Does more water mean better sorghum yields in the Sahelian Parklands? Not always, say scientists

Increased availability of water boosts tree canopy growth, reducing available light for crops and leading to lower grain yields, say Coulibaly et al in a paper published in Agroforestry Systems. Precipitation does have a key effect on grain yields but tree root patterns and management techniques such as tree density, pruning and mulching also play a major role. Simulating the effects of different management options under current and future climates still requires further empirical corroboration and model improvement. In the meantime, tree canopy pruning to reduce shading while concurrently maintaining tree root function —although carrying the fundamental challenge that trees always strive for equilibrium in above- and below-ground biomass—is probably key to parkland adaptation to a changing climate.Parkland system in Burkina Faso with scattered trees and crops. Tree species such as néré, karité and baobab provide shade for crops, along with wood, leaf and fruit products. The baobab is a giant tree with a big trunk but relatively loose canopy.

Parkland system in Burkina Faso with scattered trees and crops. Tree species such as néré, karité and baobab provide shade for crops, along with wood, leaf and fruit products. The baobab is a giant tree with a big trunk but relatively loose canopy.

 

Parkland agroforestry systems are very common in the drylands of the western African Sahel, often with sorghum and millet as the major crops. Trees are preserved in naturally established, non-systematic patterns with relatively low tree density. Beyond direct competition for water and nutrients, trees provide shade and protection from supra-optimal temperatures and support the maintenance of soil organic matter.

Several studies show a decrease of cereal crop yields under trees such as Parkia biglobosa (néré) and Vitellaria paradoxa (karité or shea). Other studies find no difference between under canopies and in open areas during a good rainfall season. These contradictory results indicate a need for research that will provide a better understanding of the conditions that affect cereal production under canopies. Such research gains more importance in light of current climate variability and climate change, which necessitate adjustments in tree species selection and tree management. The adaptation of parklands systems to climate change is critical if these systems are to continue playing a major role in food security and household economy.

Due to the complexity of interactions in agroforestry systems, modelling is a faster and cheaper way to study them than empirical approaches. The Water, Nutrient and Light Capture in Agroforestry Systems (WaNuLCAS) model has been used for exploring tree-soil-crop interactions in the parkland systems of Burkina Faso, simulating systems where sorghum is grown together with multipurpose trees such as baobab, néré and karité under different management options and climate scenarios.

In the present study, field experiments were conducted in three climatic zones to assess sorghum biomass, grain yield and harvest index in the parklands under different rainfall patterns and root systems, and compared to simulations of sorghum biomass and grain yield with the WaNuLCAS model. Management options studied included tree densities, tree leaf pruning and mulching. Eight trees were sampled for each species in each site with canopy radius ranging between 5–8 m.

The results revealed that sorghum biomass and grain yield was more negatively affected by Parkia biglobosa (néré) compared to Vitellaria paradoxa (karité) and Adansonia digitata L. (baobab), the three main tree species of the agroforestry parkland systems. Sorghum biomass and grain yield was affected not just by precipitation but also by tree canopy density. The harvest index (grain as part of total biomass) was highest under the tree canopy and in the zone furthest from the tree.

Sorghum performance cannot be solely explained by rainfall. Factors like soil fertility, rainfall distribution, crop variety and farmer’s management also play a role. In agroforestry parkland systems more water does not in all cases lead to a better sorghum grain production. In fact, sorghum harvests under canopy in drier sites of the present study were higher than that at the wetter sites. Increasing rainfall is associated with increasing tree canopy that will cause severe shade. Thus lower rainfall means less shade, which in turn means better access to light and better cereal performance.

Some recommendations arising from the study:

  • slow-growing sorghum varieties remain more stable than fast-growing ones, even without any fertilisers, and could be cultivated on a long-term basis in agroforestry parklands;
  • farmers should carry out crown pruning operations at the beginning of the rainy season
  • assisted tree regeneration and mulching also improve crop productivity;
  • pay specific attention to species root architecture when selecting species for reclaiming degraded lands; farmers must also keep in mind that pruning leads to a corresponding reduction in root biomass, and therefore root function.

Agroforestry systems using trees with dynamic canopy and deep root systems are a good option for coping with extreme events like flooding and drought. The WaNuLCAS model tested management options of the agroforestry system under current and projected climate, and is a good planning and testing tool. However, further validation is needed before model scenarios can be trusted to reflect the real choices farmers have. Longer-term monitoring is also required if we have to come up with long-term patterns and trends in link with climate variability and change.

Read the full paper: Coulibaly et al

Coulibaly YN, Mulia R, Sano J, Zombre N, Bayala J, Kalinganire A, van Noordwijk M. 2013. Crop production under different rainfall and management conditions in agroforestry parkland systems in Burkina Faso: observations and simulation with WaNuLCAS model. Agroforestry Systems Online first. 10.1007/ s10457-013-9651-8

Exploring possibilities for forests, trees and agroforestry to play a role in climate change mitigation and adaptation is a key focus of the CGIAR Collaborative Research Programme on Trees, Forests and Agroforestry of which the World Agroforestry Centre is a key partner.

Rebecca Selvarajah

Rebecca Selvarajah

Rebecca is a science writer, manager of publishing projects, trainer in science writing, and novelist — working partly from Nairobi, Kenya and partly from Morwell, Australia. With over 15 years of experience in writing, advertising/marketing, publishing and social media, she takes on varied assignments, travelling, if needed, to conduct relevant research and interviews. Originally from Sri Lanka, Rebecca holds a BA honours in Psychology, with minors in Gender Studies and Sociology. Email Rebecca on r.selvarajah@cgiar.org

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