The Ecological Significance Of Spatial Variation And Environmental Adapattion Of Functional Traits Of Robinia Pseudoacacia In Loess Plateau

Robinia pseudoacacia is a pioneer tree species in loess plateau was because its fast growth, resistance to the arid and update the germination ability and other characteristics, so as to get a massive forestation in the loess plateau. The emergence of large area of little old tree, the reason was that the restriction of region environment or site conditions, and characterized by slow growth, trees top died and undergrowth plant update slowly, etc. It is difficult to produce the ecosystem of function into fully paly because of the stability of Robinia pseudoacacia decline. Therefore, this article analyzed the space variation of functional traits of Robinia pseudoacacia from spatial scale in region and slope scale in basin, and contrast the functional traits between Robinia pseudoacacia and local plant community. To understand the primary reason of the "little old tree" through the variation characteristics of functional traits of Robinia pseudoacacia and correlation of each trait. Results shows that:(1) Different slope aspects and slope positions has different environment in slope scale in loess hilly-gully region, and each functional trait have significant difference between slope aspects and slope positions. The results showed that there is significant difference between the sunny slope and shady slope of nitrogen content of leaf and root, phosphorus content of root, leaf thickness, specific leaf area, leaf tissue density and specific root length(P<0.05). In addition to the phosphorus content of leaf has significant difference(P<0.05) in middle slope and lower slope, other functional traits have no significant difference in slope positions(P>0.05). Adjust the adaptation to external environment by the different degree correlation of each functional trait.(2) Soil nutrient content all shows sunny slope is higher than shady slope, and lower slope has the highest content, and upper slope has the lowest content through the study between environment of slope scale and functional traits of Robinia pseudoacacia. Soil carbon content has a significant effect to phosphorus content of leaf and specific root length(P<0.05), soil phosphorus content only has positive correlation relationship with leaf tissue density(P<0.05). The shady slope owns the highest soil moisture, and the lower slope owns the highest soil moisture. It can be see, the negative phenomenon of growth of Robinia pseudoacacia may be caused by lack of the phosphorus elements and the serious deficit of soil moisture.(3) Loess plateau was divided into deciduous broadleaf forest region, forest steppe and desertification steppes according to the vegetation coverage. On the three vegetation areas, the Robinia pseudoacacia in broadleaf forest region growth very well, but there were different degrees of “small old trees “phenomenon in forest steppe in middle part and desertification steppes in north part. To adapt to the water stress conditions to reduce the moisture loess and maintain nutrients in plant by the thicker leaves, smaller specific leaf area, reduce the leaf blade volume and increase the leaf tissue density because of the influence of the important factors such as drought. Therefore, leaf in Robinia pseudoacacia also has stronger drought tolerance and defense forces in arid environment. In addition, Robinia pseudoacacia increase the absorption of nutrients and moisture efficiency by increase the specific root length to adapt to the environment conditions of drought.(4) Study found that the main soil factors affecting the functional traits of Robinia pseudoacacia was the nitrogen content of soil in layer of 0 to 20 centimeter and carbon content of soil in layer of 0 to 10 centimeter. The main factors affecting the growth of Robinia pseudoacacia was the soil nutrient which rang to fewer rainfall, lower temperature, it is possible that climate changes indirectly affect function traits. Therefore, Robinia pseudoacacia becamed the “small old trees” were the common role of aoil nutrients, soil moisture and the climate factors.(5) The study of difference of Robinia pseudoacacia and local plant community showed that, in the arid environment, Robinia pseudoacacia had the higher leaf and root nutrients, and higher specific leaf area and specific root length, showing a higher competitive ability for soil water and nutrients use. Local plant community had smaller specific leaf area, larger leaf and root tissue density to maintain the balance of water and nutrients, forming adaptive strategies to local environment. However, local plant community had the lower competitiveness for water and nutrients compared with Robinia pseudoacacia. Therefore, we should fully consider the adaptability of species to the environment and effect of the adaptability on species growth, a balance between the species adaptability to environment and their health growth should be maintained in vegetation restoration in the loess hilly-gully region.