Abstract:

A field study was conducted to examine light interception and water use between upperstorey
trees (Alnus acuminata, Casuarina equisetifolia, Grevillea robusta, Maesopsis
eminii and Markhamia lutea) and annual crops, maize (Zea mays) and beans (Phaseolus
vulgaris), in a sub-humid environment in Uganda, East Africa from 1996 to 1998.
The presence of the trees significantly (p<0.001) reduced the yield of the associated crops
when compared to sole crops. Maize yields were reduced to a distance of 5 m from the
tree lines in the last season (August-November 1998) by 59, 58, 49, 33 and 25% in the
Casuarina, Maesopsis, Markhamia, Grevillea and Alnus plots respectively. Bean yield
reductions during the August-November 1997 season were 60, 43, 27, 22 and 23% for
Maesopsis, Markhamia, Casuarina, Grevillea and Alnus plots respectively.
The fraction of the incoming photosynthetically active radiation (PAR) incident on the
maize crop was significantly (p<0.001) reduced by the presence of the trees to a distance of
5 m from the tree lines in the last season. Maesopsis, Casuarina, Markhamia, Grevillea
and Alnus reduced the fraction of the incoming PAR by 59, 31, 31, 29 and 23%
respectively. Mean fractional PAR interception by the maize crop associated with the
different tree species, to 5 m distance from the trees, was 0.28, 0.32, 0.35, 0.54 and 0.53 in
the Maesopsis, Markhamia, Casuarina, Grevillea and Alnus plots respectively. The lateral
extent of tree shading can be predicted from knowledge of tree height, canopy height and
spread. The modeled shading effects of the trees showed that tall trees (Casuarina) or
trees with wide canopies (Maesopsis, Casuarina) have significantly bigger shading effects
on associated crops than short or trees with narrow canopies (Markhamia). The
relationship between the amount of the incoming PAR intercepted by the maize crop and
the biomass produced showed a significant positive correlation.
Tree root densities decreased with increasing distance from the tree lines and with depth.
The tree species showed varying patterns of root development. The proportion of tree
roots within 2 m distance from the tree lines were 83, 77, 70, 69 and 58% for Alnus,
Casuarina, Markhamia, Grevillea and Maesopsis respectively. The mean number of roots
within the top 90 cm of the soil were found to be 79, 77, 75, 71 and 64% for Markhamia,
Grevillea, Alnus, Casuarina and Maesopsis respectively. The presence of the trees
significantly (p<0.01) reduced maize root densities upto 4 m (Maesopsis, Markhamia), 2 m (Casuarina, Alims') and I m (Grevillea) distances from the tree lines.
Water uptake patterns by the tree species closely followed their rooting patterns. The tree
plots significantly (p<0.05) lost more water than sole maize plots. Plot averages of
seasonal water loss were 590, 630, 640, 662, 682 and 698 mm for sole maize, Alnus,
Markhamia, Grevillea, Maesopsis and Casuarina plots respectively. These high water
losses from the trees were associated with greater soil water depletion than in sole crop
plots. Maximum soil water depletion in the upper 1.8 m of the soil profile during the last
season was 160, 158, 148, 133, 84 and 100 mm for Casuarina, Maesopsis, Grevillea,
Markhamia, Alnus and sole maize respectively. The trees significantly (p<0.01) reduced
water use by maize plants upto 5 m distance from the tree lines with the effect varying
according to species. Casuarina and Maesopsis had the greatest effect followed by
Markhamia. Grevillea and Alnus had the least effect on maize water use. Water use by
the trees decreased with increasing distance from the tree lines and in the last season,
individual trees used 134, 124, 112, 113 and 85 kg of water day'1 for Maesopsis,
Casuarina, Grevillea, Markhamia and Alnus respectively.
Total soil evaporation (Es) was greatly reduced by the tree canopies compared to under the
maize canopy and bare soil. Total Es under the tree canopy was 49% of bare soil, while
under the maize canopy it was 71 % of bare soil.
The evaluation of this agroforestry system (three years of intercropping) in terms of
resource use indicated that interplanting Maesopsis and Grevillea with maize had a
marginal advantage over sole cropping in terms of water use and light utilization.