Productivity, pest tolerance and carbon sequestration of Khaya grandifoliola in the dry semi-deciduous forest of Ghana: a comparison in pure stands and mixed stands

Abstract

In order to restore biodiversity in the degraded forest landscape and to use forest plantations for climate change mitigation, experimental plantations of indigenous trees (including mahogany species) and important exotic trees species like Tectona grandis have been established in pure and in mixed stands in the degraded Tain Tributaries Block II Forest Reserve in the dry semi-deciduous forest zone of Ghana. This study assessed the performance of an important indigenous species, Khaya grandifoliola, in pure and in mixed stands, and compared its performance to the exotic tree species, T. grandis. The results from the study indicated that after 4 years, there was a significant difference in the diameter of K. grandifoliola (P = 0.001) between pure and mixed stands with the pure stands having an average diameter of 9.15 ± 0.19 cm compared with 7.81 ± 0.33 cm for mixed stands. Pure stands had a correspondingly higher basal area at breast height for individual trees in pure stands compared with mixed stands. K. grandifoliola also recorded average total height of 5.50 ± 0.13 m and merchantable height 3.63 ± 0.09 m in pure stands, compared to total height of 5.04 ± 0.24 m and merchantable height of 3.52 ± 0.18 m in mixed stands. However, these values were not significantly different between the stands (P > 0.05). Basal area at breast height for K. grandifoliola grown in pure stands was 5.5 ± 0.3 m2/ha at age four, which was significantly higher than the basal area of 1.1 ± 0.4 m2/ha at breast height for K. grandifoliola in mixed stands. Also total volume per hectare was higher in pure stands (17.8 ± 0.9 m3/ha) than in mixed stands (3.4 ± 0.6 m3/ha). Consequently, K. grandifoliola accumulated more carbon in pure stands (10,126 ± 557.2 kg/ha) than in mixed stands (1,976 ± 335.1 kg/ha), but the mixture of the tree species, including K. grandifoliola, accumulated more carbon (11,929 ± 401.3 kg/ha) than the K. grandifoliola in pure stands though not statistically different. Overall, K. grandifoliola performed better in pure stands than in mixed stands. With regards to the tolerance to pest attacks on K. grandifoliola, it was more tolerant to pests’ attacks in mixed stands than in pure stands. There was no statistical difference in diameter growth between K. grandifoliola (9.15 ± 0.19 cm) and T. grandis (9.61 ± 0.26 cm) in pure stands. The values of total height, merchantable height and total volume for T. grandis were 8.22 ± 0.20 m, 5.38 ± 0.18 m and 22.5 ± 1.7 m3/ha respectively which differed significantly from 5.50 ± 0.13 m, 3.63 ± 0.09 m and 17.8 ± 0.9 m3/ha for K. grandifoliola for the same parameters (P < 0.05). However, there was no significant difference between the two species with respect to basal area per hectare (P = 0.189); K. grandifoliola grew to 5.5 ± 0.3 m2/ha and T. grandis grew to 4.8 ± 0.3 m2/ha. T. grandis in pure stands accumulated slightly more aboveground biomass than K. grandifoliola in pure stands after 4 years though not statistically different.