Nitrogen availability limits plant growth and carbon storage in many temperate and boreal forests. Biological nitrogen fixation, the conversion of atmospheric nitrogen gas into a biologically useful form, has the capacity to alleviate nitrogen limitation in these ecosystems, yet nitrogen fixing trees are only present in young forests, while nitrogen limitation persists in old forests. Why are nitrogen fixing trees excluded early in temperate and boreal forest development? Recent theory, including a mathematical model developed by the investigators, has revealed a number of possible explanations for this seeming paradox. There is empirical support for some of these explanations, yet few have been tested experimentally. This proposal describes a highly controlled experiment to evaluate four of the key issues highlighted by theory. This experiment will focus the development of new theory and field-based studies, and this combination of approaches will unravel the seeming paradox of sustained nitrogen limitation.
