Woody species diversity in a changing landscape in the south-central highlands of Ethiopia

doi:10.1016/j.agee.2008.05.001

Agriculture, Ecosystems & Environment

Volume 128, Issues 1–2, October 2008, Pages 52-58

https://doi.org/10.1016/j.agee.2008.05.001 Get rights and content

Abstract

Ethiopia hosts one of the richest flora and fauna resources in tropical Africa. However, this rich bioresource is decreasing due to extensive deforestation. The objectives of this study were to (i) investigate woody species diversity on smallholder cultivated land (crop fields and homegardens) and its implication for biodiversity changes over time; (ii) assess the dynamics of woody species diversity, density and structure with age of crop fields since conversion from natural forest using a chronosequence of farm fields; and (iii) assess the effects of some household and homegarden characteristics on woody species diversity around homegardens in the south-central highlands of Ethiopia. Woody species diversity in the adjacent natural forest was used as a reference. Systematic sampling was used to collect vegetation data from crop fields and natural forest, while simple random sampling within wealth categories was used to select sample households and their homegardens. In total, 70 woody species were recorded. The highest number of woody species (64) was recorded in homegardens, followed by crop fields (32) and the lowest number (31) in remnant natural forest. Despite the low species number, natural forest showed higher Shannon and Simpson diversity indices and Shannon evenness than crop fields and homegardens. This was due to the uniform distribution of species in natural forest compared with homegardens or crop fields. The diversity and density of woody species declined with increasing age of crop fields, while diversity of woody species increased with increasing age and size of homegardens. Wealth status of the households also affected species diversity in homegardens. Rich households kept a greater number of woody species, probably due to their larger-sized homegardens. Generally, the study showed that conversion of natural forest into cultivated land (mosaics of homegardens and crop fields) typical of the smallholder system in the highlands of Ethiopia does not result in a dramatic loss of species but that the spatial distribution, density and species composition may be altered.

Introduction

Tropical ecosystems are renowned for their rich biological diversity. However, population growth and the resulting expansion of cultivated land are threatening the sustainable management and use of the rich biological resources in the tropics. In Ethiopia in particular, deforestation for farming purposes and the subsequent habitat loss and fragmentation are major threats to the biodiversity resources of the country (Ayele, 2003, Lemenih and Teketay, 2004). A report by Reusing (1998) showed that the annual rate of deforestation in Ethiopia is 163,600 ha, which is one of the highest in Africa. Such massive forest degradation is always accompanied by loss of genetic resources (Woldemariam and Teketay, 2001, Woldemariam et al., 2001, Woldemariam et al., 2002).

Although agricultural expansion is often alleged to be the major driver of biodiversity losses, there are variations among farming practices with respect to their impacts (Hamito and Abate, 1994, Harvey and Haber, 1999). For instance, intensive commercial monocropping is likely to result in low species diversity, while some of the traditional farming practices common to the tropics are known to support a high level of diversity (Harvey and Haber, 1999, Abebe, 2005).

Some studies have shown that cultivated land in different parts of Ethiopia is characterised by a high diversity of woody species and thus provides a refuge for native woody species (Hamito and Abate, 1994, Asfaw, 2003, Abebe, 2005). The practice of managing woody species in the cultivated landscape can play a key role in the maintenance of biodiversity (Harvey and Haber, 1999, Nikiema, 2005), but is relatively less studied (Polasky et al., 2003, Nikiema, 2005). Most studies that have considered the role of woody species in farming landscapes in Ethiopia have focused on their impact on soil properties and crop production (e.g. Yadessa, 1998, Hailu et al., 2000, Asfaw, 2003). Fewer studies have considered their role in biodiversity conservation (e.g. Asfaw and Nigatu, 1995, Asfaw and Woldu, 1997, Asfaw, 2003, Abebe, 2005). The objectives of this study were to (i) investigate woody species diversity changes over time on cultivated land (crop fields and homegardens); (ii) assess the dynamics of woody species diversity, density and structure with age of crop fields since conversion from natural forest using a chronosequence method; and (iii) assess the relationships between household or homegarden characteristics and woody species diversity in homegardens.

Section snippets

Site description

The study was conducted in Beseku-Ilala Peasant Association (PA), Arsi Negelle district, south-central Ethiopia. Beseku is located between 7°20′ and 7°25′N and 38°45′ and 38°50′E. Beseku belongs to the warm sub-humid eco-climatic zone (Lemenih, 2004). The rainfall in the area is bimodal, with the main rainy season from July to October and the short rainy season from March to May. Mean annual rainfall amounts to 1200 mm (Anonymous, 1990). Mean annual minimum and maximum temperatures vary between

Woody species diversity

A total of 70 woody species were recorded in the three land uses crop fields, homegardens and adjacent natural forest. The number of woody species recorded (species richness) in the homegardens (64) was much higher than in the crop fields (32) or in the adjacent natural forest (31) (Table 1). However, Shannon's diversity index (H′), Simpson's diversity index (D) and species evenness (E) were highest for the natural forest and lowest for the homegarden (Table 1), while Fisher's α value was

Discussion

This study showed that cultivated land in the Ethiopian highlands supports a high number of woody species. Homegardens were found to host a particularly high number of woody species compared with natural forest and crop fields. Several species occurred in the homegardens that were either rare or non-existent in the adjacent natural forest. Some of these species were exotic trees such as Eucalyptus spp. and Cupressus spp., fruit trees such as citrus, mango and avocado or cash crops such as chat (

Conclusions

The results of the present study confirm that smallholder farmland maintains a high proportion of native woody species, even several decades after deforestation. On the other hand, the decline in number of woody species on the crop fields with age shows that this species preservation is only temporary unless agroforestry systems that are acceptable to the community and that builds on the existing traditional parkland agroforestry system are introduced. Homegardens, where the farmers have a

Acknowledgements

This work was partially funded by Wondo Genet College of Forestry through its DOIT-AR programme and partially by the project Making soil quality last – participatory soil fertility management in the highlands of Ethiopia funded by Sida-Sarec.

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Agriculture, Ecosystems & Environment

Abstract

Introduction

Section snippets

Site description

Woody species diversity

Discussion

Conclusions

Acknowledgements

References (34)

Climate change and biodiversity conservation in Great Plains agro-ecosystems

Global Environ. Change

Amelioration of soil fertility by woody perennials in cropping fields: evaluation of three tree species in the semi arid zone of Nigeria

J. Arid Environ.

Soil organic matter dynamics after deforestation along a farmland chronosequence in the Highlands of Ethiopia

Agric. Ecosyst. Environ.

Site index equation for Cupressus lusitanica stands in Munessa Forest

Ethiopia. For. Ecol. Manage.

Munessa Shashemene State Forest Project Management Plan

Homegardens in Ethiopia: characteristics and plant diversity

SINET: Eth. J. Sci.

Crop association of homegardens in Welayita and Gurage in Southern Ethiopia

SINET: Eth. J. Sci.

Flora of Ethiopia and Eritrea, vol. 2. Part 2. The National Herbarium

Flora of Ethiopia and Eritrea, vol. 6. The National Herbarium

Flora of Ethiopia and Eritrea, vol. 2. Part 1. The National Herbarium

Millettia ferruginea from southern Ethiopia: impacts on soil fertility and growth of maize

Agrofor. Syst.

Forestry Research Directory

Cited by (62)

Woody species diversity potential and population structure across the small-scale agroforestry farming system of the Midakegn District, West Shewa Zone, Ethiopia

Land use change impose variation on woody fodder species composition, and production in the dryland ecosystem of West Guji southern Ethiopia

Support vector machine-based spatiotemporal land use land cover change analysis in a complex urban and rural landscape of Akaki river catchment, a Suburb of Addis Ababa, Ethiopia

Management strategies and floristic diversity in agroforestry practices of northwestern Ethiopia

Land use/land cover dynamics driven changes in woody species diversity and ecosystem services value in tropical rainforest frontier: A 20-year history

Diversity of woody species in traditional agroforestry practices in Wondo district, south-central Ethiopia