Plant productivity, radiation interception and water balance as indicators of tree-crop interactions in hedgerow intercropping systems : a Jatropha - Kikuyu case study

The potential of agroforestry to alleviate problems related to scarcities of arable land, water, food and fuel wood is subject to understanding system functioning and implementing and managing an efficiently designed system. The objectives of this study were to understand interactions and productivity of a hedgerow intercropping system with reference to water and radiation use, and analyse system design and management scenarios in order to enhance returns. Field trials monitoring soil water, solar radiation and plant productivity were conducted during 2006-2008 at Ukulinga Research Farm (KwaZulu Natal, South Africa) using a Jatropha-Kikuyu (Pennisetum clandestinum) hedgerow intercropping system as case study. In order to extrapolate results, a process-based hedgerow intercropping model was developed by building intercropping and tree growth into the SWB-2D model. Data collected from the field trials were used to parameterise and evaluate the model, which was used to analyse hedgerow orientation and spacing to determine income scenarios of virtual system and to help develop design criteria. Allometric relationships of Jatropha using basal stem diameter and crown width as predictor variables were found to be very reliable. Stem diameter was linearly related with wood and branch proportions and inversely proportional to foliage. Neither below-ground (BG) interspecies competition nor tree spacing had any significant effects on allometry. Allometric equations were proven valid for accurate, non-destructive and rapid predictions of tree growth under various growing and non-destructive canopy management conditions. When interspecies competition was present, none of the tree spacing/arrangement options tested resulted in consistently highest tree relative growth rates (RGR). Treatments had no effect on tree RGR when high water availability and kikuyu dormancy coincided. The single-row treatment (SR) produced the shortest trees, but generally had the highest stem RGR during low rainfall periods. The standard-spacing treatment (SS) had the highest RGR during the spring and summer seasons. Jatropha-only treatment (JO) trees were the tallest and biggest. Treatments affected post-pruning tree height increase, even when rainfall was high. Length of tree-crop interface (TCI) generally decreased tree yield, especially as trees matured toward their maximum-yield age (4-5 years). SR trees showed slow response to pruning due to a high TCI. They, however, exhibited compensatory growth during May to August, when competition for water with grass was low. BG competition reduced tree nut yield more than tree biomass. Tree spacing/arrangements had no effect on tree harvest index. Soil water varied among treatments and was asymmetrically distributed across tree hedgerows. System ET was generally the highest in SR and lowest in the double-row treatment (DR). Differences were mainly due to transpiration. Treatments affected tree root distribution, which was inferred using correlations between tree RGR and soil water deficit (SWD). In JO and SR, fine tree roots were asymmetrically distributed. Their distribution in DR was essentially symmetrical. Strong vegetative RGR-SWD correlations during the 2007/08 season indicated that tree growth was mainly water-limited. Though DR and SR had comparable tree RGRs, DR produced less grass than SR. This implied DR had more intensive BG competition than SR. Interspecific competition was severe due to a lack of temporal complementarity between Jatropha and kikuyu and a shallow soil profile (0.6 m). Tree water uptake predominantly came from the 0.2 – 0.6 depth, which had about 8.6% of the total root biomass in the profile. There was no clear relationship between intercrop growth and root distribution. Radiation use efficiency of kikuyu decreased towards tree hedgerows possibly due to preceding interaction of the irradiance with tree canopy reducing photosynthetically active radiation. The effect of radiation distribution on tree-crop (T-C) interactions was mainly to magnify effects of water. Finally, tree spacing/arrangement could be manipulated to optimise radiation and soil water distribution and intercrop growth. Predictions of solar radiation distribution, profile water content and tree water use were quite accurate. In general, intercrop productivity simulations were acceptable. Intercrop growth was overestimated when rainfall was high and underestimated when rainfall was low. During model calibration, tree woody biomass, leaf area index, crown width and nut yield were predicted adequately, while leaf dry mass was overestimated. During model validation, woody biomass and crown width were simulated reasonably well. However, foliage biomass, leaf area index and nut yield were overestimated. Overall, adequacy of the model for simulating tree productivity was established. Using scenario modelling, model capabilities to facilitate design/planning and management of hedgerow intercropping systems and interpretation of model outputs were demonstrated. The model can be used to determine the T-C trade-off that yields maximum income. By selecting best-case row orientation and spacing scenarios using the model, and keeping in mind values of tree and intercrop yields, system returns can be maximised. Tree crown growth can also be predicted in order to decide on the extent and timing of pruning. The present model is applicable to any potential tree-intercrop combination. It should be linked to a nutrient simulator of SWB, its component, and appraised further by considering shade-intolerant and shade-loving crop species, along with evergreen and deciduous tree species. This provides model users with numerous T-C combinations to choose from. Various tree spacing/arrangement options can also be explored using the model in order to realise the full potential and implications of the experimental findings of this study and others.