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

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. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=102 SRC="FIGDIR/small/719128v1_ufig1.gif" ALT="Figure 1"> View larger version (36K): org.highwire.dtl.DTLVardef@12f833forg.highwire.dtl.DTLVardef@51a1cforg.highwire.dtl.DTLVardef@1562461org.highwire.dtl.DTLVardef@6e652b_HPS_FORMAT_FIGEXP M_FIG C_FIG We reported that over 40 years of sika deer overbrowsing and subsequent soil erosion severely degraded the net ecosystem carbon balance (NECB) of mountain forests in Japan. The loss of understory vegetation drove the transition of intact stands into degraded states (no-understory, shrub-dominated, or canopy gaps). Based on field measurements, we quantified that this structural alteration suppressed net primary production (Pn) while increased both heterotrophic respiration (Rh) and lateral carbon loss via soil erosion (Se). Consequently, the forest shifted from a net carbon sink (+307 g C m-{superscript 2} yr-{superscript 1}) to a source (up to -894 g C m-{superscript 2} yr-{superscript 1}). These findings provide compelling empirical evidence that increasing ungulate populations, compounded by the rising frequency of heavy rainfall, may severely undermine the carbon sequestration functions traditionally expected of natural forests.