Characteristics of the root system of four tree species planted on Plinthosols in Burkina Faso: application of the soil-briquette method

Abstract

We observed the growth of saplings of four common tree species (Sesbania sesban, Leucaena leucocephala, Piliostigma reticulatum, and Parkia biglobosa) in Burkina Faso after planting on degraded land with Plinthosols. The saplings were raised by two methods as treatments. The soil briquette (SB) method (or “HOIKU Block” in Japanese), which has been proven to be effective in regreening trials in Japan, emphasizes vertical root development to promote rapid establishment after transplanting. We applied this method to raise saplings for comparison with the conventional plastic pot (PP) method used in Africa. In total, 288 saplings were planted at the study site in July 2017. The survival of SB saplings was approximately 90% at 29 months after planting, excluding mortality caused by disease and insect damage. The mean basal diameter and tree height of SB saplings were slightly higher than those of PP saplings for the four species at 29 months after planting, especially for P. reticulatum, and significant differences were detected in certain cases. Two destructive surveys at 12 months (61 trees sampled in August, 2018) and 39 months (65 trees sampled in October, 2020) after planting revealed that the belowground biomass (BGB), aboveground biomass, and total biomass of SB saplings exceeded those of PP saplings for three species, excluding P. reticulatum, and significant differences were detected in some instances. However, maximum root depth (MRD) and BGB were not correlated at either 12 or 39 months after planting. Approximately 70% of MRD values at 12 months (mode=42 cm) were distributed between 40 and 55 cm soil depths, and a similar percentage was observed within the same range of soil depths at 39 months (mode=40 cm). Hardpan depths measured by mechanical auger excavation at 162 sampling points adjacent to the experimental plot revealed that the mode was at 40 cm depth, which is relatively shallow and thus may hamper root growth. The BGB increased by 2.4-9.8 times (for the four species, and for SB and PP saplings) between the two destructive surveys, inferring that the depth of the hardpan strongly limited the vertical growth of straight roots, and that most roots expanded and increased biomass only at shallower depths above the hardpan layer, regardless of tree species and sapling type. In this study, the SB method did not notably promote root penetration through the hardpan layer. However, the SB method was confirmed to be applicable as a nursery and regreening method in Africa, given its suitability for local materials and the rapid growth at the planting stage. Optional planting improvements, such as mechanical boring, may be helpful to induce straight root growth of trees in areas where a hardpan layer exists.