Spatial pattern of paulownia witches' broom and its effects on the growth of Paulownia elongata in Shaanxi Province, China

The spatial pattern of paulownia witches' broom was investigated using an Iwao's (m^*-m) model in Shaanxi Province, China. Iwao's (m^*-m) model revealed that spatial pattern of paulownia witches' broom exhibited a uniform distribution in sample plots. In this model, characteristics of contagiousness coefficient (α) justified the probability of infection by virus-transmission vectors. For tree age less than six, susceptible individuals were aggregated as clusters in the plots and then mutually repelled over age. A preliminary survey of stand volume show that from age five to six, stand volumes of disease grade Ⅱ showed excessive vegetative growth and returned to normal level between ages ten and 11. Loss rate of volume increment with disease grade Ⅱ decreased, and then returned to raising trend both at grades Ⅲ and IV. This phenomenon is possibly caused by an over-compensation effect and by environmental factors. The redundancy of growth hormones in branches, shoots, leaves, buds and roots increased the stem volume of entire trees.