C.K. Ong

Utilisation of light and water in tropical agriculture

The resource capture approach developed by John Monteith has been applied in studies of a wide variety of plant species and cropping systems in the tropics over the past 18 years. The purpose of this review is to highlight the progress made and the new challenges which lie ahead. The foundation for this approach was the establishment of ‘response surfaces’ for the development and growth of tropical crops using controlled-environment facilities. The concepts of light interception and thermal time developed were then used to investigate the mechanisms responsible for overyielding in intercropping systems and genotypic differences in the drought adaptation of crops in the semi-arid tropics. The most significant achievements were in the understanding of temporal and spatial complementarity in intercropping and agroforestry systems and the development of plant growth models. More recently, the same concepts have been extended to the capture of below-ground resources in agroforestry systems and rain forests. The most serious remaining challenge is to extend this approach to studies of complex multispecies systems in the humid tropics.

Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: II. Crop growth and system performance.

Maize and cowpea were grown as sole stands or in agroforestry systems containing grevillea trees (Grevillea robusta A. Cunn.). Crop and system performance were examined over a 4.5-year-period (nine growing seasons) commencing in October 1991; failure of the rains caused the loss of one cropping season. A rotation of cowpea (Vigna unguiculata L. Walp.) and maize (Zea mays L.) was grown during the first five seasons after planting the trees, while maize was grown continuously during the final four seasons. Sole maize was also grown under spectrally neutral shade netting which reduced incident radiation by 25, 50 or 75% to establish the relative importance of shade and below-ground competition for water and nutrients in determining the performance of understorey crops. The above-ground biomass and grain yield of understorey crops were not significantly affected by the presence of grevillea during the first four seasons, but were greatly reduced in subsequent seasons as the trees became increasingly dominant; maize yields reached 50% of the sole crop values only once during the final four seasons, when rainfall was unusually high. The hypothesis that competition for water was the primary limiting factor for understorey crops was supported by the observation that above-ground biomass and grain yield were greater in the shade net treatments than in agroforestry maize, demonstrating that shade was not solely responsible for the substantial yield losses in the latter treatment. Performance ratios (ratio of values for the agroforestry system relative to sole stands) for total above-ground and trunk biomass in grevillea were initially low, reflecting the impact of competition with associated crops during tree establishment, but increased to unity within 2.5 years. Performance ratios for the understorey crops exhibited the reverse trend, initially being close to unity but approaching zero for three of the final four seasons. Performance ratios were never close to unity for both trees and crops during the same season, indicating that there was always competition for available resources irrespective of crop species or tree size. The implications for agroforestry system design and future research are discussed. # 2000 Elsevier Science B.V. All rights reserved.

Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: I. Tree growth

Abstract The work reported here formed part of a multidisciplinary project to examine the changing tree/crop interactions, expressed in terms of resource capture, tree and crop growth and system performance, that occur as the trees in semi-arid agroforestry systems establish and mature. Grevillea robusta (A. Cunn.; grevillea) trees were grown in a dispersed planting arrangement, either as sole stands or in combination with cowpea (Vigna unguiculata L.) or maize (Zea mays L.). Allometric approaches were used to monitor leaf area and leaf, branch and trunk biomass over a 4.5-year period. Seasonal and annual growth increments and the corresponding mean daily growth rates were calculated; crop growth, development and yield were also determined over nine growing seasons. Tree height, leaf area and trunk and branch biomass were reduced in the agroforestry treatment (CTd) during the first 600 days after planting as a result of competition with the associated crops. Tree height subsequently increased more rapidly in the agroforestry treatment (CTd) and regained parity with the sole grevillea treatment (Td) by ca. 820 days after planting. However, although seasonal and annual biomass increments were comparable in both treatments once the trees became the dominant component of the system, above-ground biomass in CTd grevillea never recovered fully to Td values during the observation period; trunk volume and biomass were consistently lower in CTd than in Td grevillea, reducing their economic value. Tree growth continued throughout the dry season, sometimes at rates similar to or greater than observed during the cropping seasons, indicating that the trees were able to utilise residual soil moisture or deep reserves within the profile. This finding is discussed in relation to recent interpretations of niche separation in savanna vegetation. The factors responsible for the observed reductions in tree growth in the agroforestry system are examined; effects on crop yields and system performance are considered in Part II.

Water use in a Grevillea robusta–maize overstorey agroforestry system in semi-arid Kenya

Abstract Novel approaches involving a combination of sap flow measurements of transpiration and allometric estimates of biomass production were used to determine seasonal water use by trees and crops in agroforestry systems. The results were used to test the hypothesis that agroforestry may improve productivity by capturing a greater proportion of annual rainfall than annual crops. Grevillea robusta A. Cunn., which is reputed to have a deep rooting habit, was grown in semi-arid Kenya either as sole stands or in combination with maize (Zea mays L.). Water use by individual trees and maize plants was determined using constant temperature heat balance gauges and scaled to provide stand-level estimates of transpiration based on linear relationships (r2>0.70) between sap flow and leaf area across a range of tree ages and environmental conditions. Maximum stand-level transpiration rates for grevillea ranged from 2.6 to 4.0xa0mm per day, consistent with previous studies in similar environments. Biomass production by grevillea was closely correlated with stand-level transpiration (r2>0.69–0.74), suggesting that non-destructive estimates of biomass increments can be used to provide reliable estimates of seasonal transpiration. Cumulative water use by grevillea over the 4.5-year observation period was comparable in the sole tree and agroforestry treatments, reaching a maximum utilisation of annual rainfall of 64–68% 3–4 years after planting. Approximately 25% of the water transpired by the trees was used during the dry season, indicating that they were able to utilise off-season rainfall, comprising 16% of the total annual rainfall, and residual water remaining in the soil profile after the cropping period. During the 1995 long rains, when 221xa0mm of rain was received, transpiration by sole maize was