| Human disturbance factors including over-grazing,mining,and inappropriate agricultural management practices have combined with physical factors repeated drought in the southeastern edge of Mu Us Sandyland in northern part of Shaanxi,Northwest China,to cause serious desertification and vegetation degradation.This landscape-scale degradation has caused a significant decline in the possibility of natural ecosystem recovery.Seedling establishment is the most crucial stage in a restoration project.In the southeastern edge of Mu Us Sandyland,restoration efforts may be largely limited by a deficiency in soil moisture,fluctuations in temperature,high light intensities,low soil fertility,competition,allelopathy,herbivory,and wind.Historically,several restoration techniques have been used in attempts to alleviate these problems.Nevertheless,the use of any of these techniques is restricted by low seedling and plant survival as well as high cost.Thus,a need exists to develop novel,low-cost,and efficient restoration techniques in an effort to re-develop sustainable ecosystems and accelerate vegetation restoration.Interaction effects provided by nurse plants can significantly increase seed germination rates and seedling establishment under their canopies.Can the nurse plant be successfully used in the vegetation restoration in the southeastern edge of Mu Us Sandyland?The facilitation provided by nurse plant was the key of which the theory can be successfully used in the vegetation restoration in this sandyland.The dominant vegetation type was shrub in the southeastern edge of Mu Us Sandy Land.Therefore,this paper chose dominant shrubs Caragana korshinskii,Amorpha firuticosa and Salix psammophyila as nurse plant according to the densities and biology characteristics of shrubs.(1)We assessed the difference of biomass and species richness of all target species in the absence and presence of dominant shrubs at community levels by the field removal of shrub experiment.(2)We chose two-year-old pine seedlings,Pinus tabulaeformis and Pinus sylvestris as target species in the southeastern marginal of Mu Us Sandland.Two pine seedlings independently were planted under the canopies of shrub species and open areas to investigate its survival rates and the leader shoots length within three consecutive growing seasons,to test nurse effect during seedling establishment at population levels.(3)We simulated the different light and water level in the present and removal of shrubs,assessed the difference of growth index(Height and basal diameter)and biomass index(root,stem,leaf and total biomass of Pine seedling),indicate the mechanism of the nurse effect in the southeastern edge of Mu Us Sandy land.The primary aim of the present experiment was to test the ability of shrubs to provide a restoration tool in the southeastern edge of Mu Us Sandy Land.This study will supply the new proof to nurse plant theory,and deepen understanding of plant interaction and its mechanism.Then,they will provide theoretical basis and pratical support for developing the efficient restoration techniques in poor and degenerated environment areas,to accelerate the application of this technique in the vegetation restoration of our country.The following results and conclusion were obtained:Results:(1)The total biomass and species richness with dominant shrubs were significant greater than that without dominant shrubs(P<0.05).Differences were stronger and more significant for total biomass than for species richness.The total biomass under the canopy of legume C.korshinskii and A.fruticosa were significant greater than that under the canopy of S.psammophyila(P<0.05).The species richness under the canopy of native C.korshinskii and A.fruticosa were significant greater than that under the canopy of exotic S.psammophyila(P<0.05).C.korshinskii,A.fruticosa and S.psammophyila had significant positive effects on biomass of legumes,graminoids and forbs plant functional groups(P<0.05).Differences were significant with three functional groups(P<0.05).The nurse effect on biomass of legumes were significant greater than that of graminoids and forbs plant functional groups growing under the canopy of S.psammophyila(P<0.05).(2)The survival rates of P.tabulaeformis seedlings growing under the canopy of the shrub species were higher than those of the seedlings growing in the control area(P<0.05).However,the survival rates of P.sylvestris seedlings were only higher under the canopy of A.fruticosa(P<0.05).The survival rates of P.tabulaeformis and P.sylvestris seedlings were the highest under the canopy of A.fruticosa(P<0.05).The shade-tolerant species,P.tabulaeformis,had a higher seedling survival rate than that of P.sylvestris(P<0.05).The annual survival rates of pine seedlings growing under the canopy of A.fruticosa were higher than those of the seedlings growing in the control area(P<0.05).There was no significant difference in annual survival rates between growing under the canopy of C.korshinskii and S.psammophyila and the control area at second and three growing seasons.There was no significant difference in the leader shoot lengths of P.tabulaeformis seedlings growing different nurse shrubs.The leader shoot lengths of P.sylvestris seedlings were highest under the canopy of A.fruticosa and open(P<0.05).Light intensity and soil moisture as important factors that controlled their leader shoot lengths(P<0.05),but there was no significant correlation between light and leader shoot lengths of P.tabulaeformis seedlings.Increased plant height and canopy area of nurse shrubs might improve the establishment of pine seedlings(P<0.05).S.psammophila also facilitated seedling establishment,but allelopathy led to a high mortality rate for seedlings growing under their canopy(P<0.05).(3)The near-surface light intensity as well as air and soil temperatures were significant lower under the shrubs,which led to higher soil moisture(P<0.05).The present of the dominant shrubs can improve the under-story(0?20 cm)soil organic matter,available potassium,available nitrogen,pH and the electrical conductivity of the soil.Interaction of light and water had effect on the growth and biomass index(P<0.05).In shade conditions,there was no significant difference in growth and biomass of pine seedling in different water condition.In drought condition,there was no significant difference in growth and biomass of pine seedling in different light condition.Conclusion:(1)The dominant nurse shrubs C.korshinskii,A.fruticosa and S.psammophyila significantly increased in the total biomass and species richness of all target species and survival rates of pine seedlings under the their canopy.A noteworthy finding was that the growth of the two pine species was not inhibited when they were planted under the three nurse shrubs.Therefore The existence of dominate shrubs had a nurse effect on plant under their canopy in in the southeastern edge of Mu Us Sandyland.(2)Different shrub type including ecological impact(N-fixing and allelopathy),Morphological character and origin(exotic and native species)had the different magnitude of nurse effect on all target species growing below their canopy.The leguminous shrubs(C.korshinskii and A.fruticosa)showed continuous facilitation of seedling establishment,but the presence of a dense and small shrub,C.korshinskii,and reduced seedling establishment possibly because of the competition for light.The allelopathy shrub(S.psammophyila)greatly inhibited the survival and growth of pine seedlings.Therefore the nurse shrub should choose the N-fixing shrubs,no allelopathy shrubs.The highest and largest canopy area of A.fruticosa led to the largest survival rate of pine seedlings,but had lower positive effect on the plant diversity.The magnitude of shrub facilitation significantly differed among three functional groups which beneath dominant shrubs,while there was strong variation within all species.The magnitude of nurse effect was weaker when nurse and target plant all were legumes plant.These were also the different magnitude of nurse effect on target species within different light adaptation ability.P.tabulaeformis seedlings with larger light adaptation ability should be chose target species.(3)The existence of dominate shrubs had a nurse effect on microenvironment.The most obvious amelioration of microenvironment was light intensity and soil moisture.The content of soil organic matter and available nitrogen came second.The shade alleviate adverse impact of drought,drought alleviate adverse impact of shade.This was the reason of formation of nurse effect.This was why the dominant shrubs had positive effect on the different light adaptation pine seedlings.On the whole,nurse-shrub facilitation can be used as an effective restoration strategy in the southeastern marginal Mu Us sandy land.However,the successful use of shrubs as nurse plants depends on their origin and morphological characteristics and ecological effects;the selection of target species depends on their tolerance of shade and their interaction with nurse plants. |
PDF Full Text Request