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The Michigan Gradient Study: 


Long-term Investigations of Ecosystem Processes in Northern Hardwood Forests



Principal Investigators: 


Kurt S. Pregitzer (lead PI), Ecosystem Science Center, School of Forest Resources & Environmental Science, Michigan Technological University


Donald R. Zak, School of Natural Resources & Environment and Department of Ecology and Evolutionary Biology, University of Michigan


Andrew J. Burton,  Ecosystem Science Center, School of Forest Resources & Environmental Science, Michigan Technological University


Erik A. Lilleskov,  USDA Forest Service, North Central Research Station





The Michigan gradient study was established in 1987 to examine the effects of climate and atmospheric deposition on forest productivity and ecosystem processes in the Great Lakes region.  Four intensively-monitored northern hardwood study sites are located along a 500 km climatic and pollutant gradient extending from southern Lower Michigan to northwestern Upper Michigan.  Primary funding for the study has come from the National Science Foundation (DEB Grants 9221003, 9629842, 0075397, and 0315138) and the USDA Forest Service (Northern Global Change Program and Eastern Hardwoods Research Cooperative of the Forest Response Program). 


The influence of climate on ecosystem processes along the gradient is assessed by determining the effects of annual variation in precipitation and temperature at each site, and then comparing sites to determine if they respond in the same way to climatic variation.  The impact of pollutant deposition along the gradient was addressed by comparing rates of ecosystem processes at sites receiving historically- different levels of pollutants and by examining the effects of experimentally enhanced NO3- deposition at all sites, as described below.  Long-term records of key variables along the climatic-pollutant gradient gave us a unique opportunity to study ecosystem processes under real-world environmental conditions.  The long-term data base includes the following parameters:  forest productivity (basal area growth, height growth, biomass increment, individual tree vigor and mortality); wet deposition, soil solution chemistry, site water balance and leaching losses, soil and air temperature, soil moisture availability, above and below-ground litter inputs, seed production, leaf and fine root nutrient contents, leaf area index, canopy transmittance, insect defoliation, and soil chemical and physical properties.  Some measurements have been continuously recorded since 1987, and in many instances, 16 to 18 years of data exist.


Research designed to investigate the effects chronic N additions on belowground processes was initiated at the sites in 1993.  Three additional 30 x 30 m plots were established at each site, and since 1994 they have annually received NO3- additions of 3 g N m-2.  These are applied in 0.5 g  N m-2 increments at six times during the growing season to simulate chronic atmospheric additions.  Additional measurements made during this phase of the Michigan Gradient Study included soil and root respiration, leaf and root nitrate reductase activity, microbial biomass, N mineralization, nitrification, microbial community composition, and soil solution DOC and DON.