functional diversity of weeds in crop fields and young fallows should not dis-
tract us from the fact that their value from the point of view of taxonomic bio-
diversity conservation is very low.
All consideration of the utilitarian or conservation value of noncrop plant
species in the context of shifting cultivation must eventually return to the fact
that a weed is, by definition, a plant that is undesirable to the farmer. The
degree to which weed control is attempted during the growth of a crop is
variable and may be low, although this activity seems bound to limit the accu-
mulation of species diversity during the cropping phase, especially when
accompanied by burning after harvests during that phase (Watters 1971; Lam-
bert and Arnason 1989; de Rouw 1995).
Plant Biodiversity of Fallows in Shifting Cultivation
In terms of taxonomic and functional composition of woody species, fallows
and the secondary forest arising from them share many characteristics with
secondary vegetation developing in other situations. A low representation of
forest-dependent plant species, other than those conserved by farmers for
some reason, is a universal characteristic of fallows and therefore a major lim-
itation on their conservation value.
Among pioneer trees, the first ones to dominate fallows, numerous genera
and species are widely distributed in secondary lowland tropical moist vegeta-
tion. Examples for the neotropics (Finegan 1996) are Cecropia, Croton, Helio-
carpus, Trema, and Trichospermum with short-lived species and Cordia,
Guazuma, Rollinia, Spondias, and Vochysia with long-lived species. Didy-
mopanax(Schefflera) morototoni, Jacaranda copaia,and Simarouba amaraare
additional examples of long-lived pioneer species. This is also true for the pale-
otropics. Examples of short-lived pioneer genera and species in Africa are
Anthocleistaspp., Harungana madagascariensis, Macarangaspp., Musanga cecro-
pioides,and Solanum verbascifolium.Long-lived pioneer species are represented
by Albiziaspp., Aucoumea klaineana, Milicia(Chlorophora)spp., Ricinodendron
heudelotii, Terminalia superba, and Triplochyton scleroxylon. Several of these
species are major timber species in Africa, and Finegan (1992) and Chazdon
and Coe (1999) have pointed out the prevalence of desirable characteristics
among the woods of the neotropical long-lived pioneers. In Asia (Whitmore
1983, 1984; Riswan and Kartawinata 1991) we again find similarly widespread
genera of short-lived pioneers, such as Melastoma spp., Macaranga spp., and
Tr e ma spp., accompanied by long-lived pioneer species such as Anthocephalus
chinensis, Duabanga molucana, Endospermum spp., and Octomeles sumatrana.
In general, the species richness and diversity of fallows are initially low but
higher than in crop fields, and they increase over time as the vegetation devel-
ops; several decades may elapse before these parameters approach values simi-
lar to those of primary forest in small sample plots (Lescure 1986; Saldarriaga
172 III. The Biodiversity of Agroforestry Systems