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[AHW23-P03] Effect of artificial thinning on transpiration of a Japanese cedar stand and its trend after the treatment
Keywords:artificial thinning, Japanese cedar forest, transpiration, sap flow, leaf area index
Japanese cedar (Cryptomeria japonica) is the most popular coniferous species planted in Japan. Although most parts of planted forest have been matured, they have not been utilized for timber production because of the worse financial situation of forestry, resulting in increase of unmanaged and high density stands in Japan. On the other hand, Kyoto Protocol determined that CO2 absorption by forests is counted as the reduction of greenhouse gas emission with suitable treatments of artificial thinning. As a result, in 2015, forests of 480000 ha had been thinned in Japan (Ohmasa et al., 2018). Forests affect the water balance mainly through the evapotranspiration including transpiration, and, thus, detailed investigations for effects of thinning on transpiration process are required. The changes in transpiration caused by the thinning were reported for Japanese cedar stands (Komatsu et al., 2013; Tateishi et al., 2015) and Japanese cypress stands (Sun et al., 2014; Tateishi et al., 2015) based on the comparisons of sap flow between pre- and post-thinning conditions. These papers commonly showed the decrease in transpiration just after the thinning, however, it has been still unknown that the trend of transpiration after the treatment. We measured transpiration of a Japanese cedar stand for five years, and evaluated the decrease in transpiration just after the thinning and the trend for three years after the treatment.
The measurements were conducted in a Japanese cedar forest, whose age is 59 years in 2022, located in Nagasaka Experimental Watershed (40°16'N, 140°24'E). We set the measurement plot including 16 trees of Japanese cedar. The mean diameter at breast height and tree height was 26.8 cm and 20.6 m in 2016, respectively. Our measurements were conducted from 2015 to 2019, during which the thinning was treated in February 2017. 10 trees remained after the operation (thinning ratio was 38%). We used thermal dissipation method (Granier, 1985) to measure sap flux densities at each tree. The amount of transpiration from a tree was calculated as the product of sap flux density and sapwood area. Finally, stand transpiration was obtained as dividing total transpiration of all trees by the plot area. The canopy conductance was calculated by the simplified form of the Penman-Monteith equation (Kumagai et al., 2008). Leaf area index (LAI) was measured at 22 points in the stand by LAI-2000 (Li-cor, USA) from 2016 to 2020.
The stand sapwood area was decreased by the thinning as 3810 to 2384 cm2, and decline ratio was 37%. We obtained the linear regression equations between vapor pressure deficit and canopy conductance for the pre-thinning period of 2015-2016, and the post conditions of 2017, 2018 and 2019, then calculated the change of transpiration by the thinning with the same input data to exclude effects of year-to-year difference in environmental condition. As a result, the stand transpiration in 2017 was 71.5% of pre-condition, and the decrease ratio of transpiration (28.5%) was smaller than that of sapwood area. After that, the transpiration increased as 98.7% in 2018 and 106.6% in 2019, suggesting that the decrease in transpiration recovered in three years after the treatment. On the other hand, LAI decreased to 24.7% in 2017 compared to the pre-condition, and kept relatively small value of 42.9% in 2020. These results strongly imply that the higher light condition caused by the thinning enhanced the activity of Japanese cedars remained after the thinning treatment.
Acknowledgements
This study was partially supported by the Global Environmental Research Coordination System from Ministry of the Environment of Japan.
Cited papers:
Granier (1985) Ann. Sci. For., 42, 193-200.
Komatsu et al. (2013) HRL, 7, 60-65.
Kumagai et al. (2008) Agric. For. Met., 148, 1444-1455.
Ohmasa et al. (2018) J. Japan Soc. Hydrol. Water Resour., 31, 414-427.
Sun et al. (2014) Agric. For. Met., 197, 123-135.
Tateishi et al. (2015) Hydrol. Process., 29, 5088-5097.
The measurements were conducted in a Japanese cedar forest, whose age is 59 years in 2022, located in Nagasaka Experimental Watershed (40°16'N, 140°24'E). We set the measurement plot including 16 trees of Japanese cedar. The mean diameter at breast height and tree height was 26.8 cm and 20.6 m in 2016, respectively. Our measurements were conducted from 2015 to 2019, during which the thinning was treated in February 2017. 10 trees remained after the operation (thinning ratio was 38%). We used thermal dissipation method (Granier, 1985) to measure sap flux densities at each tree. The amount of transpiration from a tree was calculated as the product of sap flux density and sapwood area. Finally, stand transpiration was obtained as dividing total transpiration of all trees by the plot area. The canopy conductance was calculated by the simplified form of the Penman-Monteith equation (Kumagai et al., 2008). Leaf area index (LAI) was measured at 22 points in the stand by LAI-2000 (Li-cor, USA) from 2016 to 2020.
The stand sapwood area was decreased by the thinning as 3810 to 2384 cm2, and decline ratio was 37%. We obtained the linear regression equations between vapor pressure deficit and canopy conductance for the pre-thinning period of 2015-2016, and the post conditions of 2017, 2018 and 2019, then calculated the change of transpiration by the thinning with the same input data to exclude effects of year-to-year difference in environmental condition. As a result, the stand transpiration in 2017 was 71.5% of pre-condition, and the decrease ratio of transpiration (28.5%) was smaller than that of sapwood area. After that, the transpiration increased as 98.7% in 2018 and 106.6% in 2019, suggesting that the decrease in transpiration recovered in three years after the treatment. On the other hand, LAI decreased to 24.7% in 2017 compared to the pre-condition, and kept relatively small value of 42.9% in 2020. These results strongly imply that the higher light condition caused by the thinning enhanced the activity of Japanese cedars remained after the thinning treatment.
Acknowledgements
This study was partially supported by the Global Environmental Research Coordination System from Ministry of the Environment of Japan.
Cited papers:
Granier (1985) Ann. Sci. For., 42, 193-200.
Komatsu et al. (2013) HRL, 7, 60-65.
Kumagai et al. (2008) Agric. For. Met., 148, 1444-1455.
Ohmasa et al. (2018) J. Japan Soc. Hydrol. Water Resour., 31, 414-427.
Sun et al. (2014) Agric. For. Met., 197, 123-135.
Tateishi et al. (2015) Hydrol. Process., 29, 5088-5097.