| Traditional plantation management,with the sole aim of maximum timber production,has been changing to multiple ecosystem services oriented management.This requires tradeoffs for an optimal and sustainable supply of the competing services from forest ecosystem.Understory vegetation is a decisive component in forest ecosystems,since it can sustain the forest ecosystems stability,influence numerous forest ecosystem services(FES)and other biodiversity components,as well as contribute to the total forest ecosystem carbon density.However,the understory vegetation is often ignored in traditional plantation management.The purpose of this study was to design a tradeoff approach,to determine the proper tree densities at different ages for balancing the competition between overstory tree layer and understory vegetation layers,within the larch(Larix Principis-rupprechetii Mayr)plantation(LP)in Liupan Mountains(LPM),northwestern China.The data of 82 plots of LP were used for field survey in this study.Firstly,the effects of stand structure(tree density,age and canopy density)on the individual services(functions)with varying stand ages were quantified with fitted equations.The services of concern for this study include the overstory timber production,understory vegetation species diversity,overstory and understory vegetation biomass,and the ecosystem carbon density.Secondly,the competing relations between these individual services were analyzed,and the tradeoff principles were determined.Finally,the optimal stand structure index was determined through tradeoffs among the individual services,and the tradeoffs approach for a multifunctional and integrated management of the overstory and understory related services were developed.The main results of the study are following:1.Optimal stand structure for balancing timber production and understory biodiversity:The aim of this study part was to determined the optimal stand structure to balance the competing services of overstory timber production and the conservation of understory vegetation species diversity(expressed by understory vegetation species number/UVSN).It was found that the variation of stand structure parameters of timber volume with age and tree density can be well described by the logistic growth model(LGM)function of age and the power function of tree density.The models were coupled and fit with field data from 82 plots of LP within the studied age range of 12-33 years.The variation of UVSN with canopy density and tree density w ere also quantified with regression analysis.The results showed that with rising tree density,the single tree timber volume decreased but the stand timber volume increased;with rising canopy density,the UVSN increased until its maximum at the canopy density range of 0.6-0.7.then decreased quickly.Keeping the optimal canopy density of around 0.7 was determined as the main principle to balance the timber volume production and UVSN conservation.The fit models and relations were used as decision tools for estimating the proper tree densities at varying ages.2.Optimal stand structure for balancing overstory and understory vegetation biomass:The aim of this study part was to determined the optimal stand structure to balance the competing overstory biomass(OB)and understoiy vegetation biomass(UVB).The effects of stand structure(tree density,age and canopy density)on the OB and UVB were analyzed.It showed that:1)With rising tree density at given age,the single tree biomass decreased,but the total stand tree biomass increased.2)The UVB decreased significantly with rising OB.All these relations were quantified with fit models and regression equation.3)As a key parameter of overstory structure,the canopy density influences both the OB and UVB.An optimal canopy density of 0.7 was suggested to balance the competing OB and UVB.The fit models and relations were used as decision tools for estimating the proper tree densities at varying ages.3.Effect of stand structure on carbon density of the larch plantation ecosystem:The plot mean carbon density(t/ha)contained in tree biomass increased with stand ages,such as 6.20,12.61,29.31,49.06 and 72.29 for the age-classes of 11-15,16-20,21-25,26-30 and 31-35 years,respectively.In contrast,the carbon density(t/ha)contained in understory vegetation decreased with age-classes(e.g.,1 1-15,16-20,21-25,26-30 years),such as for shrubs layer(4.71,4.01,2.93.1.56)and for herbaceous layer(1.15,0.96,0.67,0.59)respectively.The total vegetation carbon density were 12.05,17.58,32.91,51.21 t/ha respectively in each age-class,and increased with tree density and canopy density upto certain threshold of 1811 trees/ha and 0.7 canopy density,thereafter levels off.The carbon density(t/ha)in humus layer increased with stand ages(4.08,6.05,6.66,9.58).The soil carbon density(t/ha)within the soil layer of 0-100 cm firstly increased and then decreased with stand ages(147.95,213.53,203.85,197.26),probably because of a net soil carbon loss as a result of plantation management disturbance compared with the original natural forests.The overstory tree carbon density and canopy density(after 0.7)can affect the humus and soil layers carbon density.Therefore an optimal canopy density of 0.7 was suggested to get maximum carbon density of vegetation,humus and soil layers.However the total ecosystem carbon density of LP increased with stand ages,amounted to the values of 164.08,237.16,243.42 and 258.05 t/ha for the age-classes of 11-15,16-20,21-25,26-30 years,respectively.Our results suggested that stand age,tree density and canopy density are important stand structure parameters(along with site factors)affecting the composition of carbon density in different pools of plantation ecosystem during stand development.A proper tree density under optimal canopy density of 0.7 is required for LP ecosystem to get the maximum carbon density in each carbon pool.4.Tradeoffs approach and integrated management for multiple services:A tradeoff approach was developed in this study with the principles of achieving:(1)possibly higher UVSN with adequate timber production,and(2)possibly balanced OB and UVB.This can be realized by the tree density regulation at different stand ages for keeping the optimal canopy density around 0.7.In case of the larch plantation studied,the optimal tree densities were found to be around 2600,2000,1600,1250,1000 trees/ha for the ages of 15,20,25,30,35 years,respectively.Under the proper tree densities,the overstory timber volume and biomass production decreases with a relatively lower ratio,while the UVSN and UVB increases at a higher ratio,with an improved single tree size and timber quality.The tradeoff approach developed here can be used as a reference for future studies to guide the rehabilitation and multifunctional management of services-degraded plantation in the study region or similar regions. |
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