5:15 PM - 6:45 PM
[ACG37-P02] Net ecosystem carbon balance reduced by sika deer-induced stand structure alternations in cool-temperate forests
Keywords:Montane forests, Understory degradation, Forest type, Succession
Introduction
Forest ecosystems play a vital role in absorbing and storing carbon (C) from atmosphere. The C sequestration ability varies with forest stand structure. Recent population increase of ungulates severely impacts stand structure. Their feeding leads the loss of understory vegetation, the increase of unpalatable plant species, and the retention of gap area due to hindered tree regeneration. Our previous research revealed that such heterogeneous stand structural alternations could halve above-ground C stocks through loss of accumulated C in overstory trees. This reduction cascaded into reduced C stock of soil organic matter (SOC), probably due to changes in litter production and decomposition rare as well as increased soil erosion rate. However, how stand structural alternations degrade the C balance remains unclear. Here, we aimed to address this gap in a cool temperate broadleaf–conifer mixed forest in southern Kyushu, Japan, where vegetation feeding by sika deer has continued over 20 years.
Methods
We installed four replicate survey plots in four stand types: stands with presence of understory vegetation (PU), stands with no understory vegetation (NU), stands with dominance of unpalatable shrubs, Asebi (Pieris japonica) (shrublands; SR), and stands with canopy gap areas hindering tree regeneration (CG). In each plot, net ecosystem C balance (NECB, g C m-2 yr-1) was evaluated. In this study, NECB was determined by subtracting heterotrophic respiration (Rh) and C loss via soil erosion (Se) from net primary production (Pn). To estimate Pn, we employed a biometric method, which includes plant biomass increment, litter production, and fine root production over one-year-period. We estimated Rh by applying yearly soil temperature data to a CO2 efflux model based on temperature (the Q10 model), developed from field measurements using the collar trenching method. Yearly soil temperature data were recorded every 30 minutes in one plot of each stand type with a thermometer at 5 cm depth. We estimated Se by multiplying annual eroded soil depth (Eyr; cm-1 yr-1) and SOC (g C m-2 cm-1). Eyr was measured by tracking eroded depth of 16 iron pins buried in each plot after one-year-incubation. All data were collected between 2022 and 2024. We assumed the PU as baseline stands to evaluate changes in NECB and its components as stands altered from PU to NU, SR, and CG. This comparison was made using a two-sided Dunnett test, considering a p-value < 0.05 as statistically significant.
Reuslts and Disscussions
Pn in NU (347.5 ± 99 g C m-2 yr-1) and CG (52.8 ± 14.1) were 44 % and 92 % lower than that in PU (624.2 ± 222.5), respectively. Pn in SR (436.1 ± 81.8) was comparable to PU. The reduction of Pn in NU and CG was due to the decreased biomass increment of both understory vegetation and overstory trees. Instead, the growth of unpalatable shrubs (i.e., Asebi) in both understory and overstory layer offset decreased biomass increment of other plants in SR. Rh was highest in PU (146.4 g C m-2 yr-1), followed by CG (135.0), NU (129.8), and SR (107.6). The temperature-independent CO2 efflux (i.e., the intercept term in the Q10 model) was highest in PU, resulting in the highest Rh. The Eyr in NU, SR, and CG were significantly higher than that in PU. Reflecting higher Eyr, Se in CG (501.9 ± 218.8 g C m-2 yr-1) was 2-fold higher than that in PU (238.8 ± 97.7), whereas Se in NU (327.7 ± 72.6) and SR (287.9 ± 101.5) were comparable to PU. Finally, NECB was highest in PU (244.8 ± 209.9 g C m-2 yr-1), following SR (102.1 ± 176.8), NU (-110 ± 98.9), and CG (-583.6 ± 224.5). The reduction of NECB was mainly driven by the decreased Pn and increased Se. This reduction of NECB underscores the potential for deer-induced stand alternations to turn forests from C sinks to C sources. We argue that vegetation protection and soil erosion control is urgent to maintain the function of absorbing and storing C in forests against both increasing ungulate populations and global warming.
Forest ecosystems play a vital role in absorbing and storing carbon (C) from atmosphere. The C sequestration ability varies with forest stand structure. Recent population increase of ungulates severely impacts stand structure. Their feeding leads the loss of understory vegetation, the increase of unpalatable plant species, and the retention of gap area due to hindered tree regeneration. Our previous research revealed that such heterogeneous stand structural alternations could halve above-ground C stocks through loss of accumulated C in overstory trees. This reduction cascaded into reduced C stock of soil organic matter (SOC), probably due to changes in litter production and decomposition rare as well as increased soil erosion rate. However, how stand structural alternations degrade the C balance remains unclear. Here, we aimed to address this gap in a cool temperate broadleaf–conifer mixed forest in southern Kyushu, Japan, where vegetation feeding by sika deer has continued over 20 years.
Methods
We installed four replicate survey plots in four stand types: stands with presence of understory vegetation (PU), stands with no understory vegetation (NU), stands with dominance of unpalatable shrubs, Asebi (Pieris japonica) (shrublands; SR), and stands with canopy gap areas hindering tree regeneration (CG). In each plot, net ecosystem C balance (NECB, g C m-2 yr-1) was evaluated. In this study, NECB was determined by subtracting heterotrophic respiration (Rh) and C loss via soil erosion (Se) from net primary production (Pn). To estimate Pn, we employed a biometric method, which includes plant biomass increment, litter production, and fine root production over one-year-period. We estimated Rh by applying yearly soil temperature data to a CO2 efflux model based on temperature (the Q10 model), developed from field measurements using the collar trenching method. Yearly soil temperature data were recorded every 30 minutes in one plot of each stand type with a thermometer at 5 cm depth. We estimated Se by multiplying annual eroded soil depth (Eyr; cm-1 yr-1) and SOC (g C m-2 cm-1). Eyr was measured by tracking eroded depth of 16 iron pins buried in each plot after one-year-incubation. All data were collected between 2022 and 2024. We assumed the PU as baseline stands to evaluate changes in NECB and its components as stands altered from PU to NU, SR, and CG. This comparison was made using a two-sided Dunnett test, considering a p-value < 0.05 as statistically significant.
Reuslts and Disscussions
Pn in NU (347.5 ± 99 g C m-2 yr-1) and CG (52.8 ± 14.1) were 44 % and 92 % lower than that in PU (624.2 ± 222.5), respectively. Pn in SR (436.1 ± 81.8) was comparable to PU. The reduction of Pn in NU and CG was due to the decreased biomass increment of both understory vegetation and overstory trees. Instead, the growth of unpalatable shrubs (i.e., Asebi) in both understory and overstory layer offset decreased biomass increment of other plants in SR. Rh was highest in PU (146.4 g C m-2 yr-1), followed by CG (135.0), NU (129.8), and SR (107.6). The temperature-independent CO2 efflux (i.e., the intercept term in the Q10 model) was highest in PU, resulting in the highest Rh. The Eyr in NU, SR, and CG were significantly higher than that in PU. Reflecting higher Eyr, Se in CG (501.9 ± 218.8 g C m-2 yr-1) was 2-fold higher than that in PU (238.8 ± 97.7), whereas Se in NU (327.7 ± 72.6) and SR (287.9 ± 101.5) were comparable to PU. Finally, NECB was highest in PU (244.8 ± 209.9 g C m-2 yr-1), following SR (102.1 ± 176.8), NU (-110 ± 98.9), and CG (-583.6 ± 224.5). The reduction of NECB was mainly driven by the decreased Pn and increased Se. This reduction of NECB underscores the potential for deer-induced stand alternations to turn forests from C sinks to C sources. We argue that vegetation protection and soil erosion control is urgent to maintain the function of absorbing and storing C in forests against both increasing ungulate populations and global warming.