An index for quantifying the trade-off between drainage and productivity in tree–crop mixtures

The introduction of deep-rooted perennial species into catchments dominated by annual crops and pastures forms part of the strategy for managing dryland salinity in south Australia. This paper provides a methodology for determining whether it is better to mix trees and crops (agroforestry), or segregate them into plantations and monocrops, when attempting to achieve specified drainage and productivity targets. We introduce an index that quantifies the complementarity or competition for resources between the trees and crops. Data required to calculate this index include crop yield with distance from the tree belt and leaf area of the tree belt compared to the leaf area of a native stand. The method allows for a simple assessment of the most promising tree/crop mixtures. Such an assessment is needed because of the wide range of possible tree–crop–soil–climate combinations and the hydrological complexity of the tree/crop interface. Examples are given which make cases for either separating or mixing trees and crops. We predict that the success of a tree/crop mixture becomes less likely with declining crop season rainfall and increasing seasonal variability and more likely when the tree products have a direct economic benefit.