Tree–crop interactions: agroforestry in a changing climate
Twenty years after the first edition of the standard book on tree–crop interactions, edited by Peter Huxley and Chin Ong, we now have a second edition. The second edition has explicit attention to climate change, with chapters on microclimate effects and consequences for the various terms of the water balance.
The primary strength of the book remains the focus on a process-level understanding and, as such, on results beyond the location-specific empirical selection of best practices in a given context. The various chapters help in reasoning how changing conditions may have to be accompanied by changing practices, based on what we know of the balance between competition and complementarity. Apart from a chapter on water and two on roots, the focus remains on aboveground interactions; a volume with deeper analysis of belowground interactions had been published ten years ago.
Research traditions in agroforestry keep oscillating between direct empirical work on ‘options in context’ and efforts to build more fundamental understanding that can be used by linking databases of tree properties to quantified environmental conditions.
The book starts with a framework for quantifying the various effects of tree–crop interactions that relates the net effects of interaction on growth to the degree of nutrient and water limitation of the crop when grown as monoculture. Benefits of agroforestry may, thus, be expected to be clearest in conditions of intermediate productivity. Even when approximating potential yields, however, some degree of ‘over-yielding’ in a mixture is possible, as reflected in ‘land equivalent ratios’ (LER) above 1.0. In current terminology, such situations reflect a ‘negative yield gap’, as yield gap is 1 – LER. From this perspective, agroforestry remains a primary mode to achieve ecological intensification. From the crop’s perspective, trees provide a more buffered climate where higher water-use efficiencies partially compensate for competition.
The final chapter of the book considers a number of emerging topics and key challenges for agroforestry in the future: progressive climate change, rediscovered effects of trees on rainfall, evidence for the increase of tree cover on farms across the various ecological zones, renewed interest in trees as sources of biofuel and the continued relevance of tree diversity to spread risk.
Hopefully, this second edition will soon be surpassed by new research findings but the book provides solid shoulders for young scientists to climb on and reach a wider vision.
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