Impact of feral pig (Sus scrofa) removal on soil carbon fluxes in Hawaiian tropical montane wet forest

Background/Question/Methods

Native Hawaiian wet forests are heavily impacted by nonnative feral pigs (Sus scrofa; wild boar) which decrease native understory plant and forest floor cover, and mix soil horizons. Together, these activities likely influence soil carbon (C) fluxes, but very few studies have quantified the impact of feral pigs on soil biogeochemistry. In this study, we quantified the impacts of feral pig removal on soil C fluxes, including soil-surface CO₂ efflux (F_S), aboveground litterfall (F_L), and total belowground carbon flux (TBCF; the total autotrophic flux of carbon belowground to support root growth and respiration, exudates, and mycorrhizae). Annual F_S, F_L and TBCF (where TBCF=F_S-F_L) were quantified in 2011 inside and outside of a chronosequence of feral pig exclosures in each of five paired sites (pig removal vs. pig present; four replicate plots in each site) ranging from 6.5 to 18.5 years since feral pig removal in native tropical montane wet forest on Hawaii Island. We hypothesized that: (i) F_S, F_L, and TBCF would increase with pig removal due to native vegetation regeneration and increased ecosystem productivity; and (ii) the relative difference in F_S, F_L, and TBCF between pig removal and pig present sites would increase with time since pig removal.

Results/Conclusions

Results partially supported our hypotheses. Across all sites, F_S was 22% higher with feral pig removal (P<0.05), averaging 3.91 and 3.21 umol CO₂ m^-2s^-1 in pig removal and pig present sites, respectively. The relative difference in F_S between pig removal and pig present sites increased linearly and positively with time since feral pig removal, from 4% at 6.5 years to 62% at 18.5 years (r²= 0.90, P<0.01). F_L did not differ between treatments (P=0.39), or with time since feral pig removal (P=0.70). TBCF increased marginally following feral pig removal (P=0.05), with four of five pig removal sites having greater TBCF than their pig present pairs. The relative difference in TBCF between pig removal and pig present sites increased linearly and positively with time since feral pig removal, from 4% at 6.5 years to 102% at 18.5 years (r²= 0.86, P<0.05). These results suggest that feral pig removal increases both the flux of C to the atmosphere (F_S) and the autotrophic flux of C belowground (TBCF). Therefore, feral pigs (and their removal) have the potential to alter soil C storage in native Hawaiian tropical montane wet forests, as well as other feral pig invaded forests around the world.