization of Terra Preta soils would decrease the pressure on primary forests that are being extensively cleared for agricultural use with only limited fertility and sustainability and, hence, only providing a limited time for cropping. This would maintain biodiversity while mitigating both land degradation and climate change. However, it should not be overlooked that the infertility of most tropical soils (and associated low population density) is what could have prevented tropical forests undergoing large-scale clearance for agriculture. Increased fertility may increase the populations supported by shifting cultivation, thereby maintaining and increasing pressure on forests.

/pdf/prehistorically-modified-soils-of-central-amazonia-a-model-39wzydp7gg.pdf

Prehistorically modified soils of central Amazonia: a model for sustainable agriculture in the twenty-first century.

The rate and extent to which biophysical resources are captured and utilized by the components of an agroforestry system are determined by the nature and intensity of interactions between the components. The net effect of these interactions is often determined by the influence of the tree component on the other component(s) and/or on the overall system, and is expressed in terms of such quantifiable responses as soil fertility changes, microclimate modification, resource (water, nutrients, and light) availability and utilization, pest and disease incidence, and allelopathy. The paper reviews such manifestations of biophysical interactions in major simultaneous (e.g., hedgerow intercropping and trees on croplands) and sequential (e.g., planted tree fallows) agroforestry systems.

/pdf/biophysical-interactions-in-tropical-agroforestry-systems-26l8aikipf.pdf

Biophysical interactions in tropical agroforestry systems

In response to the growing interest in estimating carbon stocks in forests, available allometric equations have been compiled for sub-Saharan Africa. Tree, sprout and stand volume and biomass equations were reviewed. The 850 equations and 125 related references were incorporated into an open-access database on the Carboafrica website (http://www.carboafrica. net). The collected information provides a basic tool for the estimation of biomass and carbon stocks and other purposes, such as bioenergy and fodder supply assessment. A Tier-method approach was developed to illustrate the possible use of the equations. Current available biomass expansion factors that are used to convert a volume to the total aboveground biomass appear to be limited; incomplete species-specific allometric equations are preferred to generalised equations. The analysis of the database highlighted important gaps in available tools to assess forest carbon stocks and changes in these stocks. A quality control assessment revealed that 22% of the equations were misreported and recommendations were proposed to guide further research. Further statistical analyses, such as the Bayesian approach, would help to produce more accurate biomass estimates. (Resume d'auteur)

/pdf/estimating-tree-biomass-of-sub-saharan-african-forests-a-2okj3wwggg.pdf

Estimating tree biomass of sub-Saharan African forests: a review of available allometric equations.

This chapter reviews the advances made in our knowledge of the effects of termites on the physical, chemical and biological properties of soils. Emphasis has been placed on more recent contributions, particularly those that explore new concepts in the ecology of termites and soils. There are sections dealing with the effects of termite activity on soil profile development, soil physical properties, soil chemical properties, soil microbiology and plant growth. The physical effects of termites on soils range from micromorphological to soil profile evolution and structure. Recent evidence points to the substantial positive influence of termites on soil hydraulic conductivity and infiltration rates. Their influence on organic matter decomposition and nutrient recycling rates are well recognized and in some landscapes termite mounds act as foci for nutrient redistribution. New information on the microbiology of termite mounds suggests that most are sites of diverse bacterial and fungal activity. Furthermore, the association between mound-building termites and the microbial population present in the structures has a synergistic effect on organic matter decomposition and hence nutrient cycling and availability. Examination of the effects of termite activity on plant production generally indicates a positive influence.

Termites and Soil Properties

La rarefaction des ressources hydriques conduit la vegetation des zones saheliennes a se "contracter" pour former des structures originales, qu'il convient d'etudier et de comprendre afin d'en tirer le meilleur parti. Tous les ecosystemes contractes, c'est-a-dire formes d'une juxtaposition de zones de vegetation et de sol nu, ne sont pas degrades et donc ne justifient pas les projets de reboisement couteux qui peuvent aller parfois a l'encontre du but recherche. "Comprendre pour mieux gerer les forets saheliennes" est le theme de l'Atelier international qui s'est tenu a Niamey, Niger du 20 au 25 novembre 1995. Organise par la Direction de l'Environnement du Ministere de l'Hydraulique et de l'Environnement du Niger, le CIRAD-FORET et l'ORSTOM, rassemblant chercheurs et operateurs du developpement, il a permis de confronter les acquis scientifiques sur ces ecosystemes forestiers contractes avec les politiques mises en oeuvre au Niger. (Resume d'auteur)

/pdf/fonctionnement-et-gestion-des-ecosystemes-forestiers-1vdryr8wru.pdf

Fonctionnement et gestion des écosystèmes forestiers contractés sahéliens

Dans les zones soudaniennes du Nord-Cameroun, l'arrivee continue de migrants entraine le defrichement de vastes etendues de savanes. La densification du parc arbore relictuel repondrait, a la fois, a des besoins agronomiques, economiques et environnementaux. Une methode d'evaluation du carbone stocke dans la biomasse aerienne est proposee, pour un parc a karites, Vitellaria paradoxa, sur un terroir villageois. Il s'agit de mettre en place des projets agroforestiers eligibles au Mecanisme pour un developpement propre (Mdp), a partir de 2012. De plus, une evaluation de la productivite en bois du parc a karites, gere par emondage, a ete realisee durant trois annees, pour une trentaine d'arbres. Une serie d'equations allometriques a ete developpee (sur six individus pour le karite et sur un seul arbre pour Anogeissus leiocarpus et Combretum nigricans), qui met en relation la biomasse aerienne en fonction du diametre (du tronc a hauteur de poitrine ou de la base des branches) ou de la surface du houppier. Ainsi, les stocks de carbone par arbre, puis par parcelle ont pu etre estimes. Face a la pression anthropique exercee sur ces parcs, il a ete etabli qu'une rotation de huit ans entre les emondages serait un bon compromis entre les divers interets. Ces travaux constituent une premiere approche qui devra etre consolidee avec le recul du temps. (Resume d'auteur)

/pdf/evaluation-du-stock-de-carbone-et-de-la-productivite-en-bois-4oa9ctc8y6.pdf

Evaluation du stock de carbone et de la productivité en bois d'un parc à karités du Nord-Cameroun

En Republique democratique du Congo, Kinshasa, la capitale accueillant les populations issues de l'exode rural et de l'insecurite accrue, concentre environ huit millions d'habitants. La ville est principalement entouree de savanes et d'ilots forestiers degrades. La consommation en bois energie est estimee dans une fourchette de trois a six millions de tonnes d'equivalent bois energie par an (ce qui representerait 0,6 a 1,2 million de tonnes de charbon de bois par an si tout le bois etait carbonise). Le projet Mampu a ete concu comme la phase pilote d'un projet de reboisement de cent mille hectares sur les sols sableux du plateau Bateke pour faire face a la penurie de bois et de charbon. Malgre les conflits, environ huit mille hectares de plantations d'Acacia auriculiformis ont ete principalement realisees de 1987 a 1993. A partir de 1998, la plantation de Mampu a ete divisee en lots de 25 ha attribues a 320 familles d'agriculteurs. La culture se fait suivant un modele agroforestier de jachere amelioree, inspire du modele traditionnel de culture sur brulis. La production totale de charbon de ce massif varie de 8 000 a 12 000 tonnes annuelles, a laquelle il faut ajouter 10 000 tonnes de manioc, 1 200 tonnes de mais et 6 tonnes de miel. Pour le seul charbon, cela correspond a un revenu brut annuel de 2,6 millions de dollars US pour le pays. Au moins un quart revient aux proprietaires agrisylviculteurs. Le succes du projet incite a appliquer ce modele sur les savanes des plateaux Bateke, en prenant en compte les droits fonciers traditionnels et en poursuivant la diversification et la transformation locale des produits. Cela contribuera a couvrir une part plus importante des besoins urbains en energie renouvelable, tout en creant des emplois ruraux. Cependant, d'autres modeles de systemes agroforestiers meritent d'etre testes ou developpes dans les autres conditions ecologiques et socio-economiques du pays, comme la gestion du recru naturel d'especes locales a usages multiples appliquee dans le systeme traditionnel de jachere enrichie (nkunku) du Bas-Congo. (Resume d'auteur)

/pdf/plantations-industrielles-et-agroforesterie-au-service-des-bvdhqhfb1s.pdf

Plantations industrielles et agroforesterie au service des populations des plateaux Batéké, Mampu, en République Démocratique du Congo

Interactions betweenGliricidia sepium (Jacq.) Steud., the crops maize and upland rice, and soil conditions were investigated in an alley cropping experiment on aFerralic Cambisol/Plinthic Lixisol in the rainforest area of the Cote d'Ivoire. Conventional and mulched sole cropping treatments were included for comparison. Hedgerow integration significantly reduced N and Mg losses from the soil. These effects were neither observed in the mulched sole cropping treatment nor reduced by hedgerow root barriers, suggesting that they were not direct consequences of the input of hedgerow biomass into the soil. Crop yields were increased in the central part of the alleys as compared to sole cropping, but the effect was offset by the loss of cropped surface which was occupied by the hedgerows and by yield depressions at the tree-crop interface.Gliricidia mulch increased foliar nutrient concentrations of maize; however, unlike mineral fertilizer the mulch had no influence on the imbalance between low P and high K levels in maize leaves. Nutrient accumulation in the dry season vegetation tended to be higher in alley cropping than in sole cropping. Weeds were particularly efficient in accumulating those nutrients which were relatively low in the hedgerow biomass. Necessary improvements to the cropping system are discussed. The optimum composition of fertilizers in mulching systems deserves attention.

Alley cropping with Gliricidia sepium on a high base status soil following forest clearing: effects on soil conditions, plant nutrition and crop yields

Au Nord-Cameroun, comme dans l’ensemble des zones semi-arides africaines, les agriculteurs ont depuis longtemps defriche les savanes arborees pour les mettre en culture Ce defrichement a souvent ete selectif : les cultivateurs ont conserve les arbres peu genants ou utiles Par la suite, ils ont parfois enrichi ces systemes agroforestiers en introduisant de nouvelles especes ou en conservant une partie de la regeneration naturelle En fonction de la composition du peuplement arbore d’origine, des conditions ecologiques, des savoirs et des besoins des populations et de leur environnement socio-economique, differents types de parcs arbores se sont ainsi construits, dont les plus connus sont les parcs a faidherbia (Faidherbia albida) et les parcs a karite (Vitellaria paradoxa) De 1950 a 1990, les Administrations forestieres ont surtout cherche a proteger les arbres situes dans les champs et dans les formations naturelles, en interdisant la coupe Ces politiques de conservation, si elles ont permis le maintien des parcs, ont eu aussi un effet contre-productif car les agriculteurs se sont sentis depossedes de la gestion de leur patrimoine, si bien qu’ils n’ont plus conserve ou plante de jeunes arbres et que les peuplements ont vieilli Des actions de recherche-developpement montrent qu’il est possible de re-dynamiser la gestion de ces systemes agroforestiers, a l’echelle des exploitations agricoles et a celle des communautes villageoises  Cela suppose une securisation des droits sur l’arbre qui peut passer par la creation de "forets communautaires", instituees par la loi forestiere camerounaise de 1994, et pouvant inclure le parc agroforestier villageois Des lors, il y a transfert de propriete des arbres de l’Etat vers la communaute, ce qui ouvre le droit a leur exploitation dans le respect d’un plan de gestion Afin de preparer cette demarche, les pratiques traditionnelles raisonnees de gestion des arbres ont ete decrites ; une methode d’emondage conciliant conservation et developpement du tronc, production de feuilles, de fruits et de bois a ete testee, ce qui a permis de mesurer le stockage de biomasse sur pieds et la production annuelle Une reflexion sur la combinaison des techniques de gestion dans le cadre d'un amenagement des parcs a ete conduite et des pistes pour des modeles d’amenagement simplifie des parcs ont ete proposees Ces amenagements permettraient de concilier notamment la production par emondage de bois de feu, ressource rare, et la perennite des parcs Cette demarche recueille aussi l’appui des hauts responsables de l’Administration et des organismes de recherche et de developpement, malgre la reticence d’une partie des agents administratifs de base qui craignent d’y perdre leur pouvoir et leurs avantages Ces enjeux de changements demandent donc a etre soutenus et devraient trouver un appui favorable aupres du Fonds pour l'Environnement Mondial puisqu'ils rentrent dans le champ de plusieurs conventions internationales : convention pour la diversite biologique, convention sur le changement climatique, convention de lutte contre la desertification

https://hal.archives-ouvertes.fr/hal-01420215/document

Régis Peltier

Papers

Fonctionnement et gestion des écosystèmes forestiers contractés sahéliens

Evaluation du stock de carbone et de la productivité en bois d'un parc à karités du Nord-Cameroun

Plantations industrielles et agroforesterie au service des populations des plateaux Batéké, Mampu, en République Démocratique du Congo

Alley cropping with Gliricidia sepium on a high base status soil following forest clearing: effects on soil conditions, plant nutrition and crop yields

Parcs agroforestiers sahéliens : de la conservation à l’aménagement