Dynamic biogeochemical shifts after conversion from natural forests to exotic pine plantations.

Conversion from natural temperate forests to exotic plantations can substantially alter carbon ecosystem dynamics. We assessed the biogeochemical consequences of forest conversion by establishing 10 paired natural Nothofagus spp. and Pinus spp. plantation sites across contrasting soils and quantified aboveground tree biomass and total C stocks. We also measured forest carbon dynamics such as litterfall, fine root biomass, and soil CO₂ efflux for three years. C inputs were significantly higher in natural systems than in plantations (5.17 ± 1.19 vs. 3.19 ± 0.99 Mg C ha⁻¹ year⁻¹). Natural forests exhibited higher total C losses (-3.19 ± 1.68 Mg C ha⁻¹ year⁻¹) than pine plantations (-2.54 ± 1.52 Mg C ha⁻¹ year⁻¹). Yet the balance between input and losses was still greater in the native forest. Native forests had greater root production, while plantations had higher litterfall and decomposition rates, potentially accelerating C mineralization.  Across sites, native forests stored an average of over twice the aboveground carbon and maintained significantly higher total above and belowground C than adjacent plantations. Despite the generally recognized higher early-stage aboveground carbon accretion in fast-growing plantations, our adult natural forest sites showed a more balanced allocation of C between above and belowground compartments. Exotic forest plantation biogeochemical fluxes differ from those of native forests, potentially affecting ecosystem benefits related to soil carbon storage. Our findings also highlight that the response of carbon dynamics after conversion is strongly modulated by soil type, emphasizing the need to consider soil/vegetation interactions when assessing carbon benefits or designing management strategies.