Effects of Bark Harvesting Intensity on the Phytochemical Composition of Vitellaria paradoxa and Prosopis africana in Selected Local Government Areas of Jigawa State, Nigeria

Bark of Vitellaria paradoxa and Prosopis africana is widely harvested for traditional medicine in northern Nigeria, yet the effect of harvesting intensity on its phytochemical composition is poorly documented. This study compared the phytochemical composition of bark from overharvested (OH; ≥50% bark removed in the preceding two to three years) and minimally harvested (MH) trees in Kazaure, Gwiwa, Roni and Yankwashi Local Government Areas (LGAs) of Jigawa State, Nigeria. Seventeen bark samples (13 V. paradoxa, 4 P. africana) were screened qualitatively for eight phytochemical classes, and saponins, flavonoids, tannins, alkaloids and cardiac glycosides were quantified. All eight classes were present in every sample. In V. paradoxa, overharvested trees had significantly higher saponin (10.67 ± 1.59% vs 8.24 ± 1.85%; p = 0.028) and cardiac glycoside (0.86 ± 0.10% vs 0.73 ± 0.08%; p = 0.032) contents than minimally harvested trees, whereas flavonoid, tannin and alkaloid contents did not differ significantly. No significant differences were detected in P. africana, which was sampled in Kazaure only (n = 2 per group). Among LGAs, flavonoid content in V. paradoxa differed significantly (p = 0.017), being higher in Kazaure (17.50 ± 2.65%) than in Gwiwa (10.00 ± 1.73%; Tukey p = 0.035); the other compounds did not differ among LGAs. Saponin and cardiac glycoside contents were higher in overharvested than in minimally harvested trees in all four LGAs. Bark removal did not eliminate any screened phytochemical class and was associated with higher saponin and cardiac glycoside concentrations in V. paradoxa. Replicated sampling at larger scale is required to confirm these patterns and to relate them to tree health and sustainable harvesting.