Abe, H.; Fu, D.; Kume, T.; Katayama, A.

Although natural forests sequester carbon, this function may decline under chronic herbivory by abundant ungulates (hereafter overbrowsing). Specifically, overbrowsing alters stand structure, potentially impair carbon exchanges related to the vegetation. Further, overbrowsing may also accelerate soil erosion, especially in heavy-rainfall regions like Monsoon Asia. We quantified these impacts by estimating net ecosystem carbon balance (NECB; g C m -2 yr-1) by subtracting heterotrophic respiration (Rh) and lateral carbon export via erosion (Se) from net primary production (Pn) in southern Kyushu, Japan. Here, about 40-years of overbrowsing by sika deer (Cervus nippon) altered mixed broadleaf-conifer stands with presence of understory (PU) into stands with no understory (NU), then further altered into stands dominated by unpalatable shrublands (SR) or stands with canopy gaps (CG). The PU maintained a positive NECB (plot mean = 307.0 g C m -2 yr-1) because high Pn (721.9) exceeded the sum of Rh (175.4) and Se (239.5). Alteration from PU into NU converted NECB to negative (-98.2 g C m -2 yr-1). This was because the suppressed Pn (400.2 g C m -2 yr-1) could not offset the sum of Rh (170.6) and Se (327.7). Further degradation into CG caused a profound negative NECB (-894.4 g C m -2 yr-1), where Pn (71.9) offset only 7% of the sum of surging Rh (464.8) and Se (501.5). Alteration into SR showed a partially recovered NECB (97.3 g C m -2 yr-1), driven by shrub growth (Pn; 554.5, Rh; 175.4, Se; 239.5). However, this recovery is still limited given that lowered shrub biomass and prior topsoil loss via erosion. Our results validate previous findings that stand alteration from PU to SR or CG through NU leads to up to a 49% loss of ecosystem carbon stocks. Preventing stand alteration and soil erosion are key countermeasures against chronic overbrowsing and subsequent erosion.