Data for: Soil moisture, N, P, and forest cover effects on N fixation in alders in the southern boreal forest

Outside of tropical and arid regions, plant productivity is limited by the availability of soil N. Ironically, these are the regions where N-fixing plants are restricted to early successional habitats. The energy cost of N fixation has been used to explain the paucity of N-fixing plants in late successional ecosystems, and models predict reduced light and increased soil N availability will reduce N-fixing plant success. The lack of success of N-fixing plants has also been explained by their greater need for some elements, especially phosphorus. We examined the effect of N and P fertilizer over four years on N fixation in Alnus alnobetula spp. crispa in a mature Pinus banksiana forest in the southern boreal forest, and an adjacent site where wildfire had removed the tree canopy. We also monitored N fixation throughout two growing seasons examined the effect of artificial shading on photosynthesis. The addition of N or P fertilizer resulted in a 60% decrease in plant nodule nitrogenase ac...