Adaptive Strategies And Responses Of Robinia Pseudoacacia Functional Traits To Environment In Loess Plateau, North Shaanxi, China

The hilly area of the Loess Plateau is one of the most serious soil erosion areas in Chinaand even in the world, which has complex climatic conditions and fragile ecologicalenvironment. Ecological restoration and conservation of water and soil has a long history andrich experiences in this region. So this area is the focus areas of vegetation restorationprograms. From the late1970s to the early1980s, lots of effort have been devoted to theplanting trees and grass in loess hilly area. Robinia pseudoacacia, as a pioneer dominantspecies, has been widely grown. But since the1990s, the degradation of Robiniapseudoacacia plantation on the Loess Plateau has been found in many areas and the shoots ofRobinia pseudoacacia trees were dried-up to different degree. The phenomenon of“small-old-trees” has appeared in many parts of the region. Due to the overuse of the soilwater,“small-old-trees” plantations make it difficulty for understory vegetation regenerate,leading to low ability to conserve water and soil. The main reason for the phenomenon of“small-old-trees” is that Robinia pseudoacacia trees were not adapted to their site conditions.In order to propose effective measures to improve restoration work, this study, based on theachievements of climatic data and Robinia pseudoacacia functional traits collected in14counties (cities, districts) from the center to the northern part of shaanxi province in the LoessPlateau, explore the co-variation of the Robinia pseudoacacia plant functional traits alongenvironmental gradients, and analyze the adaptive strategies of Robinia pseudoacacia torespond to semi-arid and arid environments. The main results are as follows:1) Ten plant functional traits value across all the sampling sties showed large variations,indicating that Robinia pseudoacacia have different performance in different environmentalgradients. Leaf nitrogen content (LNC), root nitrogen content(RNC) and root phosphoruscontent (RPC) have no significant difference among14sampling points, while other8plantfunctional traits showed significant differences across sampling points(P＜0.05). 2) We analyzed the patterns of correlations among9traits and how these traits givedifferent adaptive strategies to environmental conditions. The results showed that thevariations in the traits were closely related. For example, specific leaf area (SLA) and leafthickness(LT), leaf tissue density (LTD) and specific root length (SRL) have significantnegative correlations(P＜0.01), root tissue density (RTD)and specific root length (SRL) hasremarkable negative correlation(P＜0.01). Whereas no correlation existed among leafnitrogen concentrations (LN), root nitrogen concentrations(RN) and other traits.3) Based on the correlation analysis and stepwise regression analysis, we found thatmean annual precipitation had the greatest impact on leaf thickness, specific leaf area, leaftissue density and leaf area. Annual Sunlight ratio had the greatest impact on leaf nitrogencontent, specific root length and root phosphorus content. Mean annual temperatur is the mainmeteorological factor affecting root tissue density. Gray correlation analysis showed that theorder of effects of terrain factors on Robinia pseudoacacia functional traits is altitude> slope>slope> slope position.4) With the increase of latitude and altitude in Loess Plateau of shaanxi, water conditiondeteriorated. In order to maintain normal physiological water demand, the underground partsof Robinia pseudoacacia increased the SRL of fine roots to absorb enough water and nutrients,while the ground parts of it increased LT accordingly to minimize water loss. According toadaptive strategies to drought environment, as tissues or organs absorbing and retaining waterneed to be assigned more materials to build defensive structure. As a result, this affect thematerials input to the growth of Robinia pseudoacacia, decreasing the growth rate and heightof Robinia pseudoacacia. This could be the main cause of “small old trees” in forest of LoessPlateau. In the case of severe drought, Robinia pseudoacacia will even sacrifice some parts,forming withered tip phenomenon. Therefore,"small old trees" and "withered tip”are bothRobinia pseudoacacia’s adaptive performance to drought environment, but this kind ofadaptive performance is beyond the scope of Robinia pseudoacacia normal growth.