Agronomic and Physiological Responses of Sorghum and Sweet Potato to Shea Tree Shade in Burkina Faso

Technical Summary: Agronomic and Physiological Responses of Sorghum and Sweet Potato to Shea Tree Shade in Burkina Faso

Problem Statement
In Burkina Faso, agroforestry parklands integrating Vitellaria paradoxa (shea tree) with cereal crops are vital for food security. However, the shade cast by shea trees significantly reduces photosynthetically active radiation (PAR), leading to competition for light and a documented decline in the productivity of associated cereal crops like sorghum (Sorghum bicolor). This yield reduction often drives farmers to neglect seedling regeneration or deliberately remove trees, threatening the long-term sustainability of these parklands. While root and tuber crops are known to be shade-tolerant, there is a lack of specific data on how the dendrometric characteristics of shea trees—specifically diameter at breast height (DBH) and crown area—differentially affect the agronomic and physiological performance of sorghum versus sweet potato (Ipomoea batatas). The study addresses the need to identify critical tree size thresholds where sorghum yields become unviable and to determine if sweet potato can serve as a viable alternative to maintain agroforestry productivity without tree removal.

Methodology
The research was conducted in the Nazinon region of Burkina Faso, specifically in the municipalities of Nobéré and Saponé. The study utilized a randomized complete block design (RCBD) to evaluate two crops: a local sorghum landrace and two biofortified sweet potato varieties (Erè and Tiébélé 2).

• Tree Classification: V. paradoxa trees were categorized into three diameter classes (DC) based on DBH:
  • DC1: 15–30 cm
  • DC2: 30–45 cm
  • DC3: ≥45 cm
• Experimental Treatments: Crops were grown under the canopy of trees in each diameter class (UCDC1, UCDC2, UCDC3) and in open areas (OA) serving as controls.
• Measurements:
  • Dendrometric: Crown area and PAR interception were measured.
  • Morphological: Stem/vine length, leaf number, and collar diameter were recorded at 30, 45, 60, and 81 days after sowing/planting.
  • Physiological: Stomatal conductance, chlorophyll content (SPAD), and linear electron flow (LEF) were measured using portable devices (AP4 porometer and PhotosynQ MultispeQ).
  • Yield: Grain/panicle yield for sorghum and tuber yield for sweet potato were recorded.
• Analysis: Data were analyzed using the Kruskal–Wallis test followed by Dunn's post hoc test with Bonferroni correction at the 5% significance level.

Key Results

• Light Interception: There was a significant negative correlation between tree diameter and PAR transmission. DC1 trees (smallest crown area) transmitted the highest PAR (274 µmol photons m⁻² s⁻¹), while DC2 and DC3 transmitted significantly less (162.75 and 142.84 µmol photons m⁻² s⁻¹, respectively).
• Sorghum Performance:
  • Sorghum growth and yield were highest in the open area (OA).
  • Under the canopy, sorghum performance was significantly better under DC1 than under DC2 or DC3.
  • However, yields under DC1 were still significantly lower than in the open area (OA). The study notes that sorghum growth and production were low compared to the open area, even under the smallest diameter class.
  • Physiological data showed that while stomatal conductance was higher under the canopy (due to a favorable microclimate), the linear electron flow (LEF) and overall growth were limited by low light availability, particularly in DC2 and DC3.
• Sweet Potato Performance:
  • In contrast to sorghum, sweet potato exhibited superior growth and yield under the larger diameter classes (DC2 and DC3) compared to DC1 and the open area.
  • Morphological parameters (stem length, leaf number) and tuber yields generally increased with tree diameter class.
  • The highest tuber yields were recorded under DC3. Notably, at the Nobéré site, no tubers were produced in the open area or under DC1, whereas production occurred under DC2 and DC3.
  • While yields under DC2 and DC3 were generally higher than under DC1, statistical analysis (Table 7) indicated that the difference in yield between DC1 and DC2 was not significant (both sharing the same superscript letter), whereas DC3 yielded significantly more than the other classes.
  • Sweet potato varieties showed higher chlorophyll content under the denser shade of DC2 and DC3, an adaptation to lower light intensities.
• Site Variations: Nobéré (higher rainfall, more intensive tree pruning) generally favored sorghum, while Saponé (lower rainfall, less pruning) favored sweet potato, likely due to differences in water availability and canopy density.

Key Contributions
The study provides empirical evidence that the impact of V. paradoxa shade on associated crops is not uniform but is strictly dependent on the tree's diameter class.

1. Threshold Identification: It identifies DC1 (DBH < 30 cm) as the critical threshold where sorghum can be cultivated with *less* yield loss compared to larger trees, though yields remain significantly lower than in open fields. Conversely, DC2 and DC3 (DBH > 30 cm) cause significant yield reductions in sorghum.
2. Crop Substitution Strategy: It demonstrates that sweet potato is a highly effective shade-tolerant alternative that thrives under the dense canopies of larger shea trees (DC2 and DC3), where cereal crops fail.
3. Physiological Insight: The research elucidates that while shade improves microclimatic conditions (higher stomatal conductance), the reduction in PAR is the primary limiting factor for C4 crops like sorghum. Conversely, the C3 nature of sweet potato and its morphological plasticity allow it to capitalize on the shade-induced microclimate and lower light compensation points.

Significance and Claims
The authors claim that this research offers a practical, science-based management strategy for Burkina Faso's agroforestry parklands. By aligning crop choice with tree diameter, farmers can optimize total system productivity without resorting to tree removal or intensive pruning.

• Recommendation: The paper explicitly suggests cultivating sorghum only under DC1 trees (DBH < 30 cm) and avoiding it under DC2 and DC3.
• Alternative: For trees with DBH > 30 cm (DC2 and DC3), the authors recommend growing sweet potatoes to prevent shea production losses (which would occur if trees were pruned) and to avoid cereal yield losses (which occur if sorghum is planted under large trees).
• Conservation Impact: This approach aims to maintain or increase tree density in parklands by providing an economic incentive (sweet potato yield) to preserve larger shea trees, thereby addressing the decline in woody vegetation observed in the region.

The study concludes that managing agroforestry systems based on tree diameter classes allows for the simultaneous optimization of tree conservation and crop production, offering a sustainable pathway for food security in the Sahelian zone.