Studies On Interactions Between Platycladus Orientalis And Prunus Davidiana Mixed Forest In Early Succession Stage In The Semi-arid Naked Mountain

The semi-arid rocky mountains in China near tihe Yellow River reach which has the most severe soil and water erosions.Most of the soils are skeletal and not well-structured,and also have large content of gravels,which mAKes them vulnerable to erosion.Some researchers have proposed that vegetation restoration and construction can play an more important role in controlling soil and water loss.People usually planted seedlings to restoration,but the survival rate is low.Because trees at the seedling stage are physiological and morphological sensitive,and can easily be affected by environment stress.Identifying feasible techniques to restore degraded forests is more topical than ever.Interspecific facilitation play an important role in vegetation restoration,and nurse effect as a forest restoration tool has been widely applied.However,researchers always use the existing trees,shrubs,or even herbs as nurse plants in recovery area,while using uniform fast-growing tree as nurse plant for slow-growing tree.In addition,what would happen to nurse effect with the process of succession if the fast-growing tree can be used as nurse plant.So,in this paper we use it as an opportunity to research the mixed plantation of and Prunus davidiana at the initial succession stage,using the artificial P.orientalis forest afforested in the same year as control.By the field investigation and experimental measurement,this paper discusses the effects on growth performance and chlorophyll fluorescence parameters of P.orientalis,and the root morphological characteristics and distributions of two tree species and the physicochieinical properties of site soil.Also use the Richards model for the growth process of tree height and the Gompertz model for DBH to verify the dynamic changes and mechanisms of nurse effect.The study has some theoretical and practical significance for digging into native trees as nurse plant in degraded region.Main results for this study are as follows:?1?Mixed with P.dovzVfejna had a certain facilitation on the growth of P.orientalis,but the relationship changed with the increasing of stand age,H,DBH and P were bigger in mixed forest than pure forest,and the parameters of mixed 1 were the biggest ten years after planted.It showed that P.davidiana was more conducive to facilitate growth.The simulation of estimating the growth process of P.orientalis showed that the growth rates of H and DBH increased with years.The growth of tree height and DBH kept a comparatively high level within 25 a.The average growth rates of H and DBH showed turning point at 10 and 15 a respectively in mixed 1,while both at 6a in mixed 2 and 10 a of tree height and 6a of DBH in pure forest.The total growth and average growth rate of tree height would be greater in pure forest than in mixed forest at 25 a.From the relationship between growth and light transmittance and the development of the age and canopy structure,sunlight was no longer the only restriction factor,and they began to compete for more space.So we suggested to applying the afforesting practice at 15 a and 6a on mixed 1 and mixed 2,respectively,to accelerate the process of succession.?2?Crown shading could facilitate the photosynthesis.Chla,Chib and Chl?a+b?were highest at mixed 2 and Chla/b was lowest,which ensured P.orientalis could absorb light as far as possible in low light environment and also was high efficient response to low light quantum.Chia/b,Car and Car/Chl?a+b?were highest and Chl?a+b?was lowest at pure forest,which indicated tiiat the capacity of light energy capture and utilization reduced and higher Car played a protective role against excessive light energy in strong light condition.For the chlorophyll fluorescence parameters,the daily mean of the photochemical quenching of effective quantum yield of PSII photochemistry?OPSII?,the maximum quantum efficiency of PSII photochemistry?Fv/Fm?and nonphotochemical quenching?NPQ?had a higher value under storey?Ll,L2?than under full light?L0?,and chlorophyll fluorescence?qP?peAKed at L0 and L2 for OPSII and Fv/Fm,respectively.While the electron transport rate?ETR?and nonphotochemical quenching?NPQ?had a higher value at LI.The orders of diurnal mean values of qP and Y?NO?were pure forest >mixed l>mixed 2,the orders of OPSII,Fv/Fm,FO,Fv/FO and Y?II?were mixed 2> mixed 1> pure forest,and the order of NPQ was mixed l>mixed 2> pure forest.The diurnal curve of qP>OPSII?Fv/Fm?Fv/FO and Y?n?showed 'V' type,ETR?NPQ?FO and Y?NPQ?showed'reverse V9 type,respectively.The daily variation of Y?NO?was smaller.These results indicated that P.orientalis had a strong ability to different light environments.The reversible inactivation of PSII reaction center was caused by high light,but without substantial damage in pure forest.Higher crown shading of P.davidiana inmixed 2 inhibited the dissipation of excess light energy,and caused a decreasing photosynthetic efficiency and aggravated photoinhibition.?3?In the mixed forest,the fine-root RB, SA and RLD of P.orientalis were higher than P.davidiana and SRL was lower,while the coarse-root RB,SA,SRL and RLD were lower than P.davidiana.It is due to different tree species with different root characteristics.From the 2D contour distribution,the fine and coarse roots of P.davidiana concentrated in deeper soil depth than P.orientalis.Fine roots of P.orientalis had relatively wider distribution than P.davidiana on the horizontal plane,and a few fine roots of orz'ew/a/h concentrated at a distance of 40₅0cm from the trunk while P.davidiana almost had no distribution over 40 cm.Coarse roots of P.concentrated at a distance of 3 0-40 cm from the trunk while P.had no distribution over 30 cm.Fine-root RB,SA,SRL and RLD of P.orientalis and P.davidiana increased first and then decreased with the horizontal distance increasing,and the biggest values appeared at a horizontal distance of 10-20 cm.Coarse-root SA>SRL and RLD of P.orientalis decreased with the horizontal distance increasing except RB.For coarse root of P.davidiana,the biggest values of RB and SRL appeared at a horizontal distance of 10-20 cm,and SA and RLD appeared at 0-10 cm.In vertical distribution,fine-root RB,SA,SRL and RLD of P.orientalis decreased with the soil layer increasing,and coarse-root RB and RLD showed the same trend,while the biggest values of SA and SRL appeared at middle soil layer?10-20cm?.For,the biggest values of fine-root RB and SA and coarse-root RB and RLD appeared at middle soil layer,while fine-root SRL and RLD and coarse-root SA and SRL decreased with the soil layer increasing.The root distributions of two tree species are complementary.P.orientalis has a more dominant and explorative fine-root system than P.davidiana.In contrast,P.davidiana has an explorative coarse-root system to explore deeper soil nutrients and water.Above all,roots present lamination distributional type to avoid the excessive competition.?4?The activities of soil urease and invertase were higher in mixed forest than pure forest Also,the soil bulk density and PH of each layer decreased while soil water content,AN,AP,AK,SOC,SOM,CEC,base saturate rate,Ca2+,Mg2+ and K+all increased.Mixed with fast-growing tree could improve soil physical and chemical properties,especially the improvement of the surface soil.?5?Nineteen above-ground indexes including growth and chlorophyll fluorescence parameters were simplified into 3 principal components using principal component analysis,the results showed that Chib,Chl?a+b?,qP and ?PS ? were affected more obviously by facilitation than growth.For the underground indexes,such as soil water content,soil bulk density,PH,AN,AK, CEC,SOC,SOM,K+ and Mg2+ were affected more obviously by facilitation than root traits.