The Structure Variation And Multifunctional Effects Of Three Typical Vegetations In The Semi-arid Area Of Liupan Mountains

The change of forest/vegetation management from the single function oriented one to a multiple functions oriented one needs the multifunction management technology on the basis of in-depth description and quantitative analysis of the spatial and temporal variation of vegetation structure characteristics and their effects on the multiple functions at different spatial units.The Loess Plateau is characterized by a harsh environment,such as the severe soil erosion,frequent drought and low precipitation,and sharp conflict between forest and water,thus it is urgent to conduct a multifunctional management based on a harmonious forest-water interrelation.For this reason,the three main vegetaions on typical slopes(Larix principis-rupprechtii plantation,Ostryopsis davidiana shrub,natural grassland)were selected in a semi-arid small watershed of Diediegou,in the semiarid region of the Liupan Mountains of Ningxia,an important water source area of the Loess Plateau.The eco-hydrological processes were monitored during the growing season of 2010-2019.The data of newly surveyed and historically surveyed plots were used to analyze the spatio-temporal variation of vegetation structure characteristics and their environmental responses.The effects of site factors and vegetation structure on the multiple functions(carbon fixation and storage,timber production,plant species diversity protection,water yield,etc.)were quantified.Then,the rational forest density,which considers both the dorminant function of water yield and other functions,was determined at forest stand scale for the sites at different slope aspects,slope positions and for different forest ages.The rational arrangement of the three vegetations on slopes to meet the multifunctional management requirement was also proposed.This work will provide a scientific and technical support for the multifunctional management of forest/vegetation in the semi-arid region of Liupan Mountains.The main results are as below:1.Variation of vegetation structure with main influencing factorsThe natural grassland was distributed on all slope positions,but mainly at the sunny,semi-sunny,and semi-shady slopes,with the constructive species of Artemisia argyi,Carex tristachya,Artemisia sacrorum,Stipa bungcana,Elymus dahuricus,etc..With rising slope aspect(i.e.,the angle of deviation from the north direction),the mean plant height and leaf area index(LAI)decrease,first slowly,then rapidly after the slope aspect exceeds 90° and 120°,respectively;while the coverage is firstly stable,but then decreases slowly after the slope aspect of 110°.With rising soil thickness,the mean plant height and LAI increase first rapidly and then slowly after the soil thickness exceeds 55 and 40 cm,respectively;while the coverage increases firstly rapidly and then tends to be stabilized after the soil thickness exceeds 50 cm.The hazelnut shrub is mainly distributed at the up-slope position of all slope aspects.With rising slope aspect,the means of plant height,coverage,LAI,and leaf biomass are firstly rather stable,and then decrease slowly within the slope aspect range of 80-120°,70-130°,60-110°,and 90-130°,respectively;and thereafter decrease rapidly.With rising soil thickness,the means of plant height,coverage,LAI,and leaf biomass increase first rapidly;then slowly within the soil thickness range of 10-25,10-25,10-30,10-35 cm,respectively;and thereafter tends to be stabilized.The Larch plantation is mainly distributed in the shaded and semi-shaded slopes.With rising tree density,the tree height and DBH are firstly stable,and then decrease slowly when the density exceeds 2000 and 1500 tree/hm2,and therafter decreas rapidly when the density exceede 3000 and 2300 tree/hm2,respectively.The variation of canopy density and canopy LAI with rising density shows an opposite trend,i.e.,first increase rapidly,then increase slowly,and finally stabilized.With rising slope aspect(the angle deviation from the north direction),the tree height,DBH,canopy density and canopy LAI are firstly stable,and then decrease rapidly after the slope aspect exceeds 80°,80°,60°,and 50°,respectively.With rising soil thickness,the tree height,diameter at breast height(DBH),canopy density,canopy LAI all increase firstly,and then gradually tend to be stabilized after the soil thickness exceeds90,80,80,and 100 cm,respectively.Based on comprehensive analysis,coupled models describing the responses of vegetation structure characteristics to multiple major influencing factors were established.2.Variation and management of carbon fixation function of vegetationOnly the vegetation carbon density was investigated for the hazelnut shrub and grassland,and it decreases gradually with rising slope aspect,but increases with rising soil thickness.The order of carbon density in each vertical layer of the larch forests is: root layer(0-100 cm soil layer)> arbor layer > humus layer > understory vegetation layer,in which the ratio of root zone soil layer carbon density was as high as 88.5% of the total carbon density.The total ecosystem carbon density,the carban density of arbor layer and humus layer increase gradually with rising forest age and soil thickness,and increase firstly and then decrease with rising tree density(with the peak value at 3300 tree/hm2),and decrease gradually with rising slope aspect.Due to the interacting effects of multiple factors,there is a weak variation of the carbon density of understory vegetation with the main influencing factors.The carbon density of root zoon soil layer firstly decreases with rising stand density,and then increases gradually within the density range of 500-3300 tree/hm2,and thereafter decreases slowly;the variation with other factors shows the same trend as the carbon density of the arbor layer.3.Variation and management of timber production function of forestsThe stand timber volume and mean single tree timber volume of the larch plantation increase gradually with rising forest age and soil thickness,but decrease gradually with rising slope aspect.With rising stand density,the single tree timber volume decreases gradually,but the stand timber volume increases firstly and then decreases(with the peak value at 3100tree/hm2).Based on the variation of the relative values of the stand timber volume and single tree timber volume with the main influencing factors,the age of the quantitative maturity of the larch plantation was determined to be 27 years,but both the stand timber voluem and the single tree timber volume still increase at the age of 50 years.If implementing the multi-functional and close-to-nature forest management to pursue high-quality timber while taking into account of other functions,the rotation period should be extended to more than 50 years,or the forest should be managed with limited thinning to maintain the continuous cover of forest land.According to the variation of stand timber volume and the mean single tree timber volume with slope aspect and soil thickness,the timber production functional zones were divided,including the optimal timber production sites with a slope aspect(the angle of deviation from the north direction)below 59° and a soil thickness above 152 cm,the suitable timber production sites with a slope aspect in the range of 59-98° and a soil thickness in the range of 80-152 cm,and the unsuitable timber production sites with a slope aspect above 98° and a soil thickness below80 cm.4.Variation and management of shrub/grass or understory vegetationThe number of plant species in the grassland increases firstly and then decreases with rising grassland coverage(the peak value at the coverage of 80%).The number and cover of herbs under the hazelnut shrub decrease near linearly with rising shrub coverage.The response of the growth characteristics of understory vegetation of larch forests to the canopy LAI was evaluated using the upper-boundary line analysis.With rising forest canopy LAI,the number of understory plant species increases first,and then decreases after reaching a peak value of(32species)at the LAI of 3.23;the herb layer coverage is firstly stable around a large value(97%),then decreases gradually when the LAI exceeds 3.39;the shrub layer coverage increases firstly,then reaches a peak value(47%)at the LAI of 2.33,then decreases;the total biomass of herb layer,shrub layer and understory vegetation all shows first stable and then a decreasing trend.After assigning a weight of 0.63,0.26,and 0.11 to the number of species,coverage,and biomass of understory vegetation,respectively,the management range of forest canopy LAI suitable for understory vegetation growth is determined to be 2.65-3.25.However,the understory vegetation management also needs to consider other functions.At special sites or management periods where it is difficult to take into account of the understory vegetation,priority should be given to guarantee the dominant function or relatively important main functions.5.Variation and management of the water yield function of vegetated landsThe variation of water yield in growing season from the forested/vegetated lands in different precipitation years(dry,normal,and wet years)with main influencing factors was analyzed.The water yield from grassland is always positive for all years,and the water yield increases gradually with rising slope aspect,but decreases first rapidly and then slowly with rising soil thickness.The water yield from hazelnut shrub is negative in dry years,but positive in normal and wet years,and the water yield is firstly stable and then increases with rising slope aspect,but firstly decreases and then is stable with rising soil thickness.The water yield from larch forests is negative in dry years,but positive in normal and wet years;the water yield decreases firstly and then increases with rising forest age,gradually decreases with rising tree density and canopy LAI as well as soil thickness,and gradually increases with rising slope aspect,respectively.The water yield is the largest for grassland then for shrubs,and the smallest for forest land.In the study area,ensuring the regional water supply safety should be regarded as the dominant function and the limiting conditions for multi-functional management,but the excessive loss of other functions should be avoided;according to the potentials of various functions and their management requirement at different sites,the rational stand density was determined at the stand scale,and the rational arrangement of vegetation types was determined at the slope scale,thereby to enhance the comprehensive benefits of the services/functions of the forested/vegeted lands.6.Decision-making for multifunctional management of larch forests at stand scaleBased on the variation of all single forest functions studied with the slope aspect,soil thickness,forest age and density,a trade-off decision scheme for the multifunctional(density)management at different sites and in various forest ages was proposed: 1)The first step is to determine the maximum stand density at given forest age and site conditions.Then the density range corresponding to the reasonable canopy density(0.6-0.8),that satisfies the stand stability requirements,is to be determined as the basic density range,but this basic density range can be considered when it cannot be met at special sites or in special management periods;Thirdly,the importance order of all individual functions considered should be determined according to the function supply potential at given sites and the function requirement by regional development.The optimal tre density range for each single function is defined as the function can reach more than 90% of its maximum;2)A certain weight will be given to each function considered according to the importance order of each function.The weighted average of the medians of the optimal tree density ranges of all single functions will be calculated using the given weights,to determine the optimal multifunctional density.However,to increase the flexibility in practice,the optinal multitifunctional density will be extended 15% at both sites to form the multifunctional tree density range;Then,the theoretically calculated multifunctional tree density range should be adjusted appropriately,based on the management principle that the density range of older stands should be less than or equal to that of the younger stands,.Using the above metioned decision scheme,the multifunctional tree density ranges were determined for different forest age segments at some sites as examples,including the poor sites(upper-slope of shady slope and semi-shade slope,the down-slope of semi-sunny slope),moderate sites(middle-slope of shade slope and semi-shade slope),and better sites(down-slope od shady slope and half-shady slope)..7.Rational arrangement modes of different vegetations at slope scaleBased on the variation of individual functions of the three vegetations of larch forest,hazelnut shrub and natural grassland with site conditions and vegetation structure,the best multifunctional arrangement modes of vegetations on four typical slopes(north,east,west,south)were proposed using following scheme: 1)In the first step,based on the analysis of large number of plot data of site conditions and vegetation characteristics,each slope is evenly divided into 6 positions/segments(slope top,upper-slope,mid-up-slope,mid-down-slope,down-slope,slope foot),and the varying soil thickness along slope positions was set up for each slope based on field inventory results;2)In the second step,according to the vegetation distribution roles in the studied small watershed,the possible vegetation arrangement modes were set up at all slope positions of the typical slopes,including 3 modes on the shaded slope and sunny slope where the vegetation distribution is relatively simple,and 11 and 6 possible modes at the semi-shaded slope and the semi-sunny slope where the vegetation distribution is relatively complex;3)In the third step,all the potential vegetation arrangement modes on each typical slope were evaluated using the slope means of each function,and the appropriate vegetation arrangement mode was determined for each typical slope based on the principles of multifunctional forest management at slope scale.On a shady slope(semi-shaded slope)with good site conditions and main functions of carbon sequestration and timber production,the slope top position is suggested to be covered by the hazelnut shrubs,the upper-slope position is suggested to be covered by larch forest(grassland),and the rest slope positions are suggested to be covered by larch forests;on the sunny and semi-sunny slopes with poor site conditions and main function of water yield,the slope positions of slop top and upper-slope are suggested to be covered by the hazelnut shrub,and the other slope positions are suggested to be covered by the natural grassland.