| In the semi-arid region of northwest China,the ecological environment is fragile and the water resources is scarce.How to scientifically construct and rationally manage forest and grass vegetation to fulfill its functions of carbon sequestration,water conservation,and species diversity is the key.In this paper,four typical plant communities of Larix principis-rupprechtii(Larch)plantation,Betula platyphylla(Birch)plantation,Hippophae rhamnoides(Seabuckthorn)artificial shrub and natural grassland in the small watershed of Diediegou in Liupan Mountain,Ningxia Hui Autonomous Region were taken as research objects.In the growing seasons of2018 and 2019,28 permanent sample plots were established along typical slopes in the watershed,the characteristics of structure and ecological hydrological processes of these communities were observed.And the functions of carbon sequestration,water conservation and species diversity and its slope position difference for these plant communities were calculated and evaluated by using the level analytic hierarchy process(AHP)-entropy weight method.The results of the study not only have great significance for understanding the multifunctional service value of trees,shrubs and herb communities,but also provide scientific basis for the multifunctional optimal management and sustainable management of artificial vegetation in semi-arid mountainous areas.The main conclusions could be made as follows.(1)Carbon sequestration and its changes with slope of typical plant communitiesThe order of biomass of the four typical plant communities was as follows:Birth plantation(70 t/hm~2)>Larch plantation(51.08 t/hm~2)>Seabuckthorn artificial shrub(35.2t/hm~2)>natural grassland(6.58 t/hm~2).Regardless of arbor forests,shrubs,and herb communities,the distribution ratio of biomass of the upper layer was the largest,accounting for82-86%.The biomass of Larch plantation biomass changes with slope position as middle slope(62.8t/hm~2)>down slope(51.08 t/hm~2)>up slope(40.87t/hm~2),but the change in natural grassland biomass along with slope position follows as:down slope(6.58t/hm~2)>middle slope(4.89t/hm~2)>up slope(4.19 t/hm~2),which may be related to the difference in soil thickness and water conditions with slope position in the forest stand and grassland.The total carbon density of the four typical plant communities was as follows:Larch plantation(316.84 t/hm~2)>Seabuckthorn artificial shrub(292.11 t/hm~2)>Birth plantation(288.17 t/hm~2)>natural grassland(262.68 t/hm~2),consistent with the results of the evaluation of carbon sequestration among communities applying the AHP-entropy weight method,indicateing that the carbon sequestration capacity of Larch plantation forest is the best,while natural grassland communities is relatively poor.The change of total carbon density of Larch plantation with slope position was as follows:down slope(316.84t/hm~2)>middle slope(296.16t/hm~2)>up slope(285.36t/hm~2).The change in total carbon density of natural grassland with slope position was shown as down slope(262.68t/hm~2)>middle slope(221.35t/hm~2)>up slope(183.62t/hm~2),consistent with the results of the evaluation of the community carbon sequestration function with slope position applying the AHP-entropy weight method,which shows that the carbon fixation capacity of the two plant communities both showed a decreasing law with the rise of slope position.The ratios of organic carbon density in the soil layer(0-100cm)to total carbon density were all as high as 88.17-99.05%,indicating that soil is the main carrier of carbon sequestration and the main reason for the difference in carbon sequestration function at different slope positions in these communities.(2)The function of water conservation and its changes with slope of typical plant communitiesCanopy interception rates of the four typical plant communities were followed by Birth plantation(15.97%)>Seabuckthorn artificial shrub(12.85%)>Larch plantation(9.94%)>natural grassland(1.64%).The order of the maximum water holding capacity and effective storage capacity of litter was as follows:Birth plantation>Larch plantation>Seabuckthorn artificial shrub>natural grassland.The soil water storage and permeability of Larch plantation and Birth plantation are better than that of seabuckthorn artificial shrub and natural grassland.Using AHP-entropy weight method to evaluate the water conservation functions of the four plant communities,the results were as follows:Birth plantation(0.593)>Larch plantation(0.383)>Seabuckthorn artificial shrub(0.174)>natrual grassland(0.102),indicating that the arbor forest has the best water conservation capacity,and the broad-leaved forest is superior to the coniferous forest.The changes in canopy interception rate,maximum water holding capacity and effective holding capacity of litter,,soil capillary water holding capacity and soil field holding capacity of the Larch plantation with slope position all show as the middle slope>down slope>up slope,while the soil saturation water holding capacity and the soil infiltration rate were both shown as the changing law that decreases with the rise of slope position.The AHP-entropy weight method was used to evaluate the water conservation function of Larch plantation in different slope positions.The results were as follows:down slope(0.435)>middle slope(0.336)>up slope(0.107),indicating that the water conservation function of Larch plantation is gradually decreasesed with the rise of slope positions.The soil saturated water holding capacity and capillary water holding capacity of natural grassland were both shown as down slope>middle slope>up slope,and the field water holding capacity and soil steady permeability rate were both shown as middle slope>down slope>up slope.The AHP-entropy weight method was used to evaluate the water conservation function of the soil layer of natural grassland in different slope positions.The results were as follows:middle slope(0.635)>down slope(0.269)>up slope(0.118),indicating that the middle slope position of natural grassland has better soil infiltration and water holding capacity,and could effectively reduce surface runoff,so as to achieve optimal performance of water conservation functions.(3)Diversity of herbaceous species and its changes with slope of typical plant communitiesThe Margalef richness index,Simpson dominance index,Shannon-Wiener index and Pielou evenness index of the herbaceous plants in the four plant communities were all showed that natural grassland>Seabuckthorn artificial shrub>Larch plantation>Birth plantation.The AHP-entropy weight method was used to evaluate the species diversity functions of four typical plant communities.The results were as follows:natural grassland(0.090)>Seabuckthorn artificial shrub(0.060)>Larch plantation(0.049)>Birth plantation(0.000),indicateing that the species diversity of herbs in natural grassland was the best,followed by shrubbery,and arbor forest was relatively poor.The Margalef richness index,Simpson dominance index,Shannon-Wiener index and Pielou evenness index of the herbaceous plants in the Larch plantation were all showed a gradual increased with the rise of slope positions.The AHP-entropy weight method was used to evaluate the understory herbaceous species diversity function of Larch plantation in different slope positions.The results were as follows:up slope(0.114)>middle slope(0.035)>down slope(0.000),indicateing that the function of herb diversity in the Larch plantation gradually increases with the rise of slope position.However,in the natural grassland community,in addition to the Pielou evenness index,the Margalef richness index,Simpson dominance index,and Shannon-Wiener index of herbaceous plants all showed a gradual decreased with the rise of slope position.The AHP-entropy weight method was used to evaluate the herb species diversity function of natural grassland at different slope positions.The results were as follows:down slope(0.112)>middle slope(0.056)>up slope(0.000),indicateing that the diversity function of herbaceous plants in natural grassland gradually decrease with with the rise of slope position.The above results indicate that due to the change of canopy density with slope position leads to the change of microenvironment in the forest,which affects the distribution,quantity and growth of understory herbaceous plant species,and further leads to the change law of diversity function of herbaceous species in the Larch plantation with slope position was opposite to that of natural grassland communities.(4)Multifunctional evaluation of typical plant community and its changes with slopeThe AHP-entropy weight method was used to conduct a multifunctional comprehensive evaluation of typical plant communities and its changes with slope.The results were as follows:Larch plantation(0.714)>Birth plantation(0.648)>Seabuckthorn artificial shrub(0.409)>natural grassland(0.233),indicateing that arbor forests were significantly better than shrubs and grasslands in performing multiple service functions,and coniferous forests were better than broad-leaved forests.The multifunctional comprehensive evaluation results of Larch plantation forests at different slope positions were down slope(0.715)>middle slope(0.448)>up slope(0.274),indicating that the comprehensive ability of Larch plantation to perform multiple functions gradually decreasesed with the rise of slope positions.The comprehensive evaluation result of the multifunction of natural grassland in different slope positions was middle slope(0.776)>down slope(0.575)>up slope(0.118),indicating that the natural grassland community has the best ability to perform multiple comprehensive functions in the middle slope position,followes by down slope,and the up slope is the worst. |
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