ther. Besides its agronomic importance, fallow length clearly is an important
issue in the management of shifting cultivation landscapes for biodiversity
conservation objectives.
Mechanisms of Plant Regeneration in Fallows
In general terms, plants may regenerate in fallows from seed already present in
the soil seed bank, from seed dispersed onto the site after it is left fallow, or by
sprouting from cut but living plant parts, either above the ground or in the
form of root suckers (Kammesheidt 1998). Regeneration of fallow vegetation
immediately after the cropping period is predominantly vegetative. Practically
all the trees, shrubs, vines, and large herbaceous perennials and a majority of
the grasses regenerate from stumps, roots, or rhizomes. Factors that impede
seed regeneration of trees and shrubs include burning and weeding during the
cropping period and the short lives of seeds of some species. Burning at
the beginning of the cropping period and frequent weeding also eliminate the
seedlings of trees and shrubs, most of which have only recently germinated.
All these factors impoverish the seed bank in the soil and reduce the contribu-
tion of seed regeneration to the reestablishment of tree cover in the fallow
period (Denich n.d.; Vieira and Proctor 1998).
Few studies have directly addressed the importance of sprouting as a regen-
eration mechanism in tropical secondary vegetation (Kiyono and Hastaniah
1997; Kammesheidt 1998; Pacioreck et al. 2000; van Nieuwstadt et al. 2001).
Although in structural and floristic terms overall successional pathways in fal-
lows may strongly resemble those that follow other types of human land use,
the importance of resprouts as a regeneration mechanism is arguably one of
the defining ecological characteristics of fallow vegetation and secondary for-
est derived from it. Uhl (1987) has shown experimentally that repeated weed-
ing during cultivation can reduce the density of sprouts in fallows. In real sit-
uations, however, the short periods of low-intensity cultivation that
characterize shifting cultivation probably do not significantly reduce the
regenerative capacity of tree stumps and root fragments, which resprout when
land is left fallow.
Species individualism is an important aspect of understanding plant
regeneration and therefore plant community composition and diversity in
fallows. For example, species in communities subject to natural disturbance
differ in the relative importance of resprouts and seed as postdisturbance
regeneration mechanisms (Boucher et al. 1994). Experimental studies are
strictly required to confirm whether tree species regenerate from sprouts, but
descriptive work provides a clear pointer to the existence of patterns.
Kammesheidt (1998) showed that 28 of the 58 tree species he recorded in
small plots in fallows at his Paraguayan study site were regenerated from both
mechanisms, whereas 7 were found only as resprouts and 23 were regenerated
- The Biodiversity and Conservation Potential of Shifting Cultivation Landscapes 161