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Dryad

Kruger National Park EBP carbon

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Sep 08, 2023 version files 31.19 KB

Abstract

  1. Woody plant encroachment (WPE) is a global trend occurring in many biomes including savannas and accelerates with fire suppression. Since WPE can result in increased storage of soil organic carbon, fire management which may include fire suppression can enhance ecosystem carbon sequestration in savannas.
  2. At our study site in Kruger National Park, South Africa, we used a long-term (~70-year) fire experiment to study the drivers and consequences of changes in woody (trees and shrubs) cover and soil organic carbon (C) sequestration. We surveyed four fire manipulation treatments replicated at eight locations within the park, annual high-intensity burn, triennial high (dry season) and low-intensity (wet season) burns and fire exclusion, to capture the range of fire management scenarios under consideration. Changes in woody cover were calculated over a period similar to the experiment’s duration (~80 years) using aerial photographs (1944–2018). Soils were analysed for soil organic C (SOC) and d13C to 30 cm, under and away from tree canopies to isolate local- and landscape-level effects of WPE on SOC.
  3. The largest increases in woody cover occurred with fire exclusion. We found that plots with greater increases in woody cover also had higher SOC. Yet, trees were not the only contributor to SOC gains, such that sustained high inputs of C4-derived carbon (from grasses), even under canopies in fire suppression plots, contributed significantly to SOC. We observed little difference in SOC sequestration between the cooler triennial (wet season) burns and fire suppression.
  4. Synthesis: Grass inputs to SOC remained high across the full range of woody cover created by varying burning regimes. Total SOC stocks stored from tree inputs only matched grass-derived SOC stocks after almost 70 years of fire exclusion. Our results point to C4 grasses as a resilient contributor to SOC under altered fire regimes and further challenge the assumption that increasing tree cover, either through afforestation schemes or fire suppression, will result in large gains in C sequestration in savanna soils, even after 70 years.