Effects Of Forest Gap On Litterfall Production And Its Elements Return In An Alpine Forest

Forest litter is an important carrier of material cycle and energy conversion in forest ecosystem.It plays an important role in maintaining stability of forest soil fertility,structure and function of forest ecosystem.Litter production and element dynamics are affected by the biological and ecological characteristics of plants.Forest gap is a common form in the alpine forest ecosystem which creates micro environmental differences in the different forest gaps and significantly affects the growth of plant organ,species diversity,community distribution and forest spatial structure.On one hand,the difference between biota and climate under the action of forest gaps will inevitably affect the production process and element return characteristics of forest litter,but this effect is remains unclear and needs to be verified.On the other hand,the alpine forest gaps in western Sichuan are widely distributed.Forest litter production and element return may have a more clear response to forest gap,but their response processes are not clear and need to be studied.At the same time,the western Sichuan alpine forest,which is located on the eastern edge of the Qinghai-Tibet Plateau is an important ecological barrier in the upper reaches of the Yangtze River.Litter production and its nutrient content are the important basis of soil fertility maintenance in the alpine forest ecosystem,especially the litter production and element return under the action of natural forest gap that plays a vital role for the mechanism of soil "self-fertilization" in natural regeneration of alpine forest.However,the current studies mainly focused on the;1)basic characteristics of the alpine forest gap and its influence on litter decomposition,and 2)characteristics of litter production and element return under the influence of forest gap.This can only explain the speed of nutrient input from litter to soil under the action of forest gap but cannot fundamentally explain the production and pattern of nutrient input from litter to soil-which limits the systematic understanding of litter contribution to material cycling processes in the alpine forest.In this study,litter production and dynamic characteristics at the organ composition,species,tree versus shrub layer and community levels were monitored by litter collection method under the action of typical natural forest gap in the alpine forest.In this paper,the characteristicsand contents of 15 litter elements return in three gradient positions under the closed canopy,forest gap edge and gap center,and their responses to forest gap are were determined.The results showed that:(1)The annual total yield of litter in the alpine forest decreased as follows: closed canopy > forest gap > gap center,which are 5407±494,3529±252 and 671±119kg·hm-2·year-1,respectively.The total litter production along the three gradients were significantly different in May but there were no differences between the litter production under the closed canopy and the forest gap in September and October.The litter production was less in the early growing season of the plants compared to the early stage of the growing season-and the effect of the canopy was not understandable.During the ice and snow cover period(December to April),the canopy was closed and the total litter production at the edge of the forest gap is apparently high.The results showed that the distribution pattern of total litter was affected by the gap-but the effect was seasonal.(2)The production of litter components showed that foliar litter and twig litter accounted for majority of the litter and the gradient of each position was 85% and above.The proportion of foliar litter in total litter was the largest under closed canopy and the annual contribution rates of the forest gap and the gap center were 55%,59%,64%,respectively.The litter production of leaves,twig litter,epiphytic litter,and miscellaneous litter were all shown as follows: closed canopy > gap edge > gap center whereas the foliar litter in the closed canopy showed a "single peak" litter dynamic model.The dynamic model of "bimodal" litter is shown under the forest gap and the closed canopy,and the "single peak" dynamic model of the litter branch is shown in the three gradient positions.The results showed that the effect of forest gap on litter dynamic model of litter differed with litter components.(3)The litter production of the different layers was as follows: The litter production of the tree layer was higher than that of shrub layer under the closed canopy,forest gap edge and the gap center,and the proportion of tree/shrub was 9:1,17:3 and 7:3 respectively.The litter production of tree layer displayed the natural distribution pattern: closed canopy > gap edge > gap center.There was no significant difference in litter production at the gap edge and closed canopy-but was higher than that in the gap center.The relativecontribution of litter in the tree layer(>77%)was significantly higher than that in the shrub layer in each month while in the closed canopy,the relative contribution rate of litter in shrub layer on August(53%)was slightly higher than that in the tree layer.The results showed that the production and dynamics of tree and shrub litter were affected by the forest gap and there was a "bimodal" litter model of trees under the canopy and the edge of the canopy,but no obvious litter model in the gap center.The shrubs litter under gap center and gap edge is a "single peak",but the peak time is different from the "bimodal" litter pattern under closed canopy.The study shows that the forest gap has an obvious effect on the vertical spatial structure of the litter source,and the tree layer litter is more susceptible to the influence of the forest gap.(4)The annual litter production of coniferous species is obviously higher than that of broad-leaf species under closed canopy,and the ratio is about 4: 1.At the gap edge,the litter contribution rate of coniferous species and broad-leaf species is similar,and the ratio is about 11:9.While at the gap center,the litter contribution rate of broad-leaf species was higher,and the ratio of coniferous to broad-leaf species was 1:4.The results showed that the relative contribution of coniferous species to litter under closed canopy was higher compared to the other two types of tree species.With the exception of September,all the other months were above 70%,which was significantly higher than the relative contribution of the broad-leaf species in the corresponding periods.From November to May,the relative contribution rate of litter from coniferous species was significantly higher than that of broad-leaf species at the gap edge,but lower in August and September,and there was no significant difference in the relative contribution of the two species in the remaining months.The relative contribution rate of of broad-leaved species tree litter is larger than that of coniferous species.The results showed that the growth of coniferous and broad-leaved species was significantly affected by the gap of alpine forest,and the response of Abies mongolica litter to the gap indicated the variation of the litter of coniferous species.The broad-leaved tree species are more susceptible to environmental factors at the gap center and gap edge.(5)The annual litter production of evergreen species and deciduous species showed different changes in each gradient position.Under closed canopy,the annual litterproduction of evergreen species was significantly higher than that of deciduous species,and the ratio was about 9:1.At the edge of forest gap,the ratio is about 16:9 and the litter contribution rate of deciduous species is higher than that of evergreen species with an evergreen/deciduous species ratio of 3:17.The relative contribution of evergreen species to total litter under closed canopy forest was 80% higher than deciduous species,except September,in which was the same as deciduous species.The relative contribution rate of evergreen species was significantly higher than that of deciduous species in the period of formation,cover and melting of snow,but lower in August and September compared to the other months.The relative contribution rate of deciduous species was larger than that of evergreen species in the forest gap.The results showed that litter yield of different groups of tree species varied greatly in the different positions under the influence of forest gap.The evergreen tree species showed a "bimodal" litter model under the canopy and the edge of the canopy,but there is no clear litter model inside the canopy.The deciduous species show a "bimodal" litter model at gap center,and a "single peak" pattern under the closed canopy and gap edge.The results showed that the effects of alpine forest gap on the litter production of evergreen and deciduous species were different,and the response of litter production of deciduous species to the "edge effect" of forest gap was more clearer.(6)The litter production and contribution of common tree species in were different in the three gradients.The litter contribution of Abies faxoniana to total litter under the closed canopy,gap edge and gap center were;79%,56% and 11% respectively.Indicating that;the dominant species in the alpine forest mainly contributed to the litter under closed canopy and gap edge,but little contribution to forest litter under gap center.However,the litter in the gap center was mainly contributed by the deciduous broad-leaved species,and the largest contribution(46%)was from the Cherry species.The results showed that;effect of forest gap on species diversity was directly influenced by the distribution pattern of litter.(7)There are differences in the average contents of litter elements under the forest gap center,gap edge and the closed canopy.The content of C,P and Al in the gap center was lower than in the gap edge and closed canopy.While the content of N,K and Cu in gap center was higher than in the gap edge and closed canopy.The content of Ca,Mg,Fe,Cr and Cd in gap center was higher than in the other positions Whereas the content of Mn in gap center was lower.There was no significant difference in the content of Zn and Pb between the gap center and gap edge but both are higher than that under the closed canopy.There was no significant difference in Na content of litter species at the three gradient positions.However,the 15 elements were significantly affected by the interaction between the litter time and the forest gap.The dynamic change of stoichiometric ratio was caused by the change of the content of C,N,P and the results showed that the litter in all gradient positions displayed phosphorus limitation.These results illustrates that the response of different element content and stoichiometric ratios to the gap position was dissimilar,but its content dynamics all showed clear response to forest gap.The C/N ratio of foliar litter was showed that under canopy(45.6g·kg-1),canopy(36.0g·kg-1),canopy(27.7g·kg-1).The N/P ratio was not significantly different in three positions(18-23),The response of litter elements content and stoichiometric ratio of the litter between the tree and shrub layer to the action of forest gap of is basically the same.And,indicating that the forest space structure under the action of the forest gap had no significant effect on the content of the elements.The C/P and C/N of deciduous broad-leaved species at the gap edge were significantly lower than those of the evergreen coniferous species,which indicating that deciduous broad-leaved trees at forest gap edge have a faster growth rate and a higher decomposition rate,can effectively promote the material circulation.The litter of broad-leaved tree species at the gap edge can provide more effective P for plant growth,thus promoting material circulation.(8)The annual return of each element of the litterfall in the alpine forest is in the order;C,Ca,N,K,Mg,Fe,Al,Mn,Na,P,Zn,Cr,Cu,Pb,Cd,from high to low in the closed canopy,gap edge and gap center.In the three positions,foliar litter and twig litter contribute 80-90% of C?N?P?K?Ca?Mg,The contribution of shrubs to the return of litter elements in the canopy was significantly higher than that in the other two positions.In the middle and late growing season,the elemental contribution of broad-leaved species on the edge of forest gaps was significantly higher than that of coniferous species.Under closed canopy,the relative contribution rate of evergreen species was above 80%.And evergreen species contribute more than 90% to the return of element C.At the edge of the gap,evergreen,deciduous tree species contribute to the N?P?Ca?K?Mg element return is similar.At the gaps center,deciduous trees contribute more than 80% to each element,and more than 90% of the contribution to the return of the element K.There was no significant difference in the total return amount of K and Zn under the closed canopy.Similarly,the total return amount of other elements were distributed as follows;closed canopy > gap edge>gap center.Although each element has different return dynamics,the overall return amount of litter elements in May September and October were larger than in other months was relatively less in the early period of plant growing season,which was the same as the dynamic pattern of litter production.The overall results showed that the return of litter elements was closely related to the litter production and the elements content because the response of litter production and litter elements to the gradient positions of forest gap were different and the response of element return amount to forest gap was affected by litter production and litter elements.In summary,forest gaps in the alpine forests have an important impact on litter production and element return.The litter production in the gap center is significantly lower than that in gap edge and under the closed canopy and the contribution rate of the foliar litter to total litter in the gap center is significantly higher than that of the other two positions.With decreased distance from the forest gap,the litter contribution rate of shrub,broad-leaf species tree species and deciduous species increased gradually.The litter in the canopy was mainly contributed by the deciduous and broad-leaved species.The influence of forest gaps on species diversity is directly related to the distribution pattern of the litter.The 15 elements were significantly affected by the interaction between litter time and forest gap which indicated that;although the response of different element content to the position of forest gap is different,the dynamic response of the element content is clear.The production and the element content of litter affect the return of litter elements and litter production is the main factor.The spatial and temporal distribution pattern of forest litter and its elements in plant tree species organs,components,species,synusia and community level were studied in this study.The production process,the distribution pattern and the accumulation of forest litter were linked to forest gap.Conversely the production,distribution and return of litter element in the inference of typical natural forest gap weresystematically reflected and provides a clear understanding of material in the litter types of the alpine forest.These results generates new insights in the integrated study of forest gap ecology on litter production and element return,and can serve as baseline for future research on the;1)decomposition of mixed litter and forest regeneration in the later stage,and 2)provide basic data for soil fertility conservation of the alpine forest and forest management practice.