Initial Effects Of Two Tending Measures On Carbon Storage And Soil Active Organic Carbon Pool Of Cypress Plantation

The quality of Cupressus funebris plantations in China varies greatly,especially in the hilly region of central Sichuan,where cypress plantations created during the construction of the Yangtze River Shelterbelt Project were initially densely planted without periodic tending and management activities.As a result,the cypress trees are showing poor growth and ecological stability.Tending and management activities on cypress plantations can improve their forest structure,internal environment,growth speed,and carbon sequestration capacity,which is significant for promoting tree growth,carbon storage,and quality of soil organic carbon pools.This study relies on the World Bank Loaned Forest Ecosystem Restoration Project in the Upper Reaches of the Yangtze River and targets25-year-old cypress plantations in the Qiqushan State-owned Forest Farm and Shuidigou Provincial Forest Health Base in Zitong County,Mianyang City,Sichuan Province.The study arranged four monitoring sample plots for 30%intensity thinning after cutting(JF,1880 trees hm^-2,CK),30%intensity thinning+replanting Cinnamomum camphora seedlings(1372 trees hm^-2,BZ),and four control sample plots(1880 trees hm^-2,CK),totaling 12.The growth status of cypress trees and soil samples were collected before implementing tending measures,in the year of tending and two years afterward.Each tree was measured,and arbor layer biomass was estimated using the relative growth equation of the biomass of each cypress organ.The biomass of each vegetation layer under the forest was also measured,estimating the carbon storage of the cypress plantation based on the measured values of each organ,vegetation layer,and soil carbon content.After two years of tending,soil organic carbon content,active organic carbon pool components,and soil physical and chemical properties were measured in the sample plots during different seasons.The research findings are as follows:（1）Both tending measures had a significant impact on the growth rate of cypress（P<0.05）,exhibiting greater growth potential after implementing the 30%intensity thinning tending measure,and the growth status of camphor tree seedlings was found to be good.Within 2 years,the growth of cypress trees in each treatment was mostly as follows:JF>BZ>CK,where BZ and JF treatments had significant growth compared to CK treatment（P<0.05）.In sample plots where camphor tree seedlings were replanted,several growth indicators of camphor tree seedlings increased within 2 years,with the average crown width,average height under branches,average diameter at breast height（DBH）,and average tree height increasing by 13.16%,82.76%,67.83%,and 66.67%respectively,compared to the year of tending.Furthermore,the distribution trend of DBH in BZ and JF sample plots has changed,with the percentage of trees having a diameter above 12 cm in the 2-year range being BZ（4.77%）>JF（4.06%）>CK（3.33%）.Lastly,during 2 years,the growth of stand volume is JF(8.9 m³·hm^-2)>BZ(8.6 m³·hm^-2)>CK(6.5 m³·hm^-2).（2）Both tending measures have significantly increased integrated carbon storage inside and outside the forest（P<0.05）and have shown greater potential for carbon storage growth after implementing the 30%intensity thinning+replanting camphor seedlings tending measure.The growth of carbon storage in each organ of a single cypress plant within each treatment for 2 years was as follows:JF>BZ>CK,except for the root system,where both BZ and JF treatments had significantly higher growth rate than CK（P<0.05）.The growth of total carbon storage in tree layer,shrub layer,herb layer,and litter layer for each treatment within 2 years was as follows:BZ>JF>CK,where BZ and JF treatments were significantly higher than that of CK（P<0.05）.In contrast,the increase of soil carbon storage within 2 years showed CK>JF>BZ,and CK was significantly higher than BZ and JF（P<0.05）.Furthermore,the integrated carbon storage in and out of each treatment stand revealed that BZ>JF>CK,where BZ and JF treatments were significantly higher than CK（P<0.05）.BZ and JF have significantly changed the distribution of carbon storage above ground（P<0.05）.（3）Both tending measures have significantly increased the content of soil active organic carbon components（P<0.05）.In the 0-5 cm soil layer,there were significant differences in the contents of total organic carbon and active organic carbon components in the same season among each treatment（P<0.05）.However,in the 5-20 cm and 20-40 cm soil layers,there were mostly no significant differences in the total organic carbon contents（P>0.05）.In the 0-5 cm soil layer,the content of total organic carbon,light organic carbon,and carbon pool management index were as follows:BZ>JF>CK,where BZ and JF treatments were significantly higher than CK（P<0.05）,and BZ was significantly higher than JF（P<0.05）.The content of easily oxidized organic carbon mainly showed an increasing trend as follows:BZ>JF>CK,where BZ and JF treatments were significantly higher than CK（P<0.05）.The content of soluble organic carbon was as follows:CK>JF>BZ,where BZ and JF were significantly lower than CK（P<0.05）,and BZ was significantly lower than JF（P<0.05）.Both tending measures did not cause significant changes on the seasonal distribution of soil active organic carbon components.Soil nutrients are closely related to the stability of soil active organic carbon pools,especially the soil nitrogen and phosphorus.In summary,during the early stage of tending,both tending measures had a significant positive effect on the rapid growth of cypress trees,optimization of the stand structure,and enhancement of the microenvironment within the forest,ultimately leading to improved ecological and economic benefits of the cypress plantation.In terms of cultivating large-diameter timber,improving integrated carbon storage inside and outside the forest and the quality of soil active organic carbon pools,the tending measure of 30%intensity thinning+replanting camphor seedlings was found to be more effective in improving the overall quality of the cypress plantation in Zitong County.