Doctoral Dissertation Research: Interactions Among Forest Defoliator Outbreaks, Wildfires, Climatic Variability, and Nitrogen Availability in the Interior Pacific Northwest

伐木、放牧和对野火的抑制导致了太平洋西北内陆地区森林结构和干扰制度的广泛变化，这些生态系统范围的变化改变了氮循环动态。尽管氮在决定生态系统健康和生产力方面起着至关重要的作用，但控制氮有效性的机制尚不完全清楚。特别是，改变干扰制度对氮有效性的影响仍然不确定。本博士论文研究项目将结合树木年轮记录和统计方法重建干扰事件、森林人口统计、气候变异和氮有效性的多世纪记录。树木年轮提供了生长速率、氮同位素组成、干扰影响和林分动态的高分辨率记录。采样将在爱达荷州和蒙大拿州混合针叶林的两个地点进行。博士生将重点回答以下问题：(1)在过去的三个世纪里，干扰制度和森林组成发生了什么变化？(2)气候变率如何影响森林组成和扰动动态？(3)氮素有效性与气候变率、火灾和西部云杉budworm的发生有何关系？(4)扰动制度和森林组成的变化对氮有效性的长期影响是什么？该项目将有助于回答有关气候变化、干扰和人类活动对养分循环动力学的影响的基本问题。从树木年轮的同位素组成中重建氮有效性是一个相当新的研究领域，本项目的结果将有助于推动这一新兴领域的发展。该项目将为昆虫落叶对西部针叶林氮素有效性的影响以及落叶-氮动力学提供新的知识。项目结果将包括美国西部高分辨率氮可用性记录的长时间序列。通过本研究产生的多世纪树木年轮记录将有助于详细评估森林管理政策、气候条件、干扰事件、森林组成和养分循环之间的关系，并将为森林管理实践对森林健康的长期影响提供见解。这些知识将有助于指导面对不断变化的气候条件和土地利用目标的森林管理政策。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立研究生涯。

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.