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.
Long-Term Data Synthesis - Our experiment has provided a unique foundation to understand how forest biogeochemistry has responded to human-derived emissions of N as well as sulfur (SO2). In 1990, the US Clean Air Act was amended to reduce acid deposition originating from human-derived SO2 emissions, but this legislative act only lightly regulated N emissions to the atmosphere. In 1987, we began studying biogeochemical processes in the ambient plots of all four study sites, and since that time, we have a continuous record of elemental leaching and foliar chemistry. We queried our long-term data to understand whether modifications to the Clean Air Act had reduced the impact of SO2 emissions on forest biogeochemistry, and whether ever-present N emissions are leading to a state of N saturation (e.g., anthropogenic N in excess of biological demand, which increases inorganic N leaching). From 1987 to 2008, H+ and SO42- deposition have declined significantly across our region, whereas atmospheric N deposition remains unchanged. Consistent with this trend, the concentration of S in tree leaves has declined, with the greatest reduction occurring in the southern sites, which received the greatest historical amounts of S deposition. Because foliar S concentrations are a reflection of soil S availability, which in turn, is controlled by atmospheric S deposition, our observations provide evidence that legislative controls are reducing the impact of S deposition on forest biogeochemistry. In contrast, quantities of dissolved inorganic N and NO3- leaching from soil have significantly increased over time, reflecting the sustained atmospheric deposition of NO3- to our study sites. This relatively simple set of sustained observations has provided us with a unique ability to determine the effectiveness of public policies on abating the impact of human activity on ecosystem function. Moreover, our data reveal the societal need to extend clean air regulation to anthropogenic N emissions and its subsequent deposition in terrestrial ecosystems, from which society derives many benefits (e.g., clean ground and surface waters). Our long-term measurements of climatic variation and forest productivity on our ambient deposition plots have provided valuable information on the response of northern hardwoods to the initial stages of climatic warming. Over the last 24 years, mean annual temperature has increased significantly at all sites (+1.3 oC) and the length of annual leaf display has increased by 13 days. Through 2005, there also was a positive trend for increasing forest growth over time, but when the more recent data from 2006 through 2011 was added, this trend remains only for our southern sites. This shift corresponded to an increasing frequency of mid- and late-summer soil moisture, compared to the earlier years of the study. The best predictors of forest growth for the study sites are those that combine length of growing season and precipitation or soil moisture, clearly indicating that site water balance will be a key driver of the productivity of these ecosystems under predicted future climates. Our findings in this regard can help forest managers throughout the region anticipate how a warmer climate will influence forest growth and the products it provides to society Outreach & Education The impact of our work also goes beyond the community of scientists and students studying global environmental change. For example, the Global Change Teacher Institute will continue to be offered annually at Michgan Tech. In the most recent Institute, held from June 18–22, 2012, twelve in-service middle and high school teachers participated, bringing the total number Institute attendees since 2004 to 117. Participants are largely from the Great Lakes region, but teachers from locations such as California, New York, Mexico, and Maryland have also attended. The teachers receive an introduction to the most recent global change science and take part in hands-on activities examining the physical, chemical, and biological science of global change and its effects on forest, wetland, and stream ecosystems. The Institute includes a field visit to our Site A and draws heavily on project data during discussions of the effects of N deposition and climatic variability on ecosystem processes. National Content Standards for mathematics; life, earth and physical sciences; technology, and social studies are addressed in the Institute and participants receive credit toward a Master’s degree or recertification. The teachers then apply the knowledge gained in teaching units they develop to fulfill Institute requirements, allowing us to reach a broad K-12 audience with the latest global change science (see http://wupcenter.mtu.edu/education/Global_Climate_Change/ for examples of teaching units and lesson plans developed by participants).
