density of speargrass in the
control plot increased with the onset of rains to a peak of 450 stands per m 2
in August and declined steadily thereafter in the dry season until November
1988 when data collection was terminated (Fig. 1). Speargrass densities were
significantly lower under gliricidia and leucaena than in the control plots at
all sampling times except in November 1987. Initial increase in stand density,
starting in February (t = 3 in Fig. 1) in all treatments must have been a
response to the burning that occurred in January, 1988. The increase in
stand density peaked in April and May for speargrass in gliricidia and
leucaena plots respectively and thereafter declined sharply for gliricidia and
gradually for leucaena. This decline occurred during the rainy season when
conditions favoured plant growth and also coincided with development of
canopy cover by the hedgerows. Shading of speargrass by the hedgerows is
therefore implicated in this decline in speargrass density. In the leucaena
plots, decline in speargrass density was gradual, reflecting the more gradual
closing of canopy by leucaena trees. Shading by gliricidia and leucaena
caused a reduction in speargrass density (67% and 51%, respectively) during
the rainy season despite the favourable moisture conditions. Equipment
malfunctioning prevented measurement of light interception by the hedgerows.
The greater effectiveness of gliricidia in reducing stand density was
probably due to its greater canopy cover than leucaena. Gliricidia has
bipinnate leaves each of which is 30 50 cm long and has six or nine pairs of
leaflets that project horizontally to the rachis [Duguma, 1985]. On the
contrary, leucaena produces 15-20 cm long leaflets that are borne at an
acute angle to the rachis. Consequently, canopy of gliricidia trees were able
to overlap and this canopy must have intercepted solar radiation more than
those of leucaena trees in which there was less of foliage at identical
densities.