Assessing adaptability of planted trees using leaf traits: A case study with Robinia pseudoacacia L. in the Loess Plateau, China

Leaf trait patterns and their variations with climate are interpreted as an adaptive adjustment to environment. This study assessed the adaptability of planted black locust (Robinia pseudoacacia L.) based on the analysis of leaf traits and the comparison of its leaf traits with inter-specific ones existing in the same area. We measured some water and N use related leaf traits: leaf dry mass per unit area (LMA) and N, P and K concentrations based on both leaf area (N (area), P (area) and K (area)) and leaf mass (N (mass), P (mass) and K (mass)) of R. pseudoacacia at 31 sites along a water stress gradient in North Shaanxi Province, China. The results show that leaves of R. pseudoacacia have high N (mass) and low LMA in the study area. High N (mass) and low LMA are usually representative of luxurious resource use, and will advance plant resource competitiveness in high-resource conditions. As a whole, LMA-nutrient relationships of R. pseudoacacia display patterns that are fairly similar to the inter-specific relationships in both direction and intensity. The tendency for LMA and N (area) to increase with decreasing water availability and the positive correlation between LMA and N (area) reflect the trend for R. pseudoacacia to enhance water use efficiency (WUE) at the expense of down-regulated photosynthetic N use efficiency (PNUE) and high construction cost in dry conditions. However, the positive relationship between LMA and N (area) in high mean annual precipitation (MAP) area is either unremarkable or reversed with decreasing water availability. This implies a lower photosynthetic capacity and a higher construction cost for high-LMA leaves. The inter-specific relationship between LMA and N (area) is positive and does not change with water availability. This difference between inter-species and intra-species may be due to more diversified anatomies and more specialised structures for inter-species than intra-species. The failure of R. pseudoacacia adaption to dry conditions reflected by LMA-Narea relationship may be partially responsible for the emergence of rampike and dwarf forms found frequently in dry conditions. Incorporating intrinsic characteristics of planted trees into vegetation restoration project will be instructive and meaningful for species selection.