• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Changes in plant debris and carbon stocks across a subalpine forest successional series

    2021-10-12 08:11:40ZhihuiWangLianjunZhaoYiBaiFeiLiJianfengHouXuqingLiYuruiJiangYuyueDengBingqianZhengandWanqinYang
    Forest Ecosystems 2021年3期

    Zhihui Wang,Lianjun Zhao,YiBai,Fei Li,Jianfeng Hou,Xuqing Li,Yurui Jiang,Yuyue Deng,Bingqian Zheng and Wanqin Yang*

    Abstract Background:As a structurally and functionally important component in forest ecosystems,plant debris plays a crucial role in the global carbon cycle.Although it is well known that plant debris stocks vary greatly with tree species composition,forest type,forest origin,and stand age,simultaneous investigation on the changes in woody and non-woody debris biomass and their carbon stock with forest succession has not been reported.Therefore,woody and non-woody debris and carbon stocks were investigated across a subalpine forest successional gradient in Wanglang National Nature Reserve on the eastern Qinghai-Tibet Plateau.Results: Plant debris ranged from 25.19 to 82.89 Mg?ha?1 and showed a global increasing tendency across the subalpine forest successional series except for decreasing at the S4 successional stage.Accordingly,the ratios of woody to non-woody debris stocks ranged from 26.58 to 208.89,and the highest and lowest ratios of woody to non-woody debris stocks were respectively observed in mid-successional coniferous forest and shrub forest,implying that woody debris dominates the plant debris.In particular,the ratios of coarse to fine woody debris stocks varied greatly with the successional stage,and the highest and lowest ratios were found in later and earlier successional subalpine forests,respectively.Furthermore,the woody debris stock varied greatly with diameter size,and larger diameter woody debris dominated the plant debris.Correspondingly,the carbon stock of plant debris ranged from 10.30 to 38.87 Mg?ha?1 across the successional series,and the highest and lowest values were observed in the mid-coniferous stage and shrub forest stage,respectively.Most importantly,the carbon stored in coarse woody debris in later successional forests was four times higher than in earlier successional forests.Conclusions:The stock and role of woody debris,particularly coarse woody debris,varied greatly with the forest successional stage and dominated the carbon cycle in the subalpine forest ecosystem.Thus,preserving coarse woody debris is a critical strategy for sustainable forest management.

    Keywords:Coarse woody debris,Fine woody debris,Forest successional series,Later successional stage,Earlier successional stage,Log decay class,Diameter size

    Introduction

    Plant debris consists of woody debris (WD) and nonwoody debris (NWD),both of which play crucial roles not only in nutrient cycling and biodiversity conservation but also in the global carbon cycle (Pan et al.2011;Zhu et al.2017).In particular,WD includes fine woody debris (FWD) and coarse woody debris (CWD) (Harmon et al.1986),and the latter accounts for ca.20%–30% of global woody biomass (Pan et al.2011).CWD plays crucial roles in conserving biodiversity,forest regeneration,global carbon sinks,and soil development (Iwashita et al.2013;Russell et al.2015;B?ońska et al.2017;Prescott et al.2017).Previous investigations have documented that WD and NWD stocks vary greatly with tree species composition (Raich et al.2007;Wang et al.,2019),forest type (Moreira et al.2019),forest origin(Hagemann et al.2010;Suzuki et al.2019),and stand age (Sefidi 2010;Schilling et al.2016).Most importantly,the rate of plant debris decomposition varies greatly with debris type (Cornwell et al.2009;Harmon et al.2013).FWD and NWD,such as foliar litter,decompose faster than CWD (Müller-Using and Bartsch 2009;Berbeco et al.2012).Additionally,plant debris decomposition differs among tree species,and lower-density WD decomposes faster (Shorohova and Kapitsa 2016;Guo et al.2020).The above description implies that the composition and proportion of plant debris might control forest regeneration,biodiversity nursing,soil and water conservation,and the cycling of carbon and nutrients.Thereby,an investigation of the changes in plant debris stock characteristics with tree species composition determined by forest type is crucial to understand the process and function of forest ecosystems.

    In natural or unmanaged forests,forest succession is an important factor affecting the forest type and species diversity (Lebrija-Trejos et al.2010;West et al.2012;Taylor et al.2020).Meanwhile,the input of plant debris is determined by the death or falling of aboveground species (Tritton 1981;Sturtevant et al.1997).As a result,forest succession may affect the composition and stock of plant debris through the following aspects.First,the stock and composition of plant debris would vary to some degree with forest succession due to the differences in tree species composition and tree longevity at different succession stages.For instance,litter production and dynamics significantly vary with overstory tree species composition in lowland Costa Rica (Raich et al.2007);forest type determines the species composition and CWD stock (Moreira et al.2019).Second,tree species in the earlier succession stages may be more susceptible to disturbance and generate more CWD (Brassard and Chen,2008).However,higher and lower stocks of WD have been observed at the later and earlier succession stages due to differential WD decomposition rates(Carmona et al.2002).Zhu et al.(2017) have shown that the biomass and C storage of CWD may increase with forest succession,while a“U-shape”trend of CWD was observed along the forest successional gradient by Sefidi(2010).Idol et al.(2001) have shown a significant decrease in volume and mass of woody debris from recently harvested to mature stands.Together,the changes in plant debris with forest succession remain uncertain.Thereby,investigating the changes in plant debris stock across forest successional series will help us better understand the function of plant debris in forest ecosystems.

    In theory,the stock and composition of plant debris depend greatly on the WD decomposition,particularly that of CWD (Zell et al.2009).The decay rates of WD and NWD vary with tree species and diameter class(Cornwell et al.2009;Harmon et al.2013),while tree species composition is determined mainly by the succession stage (Taylor et al.2020).Thus,forest succession development may affect the distribution of plant debris decay classes and diameter classes through the following aspects.On the one hand,CWD at earlier successional stages might decompose faster than those at later succession stages due to tree species at earlier successional stages have shorter life cycles and lower CWD quality(Puyravaud et al.2003).For instance,the CWD of broad-leaved species decomposes faster (Yatskov et al.2003);additionally,the CWD of angiosperms decomposes faster than that of gymnosperms (Herrmann et al.2015).On the other hand,the CWD in the earlier successional forests usually have smaller diameters (Ott et al.2006),while in the later succession,the CWD usually have larger diameters (Vanninen et al.1996).Generally,larger-diameter CWD decomposes slower than smaller-diameter CWD (Harmon et al.2013).Thus,characterizing the stocks and proportions of different decay and diameter classes at different successional stages is essential for understanding the nutrient dynamics of plant debris stock and predicting forest succession rate.However,little information is available on CWD stock changes with decay and diameter classes across forest successional series.

    Plant debris represents an essential carbon pool and strongly influences on the structures and carbon dynamics in the forest ecosystems (Harmon et al.1986).The impacts of various driving forces on CWD carbon stocks at the local scale have been widely investigated (e.g.,Hall et al.2006;Woodall and Liknes 2008;Kurz et al.2009;Jonsson et al.2011;Woodall et al.2013).However,only a few studies have evaluated the dependency between the carbon storages of NWD and the stages of succession (Zhang et al.2011).Furthermore,compared with carbon stocks in live biomass or soil,the C budgets in WD and NWD production and turnover at different forest succession stages have not been simultaneously investigated.Consequently,a comprehensive assessment of the carbon budget and the relative contributions of WD and NWD to the carbon budget across forest successional series are critically important for understanding the significance of plant debris in carbon sinks in forest ecosystems.

    To understand the changes in the composition,biomass,and carbon stock of plant debris with forest succession,NWD,FWD,and CWD with different diameter classes and decay classes were investigated across a subalpine forest successional series in Wanglang National Nature Reserve on the eastern Qinghai-Tibet Plateau and the upper reaches of the Yangtze River.This region is the main body of China’s second largest forest area and plays paramount roles not only in holding freshwater,conserving water,and soil and nursing biodiversity but also in the global carbon cycle (Liu 2002;Tan et al.2014).Subalpine forest communities at different successional stages are observed in the subalpine forest region on the eastern Qinghai-Tibet Plateau due to long-term natural disturbance and the commercial logging of natural forests since the 1950s (Yang et al.1992).Although the WD stock from the gap center to the closed canopy in an over-mature subalpine coniferous forest (Xiao et al.2016),the water storage potential of WD (Wang et al.2016),and the changes in microbial biomass and epixylic plant diversity with decay classes (Wang et al.2017;Chang et al.2019) have been widely investigated in this region,little attention has been given to the changes in the composition,biomass stock and carbon storage of plant debris with forest succession.Therefore,we hypothesized that (1) plant debris stocks would increase from the earlier successional stage to the later succession stage in the subalpine forest region,(2) the stocks and proportions of different decay and diameter classes should differ at different successional stages,and (3) the C stock of plant debris would increase across the forest succession.The objectives were to (1) investigate the changes in plant debris stocks across the forest successional series located in the subalpine forest region,(2) elucidate the stocks and proportion of different decay and diameter classes at different successional stages,and (3) assess the carbon budget and the relative contributions of CWD with different decay classes and diameter classes to the carbon budget across the forest successional series.These results could help us better understand the function and significance of plant debris in carbon sinks in forest ecosystems and provide clear insight into the preservation of CWD,which is a critical strategy for sustainable forest management in the subalpine forest on the eastern Qinghai-Tibet Plateau.

    Materials and methods

    Field description

    The study region is located in the Wanglang National Nature Reserve (32°49′–33°02′ N,103°55′–104°10′ E;2300–4983 m a.s.l.),which is situating in Pingwu County,Sichuan,Southwest China (Fig.A1).The region is a transitional area between the Tibetan Plateau and the Sichuan Basin.The annual precipitation is approximately 859.9 mm,and the annual mean temperature is approximately 2.9°C,with maximum and minimum temperatures of 26.2°C and ?17.8°C,respectively (Zhang et al.2011).The tree canopy is dominated by Abies faxoniana,Picea purpurea,Sabina saltuaria,Betula platyphylla,and B.albo-sinensis.The understory shrubs are dominated by Salix wallichiana,Hippophae rhamnoides,Rhododendron lapponicum,Lonicera spp.,Sorbus rufopilosa,and Rosa sweginzowii.The herbaceous layer is dominated by Cacalia palmatisecta,Cyperaceae,Poa pratensis,and others (Taylor et al.2006).The moss layer is dominated by Thuidium cymbifolium,Hylocomium splendens,Mnium heterophyllum,and Phaeoceros laevis(Li et al.2012).

    Experimental design

    Based on our previous visits and investigations on forest vegetation and plant debris in Wanglang National Nature Reserve,we divided the different forest types into six succession stages according to Zhang et al.(2011)and through interviews with local supervisors.S1 successional stage is dominated by shrubs (e.g.,H.rhamnoides),S2 successional stage is dominated by deciduous broadleaved species (e.g.,Betula),S3 successional stage is dominated by deciduous broadleaved (Betula) and coniferous (A.faxoniana) mixed forest and later successional stages (S4,S5,and S6) are all dominated by coniferous species.Here S4 represents the earlier coniferous successional stage (A.faxoniana and P.purpurea);S5 represents the middle coniferous successional stage (A.faxoniana);S6 represents the mature coniferous successional stage (P.purpurea).The six successional forests are widely distributed in the subalpine forest region (Fig.A1).Site information and specific characteristics of each successional stage are recorded in Table A1.

    Sampling method

    Woody debris was classified into CWD with a diameter ≥10 cm and FWD with a diameter of 2 cm ≤d<10 cm (Ward and Aumen 1986;Harmon and Hua 1991).CWD includes fallen logs,snags (dead standing trees),stumps,and large branches (Harmon et al.1986).Snags refer to CWD which inclination is not more than 45°,base diameter ≥10 cm,and length>1 m.Stumps with a height<1 m were defined as CWD,which includes coarse roots above the soil surface (Harmon and Sexton 1996;Currie and Nadelhoffer 2002).Among these,dead coarse roots of fallen logs and snags were measured and recorded,and of which stumps were neglected due to they were rarely observed.Additionally,we reclassified foliar litter,dead twig,dead fine bark,and epiphytes as NWD.Three plots of 10 m×20 m of each successional forest were established for CWD and FWD investigation,and nine subplots of 1 m×1 m were established for NWD investigation.Five decay class (I-V) systems optimized according to B?ońska et al.(2018) were used to classify the decomposition degree of WD based on the morphology and hardness observed in the field (Table A2).Five diameter sizes(D1–D5) were applied to classify the WD diameter according to Harmon et al.(1986) and Xiong et al.(2016),that is,2 cm ≤d<5 cm (D1),5 cm ≤d<10 cm (D2),10 cm ≤d<20 cm (D3),20 cm ≤d<40 cm (D4) and d ≥40 cm (D5).Then,the decay class,base diameter,end diameter,and length (fallen logs and large branches) or height (snags and stumps) of the WD were recorded and measured.For CWD,the length beyond the plot,only the part within the plot be recorded.Next,for the CWD with decay classes I to III,three dish samples of each decay class with a thickness of approximately 5 cm were obtained by chain saw.For the CWD with decay classes IV and V and FWD,three samples with a weight of approximately 500 g were harvested.For NWD,all samples within the subplot of 1 m×1 m were harvested.The samples were taken back to the laboratory for stock and C concentration measurements.

    Measurement of plant debris stock and C stock

    After the dish samples of CWD from decay classes I to III were delivered to the laboratory,their diameter (d1)and thickness (h) were measured.Then,the volume (V1)and density (ρ) were calculated (Bonan 2008).After the CWD samples from decay classes IV and V and the FWD samples were delivered to the laboratory,we used the drainage method to determine the volume (V2)(Jonsson 2000).Then,we weighed these samples after oven drying at 85°C to a constant weight (m1).For NWD,the sample was weighed after oven drying at 85°C to a constant weight (m2).The volume (V) of WD in the field was calculated according to Zhu et al.(2017),and the stock (G) of WD in the field was converted by the volume (V) and the sample density (ρ).For C concentration,the most important factor determining C concentration (%) of plant debris were decay class and geographical location,and there was little difference between plant species or other factors (Zhu et al.2017).So,for WD from the different decay classes,carbon concentrations of 50.5%,50.0%,49.7%,43.3%,and 37.9%refer to Chang et al.(2015),who have investigated the changes in the C concentrations with WD decay classes in a similar subalpine forest;for NWD,a carbon concentration of 41.9% refer to Zhu et al.(2017).

    The WD density (ρ,g?cm?3) was calculated as follows:

    where m1is the dry weight of the CWD and FWD samples (g),and d1and h are the fresh dish sample diameter and thickness(cm),respectively.

    The volume of WD(V,cm3)in the field was calculated as follows:

    where d2,d3,and l are the base diameter,end diameter,and length or height of the WD in the field (cm),respectively.

    The WD stock (G1,Mg?ha?1) was calculated using the following formula:

    where θ is the slope of each plot (°),200 is the area of each plot(m2),and 100 is the unit conversion factor.

    The NWD stock (G2) was calculated using the following formula:

    where m2is the dry weight of the NWD sample (g),θ is the slope of each subplot(°),1 is the area of each subplot(m2),and 100 is the unit conversion factor.

    The carbon stocks (Mg?ha?1of C) of WD and NWD were calculated using the following formula:

    where G1and G2are the stocks of WD and NWD,respectively.

    Statistical analysis

    One-way analysis of variance (ANOVA) and least significant difference (LSD) tests were applied to examine the different significance of WD stocks and C stocks among different decay classes,among different diameter sizes,and among these successional forests.The significance was selected at the 0.05 level.The interaction effects of successional stages,decay classes,and diameter sizes on the WD stock and C stock were analyzed by Multivariate analysis of variance (MANOVA).All statistical analyses were carried out by IBM SPSS Statistics v.20 (IBM Corporation,New York,USA).

    Results

    Plant debris stock across the successional series

    Plant debris stock ranged from 25.19 to 82.89 Mg?ha?1across the subalpine forest successional series,and the highest and lowest stocks of plant debris were observed in the later successional stage (S5) and earlier forest stage (S1),respectively (Table 1).When all plant debris components were considered together,the stocks of plant debris represented a total tendency of increasing from the S1 to S6 stands except for the sudden decrease observed in S4 medium-aged earlier coniferous forest(F=6.34,P<0.01;Fig.1a).The stock was significantly higher in CWD than in FWD and NWD from all six successional stages (Fig.1b).The proportion of CWD stock ranged from 88% to 98% along the forest succession gradient.Among these,only 1%–9% and 1%–4% of the stocks were stored in FWD and NWD,respectively(Fig.1b).In detail,the stocks of different plant debris components were 22.20–80.37 Mg?ha?1for CWD,0.63–2.36 Mg?ha?1for FWD,and 0.40–1.31 Mg?ha?1for NWD (Fig.2).CWD displayed a similar trend as that of plant debris (Fig.1a),and the stock of CWD was significantly higher in S5 and S6 than in the S1,S2,and S4 successional stages (F=6.28,P<0.01;Fig.2a).In contrast,the stocks of FWD and NWD decreased from S1 to S6,and the NWD stock was significantly lower in S5 than in S1 (F=5.23,P<0.01;Fig.2c).However,slight differences in FWD stocks were observed across the forest successional series (F=0.61,P>0.05;Fig.2b).

    Fig.1 Stock of plant debris component(a)and their allocation(b)in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau.Different lowercase letters indicate that the total stock of plant debris differed significantly among successional gradient(P<0.05).Horizontal bars indicate standard errors of means(n=3)

    Table 1 The stocks and ratios of plant debris across a successional gradient in the subalpine forest on eastern Qinghai-Tibet Plateau

    The ratios of WD to NWD stocks ranged from 26.58 to 208.89,and the highest and lowest ratios of WD to NWD stocks were observed in the mid-coniferous forest(S5) and shrub forest stages (S1),respectively (Table 1).In particular,the ratios of CWD to FWD stocks varied greatly with successional stages,ranging from 10.57 in the broadleaved forest (S2) to 435.63 in the natural coniferous forest (Table 1).Furthermore,the coefficient of variation (CV) of plant debris stocks along the forest successional series reached 48.93% (Fig.2d).Meanwhile,the CV of the FWD stocks was significantly higher than the CV of the NWD stocks (P<0.05;Fig.2d).

    Fig.2 Stock of coarse woody debris CWD(a),fine woody debris FWD(b)and non-woody debris NWD(c)in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau.Figure d indicates the coefficients of variation(CV)of stock of plant debris,CWD,FWD and NWD at different succession ages.Different lowercase letters mean significant difference among different succession stages or different plant debris components(P<0.05).Horizontal bars indicate standard errors of means(n=3)

    Changes in WD stocks with decay class across the successional series

    As shown in Table 2,the highest and lowest stocks of WD were observed in decay class IV (13.67 Mg?ha?1)and decay class II (8.77 Mg?ha?1),respectively.However,a slight variation pattern of WD stock was observed across the five decay classes(F=0.542,P>0.05;Table 3).A higher proportion of WD stock was observed in higher decay classes in the later succession stage S6,although there was no significant difference (Fig.3).Actually,among the different successional forests,a significant difference between decay classes was observed only in the earlier succession stage (S1) but was not observed in the other forests (Fig.3).Similarly,the WD stock of each decay class varied slightly among the six plant communities (Fig.3).

    Fig.3 Stock of woody debris in different decay classes among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau.Different lowercase letters mean significant difference among different decay classes(P<0.05).Horizontal bars indicate standard errors of means(n=3)

    Table 2 Changes in woody debris stock with decay classes and diameter sizes in the subalpine forest on eastern Qinghai-Tibet Plateau

    Changes in WD stocks with diameter size across the successional series

    The WD stock was significantly higher in largerdiameter (D5) than finer-diameter WD (D1 and D2)(F=25.97,P<0.001;Tables 2 and 3).Consistently,the stock of the coarsest diameter (D5) was remarkably higher than that of the other diameter sizes in the older successional series S5 and S6,and presented a significant increasing tendency with diameter size (P<0.05).The highest stock at S3 and S4 stages were observed in the coarser diameters of D4 and D3,respectively (P<0.05).The stocks of diameters D3 and D5 were higher than that of the finer WD in S1.However,a slight difference in WD stock was observed between diameter sizes at the S2 stage (P>0.05;Fig.4).Meanwhile,the WD stock of D3,D4,and D5 diameters varied significantly with successional stages (Fig.4).Furthermore,the WD stock of diameter D5 at S5 or S6 stage was obviously higher than this of the earlier succession stands (P<0.05),and the highest WD stocks of diameters D4 and D3 were observed in the mid-successional forests S3 and S4,respectively (P<0.05;Fig.4).Additionally,an interaction effect was observed among successional series,decay classes,and diameter classes for the stock measured in the WD (F=3.576,P<0.001;Table 3),although the WD stock was not significantly affected by decay class (F=0.542,P>0.05).

    Fig.4 Stock of woody debris in different diameter sizes among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau.D1–D5 indicated woody debris in diameter of 2 cm ≤d<5 cm,5 cm ≤d<10 cm,10 cm ≤d<20 cm,20 cm ≤d<40 cm,d ≥40 cm,respectively.Different lowercase letters mean significant difference among different diameter sizes(P<0.05).Different uppercase letters mean significant difference among different succession stages(P<0.05).Horizontal bars indicate standard errors of means(n=3)

    The C stocks of plant debris across the successional series

    The plant debris C stocks ranged from 10.30 to 38.87 Mg?ha?1across the subalpine forest succession series,and the highest and lowest stocks of plant debris were observed in the later succession stage (S5) and the earlier forest stage (S2),respectively (Table 4),showing a global tendency of increasing from the S1 to S6 stands except for the sudden decrease observed in S4 mediumaged earlier coniferous forest (Fig.5a).Furthermore,the stock was significantly higher in CWD than in FWD and NWD at each successional stage (Fig.5).Compared with the mean proportions of C stocks of NWD (2%) and FWD (4.5%),the minimum proportion of CWD C storage is 87% in the subalpine forest ecosystem (Fig.5b).Especially in the later stages of succession,the total plant debris C pool was dominated mainly by CWD(P<0.001;Fig.5b).

    Fig.5 Carbon stock of plant debris component(a)and their allocation(b)in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau.Different lowercase letters indicate that the total carbon stock of plant debris differed significantly among successional gradient(P<0.05).Horizontal bars indicate standard errors of means(n=3)

    Table 3 Multivariate analysis of variance (MANOVA)for the effects of succession series,decay classes and diameter sizes on the stocks of woody debris and carbon

    Among plant debris components,the changes in the plant debris C stock with plant debris component and successional stages showed a similar pattern of plant debris component stock (Fig.2).The C stock of CWD ranged from 8.99 to 37.66 Mg?ha?1and showed a global increase trend (Fig.6a),which was significantly higher in the later successional stages S5 and S6 than in the earlier successional stages S1 and S2 (P<0.05;Fig.6a).Most importantly,the C stored in CWD was more than four times higher in the highest forest (S5) than in the lowest forest (S2) (Fig.6a).The C stock ranged from 0.30 to 1.17 Mg?ha?1for FWD (Fig.6b) and 0.18 to 0.55 Mg?ha?1for NWD (Fig.6c),respectively,and showed the opposite tendency to CWD.C stored in FWD and NWD (P<0.05) was lower in the S6 stage than in the S1 stage,but this difference was nonsignificant for FWD(P>0.05;Figs.6b and c).Besides,the CVs of the C stocks varied significantly with the different components of plant debris.The CV of the FWD C stock was significantly higher than the CV of the CWD and NWD stocks in the six successional stages (P<0.05;Fig.6d).

    Fig.6 Carbon stock of coarse woody debris CWD(a),fine woody debris FWD(b)and non-woody debris NWD(c)in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau.Figure d indicates the coefficients of variation(CV)of carbon stock of plant debris,CWD,FWD and NWD at different succession ages.Different lowercase letters mean significant difference among different succession stages or different plant debris components(P<0.05).Horizontal bars indicate standard errors of means(n=3)

    Both succession stage (F=7.147,P<0.001) and diameter class (F=27.185,P<0.001) had significant effects on the C concentration of WD,but decay class had a slight effect on the C stock (F=1.564,P>0.05).Furthermore,the C stock of WD was significantly influenced by the interaction among succession age,diameter size,and decay class (F=4.072,P<0.001;Table 3).

    Table 4 The carbon stock of plant debris across a successional gradient in the subalpine forest on eastern Qinghai-Tibet Plateau

    Table 5 Comparisons of plant debris stock(Mg?ha?1) and plant debris carbon stock(Mg?ha?1) among different regions

    Discussion

    Our results indicated that plant debris stocks increased from the earlier successional stage to the later successional stage except for decreasing at the S4 successional stage in the subalpine forest region,which partly supported our first hypothesis.Meanwhile,the results showed that the WD stock varied slightly at different decay classes but significantly at different diameter sizes among the six plant communities,which was partly consistent with our second hypothesis.Additionally,our results still demonstrated a global tendency of increase from the S1 to S6 stands except for the decrease observed in S4 mediumaged earlier coniferous forest,which partly agreed with the third hypothesis.This result confirmed that CWD changed greatly with forest succession and demonstrated that CWD dominated the plant debris in natural forests and that the ratios and components of plant debris fluctuated sharply with forest successional series.

    Changes in WD and NWD stocks with forest succession

    Forest succession is an important factor that affects the stocks of CWD or litter in different forest ecosystems(Idol et al.2001;Sefidi 2010;Zhang et al.2011;Aryal et al.2015).However,the tendency of the plant debris biomass and C stocks,particularly in CWD components,differed greatly from those in previous reports (Table 5).A greater amount of CWD was found in earlier-or older-growth forests than in other stands (presenting a‘U-shaped’ pattern) (Eaton and Lawrence 2006;Sefidi 2010).A monotonic increase in CWD amount or C density was found following secondary forest succession in Chiloe (Carmona et al.2002) and Chinese forests(Tang and Zhou 2005;Zeng et al.2015;Zhu et al.2017).Idol et al.(2001) have shown a large decrease in volume and mass of woody debris from recently harvested to mature stands (Table 5).However,our results suggested a global increasing curve for CWD with succession except for the sudden decrease observed in the mediumaged earlier coniferous forest (S4) (Figs.2a and 6a).In general,the accumulation of CWD depends not only on plant debris production but also on the rate of decomposition (Tritton 1981;Sturtevant et al.1997).Since CWD breaks down persistently and slowly,its production rates ultimately outpace its decomposition rates.Therefore,the later succession stages with more aboveground biomass commonly have higher CWD stocks(Smith et al.2006),while the earlier successional stands with less live biomass have lower CWD stocks.Most importantly,the difference is likely that their analyses were simply based on the data of general successional gradients that lacked consideration of earlier coniferous forests.Although the earlier coniferous forest has a higher succession stage than that of mixed forest,it has a rapid growth period for trunk biomass.Meanwhile,the CWD is almost decomposed in the earlier stage,and there is relatively little WD,especially CWD.

    Compared with the biomass and C stocks of CWD,the stocks of NWD had an opposite tendency (Figs.2c and 6c).First,we estimated that the biomass and C stocks reached a peak in the earlier succession stage (S1)and then decreased with the forest succession stage but suddenly increased in the medium stage earlier coniferous forest (S4).However,in other forest ecosystems(Table 5),the production of litter increased through the succession process (Yan et al.2009;Zhang et al.2013)or increased rapidly and then decreased with the forest stage (Aryal et al.2015).The discrepancy of rates of input (aboveground biomass) and output (decomposition)may be one of the possible reasons (Dent et al.2006;Wang et al.2007).The tree species of litter and climate of different forest ecosystems also determined the decomposition rates of litter (Müller-Using and Bartsch 2009;Berbeco et al.2012).Second,in contrast to the CWD stock,the higher stock in the young shrub forest(S1) was contributed by shrub twigs.Meanwhile,the later successional stage of the mid-and mature coniferous forests (S5 and S6) had lower stocks,which was likely related to the lower inputs of needles and the faster losses of thinner litter,which decelerated the amounts of remains and sped up the rate of decomposition.

    The effect of decay class and diameter on WD stock across the succession series

    Interestingly,decay class did not affect the storage of WD in five successional stages of forest ecosystems in our study (Fig.3).However,it has been reported that decay class IV was the most abundant decay class in a Northern Iran forest (Sefidi 2010).Yan et al.(2007) have reported that the evergreen broadleaved forests of later successional stages have a higher proportion of WD with decay classes IV and V in the subtropical region of eastern China.In contrast,Carmona et al.(2002) have reported that CWD at advanced decay levels was more abundant in earlier-stage stands,while most CWD was in the intermediate decay classes in older forests.These contradictions can be partly attributed to the difference in the vegetation composition.Additionally,all study sites were located within a nature reserve with little disturbance and management practice,which is another crucial reason for the average distribution of WD at different decay classes.

    In addition to the significant effect of the succession stage on the stock of plant debris,diameter size influenced the storage of WD (Fig.4).Our results illustrated that larger-diameter WD dominated the plant debris as the successional stage increased.Our current estimates were also confirmed by several studies investigating whether CWD was dominant in plant debris across forest succession stage (Eaton and Lawrence 2006;Van Mantgem et al.2009;Sefidi 2010;Yang et al.2010;Zeng et al.2015).Tree species at different successional stages produced different biomasses of CWD.For instance,tree species with a cohort of old and large-diameter fir and spruce coniferous species in an old-growth stand could lead to more biomass of CWD,while most were small and thin willow shrub and birch in the earlier successional forest.However,as suggested by other reports,the relative contribution of CWD to the plant debris biomass declined with forest succession (Brown and Lugo 1990;Krankina and Harmon 1995;Delaney et al.1998).The difference may be due to different standards of classification of diameter sizes and climate conditions used at different study sites.

    Plant debris carbon stock across the succession series

    Countrywide,the estimated plant debris C stock was 5.88±0.35 Mg C?ha?1in a temperate forest (Zhu et al.2017).In our study,the plant debris C stock ranged from 10.30 to 38.87 Mg?ha?1(Table 3),much higher than the average of Chinese forests but similar to some results from local forests in China (Yang et al.2010) and southern Indiana (Idol et al.2001) (Table 5).The higher C stock in our ecosystem is likely a result of natural forests with little disturbance and management practices.The source of nationwide data,including many younggrowth plantations and areas of excessive harvest,occurred in planted forests (Guo et al.2013) and resulted in a low average C stock value.In addition,the plant debris investigation in our study tended to select forests with a greater distribution of WD (Pan et al.2011),which caused the overestimates of plant debris amount and C stock.

    Conclusions

    The change in the biomass and C stock of WD represented a total tendency of increasing from the S1 to S6 stands,while the changes of FWD and NWD were decreased across the subalpine forest succession series.CWD dominated the plant debris regardless of the forest successional stages.A larger diameter size and advanced decay class tended to be abundant in the older stage of succession.Furthermore,the C stored in CWD in later successional forests was four times higher than in earlier successional forests.Together,the stock and role of WD,particularly CWD,varied greatly with the forest successional stage and dominated the C cycle in the subalpine forest ecosystem.The results suggested keeping CWD on the forest floor be a critical strategy for maintaining forest productivity and implementing sustainable forest management in the subalpine forest on the eastern Qinghai-Tibet Plateau.Additionally,CWD plays an important role in the global C cycle in subalpine forest ecosystems.

    Supplementary Information

    The online version contains supplementary material available at https://doi.org/10.1186/s40663-021-00320-0.

    Additional file 1:Fig.A1.Distribution of the study sites across a subalpine forest succession gradient in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau.Figures b and c indicate the location of study region in China and Pingwu county,Figure a indicates the distribution of study sites at Wanglang National Nature Reserve.S1:Shrub forest;S2:Broad-leaved forest;S3:Mixed forest;S4:Early coniferous forest;S5:Mid-coniferous forest and S6:Nature coniferous forest.Table A1.Site characteristics across a successional gradient in the subalpine forest on eastern Qinghai-Tibet Plateau.Table A2.Decay classes of woody debris.

    Acknowledgements

    The authors of this study would like to thank all people of Wanglang National Nature Reserve involved in the initial sampling assignments.

    Authors’ contributions

    WQY and ZHW convinced conceptualization.ZHW analyzed data.ZHW led the writing of the manuscript.WQY led the review and editing of the manuscript.All authors made contributions to data collection via fieldwork or lab work.All authors read and approved the final manuscript.

    Funding

    This work was financially supported by the National Nature Science Foundation of China (32071554,31570445).

    Availability of data and materials

    All data generated or analyzed during this study are included in this published article.

    Declarations

    Ethics approval and consent to participate

    Not applicable.

    Consent for publication

    Not applicable.

    Competing interests

    The authors declare that they have no competing interests.

    Author details

    1School of Life Sciences,Taizhou University,Taizhou 318000,China.

    2Wanglang National Nature Reserve Authority,Pingwu 622550,Sichuan,China.

    Received:3 January 2021Accepted:3 June 2021

    亚洲人成网站在线观看播放| 亚洲精品日本国产第一区| 好男人视频免费观看在线| 国产乱来视频区| 夜夜爽夜夜爽视频| 成人免费观看视频高清| 午夜av观看不卡| 亚洲欧洲日产国产| 国产成人aa在线观看| videos熟女内射| 人人妻人人爽人人添夜夜欢视频| 国产精品偷伦视频观看了| 一级a做视频免费观看| 午夜老司机福利剧场| 啦啦啦啦在线视频资源| 丝袜喷水一区| 人妻 亚洲 视频| 国产有黄有色有爽视频| 免费不卡的大黄色大毛片视频在线观看| 亚洲,一卡二卡三卡| 99久久精品国产国产毛片| 晚上一个人看的免费电影| 中文字幕久久专区| 九九在线视频观看精品| 97超视频在线观看视频| 大香蕉97超碰在线| 日韩视频在线欧美| 永久网站在线| 一边亲一边摸免费视频| 午夜激情av网站| tube8黄色片| 国产免费现黄频在线看| 久久久久久伊人网av| 少妇猛男粗大的猛烈进出视频| 婷婷色av中文字幕| 97超碰精品成人国产| 欧美精品人与动牲交sv欧美| 男女高潮啪啪啪动态图| 99热全是精品| 亚洲人成网站在线观看播放| 制服诱惑二区| 久久午夜福利片| 免费黄网站久久成人精品| 日本黄色日本黄色录像| 亚洲熟女精品中文字幕| 免费大片18禁| 晚上一个人看的免费电影| 国产探花极品一区二区| 婷婷色综合大香蕉| 美女内射精品一级片tv| 日本91视频免费播放| 夫妻性生交免费视频一级片| 免费不卡的大黄色大毛片视频在线观看| 在线精品无人区一区二区三| 国产av码专区亚洲av| 亚洲欧美成人综合另类久久久| 亚洲,一卡二卡三卡| 中文字幕亚洲精品专区| 成人免费观看视频高清| 久久久久视频综合| 国产精品一区www在线观看| 黄色欧美视频在线观看| 欧美精品人与动牲交sv欧美| 一级二级三级毛片免费看| a级毛色黄片| 蜜桃在线观看..| 亚洲精品自拍成人| 日韩强制内射视频| 丝袜在线中文字幕| 国产男女超爽视频在线观看| 五月玫瑰六月丁香| 久久精品夜色国产| 精品人妻熟女毛片av久久网站| 成人手机av| 成人18禁高潮啪啪吃奶动态图 | 久久av网站| 黄片无遮挡物在线观看| 少妇猛男粗大的猛烈进出视频| 亚洲美女搞黄在线观看| 亚洲第一区二区三区不卡| 久久久久视频综合| 下体分泌物呈黄色| 色5月婷婷丁香| 成人手机av| 国产成人91sexporn| 91在线精品国自产拍蜜月| 久久久精品区二区三区| 伦理电影大哥的女人| 熟女电影av网| 少妇丰满av| 有码 亚洲区| 热re99久久国产66热| av线在线观看网站| 亚洲av日韩在线播放| 欧美精品国产亚洲| 三级国产精品片| 欧美激情 高清一区二区三区| 最新的欧美精品一区二区| 成人18禁高潮啪啪吃奶动态图 | 欧美精品人与动牲交sv欧美| 精品少妇久久久久久888优播| 国产无遮挡羞羞视频在线观看| 精品少妇黑人巨大在线播放| 美女主播在线视频| 精品久久久精品久久久| 天天影视国产精品| 国产免费一级a男人的天堂| 大片电影免费在线观看免费| a级毛色黄片| 国产在线视频一区二区| 亚洲精品色激情综合| 一本一本综合久久| 日韩成人伦理影院| 超色免费av| 少妇丰满av| 国产精品人妻久久久影院| 中文精品一卡2卡3卡4更新| 少妇猛男粗大的猛烈进出视频| 久久久久久久久久久丰满| 99国产综合亚洲精品| 各种免费的搞黄视频| 免费日韩欧美在线观看| 久久久精品免费免费高清| 男女边摸边吃奶| 久久久精品94久久精品| 国产欧美日韩一区二区三区在线 | 五月天丁香电影| 日韩精品免费视频一区二区三区 | 国产欧美日韩综合在线一区二区| 亚洲四区av| 日韩电影二区| 大香蕉久久成人网| 精品国产国语对白av| 国产高清三级在线| 欧美日韩成人在线一区二区| 亚洲婷婷狠狠爱综合网| 99久久精品一区二区三区| 免费人妻精品一区二区三区视频| 日本爱情动作片www.在线观看| 老熟女久久久| 男女啪啪激烈高潮av片| 91精品国产国语对白视频| 极品少妇高潮喷水抽搐| 2022亚洲国产成人精品| 18禁在线无遮挡免费观看视频| 久久久国产欧美日韩av| 欧美日韩成人在线一区二区| 日韩免费高清中文字幕av| 国产免费又黄又爽又色| 欧美日韩成人在线一区二区| 夜夜骑夜夜射夜夜干| 精品视频人人做人人爽| 3wmmmm亚洲av在线观看| 国产 精品1| 欧美老熟妇乱子伦牲交| 美女国产视频在线观看| 99热全是精品| 全区人妻精品视频| 9色porny在线观看| 国产亚洲精品第一综合不卡 | av又黄又爽大尺度在线免费看| 国产精品一区www在线观看| 久久久久久久久久成人| 97超视频在线观看视频| 建设人人有责人人尽责人人享有的| 不卡视频在线观看欧美| 中文精品一卡2卡3卡4更新| 99热全是精品| 波野结衣二区三区在线| 日韩强制内射视频| 亚洲美女搞黄在线观看| 免费高清在线观看视频在线观看| 一区二区三区乱码不卡18| 午夜视频国产福利| 草草在线视频免费看| 热99国产精品久久久久久7| 考比视频在线观看| 欧美人与性动交α欧美精品济南到 | 久久精品久久久久久久性| 亚洲中文av在线| 亚州av有码| 最近的中文字幕免费完整| 亚洲在久久综合| 久久久久久久久大av| 午夜激情av网站| 成年人免费黄色播放视频| 女人精品久久久久毛片| 飞空精品影院首页| 欧美精品人与动牲交sv欧美| 精品久久久精品久久久| 极品少妇高潮喷水抽搐| 91国产中文字幕| 午夜免费鲁丝| 久久婷婷青草| 久久精品久久久久久噜噜老黄| 高清欧美精品videossex| 国产伦理片在线播放av一区| 少妇丰满av| 国产精品久久久久久精品电影小说| 黄色欧美视频在线观看| 精品人妻偷拍中文字幕| 最近中文字幕高清免费大全6| 国产精品麻豆人妻色哟哟久久| 91久久精品电影网| 王馨瑶露胸无遮挡在线观看| 亚洲性久久影院| 爱豆传媒免费全集在线观看| 欧美日韩亚洲高清精品| 91久久精品电影网| 91精品伊人久久大香线蕉| 老女人水多毛片| 免费高清在线观看视频在线观看| 成年人午夜在线观看视频| 久久精品国产亚洲av涩爱| 国产一区亚洲一区在线观看| 在线亚洲精品国产二区图片欧美 | 如何舔出高潮| 久久久亚洲精品成人影院| 久久青草综合色| 中文字幕人妻熟人妻熟丝袜美| 欧美精品人与动牲交sv欧美| 一本大道久久a久久精品| 高清视频免费观看一区二区| 少妇的逼好多水| 国产精品蜜桃在线观看| 视频在线观看一区二区三区| 欧美 亚洲 国产 日韩一| 亚洲无线观看免费| 国产一区亚洲一区在线观看| 亚洲国产精品国产精品| 国产 一区精品| 免费观看的影片在线观看| 亚洲无线观看免费| 一级毛片电影观看| 亚洲国产毛片av蜜桃av| 成人国产麻豆网| 色婷婷久久久亚洲欧美| 最近中文字幕2019免费版| 国产高清国产精品国产三级| 男人添女人高潮全过程视频| 美女国产视频在线观看| 国产爽快片一区二区三区| 22中文网久久字幕| 免费日韩欧美在线观看| 国产成人精品福利久久| 成人漫画全彩无遮挡| 国产亚洲精品第一综合不卡 | 高清不卡的av网站| 欧美老熟妇乱子伦牲交| 欧美少妇被猛烈插入视频| 亚洲美女搞黄在线观看| 国产午夜精品久久久久久一区二区三区| 国产精品熟女久久久久浪| 在线精品无人区一区二区三| 搡老乐熟女国产| 亚洲少妇的诱惑av| 制服诱惑二区| 国产日韩欧美视频二区| 欧美精品一区二区大全| 午夜老司机福利剧场| 久久精品国产亚洲av涩爱| 色哟哟·www| 老熟女久久久| 3wmmmm亚洲av在线观看| 一级毛片我不卡| 亚洲av不卡在线观看| 青春草国产在线视频| 亚洲天堂av无毛| 男女国产视频网站| 国产亚洲一区二区精品| 国产永久视频网站| 我要看黄色一级片免费的| 国产一区二区三区综合在线观看 | videossex国产| 精品少妇黑人巨大在线播放| 亚洲国产精品一区三区| 日韩视频在线欧美| 五月玫瑰六月丁香| 丝袜喷水一区| 草草在线视频免费看| 青春草国产在线视频| 国产女主播在线喷水免费视频网站| 一区二区三区免费毛片| 成人亚洲精品一区在线观看| 成人18禁高潮啪啪吃奶动态图 | 亚洲美女视频黄频| 蜜桃国产av成人99| 欧美日韩在线观看h| 国产精品嫩草影院av在线观看| av有码第一页| 久久精品久久精品一区二区三区| 久久亚洲国产成人精品v| 亚州av有码| 99热这里只有是精品在线观看| 人人妻人人添人人爽欧美一区卜| 精品一区在线观看国产| 五月天丁香电影| 欧美日本中文国产一区发布| 日本午夜av视频| 国产精品久久久久成人av| 亚洲国产毛片av蜜桃av| 久久人妻熟女aⅴ| 国产精品秋霞免费鲁丝片| a级毛色黄片| 欧美精品人与动牲交sv欧美| 国产精品一区www在线观看| 免费观看av网站的网址| 精品国产国语对白av| 国产在线免费精品| 久久精品久久精品一区二区三区| 免费看光身美女| 男女国产视频网站| 国产又色又爽无遮挡免| av天堂久久9| 午夜免费观看性视频| 十分钟在线观看高清视频www| 内地一区二区视频在线| 成人黄色视频免费在线看| 少妇 在线观看| 亚洲av日韩在线播放| av电影中文网址| 少妇高潮的动态图| 久久综合国产亚洲精品| 人妻一区二区av| 日本猛色少妇xxxxx猛交久久| av线在线观看网站| 国产精品秋霞免费鲁丝片| 免费看av在线观看网站| 91国产中文字幕| 亚洲欧美日韩另类电影网站| 亚洲精品亚洲一区二区| 国产乱来视频区| 午夜日本视频在线| 三级国产精品片| 国产黄色免费在线视频| 天堂俺去俺来也www色官网| 久久热精品热| 黄色视频在线播放观看不卡| 欧美精品人与动牲交sv欧美| 51国产日韩欧美| 寂寞人妻少妇视频99o| 中文字幕精品免费在线观看视频 | 久久久久久久久久久久大奶| 美女cb高潮喷水在线观看| 人人妻人人澡人人爽人人夜夜| 啦啦啦视频在线资源免费观看| 国产成人freesex在线| 搡老乐熟女国产| 少妇被粗大的猛进出69影院 | 中文字幕最新亚洲高清| 久久久精品免费免费高清| 高清av免费在线| xxx大片免费视频| 熟妇人妻不卡中文字幕| 亚洲国产av影院在线观看| 自线自在国产av| 成人黄色视频免费在线看| 婷婷成人精品国产| 看非洲黑人一级黄片| 亚洲国产成人一精品久久久| 少妇丰满av| 成年av动漫网址| av福利片在线| 亚洲美女搞黄在线观看| 国产成人免费观看mmmm| 国产免费又黄又爽又色| 男女边吃奶边做爰视频| 另类亚洲欧美激情| 人人澡人人妻人| 午夜福利,免费看| 国产精品女同一区二区软件| 黄色配什么色好看| 卡戴珊不雅视频在线播放| 尾随美女入室| 成人18禁高潮啪啪吃奶动态图 | 亚洲欧洲日产国产| 在线播放无遮挡| 精品久久国产蜜桃| 另类亚洲欧美激情| 在线观看美女被高潮喷水网站| 尾随美女入室| 亚洲,一卡二卡三卡| 国产精品.久久久| 一级二级三级毛片免费看| 人妻一区二区av| 久久午夜综合久久蜜桃| 99久久精品一区二区三区| 97在线视频观看| 国产精品人妻久久久久久| 一级二级三级毛片免费看| 欧美人与善性xxx| 久久精品久久精品一区二区三区| 久久精品人人爽人人爽视色| 亚洲国产最新在线播放| 中文字幕人妻丝袜制服| 五月天丁香电影| 中文天堂在线官网| 建设人人有责人人尽责人人享有的| 成人手机av| 欧美日韩亚洲高清精品| 久久久久久久久大av| 国产高清三级在线| 国产精品欧美亚洲77777| 日韩欧美一区视频在线观看| 久久午夜综合久久蜜桃| 免费大片黄手机在线观看| 高清不卡的av网站| 99视频精品全部免费 在线| 欧美 亚洲 国产 日韩一| 人妻夜夜爽99麻豆av| 国产精品久久久久久精品电影小说| 久久免费观看电影| 韩国高清视频一区二区三区| 观看av在线不卡| 亚洲人成网站在线播| 亚洲欧美成人综合另类久久久| av在线app专区| 成人国产av品久久久| 久久久久久久大尺度免费视频| 成人综合一区亚洲| 免费观看av网站的网址| 久久 成人 亚洲| 在线观看免费视频网站a站| 草草在线视频免费看| 啦啦啦啦在线视频资源| 久久久久久久久久久久大奶| 成人无遮挡网站| 91精品三级在线观看| 男女边吃奶边做爰视频| 一个人免费看片子| 国产极品粉嫩免费观看在线 | 中文字幕久久专区| 久久久久久久久大av| 亚洲av不卡在线观看| 在线观看免费日韩欧美大片 | 在线看a的网站| 亚洲成人手机| 国产成人精品福利久久| 黄片播放在线免费| 免费少妇av软件| 日韩不卡一区二区三区视频在线| 老司机影院成人| 多毛熟女@视频| 中文字幕制服av| 天堂中文最新版在线下载| 免费大片18禁| 国产精品麻豆人妻色哟哟久久| 亚洲美女视频黄频| 亚洲国产成人一精品久久久| 日产精品乱码卡一卡2卡三| 午夜激情av网站| 高清黄色对白视频在线免费看| 久久国产亚洲av麻豆专区| 91精品三级在线观看| 欧美少妇被猛烈插入视频| 国产色婷婷99| 久久久久视频综合| 国产日韩欧美亚洲二区| 欧美人与性动交α欧美精品济南到 | 人妻系列 视频| 男的添女的下面高潮视频| 日韩亚洲欧美综合| 熟女电影av网| 一区二区三区乱码不卡18| 日本免费在线观看一区| 国模一区二区三区四区视频| 女人久久www免费人成看片| 国产成人精品婷婷| 亚洲欧美成人精品一区二区| 啦啦啦视频在线资源免费观看| 亚洲经典国产精华液单| a级毛片黄视频| 亚洲美女黄色视频免费看| 亚洲美女搞黄在线观看| 国产男女内射视频| 久久久久久久久久成人| 午夜福利影视在线免费观看| 久久久亚洲精品成人影院| 如何舔出高潮| 国产成人91sexporn| 久久久久久久亚洲中文字幕| 亚洲欧洲精品一区二区精品久久久 | 免费高清在线观看视频在线观看| 自线自在国产av| 777米奇影视久久| 蜜桃在线观看..| 毛片一级片免费看久久久久| 满18在线观看网站| 在线观看www视频免费| 搡女人真爽免费视频火全软件| 日韩视频在线欧美| 亚洲精品久久午夜乱码| 全区人妻精品视频| 国产视频内射| 极品人妻少妇av视频| 一区二区日韩欧美中文字幕 | 亚洲精品日本国产第一区| 国产精品成人在线| 亚洲少妇的诱惑av| av天堂久久9| 国产精品一区二区在线观看99| 老司机影院成人| 国产精品一国产av| 老女人水多毛片| 制服人妻中文乱码| 插逼视频在线观看| 99热6这里只有精品| 久久精品国产自在天天线| 日本欧美视频一区| xxxhd国产人妻xxx| 精品熟女少妇av免费看| 99热这里只有是精品在线观看| 极品人妻少妇av视频| 青青草视频在线视频观看| 免费看光身美女| 大香蕉97超碰在线| 久久久亚洲精品成人影院| 久久精品夜色国产| 黄片无遮挡物在线观看| 国产一区有黄有色的免费视频| 黑人欧美特级aaaaaa片| 欧美日韩一区二区视频在线观看视频在线| 狂野欧美白嫩少妇大欣赏| 亚洲欧洲国产日韩| 亚洲人成网站在线观看播放| 一区二区三区免费毛片| 国产精品国产三级专区第一集| 熟妇人妻不卡中文字幕| 一级毛片电影观看| 成人亚洲欧美一区二区av| 久久精品国产亚洲网站| 我的女老师完整版在线观看| 一本一本久久a久久精品综合妖精 国产伦在线观看视频一区 | 看非洲黑人一级黄片| 美女中出高潮动态图| 亚洲,一卡二卡三卡| 人成视频在线观看免费观看| 亚洲国产欧美日韩在线播放| 国产精品人妻久久久影院| 十八禁网站网址无遮挡| 国产片特级美女逼逼视频| 午夜av观看不卡| 国产色婷婷99| 一二三四中文在线观看免费高清| 久久久亚洲精品成人影院| 老熟女久久久| 久久久久人妻精品一区果冻| 99精国产麻豆久久婷婷| 日韩一区二区三区影片| 97超视频在线观看视频| 日本黄色日本黄色录像| 国产精品一区www在线观看| 亚洲欧洲国产日韩| kizo精华| 大香蕉97超碰在线| 最黄视频免费看| 日韩电影二区| 丝袜美足系列| av在线观看视频网站免费| av在线app专区| 国产日韩一区二区三区精品不卡 | 久久国产亚洲av麻豆专区| 亚洲激情五月婷婷啪啪| videos熟女内射| 色视频在线一区二区三区| 成年美女黄网站色视频大全免费 | 久久久久视频综合| 人妻制服诱惑在线中文字幕| 国产精品99久久99久久久不卡 | 免费看不卡的av| 午夜福利影视在线免费观看| 男人操女人黄网站| 午夜免费观看性视频| 国模一区二区三区四区视频| 欧美精品一区二区免费开放| 青春草视频在线免费观看| 亚洲成人手机| 久久99蜜桃精品久久| 欧美97在线视频| 嫩草影院入口| 蜜臀久久99精品久久宅男| 一级爰片在线观看| 91精品国产国语对白视频| 国产极品天堂在线| 免费黄频网站在线观看国产| 久久97久久精品| 成人毛片60女人毛片免费| 韩国高清视频一区二区三区| 少妇丰满av| 五月玫瑰六月丁香| 极品少妇高潮喷水抽搐| 高清av免费在线| 久久av网站| 18禁在线无遮挡免费观看视频| 免费观看av网站的网址| 一级黄片播放器| 中文欧美无线码| 午夜精品国产一区二区电影| 日韩中字成人| 亚洲av在线观看美女高潮| 十分钟在线观看高清视频www| 男人添女人高潮全过程视频| 久久ye,这里只有精品| 99国产精品免费福利视频| 一区二区三区免费毛片| 国产女主播在线喷水免费视频网站| 久久99热6这里只有精品| 少妇人妻 视频| 国产成人精品一,二区| 欧美国产精品一级二级三级| 日本黄色日本黄色录像| 最近手机中文字幕大全| 久久久久久久久久久久大奶| 美女视频免费永久观看网站| 国产精品国产三级国产av玫瑰|