Understanding the causes of bush encroachment in Africa: The key to effective management of savanna grasslands (original) (raw)
Related papers
Tropical Grasslands, 2013
The increase in biomass and abundance of woody plant species, often thorny or unpalatable, coupled with the suppression of herbaceous plant cover, is a widely recognized form of rangeland degradation. Bush encroachment therefore has the potential to compromise rural livelihoods in Africa, as many depend on the natural resource base. The causes of bush encroachment are not without debate, but fire, herbivory, nutrient availability and rainfall patterns have been shown to be the key determinants of savanna vegetation structure and composition. In this paper, these determinants are discussed, with particular reference to arid and semi-arid environments of Africa. To improve our current understanding of causes of bush encroachment, an integrated approach, involving ecological and indigenous knowledge systems, is proposed. Only through our knowledge of causes of bush encroachment, both direct and indirect, can better livelihood adjustments be made, or control measures and restoration of savanna ecosystem functioning be realized. Resumen Una forma ampliamente reconocida de degradación de pasturas es el incremento de la abundancia de especies de plantas leñosas, a menudo espinosas y no palatables, y de su biomasa, conjuntamente con la pérdida de plantas herbáceas. En África, la invasión por arbustos puede comprometer el sistema de vida rural ya que muchas personas dependen de los recursos naturales básicos. Las causas de la invasión por arbustos no están lo suficientemente claras pero el fuego, los herbívoros, la disponibilidad de nutrientes y el patrón de precipitación han demostrado ser determinantes clave de la estructura y la composición de la vegetación de sabana. En este documento se hace un análisis del impacto de estos determinantes, con especial referencia a los ambientes áridos y semi-áridos de África. Para una mejor comprensión de las causas de la invasión por arbustos se propone un enfoque integrado que involucra sistemas ecológicos y de conocimiento autóctono. Solamente a través del conocimiento, tanto directo como indirecto, de las causas de la invasión por arbustos, será posible hacer los ajustes necesarios para una mejor calidad de vida o tomar las medidas de control y restauración de las funciones del ecosistema de sabanas. Ward D. 2005. Do we understand the causes of bush encroachment in African savannas? African Journal of Range & Forage Science 22:101−105. Wiegand K; Saltz D; Ward D. 2006. A patch-dynamics approach to savanna dynamics and woody plant encroachment − Insights from an arid savanna. Perspectives in Plant Ecology, Evolution and Systematics 7:229−242.
Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local woody species and/or incursion of woody species introduced from other ecosystems into semi-arid savannas and grassland ecosystems. Rangeland degradation due to bush encroachment causes several challenges, affecting the production of livestock and pastoral people livelihoods in most parts of Africa. Scientists have long been attempting to develop schematic and mathematical theories to explain the observed phenomenon of bush encroachment, and several theories were proposed and developed. The well-regarded theories include: (1) Walter's two-layer model, (2) Moir's one-layer model, (3) state-and-transition theory, (4) equilibrium theory, (5) disequilibrium theory, and (6) non-equilibrium theory. Within those theories, the most frequently-indicated driving factors that explain bush encroachment include over-grazing, availability of soil nutrient and moisture, elevated CO 2 levels, frequency and intensity of fire, spread of seeds of woody species by livestock and wild animals. It should be stressed that couplings and interactions among diverse driving factors are more often at work in determining the condition of bush encroachment. To summarize, the effort in managing semi-arid ecosystems needs critical knowledge to understand the cause-effect relationships of underlying factors through integrated approach. Therefore, future research on encroachment of woody plants should be multi-discipline oriented and multi-partnership involved. Citation: Anteneh BELAYNEH, Zewdu K TESSEMA. 2017. Mechanisms of bush encroachment and its inter-connection with rangeland degradation in semi-arid African ecosystems: a review. Journal of Arid Land, 9(2): 299–312.
Biocomplexity from the Ground Up: Vegetation Patterns in a West African Savanna Landscape
Annals of The Association of American Geographers, 2011
Most studies of vegetation in African savannas have adopted reductionist approaches to research, mostly by looking at only “natural” or “anthropogenic” spaces in focal landscapes. Few have considered how social and biophysical factors interact to create vegetation patterns across landscapes. More holistic understanding of pattern–process relationships across savanna landscapes as wholes is needed to manage the ecological connections of global and local change. To understand how process interactions create vegetation patterns in a savanna landscape, I sampled vegetation in 217 sites in southwestern Mali, where conditions are similar to many parts of West Africa. For each site, I collected site-specific data on several biophysical factors as well as histories of settlement or cultivation, based on ethnographic interviews. Hierarchical clustering, indicator species analysis, multiresponse permutation procedures, and nonmetric multidimensional scaling revealed patterns of floristic variation across the landscape that related to patterns of biophysical variation and disturbance history. The results show complexity in terms of both pattern and process. Patterns evident at different scales of organization are not necessarily congruent, and vegetation heterogeneity is greater than expected based on any subset of biophysical or social processes. Soil moisture, bedrock lithology, and disturbance history are particularly important in shaping vegetation characteristics. Complex patterns arise because vegetation shows three different responses to settlement and cultivation—turnover, homogenization, and resilience—with actual outcomes that depend on site-specific biophysical and social conditions. This research shows that research epistemology can shape physical geographic knowledge and that savanna landscapes must be seen as entities with characteristics that are maintained through human–environment interaction. La mayoría de los estudios sobre vegetación de las sabanas africanas ha adoptado enfoques reduccionistas a la investigación, principalmente por mirar solo los espacios “naturales” o “antropogénicos” en paisajes focalizados. Pocos son los que se han preocupado del modo como interactúan los factores sociales y biofísicos para crear patrones vegetacionales de un paisaje a otro. Se necesita un entendimiento más holístico de las relaciones patrón-proceso a través de los paisajes sabaneros, como un todo, para manejar las conexiones ecológicas de cambio global y local. Para entender cómo las interacciones de proceso crean patrones de vegetación en un paisaje sabanero, hice muestreos de vegetación en 217 sitios del sudoeste de Mali, donde las condiciones son similares a muchas partes del África Occidental. Para cada caso recolecté datos específicos para el sitio sobre varios factores biofísicos, lo mismo que historias de asentamiento o agricultura, a partir de entrevistas etnográficas. Agrupamiento jerárquico, análisis indicador de especies, procedimientos de permuta de respuesta múltiple y escalado multidimensional no métrico, revelaron patrones de variación florística a través del paisaje que se relacionaban con patrones de variación biofísica e historia de la perturbación. Los resultados muestran complejidad en términos tanto de patrón como de proceso. Los patrones que aparecen en diferentes escalas de organización no son necesariamente congruentes, y la heterogeneidad de la vegetación es mayor de lo esperado con base en cualquier subconjunto de procesos biofísicos o sociales. La humedad del suelo, litología del substrato rocoso e historia de la perturbación son particularmente importantes en la conformación de las características de la vegetación. Aparecen patrones complejos debido a que la vegetación exhibe tres respuestas diferentes al asentamiento y a la agricultura—rotación, homogeneización y resiliencia—con resultados reales que dependen de condiciones biofísicas y sociales específicas del sitio. Este estudio muestra que la epistemología de la investigación puede moldear el conocimiento geográfico físico y que los paisajes sabaneros deben considerarse como entidades con características que trascienden la interacción sociedad-medio ambiente.
Do we understand the causes of bush encroachment in African savannas?
2005
Bush encroachment affects the agricultural productivity and biodiversity of 10–20 million ha of South Africa. Many people believe that we understand the causes of bush encroachment. We do not. Many people believe that either fire or heavy grazing by domestic livestock is the sole cause of bush encroachment. This is wrong. Bush encroachment occurs in many arid regions where fuel loads are insufficient for fires to be an important causal factor.
Oh trilhas de rumo errante Onde pasta o manso gado Riscando o verde pujante Dos campos do meu cerrado É lá do alto da serra Daonde o sol pinta o mundo E doa aos filhos da terra Este lar rico e fecundo Abstract Neotropical savannas are diversified systems, especially those of the Brazilian Cerrado, with a vascular flora exceeding 7,000 species, 45 % of these being endemic, which makes it the richest tropical savanna in the world. This biodiversity is threatened by the expansion of the agricultural frontiers, and the invasive African grasses. The savanna's soils are fragile, have low fertility, and high Al toxicity; nevertheless, in the savannas only the subsistence agriculture and the extensive cattle rising on low quality native grasses were practiced; but there was a quick monocropping expansion, of grains and grasses, with improved agricultural methods, government subsidies, and strong investments in fertilizers and machinery, especially in Brazil and Venezuela. Savannas becam...
Land-use changes and the invasion dynamics of shrubs in Baringo
Journal of Eastern African Studies, 2016
In the semi-arid savannahs around Lake Baringo, Kenya, the recent spread of bush encroachment by the invasive alien species Prosopis juliflora and the native Dodonaea viscosa has changed humanenvironment interactions. This article suggests how the spread dynamics of Prosopis and Dodonaea have operated. It also describes the strategies Baringo's peoples have adopted in the face of this dramatic bush invasion, relates these dynamics to current invasion theory, and analyses possible implications for Baringo's social-ecological systems. It is suggested that recent increased climate variability has triggered changes in land management and livelihoods around Lake Baringo, paving the way for bush encroachment and species invasion. The extent and speed of these changes has exceeded the capacity of local communities to adapt their productive systems, destabilizing the socio-ecology of the dryland savannahs around Lake Baringo and placing them in imminent danger of collapse.
African Journal of Ecology, 2018
Evaluation of woody vegetation changes with distance from a salt crater was conducted in the semi-arid rangelands of southern Ethiopia. Data on live woody plants were collected over three seasons at 0, 1, 4, 6, 9 and 12 km from the salt crater. The density and diversity of woody plants differed significantly (p < .01) along the distance gradient. Six woody plant families were identified of which Fabaceae and Burseraceae were the dominant families. Acacia drepanolobium, Acacia nilotica, Commiphora africana and Acacia mellifera were among the severely encroaching woody species. There were high proportions of seedlings and saplings recorded closer to the salt crater showing a vigorous recruitment by woody plants. Woody plant encroachment along the 12-km transect ranged from a low to severe encroachment, which could be translated into poor rangeland condition. Changes in soil characteristics increased grazing pressure and sedentary settlement around the salt crater, and the breakdown of traditional institutions seems to be major contributing factors to these vegetation changes. We suggest that severely encroached areas could be improved through a combination of methods such as bush clearing, prescribed fire, browsing animals and proper grazing management. R esum e Nous avons r ealis e une evaluation des changements de la v eg etation avec la distance par rapport a un crat ere salin dans les etendues semi-arides du sud de l' Ethiopie. Nous avons r ecolt e des donn ees sur des plantes ligneuses vivant a 0, 1, 4, 6, 9 et 12 km du crat ere salin. La densit e et la diversit e des plantes ligneuses diff eraient significativement (p < .01) le long du gradient de distance. Six familles de plantes ligneuses ont et e identifi ees, parmi lesquelles les Fabaceae et les Burseraceae etaient les familles dominantes. Acacia drepanolobium, Acacia nilotica, Commiphora africana et Acacia mellifera comptaient parmi les esp eces ligneuses les plus gravement envahissantes. Il y avait de fortes proportions de jeunes plants et d'arbustes plus pr es du crat ere, ce qui montre un recru vigoureux de plantes ligneuses. L'envahissement des plantes ligneuses le long des 12 km de transect allait de faible a s ev ere, ce qui peut se traduire par un etat m ediocre de l'habitat. Les changements des caract eristiques du sol, une pression plus intense du pâturage et les installations permanentes autour du crat ere salin, ainsi que la disparition des institutions traditionnelles semblentêtre les principaux facteurs de ces changements de la v eg etation. Nous sugg erons d'am eliorer les zones les plus gravement envahies grâce a une combinaison de