The importance of slope aspect and stand age on the photosynthetic carbon fixation capacity of forest: a case study with black locust (Robinia pseudoacacia) plantations on the Loess Plateau

The black locust (Robinia pseudoacacia L.) is an important tree species not only for the vegetation rehabilitation but also for the photosynthetic carbon dynamics on the Loess Plateau. Slope aspect and stand age play important roles in the photosynthesis of the black locusts. To investigate the photosynthetic carbon fixation capacity (PCFC) of the juvenile and mature black locusts located on the sunny and shady slopes, we have analyzed the capacity and daily dynamics of photosynthesis of the whole canopy of juvenile (6-year-old) and mature (18-year-old) black locusts located on the sunny (southeast facing) and shady (northwest facing) slopes. Mature plantations on the sunny slopes have lower average daily E, VPD, CE, A (n), LAI and PCFC than those on the shady slopes. Juvenile plantations have higher average daily g (s), E, C (i)/C (a), CE, A (n) and PCFC compared to the mature plantations. It is concluded that the lower average daily A (n) and PCFC of the mature black locust plantations on the sunny slopes may be due to variations in the microclimatic conditions between sunny and shady slope aspects. The higher average daily A (n) and PCFC of the juvenile black locust plantations are likely associated with stand age-related differences in tree sizes.