Alternatives to Slash-and-Burn 359is time-consuming and difficult, necessitating expertise in tropical plant identifica-
tion and classification. The functional analysis uses a combination of adaptive
morphologic or functional features (leaf size class, leaf inclination class, leaf form
and type) and enables rapid characterization by people with minimal training. It
includes measures of site physical features, vegetation structure, species composi-
tion, and PFTs (Gillison, 2001, 2002). Results from the benchmark sites are found
in Gillison (2000).
Assessing diversity of below-ground biota is even more complex than above-
ground vegetation, partly because many of the species have never been identified
but also because sampling strategies that capture the spatial heterogeneity of the
different types of biota have not been developed. The ASB below-ground biodiver-
sity group designed a prototype sampling strategy and focused on assessing the
biodiversity of certain functional groups of soil biota including macrofauna (earth-
worms, ants and termites), nematodes, arbuscular mycorrhizal fungi and rhizobial
microsymbionts. Methods and results are presented in Swift and Bignell (2001).
Agronomic sustainability
The majority of soils in the humid tropics are acid and have low native fertility
(Sanchez, 1976). Crops planted after slash-and-burn benefit from the nutrients in
the ash, but rapid nutrient depletion takes place with successive nutrient removal
in crop harvests, nutrient leaching, run-off and erosion promoted by high rainfall,
and rapid decomposition of soil organic matter after burning. Soil physical proper-
ties also degrade with exposure caused by removal of the protective vegetation, and
weeds invade fields, both of which contribute to declining crop yields (Sanchez
et al, 1987; Juo and Manu, 1996). The long vegetative fallow characteristic of
traditional shifting cultivation restores soil physical properties, accumulates car-
bon and nutrients in the fallow biomass, and eradicates weed populations. But as
fallows shorten, their ability to perform these functions diminishes. The sustaina-
bility of the different land-use systems depends on the ability to maintain these
vital ecosystem functions. A set of measurements that could indicate the sustaina-
bility of the systems was developed and includes soil structure and biological activ-
ity, nutrient balances and replacement costs, and weeds, pests and diseases. These
criteria were assessed for the different land-use systems and then, based on expert
judgement, translated into scales indicating the relative degree of difficulty farmers
would face in solving the problem.
Household economic and social concerns
Regardless of the global environmental benefits or agronomic sustainability of a
land-use system, farmers cannot be expected to adopt it unless it contributes more
to meeting household objectives, does not entail excessive risks, and is compatible
with the social and cultural norms of the community. The promotion of systems
with greater environmental benefits must specifically consider the profitability,
labour needs, food security and equity issues associated with them, as well as the
institutions needed.