COS 227-5 - Afforestation changes belowground C dynamics in montane grasslands of the Northern Drakensbergs, South Africa

Background/Question/Methods

In the early 1900’s, pine (Pinus patula) and eucalypt (Eucalyptus grandis) plantations were established in montane grasslands and mesic savannas of the Mariepskop complex (northern Drakensbergs, South Africa). A century after afforestation, abandoned plantations are still drastically altered from their original grassland state. In this study, we used attributes of soil chemistry and stable isotopes (δ¹³C) of soil to (1) determine whether plantation sites were indeed historically C₄ grassland or savanna, and (2) assess long-term impacts of afforestation on soil texture, chemistry, and carbon content. Soil samples were collected from native grassland, native forest, pine plantation, and eucalypt plantations.

Results/Conclusions

Results showed that δ¹³C values from native grassland and native forest sites were consistent throughout the soil profile. Grassland δ¹³C values (-16.7‰ ±0.36 SE) were higher than native forest values (-20.1‰ ±0.24 SE), reflecting the higher contributions of δ¹³C from C₄ compared to C₃ biomass. Eucalypt sites had lower δ¹³C values at the surface (-23.7‰ ±0.37 SE) but values increased with depth until they were similar to grassland sites at 30 cm (-17.7‰ ±0.49 SE). δ¹³C values at pine sites were lower than at eucalypt sites (23.8‰ ±0.31 SE) and did not increase with depth, suggesting that either these sites did not historically have high C₄ cover, or that P. patula has altered the soil substantially enough to mask legacies of C₄ cover. Soil at pine sites also had higher percent nitrogen, higher total organic carbon, and lower pH values compared to other cover types. These data illustrate the consequences of afforestation on soil C dynamics and provide benchmarks for future restoration in abandoned plantation sites.