As the Earth's climate warms over the next century, ecosystems throughout the northern hemisphere also will be exposed to elevated rates of atmospheric nitrogen (N) deposition. Understanding this complex environmental change lies at the heart of our ability to anticipate the degree to which forests will sequester human-produced carbon dioxide from the atmosphere. An interdisciplinary team of scientists will investigate the interaction between climate warming and simulated atmospheric N deposition using a long-term, regional-based, field experiment located in sugar maple-dominated forest ecosystems common throughout eastern North America. Over the past 10 years, simulated atmospheric nitrogen deposition at rates expected to occur by 2050 have increased tree growth and slowed the decay of dead leaves and roots, increasing the amount of carbon stored in this wide-spread ecosystem. However, it is uncertain whether carbon storage will stabilize at a higher equilibrium over the long-term as atmospheric nitrogen deposition increases, or whether expected warming will counteract this effect. The proposed research will quantify the amounts of carbon stored in overstory trees, forest floor and soil over the next decade, allowing this team of scientists to test hypotheses regarding the interaction of climate warming and atmospheric nitrogen deposition on ecosystem carbon sequestration.
The results of this project will be disseminated to other global change scientists, K-12 teachers, and the general public through the development of user-friendly, web-based tools. In addition the team of investigators will continue conducting the annual Global Change Teachers Institute. Through lectures and field-based learning, middle and high school teachers develop an understanding of the causes and ecological impacts of global environmental change, thereby bringing this information into science curricula.
As the Earth’s climate changes over the next century, ecosystems in the Northern Hemisphere will be exposed to elevated rates of atmospheric nitrogen deposition. This added nitrogen could cause an increase in forest growth, helping forests to remove more carbon dioxide from the atmosphere than they currently do. However it is not known if a growth enhancement caused by elevated nitrogen deposition will be sustained in the long term and if increased growth might eventually lead to increased mortality, as faster growing trees compete for light, water and other nutrients. To address these questions, ecologists at Michigan Technological University, in collaboration with colleagues at the University of Michigan and the University of Idaho, have conducted a long-term nitrogen addition experiment in sugar maple dominated forests in Michigan. Since 1994, forests at four research locations have annually received experimental nitrogen additions at a rate of 3 grams of nitrogen per square meter. This is 2.5 to 4 times the natural rate of atmospheric nitrogen input and represents a rate of atmospheric nitrogen deposition that may occur in some locations in the future. Tree growth has been enhanced 13% by the nitrogen additions and litter decomposition has slowed, causing a gradual build of organic matter in the forest floor and surface soil. Specific objectives of the most recent five years of research at these study sites, were to: 1) determine if growth enhancement would continue; 2) determine if forests in warmer, more southern locations would respond to a greater degree than those in cooler, more northern locations; 3) determine if faster growth would lead to increased mortality and greater creation of coarse woody debris (dead trees and logs); and 4) determine if coarse woody debris would decay more slowly in the nitrogen addition treatment, leading to more carbon storage in this material. Key findings during the five year period include: 1) a continuation of growth enhancement, with no indication of declines in the positive growth response to elevated nitrogen additions; 2) greater responses to nitrogen additions at the northernmost sites; 3) a tree mortality rate for the nitrogen deposition treatment that is slightly greater, but is proportional to the growth enhancement; and 4) reduced decomposition of the dead trees and logs in the nitrogen addition treatment, leading to an accumulation of organic matter and carbon in this material. These results indicate that the forests, which have now been receiving elevated nitrogen additions for 20 years, are continuing to capture and store more carbon as a result of the extra nitrogen availability. The accumulating carbon is stored in living tree biomass (due to enhanced growth); dead trees and logs (due to proportionally greater mortality and reduced decomposition of dead wood); and the forest floor and soil (due to slower decomposition of dead roots and leaves). These results indicate that elevated atmospheric nitrogen deposition has the potential to cause the forests to produce more wood, become stronger carbon sinks, and help offset a portion of rising atmospheric carbon dioxide concentrations. In addition to these findings, we have documented reduced seedling survival for sugar maple but not for all other species present. This suggests that elevated nitrogen deposition may change the composition of future forests by altering the relative ability of the species present to successfully reproduce. During the project, we have been able to train numerous young students interested in ecological research. At Michigan Tech, these have included four undergraduate students, who performed independent research projects using funds from NSF’s Research Experiences for Undergraduates program, and one Master of Science student. Existing and future data from the project is available to the scientific community and the general public through our project’s web site. We also willingly share archived soil and plant samples with interested scientists. To reach a broader public audience, we host an annual Global Change Teacher Institute at Michigan Tech, which includes a day of discussion and activities regarding chronic nitrogen deposition and acid rain and a field visit to a study site. During the Institute, data from the project is also used during discussion of climatic change impacts on northern forests. The teachers incorporate lessons learned during the Institute into their study plans, helping us reach numerous K-12 students. For local schools, we have occasionally arranged class visits to research sites or visited the classroom to talk about our research and careers in science. Through 2013, the Institute provided 126 teachers with recent knowledge and hands-on experience regarding the physical, chemical, and biological science of global change and its effects on forest and wetland ecosystems. National Content Standards for mathematics; life, earth and physical sciences; technology, and social studies are addressed, and we continually work with the teachers to improve their teaching units and develop hands-on learning activities for their students.
