Forests in Transition
Forests in Transition
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 with time, forests can continue storing carbon for centuries after establishment. Changes in nitrogen cycling patterns and rates are needed, however, to support tree growth and carbon storage as forests shift from dominance by early to late successional tree species. This project investigates mechanisms by which nitrogen cycling limits or enhances carbon uptake using stable nitrogen isotope studies in a 72-acre experimental forest 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 using biometric and atmospheric measurements by collaborators 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 “K-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.
Study Site Overview
The UMBS Forest Ecosystem STudy (UMBS-FEST) occupies >200 ha of mostly forested landscape along the south shore of Douglas Lake, Michigan (Cheboygan County). Within this landscape, ~35 ha of forest has been experimentally manipulated to accelerate a successional change already occurring on the landscape: the senescence of early-successional aspen and birch trees, and their replacement by longer-lived oak, maple, and pine. Aspen and birch were the principal colonizers of extensive cutover and burned lands throughout the upper Great Lakes region about a century ago, and today forests dominated by these species cover >100,000 km2 in the region.
Most of our data collection occurs in permanent plots distributed among reference (unmanipulated) and treatment (accelerated succession) areas of the landscape. Plots in the treatment area are quantitatively paired with reference plots of similar tree species composition, based on a principal components analysis of leaf litterfall. See the map below, which identifies these plot pairs with common symbols.
The central, 1.1 ha plots surrounding the two eddy flux towers accommodate our most intensive measurements. Line power supports an array of electronic equipment continuously measuring air and soil climatic parameters, incoming radiation and other components of the energy budget, and water vapor and CO2 gradients extending from the forest floor up through the forest canopy. The 1.1 ha plots are also home to the most intensive biometric measurements in our studies. Within these two plots, we make measurements of tree biomass production, chemistry, and belowground processes on and adjacent to individual trees, in order to understand fine-scale spatial variation in the processes that scale up to drive stand- and landscape-level ecosystem functions like water use, C storage, and N retention. We replicate many process measurements within and between the spatially extensive 0.08 ha plots to quantify stand-level variation in biogeochemical cycling, and to understand the aggregated effects of forest succession on biogeochemistry across multiple stand types.