Nutrients and Carbon in the Stemwood and Stembark of Khaya spp. Plantations

Nutrient removal through timber harvesting can affect the long-term sustainability of forest plantations, particularly when species differ in biomass allocation and nutrient use. Understanding these differences is therefore essential for developing species-specific nutrient management strategies. This study evaluated differences in stem biomass allocation, nutrient and carbon stocks, and nutrient use efficiency (NUE) among 8.6-year-old Khaya grandifoliola, K. ivorensis, and K. senegalensis. This study was conducted in a plantation located at Reserva Natural Vale, Brazil, where five trees per species were sampled. Discs collected from the base, middle, and top of the commercial stem were separated into stemwood and stembark for chemical analyses. Biomass, nutrient and carbon stocks, and NUE were estimated, and differences among species were assessed using univariate and multivariate analyses. Macronutrient concentrations in the stembark followed the order Ca > N > K > Mg > S > P. K. senegalensis had the highest proportion of stembark (24.13%) and accumulated greater stocks of several nutrients and carbon in this compartment, as well as higher N, P, and Mg stocks in stemwood. In contrast, K. ivorensis showed the highest K and Fe stocks in stemwood, whereas K. grandifoliola accumulated the greatest amount of Ca. Stemwood carbon stocks averaged 11.77 Mg ha−1 and did not differ among species. Nutrient use efficiency also differed among species, with K. ivorensis showing greater efficiency for N, Ca, Mg, and Cu, K. grandifoliola for K, Zn, and Mn, and K. senegalensis for Fe. The results revealed species-specific patterns of biomass allocation, nutrient accumulation, and nutrient use efficiency. K. senegalensis, which had the highest proportion of stembark, also accumulated greater stocks of several nutrients and carbon in this compartment, highlighting the potential value of retaining bark after harvesting to support nutrient return to the soil. However, the species-specific patterns of nutrient accumulation indicate that the implications for nutrient export depend on the element considered. These findings can inform species selection and harvesting practices aimed at improving nutrient conservation in Khaya spp. plantations.