Abstract:

A field study was conducted to examine light interception and water use between upper storey 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 up to 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 up to 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.