THE IMPORTANCE OF EARLY SUCCESSIONAL SHRUBS
FOR RECRUITMENT OF AFRICAN WILD OLIVE
(OLEA EUROPAEA SUBSP. CUSPIDATA) IN CENTRAL
TIGRAY, ETHIOPIA
R. AERTS
1,3
, I. VAN DER BORGHT
1,3
, E. NOVEMBER
2,3
, MINTESINOT BEHAILU
3
,
M. HERMY
1
& B. MUYS
1,
*
1
Laboratory for Forest, Nature and Landscape Research, Katholieke Universiteit
Leuven, Vital Decosterstraat 102, B-3000 Leuven, Belgium
2
Laboratory for Soil and Water, Katholieke Universiteit Leuven, Vital Decosterstraat
102, B-3000 Leuven, Belgium
3
Land Resources Management and Environmental Protection Department, Mekelle
University, P.O.Box 231, Mekelle, Ethiopia
*Corresponding author: bart.muys@agr.kuleuven.ac.be
Abstract
Recruitment of a late successional tree species, African wild olive (Olea europaea
subsp. cuspidata; Oleaceae), was examined in protected and grazed areas in the Geba
river catchment of central Tigray, northern Ethiopia, using T-square plotless
sampling. The following hypothesis was tested: recruitment of O. europaea is higher
under certain early colonizing shrubs compared to within gaps and under other
shrubs. Both in grazing land and protected areas, recruits were exclusively found
under shrubs, predominantly under Euclea schimperi, although Acacia etbaica was
dominant by far in all sampled land uses. Structural traits of shrubs explain this
disparity. However, early results of further research suggest that E. schimperi shrubs
probably act as preferential perching sites for frugivorous birds and that dispersal of
Olea propagules is directed to dense E. schimperi shrubs with a high number of stems
at ground level. The application of appropriate protection of early successional shrubs
combined with simple silvicultural measures can therefore significantly enhance
natural regeneration of indigenous woodlands in the region without having to rely on
expensive plantation activities.
Key words: facilitation, Olea, recruitment, shrubs, succession
Résumé
Importance des arbustes pionniers pour le recrutement de l’olivier sauvage africain
(Olea europaea subsp. cuspidata) dans le Central Tigray, Ethiopie. L’olivier sauvage
africain (Olea europaea subsp. cuspidata; Oleaceae), qui apparaît tardivement dans la
succession, a été examiné dans des zones protégées et pâturées du bassin de la rivière
Geba dans le Central Tigray au nord de l’Ethiopie, sur base d’un échantillon T-square
plotless. L’hypothèse suivante a été testée: le recrutement de O. europaea est plus
important sous certains arbustes colonisateurs précoces que dans des clairières et sous
1
Aerts, R., van der Borght, I., November, E., Behailu M., Hermy, M. & Muys, B. (2005). The
importance of early successional shrubs for recruitment of African wild olive (Olea europaea subsp.
cuspidata) in central Tigray, Ethiopia. In: S.A. Ghazanfar & H. Beentje (eds) African Plants:
Biodiversity, Ecology, Phytogeography and Taxonomy, pp. 00–00. Royal Botanic Gardens, Kew.

2African Plants: Biodiversity, Ecology, Phytogeography and Taxonomy
d’autres arbustes. Tant en zone pâturée que dans des régions protégées, les recrûs ont
été exclusivement trouvés sous des arbustes, principalement sous Euclea schimperi, alors
que Acacia etbaica était de loin l’arbuste dominant dans tous les placeaux
échantillonnés. Les différences de structure des arbustes expliquent cette disparité.
Néanmoins, les premiers résultats des recherches suggèrent que E. schimperi agit
préférentiellement comme perchoir pour les oiseaux frugivores et que la dispersion
des diaspores de Olea est dirigée vers les peuplements denses de E. schimperi. Une
protection appropriée des arbustes pionniers combinée à des mesures sylvicoles
simples pourrait par conséquent accroître la régénération naturelle des forêts claires
indigènes sans avoir à recourir à de coûteuses plantations.
1 Introduction
The ability of rehabilitation areas to recruit and sustain new life forms is a true
measure of their contribution to biodiversity and forest resource conservation (Tucker
& Murphy, 1997). In Africa, relatively few studies have evaluated the effect of early
colonizing shrubs on woody seedling establishment. Shrubs might facilitate, tolerate or
inhibit different stages of tree establishment in abandoned pastures or croplands
(Holl, 2002). Moreover, little is known about the specific consequences of avian seed-
disperser activity and their interaction with perch and non-perch sites for plant
demography (Rey & Alcántara, 2000).
Most research on the ability of trees and shrubs to promote recruitment of woody
plants by attracting seed dispersers has been conducted in the temperate zones of
the northern hemisphere, in the neotropics and in southeast Asia and tropical
Australia (see McDonnell & Stiles, 1983; Guevara et al., 1986; McClanahan & Wolfe,
1987; Guevara & Laborde, 1993; Parrotta, 1993; Debussche & Isenmann, 1994;
Vieira et al., 1994; Kollman & Pirl, 1995; Nepstad et al., 1996; Tucker & Murphy,
1997; Holl, 1998; Ferguson & Drake, 1999; Toh et al., 1999; Alcántara et al., 2000;
Galindo-Gonzalez et al., 2000; Slocum & Horvitz, 2000; Holl, 2002), but Africa, and
the Ethiopian highlands in particular, have received little or no attention so far (but
see Duncan & Chapman, 1999).
It has been observed that seed rain of animal-dispersed trees was higher below shrub
patches (Holl, 2002), scattered, isolated trees and saplings (Debussche & Isenmann,
1994; Duncan & Chapman, 1999; Toh et al., 1999; Slocum & Horvitz, 2000), logs and
fern patches (Slocum, 2000) and artificial perches (McClanahan & Wolfe, 1987; Holl,
1998) than below grass or in open pasture. On the other hand, predation of animal-
dispersed seeds was found to be significantly higher below shrub patches compared
with grass but then again, seedling survival was found to be higher under shrubs (Holl,
2002). The identity of trees or shrubs and whether or not they offer a fruit reward
appears to matter less than their structure and suitability as a bird perch. The
characteristics that make some trees or other structures better recruitment foci than
others, however, are poorly understood (Slocum & Horvitz, 2000).
In Tigray, the northernmost, highly degraded region of Ethiopia, regrowth of woody
vegetation in areas recently closed off to cattle (protected areas or grazing exclosures)
is often striking. The recovery process of the vegetation usually starts with the increase
in number and cover of grassy species. After some years shrubs and small trees develop
and depress the grass component. These shrubs and trees are considered as a key
element in forest rehabilitation since it has been shown that grasses present a
competitive barrier against the establishment of woody species (Aide et al., 1995).
Moreover, grasses tend to attract cattle that may trample and consume recruits