PHOTOSYNTHETIC CARBON FIXATION CAPACITY OF BLACK LOCUST IN RAPID RESPONSE TO PLANTATION THINNING ON THE SEMIARID LOESS PLATEAU IN CHINA

Black locust (Robinia pseudoacacia L.) has been widely planted throughout the semiarid Loess Plateau in China. However, black locust plantations likely induce stand degradation and soil desiccation because of high water consumption, high stand density, and insufficient stand management. In this study, five kinds of density types in 19-year-old black locust plantations, including three intact non-thinned control plantation types with different stand densities (1800, 2700, and 3600 trees ha(-1)) and two thinned-treatment plantation types with the same stand densities (1800 trees ha(-1)) were selected to calculate the responses to thinning intensity in the Maliantan catchment within the central Loess Plateau. Gas exchange, leaf area index, associated changes in the photosynthetic carbon fixation capacity (PCFC), and environmental variables (light radiation, air temperature, and soil moisture) were determined throughout the growing season in 2009. The growth, daily leaf-level net CO2 assimilation rate (An), and tree-level PCFC were significantly different among the three intact black locust plantations in their annual growth period. An and PCFC increased as the stand density decreased because of enhanced environmental resource availabilities per individual tree, including the increased exposure to light radiation, canopy air temperature, and soil moisture level. Comparing thinned and non-thinned black locust plantations, we provided ecophysiological evidence that black locust trees could actively adjust their photosynthetic functions from the beginning of the first growing season after thinning to enhance their height, diameter, and canopy diameter during growth and development. This mechanism indicated that the rapid adaptability of black locust in response to plantation thinning on the semiarid Loess Plateau. Based on the same stand density (1800 trees ha(-1)), the low thinning intensity of 33% was more suitable for the sustainable management and increased of photosynthetic carbon fixation capacities of black locust trees on the Loess Plateau in China. These findings can enrich our knowledge of forest plantation dynamics and provide valuable information regarding sustainable plantation management in this ecological fragile region.