Logging, grazing, and the suppression of wildfires have led to widespread changes in forest structure and disturbance regimes in the interior Pacific Northwest, and these ecosystem-wide changes have altered nitrogen cycling dynamics. In spite of the crucial role of nitrogen in determining ecosystem health and productivity, the mechanisms controlling nitrogen availability are not fully understood. In particular, the impact of altered disturbance regimes on nitrogen availability remains uncertain. This doctoral dissertation research project will use a combination of tree-ring records and statistical methods to reconstruct multi-century records of disturbance events, forest demographics, climatic variability, and nitrogen availability. Tree rings provide high resolution records of growth rates, nitrogen isotopic composition, disturbance impacts, and forest stand dynamics. Sampling will be conducted at two sites in mixed-conifer forests in Idaho and Montana. The doctoral student will focus on answering the following questions: (1) What changes in disturbance regimes and forest composition have occurred over the last three centuries? (2) How does climatic variability influence forest composition and disturbance dynamics? (3) What are the relationship among nitrogen availability and climatic variability, fires, and outbreaks of the western spruce budworm? (4) What long-term impacts on nitrogen availability have resulted from changes in disturbance regimes and forest composition?
This project will help to answer fundamental questions regarding the impacts of climatic variability, disturbances, and human activities on nutrient cycling dynamics. Reconstructing nitrogen availability from the isotopic composition of tree rings is a fairly new field of inquiry, and the results of this project will help to advance this emerging field. This project will provide new knowledge about the impacts of insect defoliation on nitrogen availability in coniferous western forests and about defoliation-nitrogen dynamics. Project results will include a long time series of high-resolution nitrogen availability records for the western United States. The multi-century tree-ring records produced through this research will facilitate a detailed assessment of the relationships among forest management policies, climatic conditions, disturbance events, forest composition, and nutrient cycling, and it will provide insights into the long-term effects of forest management practices on forest health. Such knowledge will help guide forest management policies in the face of changing climatic conditions and land-use goals. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.
Logging, grazing, and the suppression of wildfires have led to widespread changes in forest structure, disturbance regimes, and nutrient cycling dynamics in the interior Pacific Northwest. In this project, we used a novel combination of tree-ring records and statistical methods to reconstruct multi-century records of disturbance events and nitrogen availability. Tree rings provide high resolution records of nitrogen isotopic composition, disturbance impacts, and forest stand dynamics. We collected samples from ponderosa pine and Douglas-fir trees at two sites in mixed-conifer forests in western Idaho. This research focused on answering the following questions: 1) What changes in disturbance regimes and forest composition have occurred over the last three centuries? 2) How does climatic variability influence forest composition and disturbance dynamics? 3) What is the relationship between nitrogen availability and climatic variability, fires, and outbreaks of the western spruce budworm? 4) What long-term impacts on nitrogen availability have resulted from changes in disturbance regimes and forest composition? We reconstructed 300+ years of forest disturbances and nutrient availability at each of our sites. We found that western spruce budworm outbreaks tended to occur near the end of droughts, and that fires were most likely to occur during drought years. We found no relationship between prior outbreak activity and subsequent probability of fire occurrence. Average nitrogen availability was consistently lower at our drier, less densely vegetated site. Our preliminary results show no clear relationship between nitrogen availability and disturbances or climatic variability. Our sites did not exhibit the shift in forest composition toward a prevalence of more shade-tolerant tree species common at many other areas in western North American mixed-conifer forests. We found that after the mid- to late-1800s, fires became less frequent or entirely absent, western spruce budworm outbreaks became more frequent and long-lasting, and nitrogen availability gradually declined at both sites. Our results highlight the importance of climatic variability in driving disturbance dynamics and indicate that after the mid-1800s, following Euro-American settlement of the area, disturbance regimes and nutrient availability patterns changed in these forests. This project produced the longest high-resolution nitrogen availability records in existence in western North America. This is the first study in which multiple disturbances and nutrient availability have been reconstructed at the same location in this region.
