3:30 PM - 3:45 PM
[MIS22-07] Change in nitrate concentration in stream water during 13 years after clear-cutting and subsequent understory strip-cutting
Keywords:nitrate concentration, clear-cutting, Sasa bamboo stripe cutting, stream discharge, recovery time, conifer and broadleaved mixed forest
Nitrogen is an essential element that determines the forest productivity in temperate regions. It is known that the nitrate concentration in stream water increases after clear-cutting, mainly due to the cessation of nitrogen uptake by trees after clear-cutting, and it has been reported that the nitrate concentration recovers to its original level together with the recovery of vegetation. On the other hand, the degree of nitrate concentration in stream water after clear-cutting varies greatly depending on the ecosystem. In a cool temperate forest in northern Hokkaido, where bamboo grass grows densely on the forest floor, it was reported that the productivity of Sasa was high and contribute to mitigate nitrogen leaching after clear-cutting. However, there are few reports on long-term changes in water quality after clear-cutting and subsequent site preparation treatment. The purpose of this study was to clarify the long-term changes in nitrate concentration in stream water after clear-cutting and subsequent Sasa bamboo strip cutting.
The study site was established in a cool temperate forest in the Teshio Experimental Forest of Hokkaido University located in northern Hokkaido, Japan. The forest was conifer and broadleaved mixed forest with dense understory Sasa bamboo (Sasa species: Sasa senanensis and Sasa kurilensis). The 8-ha watershed area, except for the riparian zone, was clear-cut between January and March 2003. Sasa bamboo strip cutting was performed in October 2003. Immediately afterwards, larch seedlings were planted in the cutting row. Stream water was collected at the outlet of the watershed at 2-week intervals from 2002 to 2007, and then at 3-week intervals thereafter. Nitrate concentration and other ion concentrations in the stream water were measured using ion chromatography. In addition, the water level was continuously monitored to measure the discharge at a weir at the outlet of the catchment area. The maximum nitrate concentration before clear-cutting was 2.7 μmol L−1. The maximum concentration from after clear-cutting to before the Sasa bamboo stripe cutting was low at 1.8 μmol L−1, indicating no increase in nitrate concentration after clear-cutting. On the other hand, after the Sasa bamboo stripe cutting, the nitrate concentration rose to 15.1 μmol L−1 in maximum in the following year with a significance, suggesting that the Sasa bamboo contributes to nitrogen retention even after clear-cutting. High concentrations were observed in a pulse-like manner afterwards, and the concentration fluctuated greatly. The average nitrate concentration before the Sasa bamboo stripe cutting was 0.7 μmol L−1, while it was 4.6 μmol L−1 after the Sasa bamboo stripe cutting with a significant difference between the times (P < 0.001). Even after 13 years after the disturbance, the concentration did not decrease and did not return to the pre-disturbance level. As for the relationship with the discharge, even after the disturbance, the concentration was low at 0.1 μmol L−1 or less near the base flow where the discharge was less than 0.1 l/s. However, the concentration fluctuated widely at discharge above that level. This is considered to be because the nitrate concentration increases during flood events even at relatively low discharge, while the concentration decreases due to dilution if flooding continues. It is possible that the inorganic nitrogen pool in the ecosystem increased after the disturbance and continued to supply nitrate during flooding for a long period of time. On the other hand, the maximum concentration during that period was 33.6 μmol L−1, which was a relatively small response of the disturbance. These results indicate that in the cool-temperate forest ecosystem in northern Hokkaido, the response of nitrate leaching to stream to disturbance such as clear-cutting and subsequent Sasa bamboo strip cutting is small, while the recovery time after disturbance is long.
The study site was established in a cool temperate forest in the Teshio Experimental Forest of Hokkaido University located in northern Hokkaido, Japan. The forest was conifer and broadleaved mixed forest with dense understory Sasa bamboo (Sasa species: Sasa senanensis and Sasa kurilensis). The 8-ha watershed area, except for the riparian zone, was clear-cut between January and March 2003. Sasa bamboo strip cutting was performed in October 2003. Immediately afterwards, larch seedlings were planted in the cutting row. Stream water was collected at the outlet of the watershed at 2-week intervals from 2002 to 2007, and then at 3-week intervals thereafter. Nitrate concentration and other ion concentrations in the stream water were measured using ion chromatography. In addition, the water level was continuously monitored to measure the discharge at a weir at the outlet of the catchment area. The maximum nitrate concentration before clear-cutting was 2.7 μmol L−1. The maximum concentration from after clear-cutting to before the Sasa bamboo stripe cutting was low at 1.8 μmol L−1, indicating no increase in nitrate concentration after clear-cutting. On the other hand, after the Sasa bamboo stripe cutting, the nitrate concentration rose to 15.1 μmol L−1 in maximum in the following year with a significance, suggesting that the Sasa bamboo contributes to nitrogen retention even after clear-cutting. High concentrations were observed in a pulse-like manner afterwards, and the concentration fluctuated greatly. The average nitrate concentration before the Sasa bamboo stripe cutting was 0.7 μmol L−1, while it was 4.6 μmol L−1 after the Sasa bamboo stripe cutting with a significant difference between the times (P < 0.001). Even after 13 years after the disturbance, the concentration did not decrease and did not return to the pre-disturbance level. As for the relationship with the discharge, even after the disturbance, the concentration was low at 0.1 μmol L−1 or less near the base flow where the discharge was less than 0.1 l/s. However, the concentration fluctuated widely at discharge above that level. This is considered to be because the nitrate concentration increases during flood events even at relatively low discharge, while the concentration decreases due to dilution if flooding continues. It is possible that the inorganic nitrogen pool in the ecosystem increased after the disturbance and continued to supply nitrate during flooding for a long period of time. On the other hand, the maximum concentration during that period was 33.6 μmol L−1, which was a relatively small response of the disturbance. These results indicate that in the cool-temperate forest ecosystem in northern Hokkaido, the response of nitrate leaching to stream to disturbance such as clear-cutting and subsequent Sasa bamboo strip cutting is small, while the recovery time after disturbance is long.