Effects Of Forest Gap Size On The Metal Elements Release Dynamics During Minjiang Fir Twig Litter Decomposition

Forest gap is an important driving force of forest nutrient cycle,energy flow and community renewal,and its formation and area change can directly and indirectly affect the litter decomposition process by changing the microclimate in the forest.The majority of nutrients absorbed by forest vegetation growth come from the recycling of decomposed litter.Although the content of metallic elements are low,they are essential for plant growth and have irreplaceable functions.Compared to foliar leaf,twig litter have lower substrate quality,slower degradation rate,and longer turnover time.However,most studies on the release of metal elements in litters are based on the decomposition of litters,and there are few studies on other components of litters(such as branches).Therefore,this study focuses on the dominant tree species Minjiang fir(Abies faxoniana)of the subalpine forest in western Sichuan by the litterbag method.We conducted an eight-year(2012～2020)in situ field decomposition experiment in the three forest gap size classes and in the closed canopy including large gaps(255～290 m~2),medium gaps(153～176 m~2),small gaps(38～46m~2),and the closed canopy(CK).This study was to investigate the effects of forest gap size on the release characteristics of both macro-metal(K,Ca,Na,Mg)and micro-metal(Zn,Cu,Pb,Al,Fe,Mn)during the decomposition of twig litter.The main research results are as follows:(1)There were differences in the micro-environment in different forest gap sizes.With the increase of forest gap size,the average annual temperature and snow cover thickness gradually increased,while the frequency of freeze-thaw cycles gradually decreased.There were significant differences in the average annual temperature,snow cover thickness and freeze-thaw frequency between the forest gap and the closed canopy.(2)During the 8-year decomposition process of twig litter,the contents of major metal elements except K showed a continuous decreasing trend,while the contents of Ca,Na and Mg showed a fluctuating increasing trend.In the first two years of decomposition,compared to the forest gap,litters in the closed canopy had lower K content and higher Ca and Mg contents,but by the end of the experiment,the contents of K,Ca and Na elements in litters were not significantly different from those in the different forest gap areas.In addition,among the three forest gap areas,the contents of K and Na elements were higher in small forest gap.(3)In the trace metals of litters,the macro-metal of Zn,Cu,Pb,Al,Fe and Mn all fluctuated and increased during decomposition,except the content of Cr gradually decreased with decomposition time.After one year of decomposition,the contents of Cr,Zn,Al and Fe in the forest gap were higher than those in the closed canopy,while the contents of Zn,Cr,Pb and Fe in the forest gap were lower than those in the closed canopy after 6 years of decomposition.(4)During the decomposition of twig litter,macro-metal element(K)showed a continuous release mode,and the cumulative release rates were about 94.72%～96.52%.At the 8th year of twig litter decomposition,while elements of Ca and Mg showed a release-accumulation-release mode as a whole.At the initial stage of decomposition,the formation of forest gaps promoted the release of Ca and Mg elements.However,after 5years of decomposition,the macro-metal elements(K,Ca,Na,Mg)in the twig litter of the closed canopy had a higher cumulative release rate than that in the forest gaps.In addition,by the end of the experiment,the cumulative release rates of Na and Ca elements in litters in small forest gap were lower than those in large forest gap and middle forest gap,respectively.(5)During the decomposition of twig litter,trace metal elements fluctuated during the8-year decomposition period.In the first year of decomposition,trace metal elements Cr,Zn,Cu and Pb were released in the forest gap and in the closed canopy.In addition of Al,Fe and Mn were enriched,and the enrichment rate was lower in the closed canopy than in the forest gap.The element enrichment rate increases with the decrease of forest gap area.By the 8th year of decomposition,all trace elements were released.In addition,most trace elements in the closed canopy had a low cumulative release rate.Among different gap areas,the cumulative release rate of other micro-metal elements(Cr,Zn,Cu,Pb,Mn)in litters in small forest gap was significantly lower than that in large forest gap or middle forest gap.(6)The correlation analysis results showed that the snow cover thickness was positively correlated with the contents of Na,Ca,Mg,Cr and Al,and negatively correlated with the release rate of Mg and Zn.The freeze-thaw frequency was negatively correlated with the contents of Ca and Mg,and positively correlated with the release rate of Mg and Al.The release rate of Cu,Pb,Al,Mn was negatively correlated with the mass residual rate.In conclusion,the formation and size change of forest gaps in the subalpine forests of western Sichuan significantly make difference to the characteristics of climate microenvironment in the forest.The formation of forest gaps promoted the release of major elements(Ca and Mg)and trace metal elements(Cu and Pb)in twig litter at the early stage of decomposition(2013～2014),but had no significant promoting effect except for some trace elements(Pb,Zn and Fe)in the later stage(2019～2020).Among the three forest gaps,the larger forest gaps are more conducive to the release of metal elements.The results of this study contribute to the in-depth understanding of the material cycle in the subsurface forest ecosystem and provide scientific data for the sustainable management of subalpine forests.