Transformation of the net ecosystem carbon balance in cool-temperate forests by sika deer-induced alteration of stand structure and subsequent soil erosion

Natural forests are major carbon sinks, but chronic herbivory by over-abundant ungulates (hereafter overbrowsing) and resulting soil erosion can compromise this function. We quantified the net ecosystem carbon balance (NECB) in southern Kyushu, Japan. Here, approximately 40 years of overbrowsing by sika deer ( Cervus nippon ) altered mixed broadleaf–conifer stands with an understory (PU) into stands lacking an understory (NU), and further into stands dominated by unpalatable shrublands (SR) or stands with canopy gaps (CG). PU stands maintained a positive NECB (four plot mean; 317.6 g C m −2 yr −1 ) because high net primary production ( P n ; 721.9 g C m −2 yr −1 ) exceeded the sum of heterotrophic respiration ( R h ; 164.8 g C m −2 yr −1 ) plus lateral carbon export via soil erosion ( S e ; 239.5 g C m −2 yr −1 ). Alteration from PU to NU resulted in a negative NECB (−80.1), because the P n (400.2) did not offset the sum of R h (152.5) plus S e (327.7). Further degradation to CG caused a profoundly negative NECB (−749.3), where P n (88.2) offset only 11% of R h (335.9) plus S e (501.5). Alteration to SR led to a partially recovered NECB (122.6) driven by shrub growth ( P n 554.5; R h 143.9; S e 287.9), but recovery was limited given the lowered shrub biomass and topsoil loss via erosion. The results are consistent with previous findings that stand alteration from PU toward SR or CG through NU leads to as much as 49% loss of ecosystem carbon stocks. Preventing stand alteration and soil erosion are critical countermeasures against overbrowsing and subsequent erosion.