| Forest multifunctionality is the ability of forest ecosystem to provide multiple functions simultaneously,which reflects the important value of forest and has been widely concerned in the field of ecology.It involves many types of functions,a wide range of influencing factors,and complex interactions between functions,and it is a current research hotspot.Most of the studies have based on grassland ecosystems and temperate forests with flat topography.However,the vast subtropical forests in China have complex topography,and the spatial variation of their multifunctionality is not clear at present.Different components of plant diversity(species diversity,functional trait diversity,and structural diversity)are important factors affecting forest multifunctionality,but their relative contributions compared with environmental factors such as topography and soil to forest multifunctionality are still controversial in the study of different regions.More importantly,trade-offs or synergies between functions are the key to the realization of high forest multifunctionality,and are also directly related to the sustainability of forest.Exploring how environmental factors(topography and soil)and plant diversity affect the relationships between functions will help us further understand the influencing mechanisms of forest multifunctionality,and provide theoretical basis for the sustainable management of forests.This study was conducted on a 20 ha evergreen broad-leaved forest in Tiantong,Zhejiang.Based on the survey data(topography,soil,vegetation and litter data)of 166 quadrats in it,we selected eight forest functions: aboveground biomass(AGB),productivity(Pro),growth of saplings(Gro),recruitment of saplings(Rec),soil nitrogen stock(SNstock),soil phosphorus stock(SPstock)soil organic carbon stock(SOCstock),and average annual litter production(Lit).Then we calculate forest multifunctionality index by using the averaging method(averaged forest multifunctionality,FMA)and threshold method(forest multifunction at 50% threshold level,FMt50)to represent forest multifunctionality.In order to reveal the influencing mechanism of subtropical evergreen broad-leaved forest multifunctionality in Tiantong,firstly,we analyze the spatial variation pattern of forest multifunctionality.Then we discuss the relative impact of environmental factors and plant diversity on forest multifunctionality,and further explore the effects of environmental factors and plant diversity on the trade-off or synergistic relationships between functions.Our paper aims to answer the following scientific questions: 1)What is the spatial variation pattern of subtropical evergreen broad-leaved forest multifunctionality in Tiantong? 2)What are the impacts of environmental factors(topography and soil)and plant diversity on forest multifunctionality? And what is their relative importance? 3)How do environment(topography and soil)and plant diversity affect the relationships between functions?The major results and conclusions are outlined below:(1)Overall,the multiple single functions and forest multifunctionality had great spatial heterogeneity,and varied with topography.The spatial variation of Rec was the largest,and the spatial variation of Lit was the smallest.Higher Pro,higher Rec and higher Gro drove higher FMA in the high elevation,but FMt50 had no significant difference between the high elevation and the low elevation.Compared with the valley and slope,higher Pro,higher Rec and higher SOCstock drove higher forest multifunctionality(FMA and FMt50)in the ridge,which indicated the ridge was “forest multifunctionality hotspot” in Tiantong 20 ha plot.The relationships between functions showed that six pairs were synergistic and seven pairs were trade-off.The relationships between AGB and Pro,AGB and Lit,Pro and Gro,Gro and Rec,SNstock and SPstock,and SNstock and SOCstock were synergistic.On the contrary,the relationships between AGB and Gro,AGB and Rec,Pro and both SNstock and SPstock,Gro and both SNstock and SPstock,and Rec and SPstock were trade-off.In addition,compared with the low elevation,the overall synergy degree among functions in the high elevation was significantly lower,which indicated high forest multifunctionality in the high elevation was more difficult to achieve.(2)The effect of plant diversity on forest multifunctionality(FMA and FMt50)was higher than that of topography and soil factors.And the effect of plant diversity on FMA and FMt50 accounted for 54.8% and 73.6% of the total effect,respectively.Among them,the effect of structural diversity on forest multifunctionality(FMA and FMt50)was higher than that of species composition and functional trait diversity.Convexity can indirectly drive the improvement of forest multifunctionality(FMA and FMt50)by affecting soil p H as well as species composition,functional traits diversity and structural diversity.Elevation and humus depth were positively correlated with FMA,while soil p H was negatively correlated with FMA.Large-diameter dominant species combination with lower leaf phosphorus content and the structural diversity characterized by coefficient of DBH variation were favorable for improvement of forest multifunctionality,while high functional trait diversity was unfavorable for its improvement.These results indicated that that environmental filtering,selection effect of species composition and niche complementary effect of structural diversity jointly drive the change of forest multifunctionality.(3)Topography indirectly regulated the trade-offs/synergies between functions mainly by affecting different components of plant diversity(species composition,functional trait diversity and structural diversity).Elevation,slope and convexity significantly changed about 70%(20/28)relationships between functions.The effects of different components of plant diversity on trade-offs and synergies between functions were significantly different and changed about 40%(12/28)relationships between functions.Among them,resource-conservative dominant species combination(high twig density and leaf dry matter content)affected the maintenance of forest multifunctionality mainly through the way or strategy of strengthening the trade-offs between functions.It strengthened the trade-offs between Gro and SPstock,Rec and SPstock,and Rec and Lit,but strengthened the synergy between SPstock and Lit.Largediameter dominant species combination with lower leaf phosphorus content,community-weighted means of maximum potential DBH,and structural diversity represented by the coefficient of DBH variation improved multiple functions simultaneously and strengthened the synergies between functions.Functional trait diversity strengthened the synergy between aboveground biomass and recruitment of saplings by reducing them simulataneously.In addition,corresponding to the results of pattern analysis,topographic factors,species composition,functional trait diversity,and structural diversity jointly drove the trade-offs between Pro and both SNstock and SPstock,Gro and both SNstock and SPstock,and Rec and SPstock.Resourceconservative dominant species combination and lower structural diversity drove a lower overall degree of synergy among functions in the high elevation,which in turn made it more difficult to achieve high ecosystem multifunctionality in the high elevation.In conclusion,this paper reveals the mechanisms of the effects of topography and plant diversity on forest multifunctionality and trade-offs/synergies between functions.Topography indirectly regulated the trade-offs/synergies between functions mainly by affecting different components of plant diversity(species composition,functional trait diversity and structural diversity),and to some extent affects forest multifunctionality,which indicates environmental filtering plays an important role in maintaining forest multifunctionality.Plant diversity also has important effects on forest multifunctionality and trade-offs/synergies between functions,but the effects of different components are significantly different.The selective effect of species composition and the niche complementary effect of structural diversity are particularly important for the maintenance and improvement of forest multifunctionality.This study is helpful for us to understand the maintenance mechanisms of forest multifunctionality more deeply.Under the background of global climate change and biodiversity loss,this study is of guiding significance for the protection,restoration and sustainable management of subtropical forests. |
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