Figure 5. Principal component analysis of fine root traits (means of total soil layers) of the four species along rock fragment content gradient and years of growth. The proportion explained of Axis 1 and Axis 2 are 45.40% and 20.82%, respectively. Solid black lines indicate six fine root functional traits. The confidence circle represents the 95% confidence interval between growth years (a), yellow is 2nd, the green is3rd and the red is 4rd. The confidence circle represents the 95% confidence interval between species (b), the red is A. (Artemisia vestita ), the green is B. (Bauhinia brachycarpa ), the yellow is S. (Sophora davidii ) and the blue is C.(Cotinus szechuanensis ). Abbreviations for root traits are as follow: βF , β of fine root; FRB, fine root biomass; RLD, fine root length density; SRL, specific fine root length; RD, root diameter; RTD, fine root tissue density.