Can productivity and post-pruning growth of Jatropha curcas in silvopastoral systems be regulated by manipulating tree spacing/arrangement without changing tree density?

Abstract

The potential of tree spacing/arrangement to alleviate effects of interspecific competition of hedgerow intercropping systems on productivity and response to pruning of Jatropha curcas (Jatropha) was investigated using a Jatropha – Pennisetum clandestinum (kikuyu) silvopastoral system at Ukulinga (KwaZulu Natal, South Africa). Treatments were differentiated by presence/absence of kikuyu and arrangement/spacing of Jatropha trees (one, two and three tree hedgerows on either side of the alley) and had the same tree density (1110 ha−1). When high water availability and kikuyu dormancy coincided, tree growth rates did not significantly differ across treatments. Trees of the treatment without interspecific competition (tree-only) were the tallest. Even when rainfall was high, post-pruning tree height growth rate was affected by belowground (BG) interspecific competition and tree spacing/arrangement. Treatment with a single tree hedgerow between alleys had the most frequent maximum stem growth rate and was the most efficient during limited water availability. Its trees showed slow initial response to pruning due to a high tree-grass interface, followed by compensatory growth when competition for water with grass was low. Generally, length of tree-grass interfaces affected yield inversely especially as trees matured toward their maximum-yield age (4–5 years). BG competition reduced tree yield more than tree biomass, while tree spacing/arrangement did not affect tree harvest index. Manipulation of tree arrangement/spacing without changing tree density had no consistent effects on tree productivity.