Vertical distribution of fine-root area in relation to stand age and environmental factors in black locust (Robinia pseudoacacia) forests of the Chinese Loess Plateau

To examine the spatiotemporal characteristics of the distribution of fine-root area and its relationship with stand age and environmental factors in black locust (Robinia pseudoacacia L.) on the Chinese Loess Plateau, black locust stands were selected at four sites along a precipitation gradient. Four stands of different ages and a transect along the hillslope were also selected at one of the sites. With increasing stand age, fine-root area at the tree level increased exponentially, and the rooting pattern tended to be deeper for trees up to 15 years old and then shallower thereafter. The temporal changes of fine-root distribution could be quantified using stand age and soil nutrients. At the hillslope scale, fine-root area index (FRAI) was lower while the rooting pattern was deeper in the middle slope than in the upper and lower slopes, and the fine-root distribution could be quantified using elevation and soil properties. At the regional scale, FRAI decreased substantially while exhibiting similar rooting patterns with decreasing soil water and nutrient availability along the precipitation gradient. Humidity index represented the regional environmental variation and could be used to quantify FRAI. These findings will be helpful for improving quantification of fine roots and enhancing the accuracy of ecohydrological models.