Effects Of Canopy Gap On Dynamics Of Carbon And Nutrients In The Decomposition Process Of Fallen Log In Subalpine Coniferous Forest

Fallen log is an important part of the forest ecosystem,of which decomposition plays an essential role in maintaining soil fertility,carbon sequestration and nutrient cycling.The formation of fallen log will be accompanied by the emergence of forest gaps.The redistribution of forest gaps on sunlight and precipitation may alter the decomposition environments of fallen log and the structure and funtion of microbial community,and then manipulate the decomposition processes of fallen log.Meanwhile,fallen log provides a suitable habitat for epiphytic plants.Epiphytes indirectly promote the decomposition of fallen log by regulating environmental factors.However,the decomposition of fallen log is a long-term process.Using conventional decomposition experiments of fallen log is difficult to obtain the results of research on the carbon and nutrient release of fallen log in a short period of time,so domestic studies is relatively few.Moreover little attention has been paid to the effects of canopy gap and epiphytic plants on the decomposition processes of fallen log.Therefore,in order to understand the impact of subalpine primary forest gaps and epiphytic plant communities on the release of carbon and nutrients during the decomposition process of fallen log,taking the fallen log in alpine forest Minjiang fir(Abies faxoniana)primary forest in the eastern Tibet Plateau and western Sichuan as the research object and combining decomposition sequences with age series,through the epiphytic plant removal experiments,we studied the effects of canopy gap on the dynamics of carbon and nutrients in the decomposition processes of different structures of fallen log,and the response of fallen log to the removal of epiphytes.The research result shows that the density of the fallen log system and its components decrease gradually as the decay classes increasing.The density of bark changes most rapidly,and the density of sapwood and heartwood have major changes in the later stages of decomposition.The degree of density reduction of each structural component of fallen log is: bark>sapwood>heartwood.The effect of gap position on the density of fallen log increases with the degree of decomposition increasing.There is no significant difference in the density changes of fallen log in canopy gap,canopy edge,and closed canopy in initial decomposition.However,there is a large difference in the density changes of fallen log in the later stages of decomposition.There may be seasonal differences in the effect of canopy gap on the density of fallen log in the later stages of decomposition,but the specific mechanism of effect remains to be further studied.The degree of change in Carbon,Nitrogen and Phosphorus content in the heartwood,sapwood and bark of fallen log are all different.It indicates that the dynamic changes of C,N and P contents of different structural components of fallen log in the decomposition process are different due to the different chemical composition and physical structure of the fallen log.With the increase of decay classes,the Carbon content of each structural component of the fallen log decrease gradually.The Carbon content of different structural components of the fallen log generally shows a trend of increasing from the canopy gap to the closed canopy at different decay classes.In addition,different structures of fallen log have the same response to gap position,which have different physical structure and chemical composition.With the increase of decay classes,the Nitrogen content of each structural component of the fallen log increases gradually.The Phosphrous content of the fallen log is stable in the early stage of decomposition,and slightly increases in the later stage of decomposition.The Nitrogen content in the bark of the fallen log in ?-? is in canopy gap<closed canopy,and the bark in III is in canopy gap>closed canopy.The Phosphorus content in the bark of the fallen log is in canopy gap<closed canopy under all decay classes.The Nitrogen and Phosphorus content of the heartwood and sapwood in fallen log shows a tendency of canopy gap>closed canopy in the I-III decay classes(except the Phosphorus content of the heartwood in ?-?decay classes),and shows a tendency of canopy gap<closed canopy in the ?-? decay classes.Epiphytic plants promote decomposition of fallen log indirectly,and promote the release of Carbon and the accumulation of Nitrogen and Phosphorus in bark of fallen log.The effects of epiphytes on Carbon and nutrient content in bark of fallen log are as follows: closed canopy>canopy gap.The interaction of epiphytic plants and gap positions have a significant effect on the decomposition of bark of fallen log(P < 0.05).In summary,Sub-alpine coniferous forest canopy gap has a significant effect on the decomposition processes of fallen log.What's more,structural components and decay classes significantly affect the dynamic changes of Carbon and nutrients in the decomposition processes of fallen log.Canopy gap promotes the release of Carbon content in fallen log,but the effect on the content of Nitrogen and Phosphorus in fallen log differs with different structural components and decay classees.Epiphytic plants promote decomposition of fallen log indirectly,and promote the release of Carbon and the accumulation of Nitrogen and Phosphorus in bark of fallen log.The effects of epiphytes on Carbon and nutrient content in bark of fallen log are as follows: closed canopy>canopy gap.The above findings provide basic data for fully understanding the important role of fallen log in forest ecosystems and its changing characteristics with the regeneration of canopy gaps,and it also provides an important scientific basis for the sustainable management of forest ecosystems.