Evaluation On Long-term Effects Of Vegetation Restoration By Afforestation With Site-preparation In The Dry Valley Of Minjiang River, Southwestern China

Background:The dry valley is a special kind of mountain ecosystem on the Tibetan Plateau, and discontinuously distributed in the valley of the middle and lower reaches of Jinshajiang River, Nujiang River,Lancangjiang River and Yalongjiang River,the middle reaches of Daduhe River and Yuanjiang River,the upper reaches of Minjiang River and Baishuijiang River.Due to the bad vulnerability of dry valley which has root in its own special geological,climatic and vegetation feature and serious anthropogenic disturbances,the ecosystem is severely degrading in the dry valley.Nowadays,the principal problem of the dry valley is that the vegetation cover is low,the soil and water loss is serious and the ecological degradation is severe.Thus,restoring and rehabilitating the vegetation,heightening the vegetation cover,preventing and controlling the soil and water loss,protecting the fragile mountain ecological environment has become the main work of ecological construction in the dry valley and also the important requirement of the national and regional ecological construction.Therefore,many scientific researches to restore the vegetation and control the ecological degradation were carried out in the past and the method afforestation with site-preparation to restore and rehabilitate the vegetation has long been used in the dry valley.The pattern of vegetation restoration "preserved belts alternating with sylvicultural belts along contours and afforestation in sylvicultural belts after the site-preparation of level trench" was broadly applied into ecological restoration in the dry valley of the middle reaches of Yuanjiang River,the middle and lower reaches of Jinshajiang River and the upper reaches of Minjiang River and also the ecological engineering of China.But due to lack of inspection and evaluation on long-term effects of afforestation with site-preparation,it has not been fully understood that,under the condition of harsh climate and poor soil of the dry valley,whether or not it was a promising measure in promoting vegetation development and improving the ability of water conservation of soil.The evaluation on long-term effects of traditional afforestation facilitates understanding the possibility and capability of restoration of the dry valley and provides the academic support and practical foundation for restoring and rehabilitating the vegetation in the dry valley and protecting the dry valley rationally and effectively.Aim and Methods:We chose the survival and growth of planted trees Cupressus chengiana S. Y.Hu and other trees,vegetation cover and soil water-physical property as the indices of evaluation on long-term effects of afforestation in the dry valley with considerations as follows.Firstly,C. chengiana was long considered as a perfect drought-resistance tree and planted in a large area. Meanwhile,heightening the vegetation cover was one of the most important objectives of vegetation restoration and rehabilitation in the dry valley and the vegetation cover closely correlated with community biodiversity and the ability of water conservation of soil and other ecological services.At last,the soil water was a dominant factor that limited the transformation of potential fertility of soil into actual fertility and affected the growth and distribution of the vegetation.The present study aiming at evaluating ecological effects of afforestation by planting a native trees(C.chengiana ) with site-preparation of contour trench was conducted at 26 sample plots with different planting years(7-24 a) at 10 afforestation demonstration areas in the dry valley of Minjiang River in Maoxian County,Sichuan,Southwestern China.We investigated the survival and growth of planted trees C.chengiana and other trees(conservation rate,height and tree ring growth process,canopy coverage,biomass allocation and fecundity),vegetation coverage(in its totality, tree,shrub,herb,lichen/moss,and litter) and soil physical properties(natural moisture capacity,soil bulk density,saturated moisture capacity,capillary moisture capacity,total porosity,and capillary porosity) in preserved and sylvicultural belts.We also analyzed the impact of the spatial-temporal pattern(altitude,aspect,slope and afforestation time) on vegetation restoration.Important findings:1) There was still a high mortality of planted trees C.chengiana during 7～16 years after afforestation.The percentage of all survival trees gradually decreased with time, and only 38%trees were alive at 16 years after afforestation.2) The tree ring growth of planted trees C.chengiana was analysed by stem analysis with tree ring measurement(WinDENDRO^TM2002b).It was found that,the tree ring growth of planted trees C.chengiana first gradually increased and then gradually decreased in 82%of sample plots,and at 5～15 years after trees C.chengiana were planted the significant decline for tree ring growth had been observed.The long-term trend of tree ring growth was fitted by non-linear trend model including Linear,Logarithmic,Inverse,Quadratic,Cubic,Power,Compound,S,Logistic,Growth and Exponential Model with or without constant.The result was that the Quadratic Model y =b₁x +b₂x² without constant could fit the tree ring growth of planted trees C.chengiana well in 95%of sample plots(p＜0.001).b₁ in the equation ranged from 0.0494 to 1.1323,and b₂ from -0.0019 to -0.1049;"y" was the width of tree ring(mm) and "x" the growth years(a).3) The height growth of age classes was compared by One-Way ANOVA in SPSS 12.0.And it was found that,the height growth of planted trees C.chengiana also first gradually increased and then gradually decreased in 68%of sample plots and at 5～19 years after trees C.chengiana were planted the significant decline for annual height increment had been observed.In addition,in 14% of sample plots,the height growth of planted trees C,chengiana gradually decreased with time.4) By the research on biomass of planted trees C.chengiana,we found that the ratio of underground biomass to aboveground biomass of planted trees C.chengiana varied from 0.12 to 0.29.The fine root(D＜0.5cm) and the leaf,which respectively functioned as nutrient absorber and photosynthesis plant,accounted for a small percentage of total biomass.The stem biomass was over 30%of total biomass and the largest part.The leaf and branch biomass followed.In all kinds of root,the stump biomass was the largest one,but its percentage in total biomass was smaller than percentages of any parts of aboveground.The biomass increment of C.chengiana planted in different time ranged from 0.011 kg to 1.335 kg.5) The ten-year C.chengiana which was planted before 8 years had born fruit in advance.The 1) to 5) results clearly suggested that planted trees C.chengiana were under stress even they could survive and they could not grow well in the present condition of dry valley.So the planted trees C.chengiana had been in the phase of growth decline earlier compared with the natural C.chengiana.C,chengiana was not the ideal tree species that could be applied into the vegetation restoration and afforestation in the dry valley of Minjiang River.6) By the comparison of vegetation covers of sylvicultural belts in sample plots of different afforestation time with One-Way ANOVA in SPSS 12.0,we found that the tree and litter cover increased with time,while the shrub and herb cover decreased;total vegetation cover and lichen/ moss cover did not changed significantly.Compared with those in preserved belts by Independent-Samples T Test in SPSS 12.0,total vegetation cover,shrub cover,herb cover,and lichen/moss cover in sylvicultural belts were significantly lower.Thus,afforestation with the site-preparation of contour trench could not accelerate the vegetation development effectively.7) The water-physical properties of soil in sylvicultural belts were worse than those in preserved belts,which was the result of Independent-Samples T Test by SPSS 12.0.Afforestation with the site-preparation of contour trench could not improve the ability of water conservation of soil effectively.Natural moisture capacity,saturated moisture capacity,capillary moisture capacity, total porosity of sylvicultural belts were lower than those of preserved belts,and soil bulk density and capillary porosity in sylvicultural belts were larger than those in preserved belts.8) The impacts of the spatial-temporal pattern on the survival of planted trees,the growth of planted trees C.chengiana,the vegetation development of preserved belts and sylvicultural belts, the relative decrease ratio of vegetation cover,and the soil quality of preserved belts and sylvicultural belts were analysed by Canonical Correlation Analysis in R 2.9.0 for Windows.It was found that,the percentage of all survival trees and annual diameter increment of planted trees C. chengiana decreased with time.Vegetation and soil quality in preserved belts and sylvicultural belts had been degraded.The vegetation in the sites of high altitude,shade aspect or small slope was good,and the soil quality in the sites of high altitude or sunny aspect was high.Conclusion and Suggestion:We concluded that planting native tree C.chengiana with the site-preparation of contour trench failed to achieve the expectant result for vegetation restoration.In the long term(7-24 a),planted trees C.chengiana could not adapt to the existing soil and climate condition in the dry valley.Its survival percentage was low and gradually decreased with time. Both its tree ring(diameter) growth and height growth declined.And planted trees C.chengiana had born fruit in advance.Meanwhile,afforestation with the site-preparation of contour trench could not effectively accelerate vegetation development and improve the ability of water conservation of soil and even decreased the vegetation cover and worsen the soil quality in sylvicultural belts.Therefore,afforestation with site-preparation could not achieve the expectant aim of vegetation restoration and even induce ecological degradation in the dry valley of Minjiang River.So we suggested that afforestation with site-preparation of contour trench was not a promising measure for ecological restoration.The present status of water resources should be primarily considered in the process of vegetation restoration in the dry valley.Afforestation with intensive site-preparation and planting trees of high transpiration would aggravate the drying of soil and go against vegetation development and tree growth.Considering the competition among plants and the low soil water, anthropogenic intervene should be used carefully and the research on soil water should be further and deeper carried out.In the future,anthropogenic disturbances of intensive site-preparation should be avoided and native shrub and herb should be considered as objective plant after the impact factors and the mechanism of vegetation restoration were understood in the process of vegetation restoration in the dry valley.By accelerating the natural vegetation development,the vegetation cover and the soil quality could be heightened and the good and interactive development of vegetation and soil would be achieved.