Re-growing forests across the North Temperate Zone presently remove about 15% of fossil fuel carbon dioxide (CO2) emissions. It is unclear, however, how long and to what extent these forests will continue to sequester CO2 and thereby offset a major fraction of human-derived CO2 emissions. Recent measurements suggest that, in contrast to the long-held idea that ecosystem CO2 uptake declines to zero over time, forests can continue storing carbon for centuries after establishment. For this to occur, nitrogen cycling patterns and rates must change as tree species composition of the forest shifts during succession. How this occurs is not well understood, however, so this project will investigate mechanisms by which nitrogen cycling influences carbon uptake. The research will be conducted using novel, multi-scale, stable nitrogen isotope approaches in a large-scale, bold, experimental forest setting in northern Michigan where losses of maturing, early successional tree species are being accelerated by stem girdling. Carbon uptake by the experimental forest is being compared to a nearby non-manipulated forest under separate funding. The NSF-supported research here focuses on nitrogen exchanges between late-successional tree species, their fungal symbionts and soils as they control forest carbon balances.
Results will provide a more complete understanding of controls on forest growth across successional stages and will improve predictions of temperate forest CO2 uptake and carbon balances. The study will serve as a resource for 'Kindergarten-to-Gray' environmental education and insights derived from this research will inform decisions of policymakers and resource managers regarding forest carbon sequestration and provision of ecosystem services.
We conducted research in northern Michigan to understand how the availability and recycling of nutrients, with a focus on nitrogen, affects the amount of carbon taken up and stored by forests in the Great Lakes region. These forests are an important part of the global terrestrial carbon sink, meaning that they actively remove carbon dioxide from the atmosphere and sequester it in soils and biomass, thus helping to offset the human-caused climate change resulting from greenhouse gas emessions that has been occuring for the last several decades. Our work used an unusual approach (killing thousands of relatively short-lived trees that have dominated the forest canopy for its first 90 years of development) to trigger advancement of the forest ecosystem to a more mature state, where longer-lived trees predominate. Our most important finding was that the death of the short-lived tree species caused longer-lived trees to increase their uptake of soil nitrogen and to use this extra nitrogen to sustain high rates of carbon sequestration. This result means that as the short-lived trees in similar forests throughout the region die off, the forests will likely remain carbon sinks. The results also indicate that 10-50% of the trees in aspen forests may be removed without decreasing the ability of the forests to act as carbon sinks. Being conducted at the site of other research projects supported by federal agencies besides NSF also meant that our project added value, insight and understanding to other research projects ongoing at our site. Our research provided hands-on training in research to nearly 3 dozen undergraduate and graduate students, was the subject of educational activities, visits and presentations for 6 tribal, student, public and civic groups (reaching >100 people), and was an integral part of activities at our University's field station that support the local economy. Employment sectors including foodservice, facilities maintenance, travel, hardware, and general merchanidse experienced tangible benefits from the conduct of our research in rural northern Michigan. Beyond its immediate benefits to science, society and the economy, our project generated datasets and archived samples, scientific results, and added to the ranks of future scientists. These outcomes represent an investment of effort, education, and economy at the present time that will provide future payoff in ways not presently anticipated. Science is a cumulative enterprise, and while an individual study rarely changes the world by itself within the short-term, the overall scientific enterprise is beneficial for all participants and members of society- even those who remain unaware of its progress.
