| Chinese fir is an important fast-growing timber species in southern China with high economic and ecological value.For a long time,unreasonable management measures had led to many problems such as the decline of stand quality and fertility of Chinese fir plantation,which seriously restricts the sustainable management of Chinese fir plantation.Chinese fir litter,as a component of Chinese fir plantation ecosystem,plays an irreplaceable role in maintaining the material and energy cycle of the ecosystem.The systematic study of Chinese fir litter is of great significance to reveal the structure of plantation ecosystem,improve the function of ecosystem and the quality of plantation soil.Based on this,the 13-year-old Chinese fir plantation was selected as the research object in this study,and a completely randomized block experimental design was adopted.Three thinning treatments were set,namely light thinning,moderate thinning and high thinning,and the un-thinning plantation was divided into control groups.To explore the effects of different thinning intensity on the surface litter stock,litter seasonal variation,litter amount,litter water retention function,litter mass decomposition rate,litter nutrient release rate and litter ecological stoichiometric ratio of Chinese fir plantation.Based on the data at the end of the survey period,a comprehensive evaluation model was established to determine the most suitable thinning intensity for the ecological function of litters,and to provide effective suggestions for the management of Chinese fir plantation.The results are as follows:(1)The effect of thinning on the undecomposed layer,half-decomposed layer and the total amount of existing litter was significant.In light thinning,the surface litter of the undecomposed layer was significantly higher than that of moderate and high thinning by 88.23%and42.85%,respectively,accounting for 28.39%of the total litter.Under moderate thinning,the surface litter of the half-decomposed layer was significantly higher than that of light,high,and un-thinned by 46.65%,113.08%,and 39.86%,accounting for 86.19%of the total litter.The total existing surface litter exhibits a pattern of first increasing and then decreasing as the thinning intensity weakens,with the largest total amount under moderate thinning,significantly higher than that under high thinning by 75.61%.The existing amounts and natural water content of litter from different decomposition layers showed that half-decomposed litter was more than undecomposed litter,and the existing amount of half-decomposed litter was 3.4 times that of undecomposed litter.(2)Under light thinning,the total amount of litter was the largest,significantly higher than that under moderate and high thinning by 41.29%and 51.84%,respectively.Under un-thinning treatment,the litter from branches and cone was the largest,but there was no significant difference between the treatments.Under light thinning,the amount of leaf litter was the highest,34.38%,38.71%,and 48.28%higher than that under un-thinning,moderate and high thinning,respectively.Among the different thinning intensities,the composition of litter was as follows:leaf litter(60.08%)>branch litter(15.53%)>cone litter(10.49%).Correlation analysis showed that the litter from branches was significantly positively correlated with precipitation and humidity(P<0.05),while the litter from leaves,total litter were significantly positively correlated with precipitation(P<0.05),and the litter from cone had no significant relationship with all meteorological factors(P>0.05).(3)The total amount of litter,branch litter,leaf litter,and cone litter all showed a trend of increasing with the decrease of thinning intensity.The total litter was significantly correlated with thinning intensity in spring,while branch litter was significantly correlated with thinning intensity in autumn,and leaf litter was significantly correlated with thinning intensity in spring and winter,but there was no significance with cone litter.The turnover period of litter was between 2.28 and 4.73 years,and under light thinning,the turnover period of litter was the shortest,while under high thinning,it was the longest.(4)The hydrologic characteristics of litters were studied by indoor immersion and natural air drying.The results showed that the relationship between water holding capacity(W),water absorption rate(V)and immersion time of Chinese fir litter was in accordance with the function models of W=a+blnt and V=kt~n.The former increased with the increase of immersion time,increased rapidly before 4 h,and reached saturation after 24 h,while the latter decreased with the increase of immersion time,decreased rapidly after 0-4 h,and reached saturation after24 h.The water capacity and water absorption rate of the half-decomposed layer were higher than those of the undecomposed layer,and the moderate thinning treatment was higher than those of the other three treatments.Thinning significantly changed the maximum water holding capacity,maximum interception capacity and effective interception capacity.Compared with the control,light and high thinning,moderate thinning significantly increased by 17.00%～68.45%,16.91%～70.88%and 16.93%～71.46%,and the water holding capacity of half-decomposed litters was greater than that of undecomposed litters.(5)The water loss rate and amount of water loss of litter increased with the extension of air-drying time,both of which were consistent with the logarithmic function model.Water loss was rapid in the first 5 h and reached saturation after 24 h.Water loss rates of undecomposed and half-decomposed litters were the highest in un-thinning.The amount of water loss of undecomposed litters was the largest under moderate thinning,and the amount of water loss of half-decomposed litters under un-thinning was the largest.The water loss rate of litters decreased gradually with the extension of air-drying time,and the relationship between the two was in line with the exponential function model.Water loss was rapid in the first5 h,and reached saturation after 24 h.The water loss rate of undecomposed and half-decomposed litters was the highest with moderate thinning.The rate of water loss in the undecomposed layer of litter was the largest in the light thinning,and the rate of water loss in the half-decomposed layer of litter was the largest in the moderate thinning,and the amount and rate of water loss in the half-decomposed layer of litter was greater than that in the undecomposed litter.The ratio of the early water loss(0.25 h)of litter to its total water holding capacity under different thinning intensities was smaller than that of the later water loss(48 h).The un-thinning treatment has the highest proportion of water loss to total water holding capacity,indicating that un-thinning was not conducive to maintaining the moisture content of litter.No matter at the early stage or late stage of water loss,the water loss under un-thinning treatment accounted for the maximum total water holding capacity.Thinning significantly changed the maximum amount of water loss of litters in the undecomposed layer and the half-decomposed layer.The maximum amount of water loss of litters in the undecomposed layer under moderate thinning was 25.06%,100%and101.70%higher than that in the un-thinning,moderate and high thinning.The maximum amount of litter water loss under moderate thinning was significantly higher than that un-thinning,light thinning and high thinning by 41.50%,15.70%and 173.63%.The maximum litter water loss under moderate thinning was significantly higher than that un-thinning and high-thinning,but there was no significant difference between light thinning and moderate thinning.(6)Compared with the un-thinning treatment,the decomposition rates of litter in Chinese fir plantations increased by 25.93%,18.52%,and11.11%under light,moderate,and high thinning,respectively,indicating that thinning can promote litter decomposition and that decomposition was fastest under light thinning.The Olson decay model showed that the time required for 50%and 95%of the decomposition of litter in the light thinning treatment was the shortest,at 2.06 years and 8.94 years,respectively.Under different intensity thinning,C content in litters of Chinese fir showed a change trend decreased firstly,then increasing and then decreasing in the whole decomposition cycle,which decreased obviously before 180 d,and then fluctuated relatively steadily.The content of N and P showed a pattern of first release and then fixation.The stoichiometric ratio of litters was different.Among them,C/N and N/P decreased rapidly in the 120 days,while C/P increased rapidly in the 120days after high thinning.Two-factor variance analysis showed that treatment and decomposition time had significant effects on the decomposition rate of litters and the release of C,N and P.Correlation analysis showed that N/P was the key factor affecting the decomposition rate of litters of Chinese fir.Precipitation and air temperature are the key factors affecting the release of P and N/P litter.(7)The results of principal component analysis and membership function for the comprehensive evaluation of different thinning intensities indicated that the ecological service function of litter was optimal under light thinning,which was most conducive to improving the quality of Chinese fir plantations. |
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