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
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.