Forest invasion inside mire caused by human activity has been widely occurred, and it is indispensable to understand the dynamics to conserve the ecosystems appropriately. In the Kushiro mire, an actinorhyzal alder Alnus japonica growing inside mire commonly forms dwarf tussock with variable multi-stemmed architecture throughout vigorous resprouting. To evaluate the effects of internal and external factors on the intra-specific variability in resprouting, we analyzed the soil water chemistry, stem dynamics and nutrient-use traits for the alders at the following two sites with almost similar age after invasion: upper site occasionally flooded, and lower site where floodwater has been seldom supplied due to the artificial bank. Though difference in the nutrient abundance of the soil water was not shown between sites, productivity was smaller in the lower site, where salt concentration was higher. Resprouting was vigorous in the upper site: more stems have been newly recruited and also dead. Na+ concentration in the soil water in spring affected to N and P resorption positively, and of these, only P resorption drove the number of dead stems positively but those newly recruited negatively. Though P resorption was negatively correlated with productivity, insignificant relationship was found with N resorption, as predicted in actinorhizal plants. Overall, nutrient availability controlled by salinity in the soil water drives the intra-specific variability in resprouting dynamics through the plasticity in nutrient resorption, rather than the nutrient abundance in the habitats.