Susy S. Ziegler Julia Rauchfuss University of Minnesota
The Big Woods hardwood forest developed along the prairie-forest border in Minnesota 300 to 650 years ago. The long-standing hypothesis to explain Big Woods' establishment has been that a change to wet conditions reduced fire activity and allowed fire-sensitive species to grow up into a forest. Recent analyses of fossil pollen, charcoal fragments, and lake-level changes, however, suggest that the development of the Big Woods may have coincided with dry conditions. This study seeks to identify the primary climatic conditions that drive establishment of trees at the prairie-forest border in Minnesota. The research will also yield a reconstruction of the disturbance history of several old-growth deciduous forests, and an evaluation of recent methods for estimating the percentage of the canopy disturbed each decade to determine their potential for streamlining classical methods. This project will use the high spatial and temporal resolution inherent to tree rings to complement and enhance recent reconstructions from lake sediments of the ecological history of the Big Woods. To achieve the objectives, the investigators will visit Big Woods remnants and extract increment cores from all trees in several 0.1-hectare plots. Measurements of tree and canopy size will be recorded. The PIs will assign accurate dates to tree rings, and will measure ring widths to calculate the canopy area that was disturbed each decade and to evaluate two methods of disturbance reconstruction (time-series analysis with intervention detection and the absolute-increase method). The investigators will test whether both methods will yield data that are comparable and statistically similar to data from already tested methods; if so, the new methods will increase efficiency compared to the classical ones. This study will support the recent lake-sediment analyses if it shows that trees in the Big Woods established during dry rather than wet periods and that the disturbance history differed throughout the Big Woods.
More complete knowledge of vegetation change in the sensitive grassland-forest transition zone will support conservation efforts in a landscape increasingly affected by humans. This research will provide theoretical insights into vegetation response to past climate change and disturbance. In turn, modelers can use this information to improve predictions of the effects of environmental change on vegetation patterns, which will help society prepare for the future. With climate conditions in Minnesota currently changing to warmer and potentially drier conditions, resolving the debate around whether forests expand during drier or wetter climatic conditions is essential for predicting how vegetation at the prairie-forest border will change as global warming continues. State and local agencies will be able to develop science-based management plans with a clear vision of the natural range of variability of disturbance in this forest type and a firm grasp of the complex climate-disturbance interactions that shape vegetation cover in the Big Woods. Assessment of recently proposed methods to reconstruct the disturbance history, and a comparison between them and classical approaches, will also refine methods of tree-ring analysis. A strong educational component of the project will not only train the graduate investigator, but through field and laboratory work will also engage undergraduates in the research. A parallel educational effort will introduce lifelong learners and K-12 students to new knowledge about the formation and value of the Big Woods, and thus help them appreciate the need to protect undeveloped land.
