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.
Industrial and transportation activities emit significant quantities of reactive nitrogen (N) into the atmosphere. Much of this nitrogen is later deposited onto temperate forests located downwind from emission sources. Because nitrogen is an essential nutrient, nitrogen deposition has the potential to alter the ecological function of temperate forests and increase ecosystem carbon (C) storage, but the extent of these impacts is unclear. To understand the impact of nitrogen deposition, and its interaction with climate change, we have conducted a long-term collaborative experiment in the north central United States. To simulate potential future increases in atmospheric nitrogen deposition, four sugar maple (Acer saccharum)-dominated northern hardwood study sites have received annual additions of nitrogen (3 g of NO3--N/m2) since 1994. During the first 14 years of this experiment, simulated nitrogen deposition increased forest productivity, while causing soil carbon dioxide (CO2) emissions (respiration) and plant litter decomposition to decline. These responses have increased the amount of carbon stored in trees and the soil. At the beginning of our LTREB award in 2008, we hypothesized that: i) simulated nitrogen deposition would continue to accelerate tree growth, ii) simulated nitrogen deposition would continue to cause surface soil carbon to accumulate at a faster rate; iii) warmer temperatures would accelerate forest productivity across the climatic gradient encompassed by the study sites; and, iv) climate warming would interact with simulated nitrogen deposition to differentially increase ecosystem carbon storage among sites. We tested these hypotheses using established long-term measurements as well as new investigations into the litter decomposition. During the past five years, forest growth has been 12% greater under simulated nitrogen deposition. Since 2006, average annual growth across all sites has been slightly lower. This is due, in part, to the occurrence of several of the driest late summers (July and August) in our years of record, in combination with significant insect defoliation events at two sites. In the absence of such conditions, these forests still have very high annual growth. For example, our southernmost site has not experienced defoliation, had sufficient moisture from 2008 through 2010, and had very high growth during those years. Simulated nitrogen deposition has caused the greatest increase in aboveground tree biomass at the coolest, northernmost site. Our long-term measurements of forest nitrogen cycling reveal that the higher levels of ambient nitrogen deposition present in the more southern sites have increased nitrogen availability, so the weaker response to simulated nitrogen deposition could be evidence that ambient nitrogen deposition has affected these sites. For soil carbon accumulation, our most recent (2009) inventory of soil carbon was consistent with the greater soil carbon storage we documented in 2004. However, it was unclear whether increased soil carbon storage resulted from slower decomposition or a reduced extent of decomposition. To determine which of these processes was important, we conducted model simulations based on both scenarios. These simulations indicated that a reduced extent of litter decay was responsible for the accumulation soil carbon under simulated nitrogen deposition. The decrease in litter decay under simulated nitrogen deposition has also thickened the forest floor, which has apparently increased the mortality of sugar maple seedlings and caused a shift in the composition of the tree seedling community. In the long-term, this may negatively impact sugar maple regeneration and favor the establishment of other species. In addition to investigating simulated nitrogen deposition, we have been able to use these study sites to understand how decadal changes in air pollution have affected forest chemistry. In response to changes in emissions regulations, acid and sulfate (SO42-) deposition declined over the past two decades. Consistent with these trends, foliar sulfur (S) concentrations have declined, particularly in the sites that historically received the greatest amounts of S deposition. Foliar S parallels soil S availability, which is influenced by S deposition. This is evidence that pollution controls are reducing the impact of S deposition on forests. During our LTREB grant, our collaborative group has together trained one research scientist, 3 postdoctoral scholars, 7 graduate students, and 4 REU students. Since 2008, 33 peer-reviewed papers have been published from our work at these study sites (see www.webpages.uidaho.edu/nitrogen-gradient/). These works have been featured by The Faculty of 1000 Biology and The Great Lakes Echo, and promoted as a National Science Foundation Discovery. Seventeen datasets from our research are available on our website and some of these data have been contributed to the University of Michigan Biological Station’s database for use in research and education. In addition, the study is visited annually by the Global Change Teachers Institute, which is sponsored by our collaborators at Michigan Technological University. This program exposes middle and high school teachers to information about a number of environmental science topics, including nitrogen deposition and climate change. The Institute also assists teachers in developing curriculum modules that they can use in the classroom.
