that there is no competition for labor time. Fourth, the data in Table 4.1 are
best thought of as snapshots of actual situations. In practice, because some
crops are more profitable than others, there will indeed be a process of substi-
tution away from less profitable crops.
Once the shadow pricing exercise is conducted, all systems become socially
profitable. This results almost entirely from the effect of pricing outputs at
border prices because these are well above domestic prices. Whereas the finan-
cial returns to gum arabic exceed those of all crops other than sesame, the eco-
nomic (shadow priced) returns to gum arabic are the lowest. The reason for
this is that the financial returns already reflect border prices because almost all
gum arabic is exported (of course, this does not apply to the fuelwood and
fodder components of A. senegalbenefits), whereas the other crops generally
are not exported.
Barbier was unable to isolate the wider environmental benefits of A. sene-
gal,so that the figures shown in Table 4.1 may well understate the true social
returns to A. senegalagroforestry. A subtle issue is how far these additional
benefits extend. For example, if they result in increased crop productivity, then
they are already reflected in the crop output benefits; they have been internal-
ized. To count them again would be double counting. Therefore, it could be
that many of the environmental benefits have already been accounted for.
Three central roles of gum arabic are thus suggested: it acts as a reasonably
secure and stable form of income, it generates positive net financial and eco-
nomic benefits, and it interacts as a support system for other crops. Barbier’s
major caution is that gum arabic makes financial sense only if the interna-
tional price of gum arabic is sustained. This is a complex issue because there
are available substitutes from synthetic starches, and the political instability in
the Sudan threatens supply stability.
Agroforestry Systems in Practice II: Mixed Crops
and Trees in Kano, Nigeria
A second example of a detailed cost-benefit assessment of agroforestry is pro-
vided by Anderson (1987, 1989) for rural afforestation schemes in the Kano
region of northern Nigeria. Anderson notes that trees on African farmlands
often have not been given any priority, with the result that tree loss has exac-
erbated soil erosion. The background to this may lie in land tenure problems:
tree growing assumes some form of longer-term tenure, low priority is gener-
ally given to the agricultural sector in macroeconomic policies in Africa, the
wider benefits of tree growing are not perceived, and there is the classic exter-
nality issue introduced before. Anderson simulated two forms of tree growing:
trees as shelterbelts on the edge of farmland and farm forestry (i.e., the mix of
crops and trees), both of which qualify as agroforestry. Simply looking at tim-
ber and crop yields fails to capture the wide-ranging benefits of agroforestry.
78 II. The Ecological Economics of Agroforestry