17:15 〜 18:45
[MIS14-P01] 冷温帯林におけるシカ採食によるササの衰退が森林の窒素循環に与える影響
キーワード：森林の窒素循環、ササ、シカ採食
Introduction
Understanding forest nitrogen (N) cycling is crucial for comprehending ecosystem function. In recent years, a high population of Sika deer (Cervus nippon) was reported throughout Japan, and this heavy grazing pressure has put the Sasa understory (Sasa spp., below Sasa) in danger of disappearing. Sasa degradation leads to changes in stand structure, and even to the emergence of “Sasa-less” forests. Short-term effects of Sasa degradation in forest stands have been reported to negatively affect the forest N cycling, such as increasing soil N availability and N concentration in the stream water, possibly due to the reduction of N uptake by Sasa. In heavy deer-grazed forest ecosystems, the complete disappearance of Sasa may significantly alter the soil environment and nutrient cycling, which in turn hinders mature overstory tree growth and possibly nutrient allocation patterns, further impacting forest N cycling. However, these complex effects in such heavily grazed forests have rarely been emphasized, as some mature tree species were considered to be unaffected by heavy deer grazing. Therefore, this study aims to investigate the impact of heavy deer grazing on forest N cycling patterns in forests with and without Sasa. We hypothesize that (1) deer grazing will decrease total N uptake by plants due to sasa loss, leading to increased soil N availability and high level of N leaching; and (2) N uptake and N use strategy of overstory trees will change, further increasing soil N availability and N leaching.
Methods and Materials
Site description
The study was conducted in mixed broadleaves and conifer forests in Shiba Research Forest (SRF) of Kyushu University, western Japan. Three plots were established in forests with every two types of stand structures: (1) Overstory trees with Sasa understory (SA), (2) No understory (NU). Each plot is basically 10 m × 10 m in size.
Plant nitrogen uptake
Plant N uptake was divided by Sasa and overstory trees. Tree N uptake was defined as the sum of the N content of litter, fine root increment, and annual wood increment (stems, branches, and coarse roots). Sasa N uptake was defined as the N content of the annual new culms.
Soil N mineralization, soil N availability, and potential N leaching
Soil net mineralization rate was measured by the “buried bag” method under plots. Soil N availability was defined as the net mineralization and the soil N content with a depth of 10 cm. Potential N leaching was evaluated by the inorganic N concentration in soil water with a depth of 50 cm by using porous cups.
Plant nitrogen use strategies
Fresh leaves of overstory trees were also collected for measuring the N content. Based on this, N resorption efficiency (NRE) of tree leaf litter was calculated by differences in N content between fresh leaf and leaf litter.
Results and discussion
Nitrate (NO3-) concentration in 50 cm soil water was significantly higher in NU than that in SA. This difference is consistent with the surface soil N content in the depth of 10cm, with all types of inorganic N components (nitrate, ammonium) significantly higher in NU than in SA. But the level of NO3- concentration (< 10 μmol·L-1, respectively) was much lower compared to some N-saturated Japanese cedar forests (> 300 μmol·L-1, respectively) in western Japan. These results indicate that deer grazing could increase the soil N availability and subsequently N leaching.
Soil mineralization rates were not different between SA and NU, indicating that the generation rates of inorganic N could be similar between SA and NU. This result, together with the lower N availability and lower N leaching than that in SA, possibly suggests that plant N uptake in NU is lower than that in SA. Annual Plant N uptake is under analysis.
At the tree species level, there was no significant difference in leaf N content between SA and NU. However, N content in leaf litter was marginally lower in SA than that in NU, and NRE was marginally higher in SA than that in NU. These results indicate that trees may provide N-rich leaf litter under heavy grazing pressure. This may be a reason for the higher soil N availability in NU than that in SA.
In summary, our results indicate that deer grazing could lead to increased soil N availability and increased N leaching, possibly due to decreased plant N uptake due to sasa loss and decreased N resorption rate of overstory trees.
Understanding forest nitrogen (N) cycling is crucial for comprehending ecosystem function. In recent years, a high population of Sika deer (Cervus nippon) was reported throughout Japan, and this heavy grazing pressure has put the Sasa understory (Sasa spp., below Sasa) in danger of disappearing. Sasa degradation leads to changes in stand structure, and even to the emergence of “Sasa-less” forests. Short-term effects of Sasa degradation in forest stands have been reported to negatively affect the forest N cycling, such as increasing soil N availability and N concentration in the stream water, possibly due to the reduction of N uptake by Sasa. In heavy deer-grazed forest ecosystems, the complete disappearance of Sasa may significantly alter the soil environment and nutrient cycling, which in turn hinders mature overstory tree growth and possibly nutrient allocation patterns, further impacting forest N cycling. However, these complex effects in such heavily grazed forests have rarely been emphasized, as some mature tree species were considered to be unaffected by heavy deer grazing. Therefore, this study aims to investigate the impact of heavy deer grazing on forest N cycling patterns in forests with and without Sasa. We hypothesize that (1) deer grazing will decrease total N uptake by plants due to sasa loss, leading to increased soil N availability and high level of N leaching; and (2) N uptake and N use strategy of overstory trees will change, further increasing soil N availability and N leaching.
Methods and Materials
Site description
The study was conducted in mixed broadleaves and conifer forests in Shiba Research Forest (SRF) of Kyushu University, western Japan. Three plots were established in forests with every two types of stand structures: (1) Overstory trees with Sasa understory (SA), (2) No understory (NU). Each plot is basically 10 m × 10 m in size.
Plant nitrogen uptake
Plant N uptake was divided by Sasa and overstory trees. Tree N uptake was defined as the sum of the N content of litter, fine root increment, and annual wood increment (stems, branches, and coarse roots). Sasa N uptake was defined as the N content of the annual new culms.
Soil N mineralization, soil N availability, and potential N leaching
Soil net mineralization rate was measured by the “buried bag” method under plots. Soil N availability was defined as the net mineralization and the soil N content with a depth of 10 cm. Potential N leaching was evaluated by the inorganic N concentration in soil water with a depth of 50 cm by using porous cups.
Plant nitrogen use strategies
Fresh leaves of overstory trees were also collected for measuring the N content. Based on this, N resorption efficiency (NRE) of tree leaf litter was calculated by differences in N content between fresh leaf and leaf litter.
Results and discussion
Nitrate (NO3-) concentration in 50 cm soil water was significantly higher in NU than that in SA. This difference is consistent with the surface soil N content in the depth of 10cm, with all types of inorganic N components (nitrate, ammonium) significantly higher in NU than in SA. But the level of NO3- concentration (< 10 μmol·L-1, respectively) was much lower compared to some N-saturated Japanese cedar forests (> 300 μmol·L-1, respectively) in western Japan. These results indicate that deer grazing could increase the soil N availability and subsequently N leaching.
Soil mineralization rates were not different between SA and NU, indicating that the generation rates of inorganic N could be similar between SA and NU. This result, together with the lower N availability and lower N leaching than that in SA, possibly suggests that plant N uptake in NU is lower than that in SA. Annual Plant N uptake is under analysis.
At the tree species level, there was no significant difference in leaf N content between SA and NU. However, N content in leaf litter was marginally lower in SA than that in NU, and NRE was marginally higher in SA than that in NU. These results indicate that trees may provide N-rich leaf litter under heavy grazing pressure. This may be a reason for the higher soil N availability in NU than that in SA.
In summary, our results indicate that deer grazing could lead to increased soil N availability and increased N leaching, possibly due to decreased plant N uptake due to sasa loss and decreased N resorption rate of overstory trees.